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

Abstract

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 ]/dt (from sunrise to daytime O 3 maxima) was highest during periods influenced by post-harvest fires in post monsoon season (9.2 ppb h À1 ) and lowest during monsoon season (4.1 ppb h À1 ). 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.

Water-Soluble Ionic Composition of Aerosols at Urban Location in the Foothills of Himalaya, Pokhara Valley, Nepal

“…A significant portion (16%) of the air mass, during pre-monsoon, originated from the IGP region. These air masses could carry the biomass burning emission from the IGP region at our site since agro residue burning is common during this season of the year over northwestern IGP region [46]. A recent study [18] has shown that the site over Himalayan foothills gets influenced by biomass burning on either side (east or west) of Indo-Gangetic Plain.…”

Organic molecular tracers in the atmospheric aerosols from Lumbini, Nepal, in the northern Indo-Gangetic Plain: influence of biomass burning

“…Extensive agricultural burning in this area lasts for more than 3 weeks during every post-monsoon season, causing the smoke to spread widely like a blanket over nearly the whole IGP (Fig. 10), with very high O 3 and CO loadings (Kumar et al, 2016) and particulate matter concentrations (Rastogi et al, 2016). Concerning the situation in Lumbini, the flow of air mass is dominated by westerlies in post-monsoon season.…”

Pre-monsoon air quality over Lumbini, a world heritage site along the Himalayan foothills

“…In the emission source region, diurnal variations of primary pollutants provide information about the time-dependent emission activities (Kumar et al, 2016b). Figure 5 shows the diurnal variation of hourly averaged concentrations of measured pollutants during the sampling period.…”

“…Morning and evening periods experienced the winds coming from the southeastern direction while the winds were predominantly from the southwestern direction during late afternoon. The increase in CO concentrations in the evening hours might be due to transport of CO from source regions upwind of Lumbini, which, along (Kumar et al, 2016b). The PBL height (in meters) was obtained from the WRF model as observations were not available.…”

“…The northern Indo-Gangetic region is characterized by fires even during the monsoon and post-monsoon season (Kumar et al, 2016b;Putero et al, 2014). These activities not only influence air quality over nearby regions but also get transported towards high-elevation pristine environments like Mount Everest (Putero et al, 2014) and Tibet (Cong et al, 2015a,b).…”