Nömrög Grassland Fire in Dornod, Mongolia: Satellite Monitoring and Weather

Around noon on 2026-10-08, a fire started near Nömrög (Нөмрөг) in Khalkhgol soum, Dornod Province, Mongolia, about 27 km from the China–Mongolia border. Two geostationary satellites put the ignition between 11:41 and 11:51, and Mongolia’s National Emergency Management Agency (NEMA) received the call at 11:55¹. The Mongolian telecom operator Mobicom is a subsidiary² of Japan’s KDDI. Its statement of 10-12³ says that a vehicle caught fire while a group was handing over telecommunications work at border outposts and the Sümber border crossing and surveying local conditions, and that this started the grassland fire. The statement⁴ of Mongolia’s General Authority for Border Protection says the vehicle caught fire “due to the carelessness of citizens who entered the border zone”. The Dornod Province police department began investigating⁵ the cause of the fire on 10-17; as of 10-19 no findings have been published.

The fire peaked twice, on the evening of 10-10 and during the day on 10-11, and on 10-11 and 10-12 its front reached the border west of Arxan City, Hinggan League, Inner Mongolia. After a cold front passed early on 10-12 the fire weakened. Residual burning continued intermittently inside the Mongolian fire area until the afternoon of 10-15, after which the satellites detected no more fire pixels; NEMA declared the fire fully extinguished at 07:00 on 10-16⁶. The burned area is about 4,900 km². Montsame, quoting the Mongolian emergency services, reports that about 80% of the Nömrög Strictly Protected Area burned⁷.

China deployed first more than 2,000⁸ and then more than 3,400⁹ personnel along the border, and its reports say the fire did not cross the border line. The satellites show no contiguous burned area extending into China, and two burned patches on the Chinese side: about 12 km² in the south section, on the East Ujimqin Banner side, consistent with the backburning operations that the East Ujimqin Banner authorities reported publicly; and about 0.9 km² west of Arxan, which is inferred to be fire that crossed the border by about 1 km and was then stopped by the firebreak that Hinggan League created by counter-burning (对冲点烧, backburning toward the oncoming fire) along the border from 10-11. Smoke reached Hailar on 10-11 and Baicheng on 10-12, and the PM2.5 peaks in the two cities on those days contain smoke from this fire.

This post records the accounts from the parties involved, the ignition time and the spread, the weather at the fire area, the burned area, the assessment of burning near the border, and the smoke transport. The confidence in each conclusion is listed at the end.

Accounts from the parties involved

Satellites can establish when and where the fire started. For what happened before and after ignition, the only sources are the accounts of the parties involved. This section records, in chronological order, the statements issued and the interviews given by Mongolian government bodies, the company Mobicom, and Chinese official sources. Each party is cited only through its own channels or through original reporting on it. As of 10-19, the Mongolian side has not published investigation findings on the cause of the fire.

Reported ignition time and place names

The Xinhua report of 10-11⁸ states that “around 13:00 on October 9, a fire broke out inside Mongolia along the border of East Ujimqin Banner, Xilin Gol League” (10 月 9 日 13 时许，锡林郭勒盟东乌珠穆沁旗边境一线蒙古国境内发生火情). This is the time when fire appeared on this border section, one day after the ignition in the satellite record. From 10-04 to 10-12 there were no fire detections of significant size across the border from East Ujimqin Banner (44–46.5°N, 114–119°E). The border along the south side of this Mongolian protrusion adjoins both East Ujimqin Banner and Hinggan League. By the geoBoundaries county-level boundaries, the Chinese side opposite the southern edge of the fire area belongs to East Ujimqin Banner. The “East Ujimqin Banner border line” (东乌旗边境一线) in the news therefore refers to the southern edge of this fire. The CNR report of 10-12¹⁰ states that “around 13:00 on October 9, a fire broke out inside Mongolia north of Mandubaolige town, East Ujimqin Banner, Xilin Gol League, and the fire line was about 23 km from the East Ujimqin Banner border line” (10 月 9 日 13 时许，锡林郭勒盟东乌珠穆沁旗满都宝力格镇北侧蒙古国境内发生火情，火线离东乌珠穆沁旗边境线约 23 公里). This bearing matches the ignition point determined from the satellites. Note: 23 km differs from the distance of about 27 km between the ignition point and the border given in this post. CNR gives the position of the fire line at 13:00 on 10-09, while this post gives the ignition point on 10-08. The two refer to different times and different features, and this post has not checked them point by point.

The report of the Hinggan League Forestry and Grassland Bureau on 10-16¹¹ is more specific: “at 12:15 on October 8, the Sanguangshan watchtower of the Bailang Forestry Bureau observed a large grassland fire inside Mongolia about 40 km from our league’s border” (10月8日12时15分，白狼林业局三广山望火楼观测发现蒙古国境内距我盟边境约40公里处发生草原大火), and “at 13:00 on October 9, the large Mongolian grassland fire spread on the northwest wind to the border of East Ujimqin Banner, Xilin Gol League” (10月9日13时，蒙古国草原大火借助西北风蔓延至锡林郭勒盟东乌珠穆沁旗边境). The sighting at 12:15 is 24–34 minutes after the ignition in the satellite record. Note: the ignition point in this post is about 27 km from the nearest point of the China–Mongolia border, and the distance to the Hinggan League section has not been calculated separately. In the satellite record, the Mongolian fire front in the south section at 12:44–13:27 on 10-09 was 5–8 km north of the border, which differs from the statement that it “spread to the border”; the report may refer to a different position, and this post has not checked them point by point.

The Mongolian official reports give the same ignition date as the satellite record. The state news agency Montsame on 10-12⁷ quotes the Eastern Regional Emergency Management Department:

Энэ сарын 8-ны өдөр Дорнод аймгийн Халхгол сумын Цогтсүмбэр багийн нутаг, Бичигтийн өндөрүүд гэдэг газарт хээрийн түймэр гарсан юм.

On the 8th of this month, a grassland fire broke out at a place called Бичигтийн өндөрүүд (the Bichigt heights) in the territory of Цогтсүмбэр (Tsogt-Sümber) bag, Khalkhgol soum, Dornod Province.

The earliest Mongolian report found for this post is from 10-11. It can therefore only be said that the ignition date in the Mongolian reports matches the satellites; when the Mongolian side first announced the fire is not known. Other Mongolian reports:

• ikon.mn 10-11¹²:

  …3 өдөр ажилласан ч цурамд оруулж чадаагүй байна.

  …after 3 days of work, it still could not be brought under control.

• The Mongolian National Broadcaster (МҮОНРТ 10-12¹³), relaying NEMA:

  …Халхгол суманд энэ сарын 08-нд гарсан хээрийн гал түймрийг өнөөдөр (2026.10.12) 00:55 цагт цурамд оруулж, бүрэн унтраахаар ажиллаж байна.

  …the grassland fire that broke out in Khalkhgol soum on the 8th of this month was brought under control today (2026.10.12) at 00:55, and work is under way to extinguish it completely.

  Note: the satellites still show a resurgence during the day on 10-12, with 196–305 fire pixels per overpass from 11:45 to 14:35 and the main fire line along the border west of Arxan.

• The Montsame 10-12⁷ headline and article:

  Нөмрөгийн дархан цаазат газрын 80 орчим хувийг түймэр хамарчээ

  Fire has covered about 80% of the Nömrög Strictly Protected Area

  Түймэр баруунаас зүүн тийш 96 км, урдаас хойш 82 км газар нутгийг хамарсан…

  The fire covered an area of 96 km from west to east and 82 km from south to north…

  Note: the bounding box of the burn scar in this post is 118.85–119.92°E, 46.61–47.43°N, about 81 km west–east and about 91 km south–north. The magnitudes agree, but the lengths in the two directions do not fully match the report, which does not say how they were measured.

The Mongolian place name near the ignition point is Нөмрөг (Nömrög), provisionally rendered in Chinese as 努木尔格. Nomonhan (Номон хан), the site of the 1939 Battle of Nomonhan, lies on the lower Khalkh River northwest of the fire area and is a different place from Nömrög.

Border crossing event and entry permits before the fire

The statement of Mongolia’s General Authority for Border Protection (reprinted in full by Eguur.mn on 10-14⁴) says that the commander of border unit 0198, acting on applications and under the relevant provisions of law, issued border-zone entry permits to the border crossing administration, Mobicom management and local residents attending the opening ceremony for the fiber connection at the Sümber border crossing. The statement does not list the names of the 13 people. The relevant passages:

хилийн Сүмбэр боомтыг өндөр хурдны шилэн кабельд холбох арга хэмжээний нээлт

opening of the event to connect the Sümber border crossing to high-speed fiber-optic cable

нийт 13 хүн (5 автомашин)-д 2026 оны 10 дугаар сарын 06-08-ны өдрүүдэд хилийн бүсэд нэвтрэх зөвшөөрлийг

permits for a total of 13 people (5 vehicles) to enter the border zone on October 6–8, 2026

For the date of the event, the statement of the General Authority for Border Protection gives October 6 (“10 дугаар сарын 06-ны өдөр”). State Great Khural member У.Отгонбаяр, who attended the opening event, also gives the morning of the 6th in an interview¹⁴ with the newspaper Зууны мэдээ; the reports by Eguur.mn on 10-12¹⁵ and urug.mn on 10-13¹⁶ give 10-07. According to this member:

Тэр арга хэмжээг Хил хамгаалах ерөнхий газар, аймгийн ЗДТГ зохион байгуулсан.

