Characteristics of Surface Ozone Levels at Climatologically and Topographically Distinct Metropolitan Cities in India

Surface ozone (O₃) data at Pune (1998–2014) and Delhi (1998–2013) are studied to examine their temporal characteristics. Study also examines role of meteorology and atmospheric boundary layer height (ABLH) in modulating surface O₃ at these sites. Using diurnal variability of surface O₃, rate of change of surface O₃, [d(O₃)/dt] is estimated to infer the nature of surface O₃ formation/destruction mechanisms. Analysis of data reveals that at both locations, surface O₃ concentrations during daytime are significantly high as compared to those during nighttime. Seasonally, at Pune averaged daytime surface O₃ concentrations are high during pre-monsoon and low in monsoon while those during winter and post-monsoon are found to be significantly higher than those in monsoon but half as compared to those in pre-monsoon. At Delhi, averaged daytime surface O₃ concentration is minimum in winter and maximum in pre-monsoon with monsoon and post-monsoon values being about 0.79–0.82 times with respect to pre-monsoon O₃ concentrations. High natural/anthropogenic pollutant concentration, abundance of ozone precursor gases, high temperature and high rate of photo-oxidation of precursor gases due to solar flux are the causal factors for increased surface O₃ concentrations in pre-monsoon season. Reduced solar flux decreases photo-dissociation of ozone precursor gases resulting in low O₃ concentration during winter season. Occurrence of low surface O₃ during early morning hours in monsoon, post-monsoon and winter seasons is because of low ABLH and low stratosphere-troposphere exchange (STE). [d(O₃)/dt] values during morning/evening at Pune and Delhi are indicative of asymmetric and symmetric nature of ozone formation/destruction mechanisms.