Ozone variability, its formation potential and crops losses in the himalayan foothills

Abstract

Surface ozone observations in Doon Valley (Dehradun: 30.3^oN, 78.0^oE, 700 m), which acts as a bridge between the Himalayas and the Indo-Gangetic Plain, showed daytime higher values, suggesting a typical urban behaviour in proximity of the Himalayas. Ozone exhibited a maximum in spring (49.2 ± 24.8 ppbv in May) with an hourly average of more than 110 ppbv, followed by a secondary maximum in autumn and the lowest level occurring in the summer-monsoon (~ 13 ppbv in July-August). Ozone levels exceeded the 8-hour National Air Quality Standard limit (50 ppbv) throughout the year, except in July-September. The observed spring maximum was found to be triggered by biomass burning, leading to 9–50% enhancement in ozone during the high-fire activity period (April-May). Using a box model, in-situ photochemical ozone production and loss were estimated at ~ 41 ppbv and ~ 8 ppbv, respectively. The model highlighted the dominant role of the HO₂ + NO reaction (85.6%) in ozone production and the O₃ + HO₂ reaction (56.2%) in ozone loss. Exposure metrics analysis (M7 and AOT40) estimated an annual loss of 27–37 kilotons of wheat and 14–32 kilotons of rice production due to elevated ozone levels. Furthermore, hazard ratios for non-methane hydrocarbons and lifetime cancer risk values for benzene and ethylbenzene exceeded the standard limits (USEPA and WHO), indicating significant health risks to the population. Model and satellite-based studies demonstrated the NO_x-sensitive behaviour of ozone production in this Himalayan region, where aromatics exhibited the maximum ozone formation potential among different NMHCs.