The event was organized by the General Authority for Border Protection and the provincial Governor’s Office.

Арга хэмжээнд Мобиком, аймгийн ЗДТГ, ХХЕГ бүгд оролцсон.

Mobicom, the provincial Governor’s Office and the General Authority for Border Protection all took part in the event.

Mobicom and KDDI

Mobicom’s statement of 10-12 (reprinted in full by ikon.mn³) says:

Халхголын хилийн отрядын бүс заставыг 4G сүлжээнд холбож, Сүмбэр хилийн боомтод шилэн кабелийн холболт нэвтрүүлсэн ажлыг хүлээлгэн өгөх ажиллагаанд оролцоод, цаашид харилцаа холбооны сүлжээ нэвтрүүлэх боломжийг судлах, газар орны нөхцөл байдалтай танилцах үеэр тээврийн хэрэгсэлд гал гарч, тухайн бүс нутагт хээрийн түймэр гарсан харамсалтай үйл явдал болсон

the group took part in the handover of the work connecting the zone outposts of the Khalkhgol border detachment to the 4G network and bringing a fiber-optic connection to the Sümber border crossing, and afterward, while studying the possibility of extending the telecommunications network further and assessing the local terrain conditions, a vehicle caught fire, and the regrettable incident of a grassland fire breaking out in that area occurred

At 20:58 on 10-14, NTV МЭДЭЭ, the news channel of the Mongolian television station NTV, uploaded a 128-second interview¹⁷ to YouTube. The interviewee’s identity follows NTV’s attribution. The title and the video description:

Мобиком корпорацийн гүйцэтгэх захирал Ясүхиро Очи-оос бид тодруулга авлаа

We sought clarification from Mobicom Corporation CEO Yasuhiro Ochi

Тэрээр Монголын ард түмнээс уучлал хүсэж, тогтоогдсон нөхцөл байдал, холбогдсон хуулийн дагуу учирсан хохирлыг бүрэн барагдуулна гэж мэдэгдлээ.

He apologized to the Mongolian people and stated that the damage caused will be fully compensated in accordance with the established circumstances and the relevant law.

According to the Mongolian legal-entity registry², Mobicom’s shareholder is KDDI Overseas Holdings B.V., which makes it a subsidiary of KDDI. As of 10-19, the press releases, group news and IR announcements in the KDDI newsroom¹⁸ contain no item related to this fire. A search of NHK, Kyodo News, Jiji Press, Nikkei, Asahi Shimbun, Yomiuri Shimbun, Mainichi Shimbun and Sankei Shimbun found no related reports as of 10-19.

Mongolian firefighting and investigation

NEMA’s daily bulletin of 10-12¹:

дуудлагыг 10.08-ны өдрийн 11:55 цагт хүлээн авсан

the call was received at 11:55 on October 8

This is 4–14 minutes after the satellites first detected warming. The containment line at 00:55 on 10-12 and the report that about 80% of the protected area burned are covered above, in the section on the reported ignition time and place names. NEMA’s bulletin of 10-16⁶:

хээрийн түймрийг өнөөдөр (2026.10.16) 07:00 цагт бүрэн унтраалаа

the grassland fire was completely extinguished today (2026.10.16) at 07:00

On the satellites, the last fire pixels of the main fire were at 14:52 on 10-15, which is consistent with this.

Montsame’s description of Nömrög from 2026-05¹⁹:

1992 онд улсын тусгай хамгаалалтад авч, 1995 онд УИХ-ын 26 дугаар тогтоолоор Дархан цаазат газрын ангилалд оруулан баталгаажуулжээ.

Placed under special state protection in 1992 and designated a strictly protected area in 1995 by State Great Khural Resolution No. 26.

587 мянган га талбайг эзлэн оршдог.

It covers an area of 587 thousand hectares.

With 587 thousand hectares, 80% is about 4,700 km², close to the burned area of about 4,900 km² in this post. This post has no protected-area boundary data and has not checked how much of the burned area lies outside the protected area.

On the investigation, Montsame on 10-12²⁰ writes:

Одоогоор түймэр гарсан шалтгаан, нөхцөл … нарийвчлан тогтоохоор ажиллаж байна

Work is currently under way to determine in detail the cause and circumstances of the fire…

The statement of the General Authority for Border Protection⁴ says:

Хилийн бүсэд нэвтэрсэн иргэдийн санамсар болгоомжгүй байдлын улмаас автомашинд гал гарч, хээрийн түймэр болон өргөжсөн

Due to the carelessness of citizens who entered the border zone, a vehicle caught fire, and the fire spread into a grassland fire

хууль хяналтын байгууллагууд албаны шалгалт явуулж байна

Law enforcement agencies are conducting an official investigation

The Dornod Province local outlet nutag.mn reported on 10-17⁵:

Түймрийн шалтгаан нөхцлийг тогтоохоор аймгийн цагдаагийн газрын ажлын хэсэг шалгалтын ажиллагааг эхлүүлээд байгаа аж.

A working group of the provincial police department has reportedly begun an investigation to determine the cause and circumstances of the fire.

As of 10-19, no official investigation findings have been published, and no information has been published on a case being opened or charges being filed.

Chinese border response

Chinese official reports do not mention the name of the protected area or the cause of the fire. Across from the southern edge of the fire area is East Ujimqin Banner, Xilin Gol League, and across from the eastern edge is Arxan City, Hinggan League; the “East Ujimqin Banner border line” (东乌珠穆沁旗边境一线) and the Arxan direction in the Xinhua report of 10-11⁸ refer to the same fire. That report said Inner Mongolia had mobilized more than 2,000 people to guard the border section by section, and that as of 21:00 on 10-11 “the fire has not yet spread into the country” (大火暂未蔓延入境). The Xinhua report of 10-13⁹ said Inner Mongolia had deployed more than 3,400 personnel from professional teams, that “this round of Mongolian grassland fire did not cross the border line” (此轮蒙古国草原火未越过边境线), and that as of 11:00 on 10-13 “intermittent fire lines remained only in parts of the Mongolian side along the border” (边境沿线仅蒙古国一侧局部尚存断续火线). The Inner Mongolia Daily report of 10-13²¹ said of Hinggan League that “as of 14:00 on October 12, the fire from outside the country had been successfully stopped outside the border” (截至 10 月 12 日 14 时，境外火势被成功阻击在边境之外). Su Fengjiao, director of the Hinggan League Emergency Management Bureau, said (Inner Mongolia News 10-13²²): “We immediately launched a Level IV emergency response for forest and grassland fires, and coordinated more than 2,000 personnel from all sides to guard the border section by section and block the fire along the whole line.” (我们第一时间启动森林草原火灾处置四级应急响应，统筹各方力量 2000 余人分段布防，全线堵截。) The Xilin Gol League Emergency Management Bureau (CCTV Finance 10-13²³) stated that “the open flames did not cross the national border and have not yet caused direct losses on our side such as damage to forest and grassland resources or casualties” (明火未越过国境线，尚未造成我方森林草原资源损毁、人员伤亡等直接损失). The People’s Daily app on 10-14 (reposted by gmw.cn)²⁴ said that “this round of Mongolian grassland fire did not cross the border line and caused no casualties on our side” (此轮蒙古国草原火未越过边境线，未造成我人员伤亡).

The report of the Hinggan League Forestry and Grassland Bureau on 10-16¹¹ records the response on the Hinggan League side. From 10-09 it “assembled a professional firefighting force of 530 and stationed it section by section along the 131 km border defense line under its responsibility” (集结530人专业扑火队分段驻守在131公里边境责任防线). At 17:00 on 10-10 “the nearest point of the fire was only 9.2 km from our league’s border” (最近火点距我盟边境仅9.2公里), and “at the peak, 680 personnel were stationed on the front line with 750 on standby” (最高峰实现一线驻防680人、备勤750人). After a sudden wind shift at 12:40 on 10-11, it “used the border patrol road to carry out precise counter-burning in the hundred-meter buffer between the border wire fence and the firebreak, building a flame-retardant barrier about 100 m wide” (依托边境巡逻路，对国境铁丝网至防火隔离带百米缓冲区开展精准对冲点烧作业，构建宽度约100米的阻燃隔离屏障), and “the backburning of the control segments was completed in full: 29 km in the Horqin Right Front Banner sector, 21 km in the Wuchagou sector, 58 km in the Bailang Forestry Bureau sector and 23 km in the Arxan City sector” (科右前旗战区29公里、五岔沟战区21公里、白狼林业局战区58公里、阿尔山市战区23公里防控区段点烧作业全面完成). On 10-13 there were 2 residual smoke points outside the country. “From 22:00 on October 13 to 01:00 on the 14th, the meteorological department carried out artificial rain (snow) enhancement, firing a total of 56 rockets and flying 2 hours 46 minutes of aircraft operations; the operation area received 1–2 mm of snowfall and 1 cm of snow cover” (10月13日22时至14日1时，气象部门开展人工增雨（雪）作业，累计发射火箭弹56枚、飞机作业2小时46分钟，作业区域形成1—2毫米降雪、1厘米积雪). “As of 13:00 on October 15, there was no smoke and no fire at all within visible range inside Mongolia across from the Hinggan League border” (截至10月15日13时，兴安盟边境对面蒙古国境内可视范围彻底无烟无火), and the remaining personnel withdrew. The report also mentions “the league’s 196 km staggered border firebreak” (全盟196 公里错落式边境防火隔离带). Note: the last fire pixels of the main fire in the satellite record are at 13:48 and 14:52 on 10-15, more than 11 km from the border, which does not contradict the statement of no smoke or fire “within visible range” at 13:00.

