Influence of post-harvest crop residue fires on surface ozone mixing ratios in the NW IGP analyzed using 2years of continuous in situ trace gas measurements

O-3, CO, and NOx affect air quality and tropospheric chemistry but factors that control them in the densely populated N.W. Indo-Gangetic Plain (IGP) are poorly understood. This work presents the first simultaneous 2year long in situ data set acquired from August 2011 to September 2013 at a N.W. IGP site (30.667 degrees N, 76.729 degrees E; 310masl). We investigate the impact of emissions and meteorology on the diel and seasonal variability of O-3, CO, and NOx. Regional post-harvest crop residue fires contribute majorly to an enhancement of 19ppb in hourly averaged ozone concentrations under similar meteorological conditions in summer and 7ppb under conditions of lower radiation during the post monsoon. d[O-3]/dt (from sunrise to daytime O-3 maxima) was highest during periods influenced by post-harvest fires in post monsoon season (9.2ppbh(-1)) and lowest during monsoon season (4.1ppbh(-1)). Analysis of air mass clusters revealed that enhanced chemical formation of O-3 and not transport was the driver of the summertime and post monsoon ambient O-3 maxima. Despite having high daytime NOx (>12ppb) and CO (>440ppb) in winter, average daytime O-3 was less than 40ppb due to reduced photochemistry and fog. Average daytime O-3 during the monsoon was less than 45ppb due to washout of precursors and suppressed photochemistry due to cloud cover. The 8h ambient air quality O-3 standard was violated on 451days in the period August 2011-September 2013. The results show that substantial mitigation efforts are required to reduce regional O-3 pollution in the N.W. IGP.