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We comprehensively integrated various remote sensing, modeling and meteorological datasets to assess and quantify the effects of Indonesia's forest fires in 2015 on the ambient atmosphere. When the forest fires occurred, the fire spots in Sumatra and Borneo increased sharply to 78,055 and fire radiative power (FRP) rose to 4.05 × 106 MW in September–October 2015. The Aerosol Optical Depth (AOD) and the retrieved concentration of PM 2.5 around these two islands also peaked during this period. With the remote sensing data from 2016 and 2017 as the background, we found that the carbon monoxide (CO) anomalies along the equatorial Indian and Pacific Oceans (25°S to 25°N and 40°E to 160°E) reached 10.32 ± 0.58 Mt. and 25.05 ± 1.35 Mt. in September and October 2015, respectively. Meanwhile, the nitrogen dioxide (NO 2) changes were not so obvious; the NO 2 anomalies within Sumatra and Borneo were only 0.46 ± 0.14 Kt and 0.49 ± 0.17 Kt, respectively. All the four indicators (AOD, PM 2.5 concentration, CO and NO 2 anomalies) revealed that the ambient air quality in October 2015 was even worse than that in September. The precipitation anomaly (PA) of these two islands decreased to −1.89 mm/day in September 2015, which is assumed to be the main meteorological factor to induce the forest fires. During the 1997/1998 El Niño event, the PA decreased to −3.04 mm/day on October 1997 and temperature anomaly (TA) increased to 1.52 °C on March 1998, the variation of which is more significant than other two El Niño events in 1982/1983 and 2015/2016. Unlabelled Image • We combined fire spots with FRP to analyze the forest fires. • The AOD and PM 2.5 reached extraordinarily high levels in October 2015. • The CO anomalies reached 25.05 ± 1.35 Mt. in October 2015. • The NO 2 anomalies are more significant in the vicinity of the fires. [ABSTRACT FROM AUTHOR]