The CCTV News report of 10-15 (reposted by gmw.cn)²⁵ says: “On October 12, a working group of the Ministry of Emergency Management went to the Inner Mongolia border to guide the fire-blocking work.” (10月12日，应急管理部工作组赴内蒙古边境一线，指导火灾堵截工作。) It continues: “After several days of coordinated operations by multiple rescue forces, more than 330 km of fire from outside the country was successfully blocked, and it did not burn into our country’s territory anywhere along the line.” (经过这几天多方救援力量协同作战，成功堵截境外火330余公里，全线未烧入我国境内。) The methods were “tactics such as patrolling in readiness to suppress, clearing while suppressing, and widening firebreaks, with 5 task areas designated” (以巡待扑、边打边清、拓宽隔离带等战术战法，划分5个任务区域). As of publication, “the fire blocking is under control overall” (火情堵截整体可控). The CCTV.com report of 10-15 22:02²⁶: “As of 12:00 on the 15th, after coordinated efforts by all forces, the grassland fire that broke out inside Mongolia has been successfully blocked at the China–Mongolia border line, and the fire line has not burned into our country’s territory. Within visible range along the border in Xilin Gol League, Hinggan League and Hulunbuir, Inner Mongolia, there is no smoke and no fire.” (截至15日12时，经各方力量协同攻坚，发生在蒙古国境内的草原火灾已成功堵截于中蒙边境一线，火线未烧入我国境内。内蒙古锡林郭勒盟、兴安盟、呼伦贝尔市边境一线可视范围内无烟无火。) “At present, the firefighting forces have shifted to routine patrol and watch, and will withdraw in an orderly manner after 24 hours depending on the situation.” (目前，扑火力量转入常态化巡护看守，24小时后视情况有序撤离。)

Southern Weekly on 10-18²⁷ interviewed Wang Yong (王永), a firefighter with the Arxan City Forestry and Grassland Bureau. On 10-10, “at around 9 p.m., the border crossings on the Chinese and Mongolian sides coordinated to let them through. Wang Yong and about 20 teammates crossed the border and, led by a Mongolian guide, went more than twenty kilometers into Mongolian territory, where they put out a fire line about 5 km long” (晚9时许，中蒙双方口岸协调放行。王永和队友约20人出境，并在蒙古国向导带领下深入其境内二十余公里处，扑灭了约5公里长的火线); on 10-11 “they crossed the border twice to put out flames” (他们两次出境扑灭火焰), and after they returned at night, “the subsequent fire situation at the border was handed over to professional firefighters” (边境后续的火情交由专业消防人员接手).

Ignition and spread

The fire started around noon on 10-08 near 46.88°N, 119.40°E. It was weak that afternoon, flared up again on the night of 10-09, and the fire area elongated northwest–southeast. It peaked twice, on the evening of 10-10 and during the day on 10-11, and on 10-11 its eastern edge reached the border. After the cold front passed early on 10-12 it weakened, then resurged during the day at a lower intensity than on the previous two days. From 10-13 only residual burning remained in the unburned island in the middle of the fire area and along its edge, and the fire went out on the afternoon of 10-15. The table below records the sequence observation by observation.

Ignition time and the early fire

The two high-resolution images closest to ignition both show no fire: Landsat 9 at 10:45 (30 m, with thermal infrared) and Sentinel-2C at 11:15 (10 m). No other polar-orbiting satellite passed over between 11:15 and 12:08. During that interval only two geostationary satellites observed the area: Himawari-9 and GK-2A of the Korea Meteorological Administration (128.2°E). Both have 2 km fire channels and take one frame every 10 minutes. The two satellites view the area from different angles, so their observations of the ignition pixel are independent.

• The ignition time is therefore narrowed to 11:41–11:51 on 10-08. A 2 km pixel is insensitive to a very small early fire, so the geostationary satellites cannot show whether a very small open flame already existed around 11:40.
• In the Himawari frame at 12:00, the ignition pixel was already about 14 K above background, and all other conditions of the contextual test were met. The only reason the pixel was not selected was that its B07 brightness temperature, 309.94 K, was 0.06 K below the 310 K candidate threshold. The first automatic detection was therefore delayed to 12:10.
• Inference: by 12:08 the smoke had drifted 3–4 km south. With the surface wind of 3–4 m/s at the time of ignition, smoke takes about 15–20 minutes to drift that far. Working back from 12:08 puts the ignition at about 11:45–11:50, consistent with the two geostationary satellites.
• In the Himawari and GK-2A visible bands there are only thin cirrus streaks near the ignition point, and no smoke plume can be distinguished.

The first three VIIRS overpasses after ignition:

• The hot area is defined as the pixels where I4 is more than 15 K above the background within 13 km of the ignition point. Within an hour the hot area stretched north–south to about 3 km, and its main part moved south and slightly east of the ignition point. The smoke plume grew from about 4 km to about 15 km, and its direction turned from south to east-southeast.
• In all three overpasses the hot area and the smoke plume were on the Mongolian side, and the plume tip was more than 20 km from the border. The smoke did not reach the Chinese side at midday on the day of ignition.
• The few single-pixel horizontal lines in the NOAA-20 false color are detector striping and do not affect the assessment.

Surface observations at the time of ignition: Arxan station (50727, about 52 km northeast of the ignition point) and the Mongolian station Khalkh-Gol (44313, about 101 km northwest) both reported north to northwest winds of 3–4 m/s, temperatures of 10–14 °C, relative humidity of 34–46% and no weather phenomena from 11:00 to 14:00 on 10-08. With this wind direction, smoke drifts south and southeast, consistent with the plume direction seen by VIIRS. Station 44313 was drier the day before: at 14:00 on 10-07 it had a south wind of 8 m/s, 21.8 °C and 21% relative humidity.

Residual burning and extinction, 10-13 to 10-15

The daytime S-NPP and NOAA-21 fire data of 10-13 (00–11 UTC) were still absent from the NASA LANCE near-real-time archive as of 10-19, and the standard product has not been released either; the only daytime overpass that day is NOAA-20 at 11:30, with 18 fire pixels and 178 MW, all on the Mongolian side.

Day-to-day change for the same satellite and the same overpass: NOAA-20 daytime from 10-12 to 10-15 is 196 pixels/6,925 MW, 18/178, 14/252 and 18/339; Terra morning on 10-13 and 10-14 is 9/448 and 2/21; NOAA-21 nighttime from 10-13 to 10-15 is 50/317, 4/13 and 7/49.

The 18 daytime NOAA-20 pixels on 10-13 fall into four groups: 8 at 47.03–47.10°N, 119.58–119.63°E, 5 at 47.18–47.20°N, 119.57°E, 4 at 47.03–47.05°N, 119.48–119.49°E, and 1 at 46.706°N, 119.905°E. The last is in the southeastern corner of the fire area, inside burn scar that already existed on 10-12, on the Mongolian side, 1.2 km from the border.

S-NPP at 14:12 on 10-14 had 31 pixels and 901 MW. Aqua at 14:14 had 6 pixels of 72–355 MW each, with confidence values of 44–93. Most were in the northern half of the unburned island, and 1 S-NPP pixel was on burn scar outside the unburned island, at about 47.07°N, 119.50°E. All were on the Mongolian side. This Aqua overpass was at the edge of the swath, with pixels of about 3.9 × 1.8 km, so its FRP has a larger uncertainty. In the same period the fire area was clear on Himawari (cloud fraction over the core 0–0.03). Each frame had 0–3 pixels with a 3.9 µm brightness temperature above 320 K, with a maximum of 348 K (13:30) against a background of about 285–297 K. This indicates sustained small-scale open flame in the afternoon, much smaller than during the day on 10-12.

