Long term trends and characteristics of surface ozone at high altitude site in the central Himalayas

Tropospheric ozone is a greenhouse gas and a secondary pollutant that has negative impact on human health, crop yield and the environment. Therefore information on the long-term trends in surface ozone, particularly in the regional representative site is crucial for assessing the impact on climate and environment. While there are extensive observations of surface ozone in Europe, North America and East Asia, there are very limited observations in South Asia, particularly in the Himalayan regions. In this reference, surface ozone observations were initiated at a mountain site in Nainital (29.40° N, 79.50° E, 1948 m amsl) situated in central Himalaya in October 2006. Here, we present the observed long-term trends in surface ozone over the central Himalayas and underlying factors influencing its diurnal and seasonal variabilities. In general, ozone diurnal variations indicates least daytime photochemical ozone buildup at this remote site, except during episodes of biomass burning in the plain regions of northern India during spring. Instead, ozone levels at this site are primarily influenced by transported air masses, mountain-valley breezes and stratospheric intrusions. The long-term trend found not to be very prominent, but it showed a very slight negative (about 1 ppbv/yr) for 2007-2014 period, while a positive trend (about 0.7 ppbv/yr) for 2014-2022 period. The negative trend during 2007-14 was more prominent in spring while positive trend was prominent in winter. Trend analysis in AIRS ozone data at different pressure levels is being studied and residence time analysis of air-masses, obtained from the back air-trajectories simulations is in progress, which will be presented during the conference. These observations will be very useful for the ongoing efforts by TOAR