Role of HCl and Cl2 in the Tropospheric Ozone Formation at Delhi, India

Delhi experiences severe air quality deterioration in winter. The role of atmospheric aerosols is well known for affecting human health, visibility, cloud formation, and other atmospheric processes. Ozone also plays a very important role in the atmospheric chemistry of any city. Previously, high ozone concentrations were observed in Delhi during the lockdown. This article investigates the concentration of ozone in the troposphere with chlorine concentrations. The main anthropogenic sources of chlorine in the atmosphere are coal combustion, biomass burning and, burning chlorinated substances, including plastic waste. To study the relationship between gaseous Cl − (Cl g )/HCl and tropospheric ozone (O 3 ), we measured particulate Cl − , Cl g , PM 10 , PM 2.5 and ozone on an 8-h interval period during severe air pollution conditions in Delhi (November–December 2023). In this study, average concentrations of ozone, particulate Cl − , PM 2.5 , PM 10 , and Cl g were recorded as 35.56 μg m −3 , 0.91 μg m −3 , 219.59 μg m −3 , 333.80 μg m −3 , and 19.06 μg m −3 respectively. Results showed a significant negative correlation between ozone and particulate Cl − (r = − 0.51) which suggested that higher concentrations of particulate chlorine were associated with lower ozone levels. The study emphasize the need for a larger program involving remote sensing data to explain recent ozone spikes reported in the Delhi region.