The three VIIRS nighttime overpasses early on 10-15 (NOAA-21 at 01:49, S-NPP at 02:26, NOAA-20 at 02:45) each had 5–8 pixels and 23–74 MW at the same location. They were in the northwest of the unburned island, at 47.12–47.16°N, 119.51–119.53°E, 10.3–14.3 km from the border.

At midday on 10-15 the fire pixels moved to the southeastern edge of the unburned island. Aqua at 14:52 had 1 more pixel at the southern edge of the check box, at 46.499°N, 119.517°E, on the Chinese side. It is about 12 km from the southern edge of the main burn scar and about 60 km from that day’s residual burning. It belongs to a separate fire and is not included in the table above; see below.

When Sentinel-2C passed over at 11:05 on 10-15, the southern edge of the unburned island was visible through a gap in the cloud:

• There are 5 clusters of saturated SWIR pixels, 164 cells of 10 m in total. Their centers are at 47.0842°N, 119.5717°E (82 cells), 47.0853°N, 119.5697°E (44 cells), 47.0838°N, 119.5753°E (18 cells), 47.0865°N, 119.5961°E (17 cells) and 47.0912°N, 119.6133°E (3 cells). B12 reflectance is 1.2–1.6, above the upper limit for surface reflectance, so these are open flames.
• The five clusters are spread east–west along 47.084–47.091°N, from 119.570°E to 119.613°E, over about 3.3 km. They lie at the boundary between the black old burn scar to the south and unburned grass to the north, and smoke is visible in the true color image.
• They are 0.03–0.42 km from the fire pixels of the three VIIRS overpasses that midday and 11.2–14.2 km from the border.
• Inference: the fire line on the southern edge of the unburned island was burning north into the unburned grass and was still burning at 11:00 on 10-15. The northern half of the unburned island is cloud-covered, so whether there was fire there cannot be seen.
• The core box of the unburned island (47.04–47.20°N, 119.48–119.66°E) is only 9% clear, and pixels that are clear in both this scene and the 10-13 scene make up only 2.7%. This scene therefore cannot quantify whether the burn scar inside the unburned island grew.

After midday on 10-15 the number of fire pixels decreased with each overpass. NOAA-21 at 13:16 still had 11 pixels and 96 MW. S-NPP at 13:48 had only 1 pixel, 4 MW, at 47.069°N, 119.625°E, 11.8 km from the border; this is the last VIIRS fire pixel of this fire. The 4 Aqua pixels at 14:52, 113 MW, are the last fire pixels of the main fire. From 14:00 on 10-15 to 14:00 on 10-18, S-NPP, NOAA-20 and NOAA-21 made about 30 overpasses of the fire area, day and night, without a single fire pixel. MODIS had none through 20:23 on 10-18. The fire area was clear during the three VIIRS overpasses on 10-16, cloud-covered on 10-17, and clear over its western half on 10-18. The zero detections on 10-16 were obtained under clear sky, and on that basis the main fire is judged to have gone out on the afternoon of 10-15.

Separate fire south of the main fire

At the Aqua overpass at 14:52 on 10-15 there was 1 fire pixel south of the main fire, at 46.499°N, 119.517°E, 14 MW, on the Chinese side, 13.6 km from the border. That evening Terra at 19:19 had 2 fire pixels about 6 km to its southeast, at 46.459–46.472°N, 119.584–119.591°E, with FRP of 49 MW and 110 MW, 14.5–15.5 km from the border. By a reverse lookup of OpenStreetMap place names, this location is in Mandubaolige town, East Ujimqin Banner, Xilin Gol League.

• VIIRS did not detect this fire. The daytime VIIRS overpasses on 10-15 were at 12:14–13:48, and the nighttime overpasses started at 01:30 on 10-16. None of them had fire pixels here.
• On 10-14 this area was clear, and the imagery shows no burn scar; on 10-15 it was cloudy. All three scenes on 10-16 show a new northwest–southeast burn scar whose two ends correspond to the Aqua and Terra fire pixels. It is about 7 km long, spans 46.45–46.50°N, 119.52–119.59°E, lies entirely on the Chinese side, and is about 12 km from the southern edge of the main burn scar.
• The area is about 10 km² in the NOAA-21 scene and about 4.5 km² in the S-NPP scene. The burn scar is only a few VIIRS pixels wide and its edge error is large, so the area is given as 5–10 km². Confirming it requires 10 m imagery.
• Inference: this is a separate ignition. It started after the S-NPP overpass at 13:48, and the fire burned about 7 km from northwest to southeast before 19:19. That day the residual burning of the main fire was at 47.06–47.10°N, about 60 km away, and this distance makes spotting from the main fire unlikely. The cause is unknown.

Weather at the fire area

The 10-07 row comes from a different spliced series of archived forecasts. The series in the figure above starts on 10-08 and does not include 10-07. Temperatures in the two series may differ by a few tenths of a degree.

Since early October the fire area has had almost no precipitation, and surface soil moisture fell from 0.22 to 0.17 m³/m³. On the pre-fire image of 10-05, the whole grassland is already dry and yellow, forming a continuous layer of fine fuel.

On 10-10 and 10-11 the cold front had not yet passed, and in the afternoon the fire area had a southwest wind of 5 m/s with gusts of 10–11 m/s, coinciding with the two outbreaks. On the afternoon of 10-11, relative humidity fell to 22–25% and the temperature was 19 °C. After the cold front passed on 10-12, the wind turned northwest. The drop in temperature reduced fire intensity, and the wind direction pointed toward the border and Arxan.

The model forecasts initialized on 10-13 gave weak north to northwest winds for 10-14 and 10-15, and precipitation from 10-17. The main fire went out on the afternoon of 10-15, before the first forecast precipitation; the forecasts have not been checked item by item against observations.

Upper-air and surface patterns

• 10-12: The cold front had moved east out of the fire area, and the fire area was in the northwesterly flow behind the front.
  • The 250 hPa jet core was at 48°N, 125°E, at 75 m/s, and the fire area was beneath the jet entrance region.
  • Downward transfer of momentum from aloft produced daytime surface gusts of 13–15 m/s.
• Sounding verification:
  • At Hailar, from 20:00 on 10-11 to 08:00 on 10-12, the 850 hPa temperature fell from 8.7 °C to −1.7 °C, and the 500 hPa height fell by 104 gpm.
  • At 20:00 on 10-13, the Hailar 500 hPa height was 5392 gpm, down 72 gpm in 12 hours (down 25 gpm in the previous 12 hours), with a temperature of −34.4 °C. At Nenjiang the height fell by 43 gpm. The low to the north was deepening and moving south.
  • At the same time, the Xilinhot 500 hPa height was 5607 gpm, up 55 gpm, with a temperature of −22.6 °C, up 7.7 K, and northwest winds of 27–44 m/s from 500 to 250 hPa. The ridge to the southwest was strengthening, and the fire area lay in the tighter northwesterly flow between the two systems.
  • Precipitable water was 4.2–7.8 mm, lowest at Xilinhot, and the 850 hPa dew point was −8.9 to −13.3 °C. The air was still very dry.
  • Compared with the 4 sounding stations, the 6–12 hour ECMWF 00Z and GFS 06Z forecasts for 20:00 on 10-13 have a mean 500 hPa height bias of −14 gpm and a maximum of 21 gpm. The 500 hPa temperature biases are no more than 1.6 K, and the 250 hPa wind direction biases are no more than 10°.

Satellite imagery

High-resolution imagery before and after ignition

Daily imagery

Himawari-9 fire channel

Himawari-9 B07 (3.9 µm) responds strongly to hot targets. It takes one frame every 10 minutes, which covers the intervals between polar-orbiting overpasses.

Himawari hotspot detection switches from daytime to nighttime thresholds when the solar zenith angle exceeds 85°, so the hotspot count jumps every day at 16:40–16:50. This jump comes from the detection method. The table below compares the same periods day by day:

Nights and the following mornings, compared with the same method:

On the night of 10-12/13 the count decreased from one period to the next: an average of 28.9 pixels from 17:00 to 20:00, 8.9 from 20:00 to 23:00, 1.4 from 23:00 to 02:00 and 0.4 from 02:00 to 05:00. In the second half of the night, the fraction of the fire core with BT14 < 265 K was 0.90–0.97, which may be low cloud, so the low values in the second half of the night may be underestimates. The 23:00 frame on 10-12 had 1 pixel on the Chinese side at 47.022°N, 119.799°E, 0.8 km from the line; an AHI pixel here is about 2.6×3.6 km, so the side of the border it is on cannot be resolved, and after 00:00 on 10-13 there were no hotspots within 5 km of the border. From 07:00 to 09:00 on 10-13 the fire area was mostly clear, and only 0–3 pixels were detected. During the day there were 9 hotspot pixels in total, at 46.97–47.20°N, 119.49–119.63°E, all on the Mongolian side, the nearest 5.2 km from the border. After 11:50 cumulus coverage was 0.42–0.87, and at 13:00 the fire core was fully cloud-covered.

