Photochemical evolution of air in a tropical urban environment of India: A model-based study.

The photochemical processes over tropical Indian region impact the atmospheric composition and air quality over local to global scales; nevertheless, studies on detailed atmospheric chemistry remain sparse in this region. In this study, we investigate the photochemical evolution of air in the downwind of a tropical semi-arid urban environment (Ahmedabad) in India using the Master Mechanism model. The 5-days long chemical evolution has been simulated for the winter conditions - when this region experiences strong ozone build up. Model environment has been set up by including the meteorological conditions, overhead ozone, and aerosol loading, etc. Nitrogen oxides (NOx), carbon monoxide (CO), ozone (O3), and several volatile organic compounds (VOCs) have been initialized in the model based on the wintertime observations. The model predicts large O3 production (∼115 ppbv) in the downwind regions, followed by a gradual decrease from the 3rd day onwards. Additionally, significant amounts of the secondary inorganics, e.g. nitric acid (∼17 ppbv), hydrogen peroxide (∼9 ppbv), and organics, e.g. ketones (∼11 ppbv), are also simulated. The noontime maximum levels of hydroxyl (OH) and hydroperoxyl (HO2) radicals are simulated to be 0.3 and 44 pptv, respectively. While the production of OH is dominated by the reaction of NO with HO2 on the first day, photolysis of O3 dominates subsequently with reduction in NOx levels. VOCs are the major OH sink during day 1, however contribution of CO is greater on further days. The air mass trajectory analysis suggests the outflow of ozone-rich air over the rural areas and the Arabian Sea, in agreement with measurements and a global model. Our study highlights the strong impact of the urban outflows on the regional atmospheric composition. The continuous measurements of VOCs and radicals are needed over tropical regions to complement the models and further improve the understanding of air chemistry.