Surface ozone over Doon valley of the Indian Himalaya: Characteristics, impact assessment, and model results

The air quality and climate of the Himalaya is found to be impacted profoundly by strong anthropogenic emissions and photochemical processes in the valley region. Considering rapid urbanization and population growth, we performed surface ozone (O₃) measurements over Doon valley of the Indian Himalaya during April 2018–June 2023, in conjunction with the analysis of satellite observations and modeling. Noontime O₃ levels are observed to be the highest during pre-monsoon (63.8 ± 15.3 ppbv in May) and lower (22.1–56.7 ppbv) during winter and monsoon seasons. Notably, the daily maximum 8-h average (MDA8) O₃ exceeds the 50 ppbv threshold for ∼60% of the days during April–June, which suggests substantial health impacts in the region. Impact of O₃ exposure on vegetation is also significant during this period of year, as reflected from high Accumulated Ozone above Threshold 40 ppbv (AOT40) and Mean of daytime 7 hours (M7) indices. The Copernicus Atmosphere Monitoring Service (CAMS) reanalysis successfully reproduced the observed variability in the noontime O₃ (r² = 0.79–0.91). Analysis of a tracer in the CAMS model shows that the mean stratospheric contributions to surface O₃ were typically smaller (up to 8%). This suggests that O₃ pollution is governed primarily by the photochemical production favored by regional emissions and meteorological conditions. Analysis combining in-situ O₃ measurements with satellite retrievals (HCHO and NO₂) revealed that the photochemical O₃ production is in the transition or VOC-limited regime, and therefore emission of both NO_x and volatile organic compounds (VOCs) are to be reduced to mitigate O₃ pollution. Finally, a statistical model considering the non-linearities was successfully applied to simulate observed O₃ variability from available satellite observations and meteorological reanalysis data (r² = 0.75, RMSE = 7 ppbv). Our study highlights the need to mitigate O₃ pollution in the Doon valley of the Indian Himalaya and also provides invaluable inputs for designing science-informed policies.