CORRELATION BETWEEN PM2.5 AND TROPOSPHERIC O3 IN DELHI: TEMPORAL VARIATIONS AND METEOROLOGICAL IMPACTS

Abstract

Ambient air pollution in India, especially in Delhi, poses severe health risks. The capital experiences alarming levels of pollutants due to vehicular emissions, industrial activities, and crop burning. This pollution leads to cardiovascular issues, respiratory problems and shorter life expectancy. The pollutant of interest in the present study is ground-level ozone or tropospheric ozone – a secondary air pollutant. Ozone (O3) is a highly oxidizing triatomic molecule that is desirable in the stratosphere but can be a lethal pollutant in the troposphere. Ecosystems and human health are both harmed by it. Research on tropospheric O3 is essential to understand its formation, impacts, and to develop effective mitigation strategies, ensuring healthier air quality and environmental protection. While extensive research has taken place on chemistry of O3 formation, there has been little work done on exploring the relationship between PM2.5 and ozone. This work explores the relationship of O3 with other gaseous pollutants, particulate matter (PM) and meteorological parameters in Delhi over a six-year period (January 2018-December 2023). Comprehensive statistical analysis for exploring factors influencing ozone formation in Delhi was done on diurnal basis and seasonal-basis using data from six CAAQMS. Data was taken at hourly intervals throughout the study period. The maximum concentration of O3 observed was 53.44 ± 28.7 µg/m³ which was experienced in Summer of 2020 (April -June 2020). A negative correlation existed between O3 and PM2.5, except during monsoon season (July September), with maximum negative correlation observed in Summer of 2020 as -0.55. This may be attributed to: higher concentrations of PM2.5 observed throughout Delhi during postmonsoon (maximum of 200.77±140.03 µg/m³ observed in postmonsoon of 2020) and winter (maximum of 210.77 ± 125.87 µg/m³ observed in winter of 2018-19) seasons that reduces solar radiation available for O3 formation (low scattering albedo) (Concentration during winter was <30 µg/m³ for all the years); along with smog, haze, low mixing heights vi and temperature inversion which exacerbates the situation. In spring and summer season, though O3 concentrations increase due to increasing solar radiation and temperature, the relative concentration of PM2.5 reduces (maximum concentration: 94.89 ± 54.77 µg/m³ observed in summer of 2022). Biogenic VOCs also contribute more to PM2.5 concentration proportion, thereby increasing O3 concentrations. However, during monsoon, a mildly positive, but statistically insignificant, correlation was generally observed. This may be attributed to cleansing effect of rainfall that diminishes PM2.5 concentration (maximum concentration of 38.41 ± 20.02 µg/m³ observed in monsoon of 2022). Also, wind, that carries relatively less dust, blows in monsoon and overcast weather reduces O3 formation. This study will aid policy makers in formulating policies to curb co-pollution of O3 and PM2.5 in Delhi. Also, studies at micro-level are deemed necessary to conclusively establish a theory explaining relationship between O3 and PM2.5 in Delhi; as similar studies in other regions show significant spatial variation in the same.