Assessment of Ozone pollution in the foothills of the central Himalayan valley

Ozone pollution has become a burgeoning issue in urban and downwind regions because of increased anthropogenic emissions of ozone precursors. Anthropogenic activities are increasing across the foothills of the Himalayas, and the region experiences elevated levels of ozone and poor air quality, which impacts the fragile ecosystem of the Himalayas. To assess ozone pollution and drive chemistry, we have conducted ground-based O3 measurements in the Doon Valley at the Graphic Era (77.99° E, 30.27° N, 600 m above mean sea level) since April 2018. Maiden measurements showed significant ozone buildup during noontime (20–100 ppbv), with the highest levels occurring during spring/premonsoon and a secondary enhancement occuring during postmonsoon. The Copernicus Atmosphere Monitoring Service (CAMS) reanalysis successfully reproduced 80–90% of the total variability in the noontime ozone. The maximum daily 8-h ozone (MDA8) index exceeded the threshold of 50 ppbv about 60% of the days during the April—June period. Such elevated levels are due to intense photochemistry owing to higher solar insolation and higher levels of precursors transported from upwind regions where biomass burning is intense during this season. The contribution of biomass burning over the Haryana-Punjab region is estimated to be ~35–56% at noontime ozone during the premonsoon season. The impacts of crop residue burning were, however, weaker during the postmonsoon season due to seasonal changes in wind circulation and lower solar radiation. An analysis with satellite observations of precursors showed that ozone formation is in the transition or VOC-limited chemical regime; hence, the emission of both NOx (oxides of nitrogen) and volatile organic compounds (VOCs) are to be reduced to mitigate ozone pollution over the foothills of the central Himalayas. Our assessment of ozone shows the intertwined role of photochemistry and transport from regional biomass burning.