From the afternoon of 10-13 the fire area was covered by cumulus. The afternoon and evening periods, compared day by day with the same method:

After 13:20 on 10-13, cloud fraction over the fire core was above 0.9, and the low values on the afternoon and evening are caused mainly by cloud. From 13:20 to 22:00 there was only 1 pixel each at 18:40 and 19:50, both inside the residual burning zone in the middle of the fire area, 10.7–11.9 km from the border.

Smoke plume

The turn of the plume matches the change in the wind field: after the cold front passed early on 10-12, the wind turned from southwest to northwest. After 11:00 on 10-13 cumulus developed widely. At 12:00 a band of smoke haze 20–25 km wide was still visible between the clouds, with its northern end about 20 km south of Arxan, and by 13:00 only scattered smoke haze remained. After 14:00 the fire area, the border and Arxan were all under cumulus and mid-level cloud, and no smoke plume could be distinguished.

Burned area

In the 10-13 NOAA-20 VIIRS scene, most of the fire area is covered by cumulus. The same method extracts only 47–219 km², broken up by cloud, which cannot be used to update the area; the clear part shows no new cross-border burn scar. On 10-13, therefore, only the Terra scene is usable. It lies at the eastern edge of the swath, where pixels are large and the image is bright. The original threshold gives only 2,046 km², split into a dozen or more pieces, so the green-channel threshold was relaxed with the other conditions unchanged. The result agrees within error with the 4,380–4,450 km² from the two 10-12 VIIRS scenes. About 110 km² inside the unburned island on the 10-13 Terra scene was not counted at the time; see below. The area is measured to the outer edge of the burn scar. It includes small unburned patches inside the fire area and excludes separate small fires outside it.

Recalculation on 10-14 and 10-16

At midday on 10-14 the S-NPP and NOAA-20 scenes were fully clear over the fire area, and the area was recalculated with the same threshold. Recalculating the 10-12 and 10-13 scenes reproduces every earlier result; for example, 10-12 NOAA-21 is still 4,450 km².

On 10-12 the unburned island in the middle of the fire area was open to the east, so it was not filled in and counted. By 10-14 it was surrounded by burn scar on all sides. The original hole-filling step would therefore also count the 357–386 km² inside it that was still unburned, which gives largest connected regions of 5,291 km² and 5,304 km² for the two scenes. Subtracting this part gives 4,905 km² and 4,948 km².

Relative to the 4,450 km² of 10-12 NOAA-21, the roughly 500 km² added consists of three parts (the two numbers are for S-NPP and NOAA-20):

Most of the increase had already burned on 10-12 but was hidden by smoke in the 10-12 NOAA-21 scene or missed by the threshold; the comparison with the NMC estimate below gives details. The roughly 190 km² that newly burned between the afternoon of 10-12 and midday on 10-14 is all inside the unburned island on the Mongolian side. About 110 km² of it already appears on Terra at 08:35 on 10-13 and in none of the five 10-12 scenes, and about 80 km² burned between the morning of 10-13 and midday on 10-14. Excluding the narrow band along the edge, the total is about 4,780 km². The best estimate is therefore revised to about 4,900 km², with a range of 4,750–5,000 km²; the lower end excludes the narrow band along the edge.

In the NOAA-21 scene at 13:16 on 10-15, the unburned island is 99% clear, and the burn scar inside it is 279 km², level with the 278 km² of 10-14 NOAA-20. Before midday on 10-15, therefore, there was no resolvable growth inside the unburned island. This scene has cloud over the southern half of the fire area (south of about 46.95°N) and cannot be used for the total area. In the 10-14 Terra and Aqua scenes, as in the 10-13 Terra scene, the burn scar appears too bright and the original threshold fails, so areas from 10-14 onward use VIIRS only.

The fire area was fully clear during the three VIIRS overpasses on 10-16, and the area was recalculated with the same threshold. The largest connected regions (without hole filling) in the NOAA-21 and NOAA-20 scenes are 4,915 km² and 4,898 km². The western half of the S-NPP scene is bright and its burn scar is broken up, giving only 4,195 km²; with the green-channel threshold relaxed to < 110 it is 4,743 km². The estimate remains about 4,900 km² (4,750–5,000 km²), with no growth after 10-14. The new burn scar south of the main fire is not included.

Method and errors

• dNBR: NBR = (B8A − B12)/(B8A + B12), and dNBR is the pre-fire NBR minus the post-fire NBR. The pre-fire image is from 10-05, with cloudy areas filled from 10-07.
• Reasons dNBR is biased low:
  • dNBR is biased low under dense smoke.
  • The Sentinel-2 scene classification often misclassifies black burned ground as “cloud shadow”, so only the cloud classes were masked.
• Error of moderate-resolution imagery: 250–375 m imagery is affected by pixel size, cloud shadow and smoke, with an error of about ±10–15%.
• The roughly 800 km² added from 10-11 to 10-12: Part of it is area that was hidden by smoke on 10-11 and became visible on 10-12. The rest is actual spread of the fire on the east side of the fire area and into unburned patches in the middle.
• The 10-13 Terra result: With the relaxed threshold it agrees with VIIRS. It is used only as supporting evidence that there was no growth and does not replace the 10-12 estimate.

Comparison with the NMC estimate

The Global Severe Weather Monitoring and Forecast Bulletin (《全球灾害性天气监测预报快讯》)²⁸ issued by the NMC and other agencies at 19:00 on 10-12 states that “based on satellite remote sensing estimates, the burned area is about 5,050 square kilometers” (通过卫星遥感估算，过火区面积约 5050 平方公里). The bulletin does not give the satellite, time or method behind this estimate; the latest image in the same bulletin is from FY-4C at 14:40. The figure of 5,050 km² is about 15% above this post’s 10-12 estimate of 4,400 km², about 3% above the recalculated 10-14 figure of about 4,900 km², and about 1% above the upper end of the range, 5,000 km². The two agree within the error of the two satellites and methods.

Of the gap of about 600 km² between 4,450 km² and 5,050 km², about half comes from the 250–300 km² missed by the single-scene method on 10-12; the rest may come from pixel size or the area definition, see below.

There are five GIBS scenes on 10-12, all earlier than 14:40. The earlier approach extracted each scene separately and used only the clearest scene, from NOAA-21, which gave 4,450 km². Each scene misses different parts of the edge. The part of each scene that extends beyond NOAA-21 is compared with the 10-14 NOAA-20 burn scar:

The NOAA-21 row is the difference between all connected regions and the largest connected region. In the NOAA-21 scene the eastern front is hidden by smoke, while S-NPP at 13:10 shows this section. The union of the NOAA-21 and S-NPP scenes is 4,724 km², of which 4,686 km² is inside the 10-14 burn scar. In the union of all five scenes, the part confirmed by 10-14 is 4,743 km²; another 98 km² lies outside the 10-14 burn scar and is probably false patches caused by cloud or smoke. Using only the 10-12 data, combining several scenes therefore gives an estimate of about 4,700 km².

A gap of about 350 km² remains between 4,700 km² and 5,050 km². There are two possible sources. Their shares cannot be determined, because the NMC has not published its method:

• Later burning: Between the afternoon of 10-12 and midday on 10-14, about 190 km² more burned inside the unburned island. About 110 km² of this already appears on Terra at 08:35 on 10-13, and about 80 km² burned between the morning of 10-13 and midday on 10-14. This burning is later than the FY-4C image at 14:40, so the 5,050 km² should not include it.
• Pixel size or burn scar threshold: An FY-4C pixel at 47°N is about 2.5–3 km, which expands the burn scar edge outward. Expanding the 10-12 VIIRS burn scar boundary outward by 0.5 km adds 285–326 km², and by 1 km adds 496–570 km². Larger pixels and a looser threshold both expand the boundary.

Two other items:

• Detached small patches: Keeping all connected regions of 5 km² or more adds only 6–45 km² (0.1–1%) to the two 10-12 VIIRS scenes, and 15.5 km² of that is false patches under smoke. The gap does not come from this item.
• Area definition: The table below gives the two 10-12 VIIRS scenes recalculated with different definitions.

The definition used in this post already includes the unburned patches fully enclosed by burn scar (334 km² and 443 km²). Not excluding unburned patches inside the fire area therefore does not by itself explain the gap.

The following is an inference, since the NMC has not published its estimation method. The 5,050 km² may have been calculated from the outer outline of the fire area, which includes unburned areas that extend into the fire area, such as the unburned island in the middle. Recalculated with this definition, the data in this post give 4,500–5,300 km², and 5,050 km² falls within that range. This definition is sensitive to the choice of indentation width. For S-NPP, going from 1 km to 2 km adds about 670 km², because the unburned island in the middle of the fire area has an opening about 2 km wide, and at 2 km the whole unburned island is counted. The convex hull gives 5,500–5,600 km², larger than 5,050 km². This inference uses single-scene burn scars and does not include the additional 250–300 km² or so from combining several scenes, described above. Pixel size and the burn scar threshold also remain possible causes.

