Tropospheric ozone enhancement during post-harvest crop-residue fires at two downwind sites of the Indo-Gangetic Plain.

In the present study, surface ozone (O3), nitrogen oxides (NOx), and carbon monoxide (CO) levels were measured at two sites downwind of fire active region in the Indo-Gangetic Plain (IGP): Agra (27.16° N, 78.08° E) and Delhi (28.37° N, 77.12° E) to study the impact of post-harvest crop-residue fires. The study period was classified into two groups: Pre-harvest period and Post-harvest period. During the post-harvest period, an enhancement of 17.3 and 31.7 ppb in hourly averaged O3 mixing ratios was observed at Agra and Delhi, respectively, under similar meteorological conditions. The rate of change of O3 was also higher in the post-harvest period by 56.2% in Agra and 39.5% in Delhi. Relatively higher O3 episodic days were observed in the post-harvest period. Fire hotspots detected by Moderate Resolution Imaging Spectroradiometer (MODIS) along with backward air-mass trajectory analysis suggested that the enhanced O3 and CO levels at the study sites during the post-harvest period could be attributed to crop-residue burning over the North-West IGP (NW-IGP). Satellite observations of surface CO mixing ratios and tropospheric formaldehyde (HCHO) column also showed higher levels during the post-harvest period. Graphical abstract.