Behavioural Chemistry of ground level ozone formation in heavily polluted environment of Delhi city

Photochemical pollutants like ground level Ozone (O₃), Oxides of Nitrogen (NO_x), and volatile organic compounds (VOCs) are among the most challenging complex environmental concerns in urban and industrial areas worldwide. Among them, ground-level O₃ is both — a greenhouse gas, and a secondary pollutant. It has emerged out to be one of the critical issue for urban air quality in highly polluted environments like in the city of Delhi, India. Despite proliferation in attentiveness about O₃ (in both science and policy communities), its increasing atmospheric abundance and severity as pollutant can be well-traced. Recent findings of extensive investigations report that O₃ pollution caused the highest number of premature deaths in India alone, and it is adversely affecting wheat crop. While efforts are made to reduce particulate matter concentrations (PM), other primary pollutants like NO_x and VOCs have risen gradually (which are precursors of O₃). During hot-summer afternoons, O₃ concentrations in many parts of Delhi are often found to exceed 200μg/m^3, against the eight-hour average safety standard provided by NAAQS (100 μg/m³). The analysis of last three years (2016-2018) ground monitored ozone and its precursors (VOCs; Benzene, Toluene and Xylene; NO and NO₂) at key hotspot sites in Delhi illuminates the underlying photochemistry. NO₂ is found to be crossing permissible daily limits (daily average of 80 ppb) for about 52% -36% days at various sites. The Analysis also revealed that NO_x has strong correlations with ozone in summer, and VOCs in post monsoon. High concentrations of O₃ can be associated with the raising temperature, peculiar night-time photochemistry and transportation from periphery of Delhi. Low O₃ concentration during monsoons can be concomitant with role of active HO_x cycle in consuming the O₃ molecules in low NO_x environment.