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One seventh of the world's population lives in the Indo-Gangetic Plain (IGP)and the fertile region sustains agricultural food crop production for much ofSouth Asia, yet it remains one of the most under-studied regions of the worldin terms of atmospheric composition and chemistry. In particular, theemissions and chemistry of volatile organic compounds (VOCs) that formsurface ozone and secondary organic aerosol through photochemical reactionsinvolving nitrogen oxides are not well understood. In this study, ambientlevels of VOCs such as methanol, acetone, acetaldehyde, acetonitrile andisoprene were measured for the first time in the IGP. A new atmosphericchemistry facility that combines India's first high-sensitivity protontransfer reaction mass spectrometer, an ambient air quality station and ameteorological station, was used to quantify in situ levels of several VOCsand air pollutants in May 2012 at a suburban site in Mohali (northwest IGP).Westerly winds arriving at high wind speeds (5–20 m s−1) in thepre-monsoon season at the site were conducive for chemical characterizationof regional emission signatures. Average levels of VOCs and air pollutants inMay~2012 ranged from 1.2 to 2.7 nmol mol−1 for aromatic VOCs, 5.9 to37.5 nmol mol−1 for the oxygenated VOCs, 1.4 nmol mol−1 foracetonitrile, 1.9 nmol mol−1 for isoprene, 567 nmol mol−1 forcarbon monoxide, 57.8 nmol mol−1 for ozone, 11.5 nmol mol−1 fornitrogen oxides, 7.3 nmol mol−1 for sulfur dioxide,104 μg m−3 for PM2.5 and 276 μg m−3 forPM10. By analyzing the one-minute in situ data with meteorologicalparameters and applying chemical tracers (e.g., acetonitrile for biomassburning) and inter-VOC correlations, we were able to constrain major emissionsource activities on both temporal and diel scales. Wheat residue burningcaused massive increases (> 3 times the baseline values) for all themeasured VOCs and primary pollutants. Other forms of biomass burning at nightwere also a significant source of oxygenated VOCs and isoprene (r2 withacetonitrile ≥0.5 for nighttime data), which is remarkable in terms ofatmospheric chemistry implications. Surface ozone exceeded the 8 h nationalambient air quality limit of 100 μg O3 m−3(~50 ppbv) on a daily basis, except for 17 May 2012, when a severe duststorm event (PM2.5 > 800 μg m−3;PM10 > 2700 μg m−3) characterized by long-rangetransport from the west impacted the site. The novel data set and resultspoint to the occurrence of high primary emissions of reactive VOCs. They alsohighlight the urgent need for establishing more comprehensive observationalfacilities in the IGP to constrain the spatial and seasonal variability ofatmospheric chemical constituents. Such efforts will enable amechanistic-level understanding of the in situ chemical processes controllingthe formation of surface ozone, a necessary step for effective ozonemitigation and improvement of the regional air quality

atmospheric chemistry and physics

Chemical composition of pre-monsoon air in the Indo-Gangetic Plain measured using a new air quality facility and PTR-MS: high surface ozone and strong influence of biomass burning