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

O 3 , CO, and NO x 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 2 year long in situ data set acquired from August 2011 to September 2013 at a N.W. IGP site (30.667°N, 76.729°E; 310 m asl). We investigate the impact of emissions and meteorology on the diel and seasonal variability of O 3 , CO, and NO x . Regional post‐harvest crop residue fires contribute majorly to an enhancement of 19 ppb in hourly averaged ozone concentrations under similar meteorological conditions in summer and 7 ppb under conditions of lower radiation during the post monsoon. d[O 3 ]/d t (from sunrise to daytime O 3 maxima) was highest during periods influenced by post‐harvest fires in post monsoon season (9.2 ppb h −1 ) and lowest during monsoon season (4.1 ppb h −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 NO x (>12 ppb) and CO (>440 ppb) in winter, average daytime O 3 was less than 40 ppb due to reduced photochemistry and fog. Average daytime O 3 during the monsoon was less than 45 ppb due to washout of precursors and suppressed photochemistry due to cloud cover. The 8 h ambient air quality O 3 standard was violated on 451 days 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.