Photochemistry of ozone over urban area: a case study for Delhi City

Ozone (O 3 ), oxides of nitrogen (NO x ), and carbon monoxide (CO) were monitored at CSIR National Physical Laboratory, New Delhi, during May and October, 2014. One-hour O 3 concentrations exceeding more than 120 ppb were observed for 9 days in May and 16 days in October months with concurrent high NO x and CO values. The higher ozone episodes in October 2014 as compared to May 2014 may be due to contribution of biomass burning. The nitrogen oxide (NO), nitrogen dioxide (NO 2 ), O 3 , and the photolysis rate of NO 2 (i.e., JNO 2 ) values of May and October, 2014, were used to evaluate the photo-stationary state ratio (or Leighton ratio, ϕ ) for clear sky conditions. Leighton ratio ranged from 0.4 to 2.6 (average values 0.89 ± 0.4) for May 2014 and from 0.3 to 3.6 (average value 1.01 ± 0.52) for October 2014. The observed NO y values in October 2014 indicated the production of O 3 through hydrocarbon-sensitive O 3 chemistry. The sum of reactive nitrogen species’ (i.e., NO y ) mixing ratios recorded during October 2014 varied from 259.4 to 10.4 ppb with an average value of 80.5 ± 39 ppb. The averaged diurnal ratio of NO x –NO y of > 0.3 indicated the presence of freshly emitted pollutants.