Southern Weekly on 10-18²⁷ quotes Bao Yulong (包玉龙) of Inner Mongolia Normal University: “Based on an estimate from the published remote-sensing fire monitoring maps, the burned area of the wildfire inside Mongolia is expected to reach about 610 thousand hectares as of October 13.” (根据公开的火情遥感监测图估算，截至10月13日，蒙古国境内的野火过火面积预计达到约61万公顷。) Note: 610 thousand hectares is about 6,100 km², about 22% above the upper end of this post’s range, 5,000 km². The report does not say which monitoring maps or which area definition were used, and this post has not checked the figure.

Burned area assessment near the border

All assessments use the OSM border, which is based on LSIB and the median line of boundary rivers. On this section, the commonly used Natural Earth 1:10m border is offset from it by a median of 2.2 km and a maximum of 4.4 km; with the fire pixels up to 09:30 on 10-12, the number of “fire pixels on the Chinese side” is 258 using the OSM line and 602 using the Natural Earth line. Along a 300 km border, a strip one 250–375 m pixel wide corresponds to an area of 75–110 km². By the OSM line, 313 FIRMS fire pixels from 10-08 to 14:35 on 10-12 fall on the Chinese side. Their median distance past the line is 0.35 km; 71 are more than 0.75 km past the line and 12 are more than 1.5 km past it. The 375 m pixel size plus a geolocation error of 0.2–0.5 km can account for most of them. After 14:35 on 10-12, none of this fire’s pixels are on the Chinese side. The 3 MODIS fire pixels on the Chinese side south of the main fire on 10-15 belong to the separate fire described above.

On the 10-13 Terra image, burn scar outside the OSM line totals 26 km², of which 3.6 km² is more than 1 km from the line and 1.6 km² is more than 2 km from it, up to 3.2 km. On the two 10-12 VIIRS scenes it is 39–40 km², up to 4.2 km from the line. The difference between them is within pixel error, and there is no new cross-border area. Apart from the south section and the section west of Arxan listed in the table below, the remaining parts outside the line are narrow margins 0.2–1.2 km wide along the line, within the error of 1–3 pixels.

On the two fully clear 10-14 VIIRS scenes, burn scar outside the OSM line is 58–60 km², about 20 km² more than on 10-12. The new burn scar on the Chinese side, outside the 10-12 burn scar, is 24.5 km² (S-NPP) and 26.7 km² (NOAA-20). It consists entirely of narrow strips within 1.3 km of the line, and most of it already appears on 10-12 S-NPP or 10-13 Terra. On the east side, at 46.85–47.13°N, 119.76–119.92°E, there are seven pieces totaling about 14 km², with median distances from the line of 0.1–0.4 km. In the south section, the pieces at 46.60–46.66°N belong to the burned area already listed for the south section in the table below. Only one piece, 1.2 km², is absent from all earlier scenes. It is at 47.102–47.129°N, 119.757–119.772°E, at the southern end of the southern cluster west of Arxan, with a median distance from the line of 0.23 km and a maximum of 0.74 km. It is at pixel scale, and confidence is low. There is no new large cross-border area on 10-14. On the Sentinel-2C image at 11:05 on 10-15, 99–100% of the Chinese side of the south section and of the section west of Arxan is cloud-covered, so this scene cannot be used to assess border crossing.

The Chinese official statements at each level are quoted above, in the section on the Chinese border response. Note: at the large scale these statements agree with the satellite assessment, which finds no contiguous burned area entering China. However, west of Arxan, VIIRS fire pixels crossed the border at midday on 10-12, up to 1.5 km past it, and 10-13 Sentinel-2B shows about 0.9 km² burned on the Chinese side. In the south section, a further 12 km² or so burned on the East Ujimqin Banner side, which is consistent with backburning. The official statements do not mention these two locations. The Xinhua report of 10-13⁹ lists the measures as “suppression and blocking along the line, widening firebreaks, and clearing combustible material” (沿线扑救堵截、拓宽隔离带、清理可燃物). The statements of the Arxan City Emergency Management Bureau and the Hinggan League Fire and Rescue Mobile Detachment quoted by the Beijing News on 10-11²⁹ mention only border deployment and blocking. The backburning on the Hinggan League side is described in the report of the Hinggan League Forestry and Grassland Bureau on 10-16¹¹: from 12:40 on 10-11 it “used the border patrol road to carry out precise counter-burning in the hundred-meter buffer between the border wire fence and the firebreak” (依托边境巡逻路，对国境铁丝网至防火隔离带百米缓冲区开展精准对冲点烧作业), including 23 km in the Arxan City sector and 21 km in the Wuchagou sector (Xinhua Inner Mongolia on 10-14³⁰ also reported the 21 km at Wuchagou). A firefighter with the Arxan City Forestry and Grassland Bureau interviewed by Southern Weekly on 10-18²⁷ said that the firebreak is backburned routinely every late autumn: “As originally planned, the backburning of the firebreak was scheduled for October 10. But that afternoon they saw a smoke column and fire glow rising in the direction of Mongolia, and immediately switched to fire watch” (按原计划，阻隔带的点烧定在10月10日进行。但当天下午，他们发现了蒙古国方向腾起烟柱和火光，随即转入火场戒备); by the Hinggan League Forestry and Grassland Bureau’s report, the backburning took place after 12:40 on 10-11. A National Forestry and Grassland Administration report of 2025-11³¹ says Hinggan League has built a 131 km firebreak, 100–300 m wide, along the China–Mongolia border.

Burned area on the Chinese side in the south section

By the geoBoundaries county-level boundaries, this burned area lies in East Ujimqin Banner, Xilin Gol League. It is 44–53 km from the boundary of Horqin Right Front Banner, Hinggan League, and 53–59 km from the boundary of Arxan City. The CNR Xilin Gol report of 10-12¹⁰ covers the response on the East Ujimqin Banner section: “relying on the existing firebreaks, firefighters were organized to continue backburning in areas where conditions allowed backburning to be done safely” (依托现有防火隔离带，组织扑火人员对具备安全点烧条件的区域，继续实施点烧作业).

• On the pre-fire images of 10-05 and 10-08, a strip of land 100–190 m wide with parallel stripes already runs along a dirt road 2.5–3.3 km south of the border. Its appearance matches a firebreak made by plowing.
• The southern edge of the burned area at 11:05 on 10-10 lies on this strip. For example, at 119.455°E the southern edge of the burn scar is at 46.6087°N and the northern edge of the strip is at 46.6085°N. To the north, the burned area is continuous with the Mongolian burn scar, with no unburned gap at the border. On the west side it is bounded by a river valley.

The shape of the burn scar alone cannot distinguish between two cases. In the first, the Mongolian fire crossed the border and was stopped by the firebreak. In the second, the Chinese side backburned north from the firebreak, and the backburn merged with the incoming fire. In both cases the southern edge is straight. The conditions during burning fit the second case better. This area of about 12 km² formed at night, with near-calm wind, relative humidity of 78–92%, and a southerly wind in the second half of the night. A southerly wind pushes fire north, opposite to the direction in which this burned area extended south. In addition, the two nighttime VIIRS overpasses detected almost no open flame. Under these conditions, it is unlikely that the Mongolian fire advanced 3 km south against the wind. The specific backburning locations and times have not been published, so they cannot be matched to the burned area piece by piece.

On the Sentinel-2B image at 11:15 on 10-13, about 56% of the Chinese side of this section is covered by cumulus and cloud shadow. Among the pixels that are clear in both the 10-10 and 10-13 scenes (34% of the Chinese side), new dNBR > 0.27 since 10-10 totals 0.14 km², all in scattered pixels of 0.01–0.02 km². The burned area did not grow south or east. The clear areas show no SWIR signal at active-fire levels.

Burned area on the Chinese side in the section west of Arxan

On the Sentinel-2B image at 11:15 on 10-13, about 77% of the Chinese side of the section west of Arxan (47.08–47.22°N, 119.66–119.84°E) is covered by cumulus and cloud shadow. The scene classification does not separate cloud shadow from burn scar, so here the two are separated using SWIR: cloud shadow has low B12 reflectance, and burn scar does not.

• The northern part, 47.155–47.19°N, 119.70–119.76°E, is 67% clear. dNBR > 0.27 east of the line covers about 0.9 km², split into several small pieces of 0.1–0.3 km². The largest piece, at 47.155–47.165°N, 119.72–119.745°E, connects to the Mongolian burn scar to the west and lies 0.2–1.3 km from the line. Its eastern edge is roughly at the river valley inside the border.
• This burned area has an irregular shape, with no straight edge parallel to the border. Its position matches the 5–18 fire pixels detected on the Chinese side in the four daytime overpasses on 10-12 (up to 1.5 km from the line). The burned area extends east from the border, and its eastern edge stops at the river valley inside the border; it has no straight edge parallel to the border like a backburn strip. The Hinggan League Forestry and Grassland Bureau reported backburning “along the border patrol road” (依托边境巡逻路) from 12:40 on 10-11, about 100 m wide¹¹; near the southern cluster, the patrol road runs along the river channel inside the border (see the table above). Inference: the Mongolian fire crossed the border by about 1 km during the day on 10-12 and was stopped at the river valley by the firebreak created by the backburning. The 10-13 imagery does not resolve a backburn strip along the valley, and most of the area east of the northern cluster is cloud-covered, so no imagery directly supports this inference.
• The narrow strip to the south, at 47.12–47.14°N, 119.76–119.78°E, is only 16% clear on the Chinese side, which is not enough to judge its shape; its cause is undetermined. The parallel dirt road 0.1–0.6 km inside the border may be the road along which the backburning was carried out, the “border patrol road” (边境巡逻路) of the report, and the strip could be a backburn strip, but most of it is cloud-covered on the 10-13 image and this cannot be confirmed. The clear fraction over the whole 47.10–47.16°N range is 13%.
• The clear areas show no SWIR signal at active-fire levels.

