Decadal Shifts in Surface Ozone Trends at a Central Himalayan Site: Revealing Contrasting Phases from 2007 to 2022.

Understanding long-term changes in surface ozone pollution across South Asia, particularly remote region, remains a critical challenge due to the scarcity of surface-based observations. Tropospheric ozone is secondary pollutant and a greenhouse gas whose higher levels pose a serious hazard to human health, crop yield, environment, and climate. In view of this, continuous surface ozone observations was initiated in October 2006 at a high-altitude site Nainital (29.25°N, 79.45°E, 1948 m amsl) in the central Himalayas. This study examines the long-term trends of surface ozone (2007–2022) at this site using ground observations, WRF-Chem model simulation, and reanalysis datasets. The long term trend analysis is done using statistical method adopted by TOAR II  which revealed a negative trend in surface ozone during 2007–2022. However, a negative trend ( about -0.5 ppbv/year) was observed from 2007-2015, and a positive trend (1.2 ppbv/year) from 2016-2022. Investigating different percentiles for such trend highlights that some specific percentiles play a pivotal role in shaping the trend rather than a uniform distribution around the mean or median. Daily peak-to-peak ozone amplitude shows positive trend, with spring exhibiting the steepest rise and autumn the least. While the annual MDA8 ozone exceedance (>50 ppbv) suggests a slight negative trend over the study period with 2022 recorded the highest exceedance. The ERA5 reanalysis ozone shows a negative trend during 2007-2015 and positive trend during 2016-2022 for the nearest pressure levels which is similar tendency as in observed surface ozone. While trend from MERRA-2, CAMS, and AIRS data showed different tendencies. Tropospheric column ozone trends from OMI/MLS indicate a modest positive trend (about 0.2 DU/year) during 2007–2020. WRF-Chem model simulation is able to produce the diurnal and seasonal variation of ozone with some overestimation. Notably, long-term trends diverge, with WRF-Chem model simulations showing positive trend while observations indicate a negative trend. An investigation into meteorological parameters provided no definitive explanation for the shift in trends across the two periods. The trend observed during 2016–2022 over the central Himalayan region underscores the need for a well-defined action plan to mitigate emissions of ozone precursor gases.