An area of 0.9 km² is smaller than 1–3 VIIRS or MODIS pixels, so moderate-resolution imagery cannot resolve burned area of this size. The 10 m imagery can resolve this burned area east of the line.

Effects on the Chinese side

On the day of ignition the smoke did not reach the Chinese side. From 10-09 the smoke plume turned southeast, east and northeast with the wind; see the smoke plume section above. Arxan City has no national air-quality monitoring station, and Hinggan League has had no data for a long time in the air-quality data used here. The effect of the smoke on the Chinese side therefore has to be assessed by combining satellite data, back trajectories, visibility at weather stations and official statements.

Weather station observations:

• The Mongolian station Khalkh-Gol (44313, northwest of the fire area) reported smoke twice: at 08:00 on 10-09 with visibility of 20 km, and at 11:00 on 10-10 with visibility of 2 km. Both times the wind was from the southeast (130–140°), blowing from the fire area toward the station.
• At Arxan station (50727), visibility was 30 km (the upper limit of the code) all day on 10-09, and there was rain and snow from the afternoon of 10-10 to the morning of 10-11. During the day on 10-11 the wind turned south-southwest to southwest, blowing from the direction of the fire area, with relative humidity of 20–45%. Visibility was 13–21 km at 11:00, 17:00 and 20:00, and at 23:00 it fell to 3.9 km with haze reported. The air was dry, which rules out fog and mist; the observations are consistent with smoke. From 10-12 the wind turned northwest at 5–8 m/s, and visibility returned to 30 km.

Smoke transport and air quality: the PM2.5 peaks at Baicheng and Hailar both contain smoke from this fire. For Baicheng there are three lines of evidence, the satellite smoke plume, the back trajectories and the model smoke plume; the model plume arrives late. For Hailar the satellite evidence is weaker, and the timing and magnitude of the model smoke plume agree with the observations. The back trajectories are computed kinematically from hourly ECMWF IFS 0.25° winds (10 m, 925, 850 and 700 hPa), without vertical motion or diffusion; over about 10 hours the error is a few tens of km.

• Baicheng (Jilin Province, about 300 km southeast of the fire area): from 16:00 to 23:00 on 10-12, PM2.5 rose from 82 to 225 µg/m³, PM10 reached 255 µg/m³, and CO rose from 0.8 to 1.1 mg/m³. This event very likely includes smoke from this fire:
  • On the Aqua true color image at about 13:30 on 10-12, the smoke plume extends from the fire area, south of Arxan, to near Ulanhot. In the two TROPOMI orbits at 11:33 and 13:12, the high-value bands of carbon monoxide and UV aerosol index have the same orientation, and high aerosol index values reach about 46.2°N, 121.7°E. MODIS MAIAC aerosol optical depth is elevated along the plume.
  • The back trajectories arriving at Baicheng at 16:00, 19:00 and 22:00 all pass over the fire area at the 10 m, 925 hPa and 850 hPa levels, and two of them pass over it at 700 hPa. The time from leaving the fire area to arrival is about 6–12 hours, which matches the peak period.
  • Within about 100 km of Baicheng, FIRMS had only scattered small fire pixels on 10-10 and 10-11, 15 in total and no more than 32 MW per overpass, and none on 10-12. This fire was at the GW level over the same period. A contribution from local burning cannot be fully excluded, but it is much smaller in magnitude.
  • The concentration jumped even though the air behind the cold front was otherwise clean. The PM2.5 to PM10 ratio was about 0.88, and CO rose at the same time, which also fits a combustion smoke plume.
• Hulunbuir (Hailar, about 260 km north of the fire area): from 07:00 to 11:00 on 10-11, PM2.5 rose to 43–75 µg/m³, compared with mostly 3–25 µg/m³ from 10-07 to 10-10, and CO rose from 0.2 to 0.5 mg/m³. This event also includes smoke from this fire. The satellite evidence is weaker than for Baicheng, and the model evidence is stronger (see the GEFS-Aerosols simulation below):
  • The 10 m and 925 hPa trajectories arriving at Hailar at 07:00, 09:00 and 11:00 all passed over the fire area about 10 hours earlier, during the period of strongest fire activity on 10-10. The airflow at 850 hPa and above came from central Mongolia and did not pass over the fire area. Arxan station had a southerly wind at 11:00 on 10-11, consistent with the low-level trajectories.
  • At 11:52 on 10-11, TROPOMI shows a tongue of high carbon monoxide extending north from the fire area to about 48.7°N, about 50 km from Hailar. This branch has no aerosol index signal. The UV aerosol index is insensitive to smoke near the ground. Inference: this is low-level smoke near the surface.
  • Within about 100 km of Hailar, FIRMS had no fire pixels from 10-10 to 10-12.
  • Uncertainties: the satellite overpass is 1–5 hours after the peak. The fire area is at about 800–1,000 m elevation, so 925 hPa is close to the ground, and the low-level trajectories have large errors. Under the morning inversion, local emissions can also raise concentrations, but the model’s no-fire control at this time is only 5–9 µg/m³.
  • Hailar had another increase from the night of 10-08 to the morning of 10-09, with PM10 of about 100 µg/m³ and CO of about 0.9 mg/m³ but PM2.5 of only 35 µg/m³. It was dominated by coarse particles, and its source is unknown.
• Hinggan League (Ulanhot) has no national monitoring station data. On the afternoon of 10-12 the satellite smoke plume passed near Ulanhot, and the back trajectories arriving at Ulanhot at 14:00 and 17:00 passed over the fire area at all four levels, taking about 3–7 hours. This is an inference from satellite data and the model; there are no ground measurements.

GEFS-Aerosols simulation

NOAA GEFS-Aerosols (the chemistry version of GEFSv12, control forecast, 0.25°, 3-hourly) takes its fire emissions from GBBEPx, which estimates them from satellite FRP. All runs from 10-09 12Z onward include emissions from this fire. The column total of organic particulate matter over the fire area at initialization was 16–21 t before 10-09 00Z, 113 t at 10-09 12Z, a maximum of 613 t at 10-11 00Z, and 107 t at 10-12 00Z. The 10-08 00Z run was initialized before ignition and does not include emissions from this fire through 10-13 00Z, so it serves below as the no-fire control. The difference between the with-fire runs and the no-fire run also includes changes in emissions from other fires and differences in forecast lead time. On 10-11 and 10-12, MODIS had only 2 fire pixels within 200 km of the two cities (32 MW near Baicheng and 46 MW near Hailar).

• Hailar: the two with-fire runs initialized at 10-10 12Z and 10-11 00Z give 93 and 106 µg/m³ at 08:00 and 71 and 112 µg/m³ at 11:00 on 10-11, with the smoke plume moving due north from the fire area over Hailar. The observations are 68 µg/m³ at 08:00, a peak of 75 at 10:00 and 59 at 11:00, with a pre-peak average of 13 µg/m³. The peak timing agrees, and the modeled magnitude is 1.0–1.5 times the observed value. The earlier runs of 10-09 12Z and 10-10 00Z, whose fire emissions were still small, give only 28–29 µg/m³. The no-fire control gives 9 and 5 µg/m³.
• Baicheng: in all four with-fire runs, the smoke plume runs southeast from the fire area through Ulanhot to Baicheng, in the same direction as observed, but it reaches the city grid point late and weak. At 20:00 the model gives 11–43 µg/m³ (observed 225), at 23:00 14–70 (observed 124), and at 02:00 on 10-13 36–96 (observed 42). The model maximum within 50 km of the city is 23–56 at 20:00 and 66–167 at 02:00. The model smoke plume is 3–9 hours late and 2–4 times weaker at the city. The plume is only one or two 0.25° grid cells wide, so a position error of one cell changes the value at the city several-fold. The no-fire control gives 6–10 µg/m³.
• Share within the model: the difference between the with-fire runs and the no-fire control is about 90–95% of the model PM2.5 at the Hailar peak, and about 85–90% after the plume reaches Baicheng. The model timing at Baicheng is late, so this share cannot be applied directly to the observed peak.
• Estimate from observations (my own calculation): with the few hours before the peak as background (Hailar 13 µg/m³, Baicheng 08:00–15:00 average 44 µg/m³), the part above background is 83% and 80% of the peak, and 80% and 71% of the peak-period average. With the weekly median as background (11 and 16 µg/m³), it is 85% and 93% of the peak. The Baicheng background of 44 µg/m³ may already contain smoke: the two with-fire runs have a small peak of 60–70 µg/m³ around 06:00 on 10-12, compared with about 20 µg/m³ in the no-fire control.
• Inference: about 70–90% of each of the two peaks came from smoke from this fire. The effect of the shallower nighttime boundary layer combined with local emissions has not been quantitatively ruled out.
• Model limitations: GEFS-Aerosols does not assimilate AOD. Fire emissions enter about 1.5 days late, as daily values without a diurnal cycle, and the injection height is parameterized. A 0.25° grid is coarse for a narrow smoke plume. In a separate evaluation of this model’s data (2026-09, against the same national monitoring stations), I found that its PM2.5 in Northeast China averaged about 0.6 times the observations, and that some pollution events were missed.

Official and media statements:

• 10-11, notice from the Arxan border crossing scenic area, issued by the Arxan Culture and Tourism Group (reposted by ifeng.com on 10-12³²): “Due to temporary border controls, the border crossing scenic area is suspending operations as of today.” (因边境临时管控，口岸景区即日起暂停营业。)
• 10-12, a staff member of Arxan Forest Park told Southern Metropolis Daily³³ that the park is “currently operating normally” (目前正常营业), “but you can still smell smoke, though the smell is not as strong as in the previous two days; earlier you could see smoke drifting over, and now there is less” (但还是能闻到烟味，不过味道已经没有前两天那么大了，早前还能看到飘过来的烟雾，现在小一点了).
• Fan Aijun of the Xilin Gol League Emergency Management Bureau (CCTV Finance 10-13²³): “Dense smoke from the fire intermittently drifted with the wind to our country’s border areas, and in some areas inside the country smoke could be seen and smelled, but the open flames did not cross the national border and have not yet caused direct losses on our side such as damage to forest and grassland resources or casualties.” (火情产生的浓烟随风向间歇性飘至我国边境沿线，境内局部区域可见烟雾、闻到烟味，但明火未越过国境线，尚未造成我方森林草原资源损毁、人员伤亡等直接损失。)

The visibility drop at Arxan station on 10-11 falls within the “previous two days” (前两天) in which, according to the Southern Metropolis Daily quote of 10-12, smoke could be seen and smelled.

Confidence in the conclusions

The following observations would change the assessments above:

• A Landsat 8 image on 10-16 or the next clear Sentinel-2 image could confirm whether the narrow strip west of Arxan formed a burn scar on the Chinese side, and show its shape (both the 10-13 and 10-15 scenes are cloud-covered).
• Clear Sentinel-2 or Landsat imagery after 10-16 could confirm the area and shape of the new burn scar south of the main fire, which VIIRS can only estimate at 5–10 km².
• If the location and time of the backburning on the East Ujimqin Banner section and the exact locations of the backburning in each Hinggan League sector are published, the causes of the burning in the south section and of the narrow strip west of Arxan can both be determined.

Sources and acknowledgments

Fire data are current to 2026-10-18 20:23 (MODIS) and 10-18 14:00 (VIIRS), and high-resolution imagery to 10-15 11:05.

• Fire detections: NASA FIRMS near-real-time active fire data: VIIRS (S-NPP, NOAA-20, NOAA-21) C2 375 m and MODIS (Terra, Aqua) C6.1 1 km, from 10-04 onward. Data and imagery from NASA LANCE/FIRMS and GIBS/Worldview.
• VIIRS 375 m L2 active fire: NASA LP DAAC (VNP14IMG, VJ114IMG, VJ214IMG v002) and the LANCE near-real-time versions, which use the same algorithm as FIRMS; used for the fire spread animation and the overpass statistics from the afternoon of 10-15 onward. L2 overpass times are granule start times and differ from FIRMS times by a few minutes; for example, the first overpass after ignition is at 12:08 in FIRMS and at 12:06 in L2.
• Terrain: Copernicus DEM GLO-90, obtained via the AWS open data registry, used for the animation base map.
• Himawari-9 AHI L1b: Observed by the Japan Meteorological Agency (JMA), distributed by NOAA on AWS (noaa-himawari9), and reprojected for this post. Full disk; B07 (3.9 µm) and B14 (11.2 µm) at 2 km, one frame every 10 minutes; B01–B04 true color, with B03 red at 0.5 km. Used to 10-13 22:00, plus 10-14 13:00–15:00.
• GK-2A AMI L1B: Observed by the Korea Meteorological Administration (KMA, 128.2°E), distributed by NOAA on AWS (noaa-gk2a-pds). Full disk; 3.8 µm at 2 km and 0.64 µm at 0.5 km, every 10 minutes; used only for the period around ignition on 10-08.
• VIIRS L1B: NASA LAADS DAAC (VNP02/03, VJ102/103, VJ202/203); the first three overpasses after ignition on 10-08 (NOAA-21, S-NPP, NOAA-20), I bands at 375 m, plus M-band true color at 750 m for NOAA-21.
• Sentinel-2 L2A: Contains modified Copernicus Sentinel data 2026, obtained via Element84 Earth Search. 10/20 m; five scenes, on 10-05, 10-08, 10-10, 10-13 and 10-15. The 10-13 11:15 scene has extensive cumulus: about 56% of the Chinese side of the south section and about 77% of the Chinese side of the section west of Arxan are covered by cloud or cloud shadow. The 10-15 11:05 scene (Sentinel-2C) is cloudier: 99–100% of the Chinese side of the south section and of the section west of Arxan is cloud-covered, and so is about 90% of the unburned island in the middle of the fire area, of which only the southern edge is visible.
• Landsat 9 C2 L2: USGS, obtained via Microsoft Planetary Computer. 30 m; one scene on 10-08.
• Daily MODIS/VIIRS imagery: NASA GIBS, 250–375 m, 10-07 to 10-18. The three scenes of 10-13 are cloudy, 10-14 and 10-16 are fully clear, 10-15 has cloud over the southern half of the fire area, 10-17 is fully cloud-covered, and the western half of the fire area is clear on 10-18.
• Sentinel-5P TROPOMI L2: Contains modified Copernicus Sentinel data 2026, obtained via NASA GES DISC. Carbon monoxide total column and UV aerosol index, 10-09 to 10-12, quality flag ≥ 0.5. Aerosol optical depth uses the MODIS MAIAC product from NASA GIBS.
• ECMWF IFS and AIFS open data (CC BY 4.0): IFS 0.25° from the 10-13 00Z run and AIFS from the 10-13 06Z run; the back trajectories use hourly IFS 0.25° winds. ECMWF does not endorse derived products.
• IFS HRES 0.1°: Via Open-Meteo (CC BY 4.0), 10-13 00Z run.
• NOAA GFS 0.25°: 10-13 06Z run.
• NOAA GEFS-Aerosols: Chemistry version of GEFSv12, control forecast, 0.25°, 3-hourly, with fire emissions from GBBEPx, obtained via AWS open data (noaa-gefs-pds); runs initialized from 10-08 00Z to 10-12 12Z.
• Open-Meteo forecast and historical forecast APIs (CC BY 4.0): used for the hourly series at the fire area and for the CMA-GRAPES and ICON point forecasts (both 10-13 06Z).
• Model fields, soundings and fire products: Read from the public STAC catalog of Xue.
• Soundings: WIS2, WMO Unified Data Policy. Hailar, Xilinhot, Tongliao and other stations, to 20:00 on 10-13.
• Surface weather reports: SYNOP from both sides of the China–Mongolia border, 10-07 to 10-13, obtained via Ogimet. The Chinese stations are automatic and report weather phenomena only when phenomena are present; the visibility code has an upper limit of 30 km.
• Air quality: National real-time city air-quality release of the China National Environmental Monitoring Centre, obtained via the quotsoft.net mirror, hourly, 10-07 to 10-14. Hinggan League (Ulanhot, Horqin Right Front Banner) has had no data in the mirror for a long time, and Arxan City has no national monitoring station.
• Border and administrative boundaries: © OpenStreetMap contributors (ODbL). The border on the weather maps and the Chinese county-level (ADM3) administrative boundaries come from geoBoundaries (CC BY 4.0); the latter are used to determine which league and banner the burned area on the Chinese side belongs to.
• Online viewers:
  • Fire replay: fire pixels, NOAA-21 false color and GFS winds: VIIRS/MODIS fire pixels from 10-08 to 10-15, with 10 m wind, gusts, temperature and dew point from the GFS 10-08 00Z run
  • Fire replay: Himawari-9 10-minute infrared: 3.9 µm and 10.4 µm, 120 hours from 08:00 on 10-08, with the same fire pixels; press H in the right panel to show hotspots
  • Himawari true color
  • ECMWF 10 m wind