Spatial and temporal variability of surface ozone and nitrogen oxides in urban and rural ambient air of Delhi-NCR, India

Ozone (O3), nitric oxide (NO), nitrogen dioxide (NO2), and nitrogen oxides (NOx) were measured continuously at three sites viz. urban background (JN), urban/traffic (CP), and rural (DP) in Delhi-NCR during the years 2013–2014. Meteorological parameters (temperature and relative humidity) were also measured in order to evaluate the relationship with targeted pollutants. The study shows that highest concentration of O3 was in summer while the lowest ones were recorded in winter and autumn for all the three sites. However, the level of NOx was observed maximum in CP (22.6 ppb) during winter and minimum in DP (5.3 ppb) during autumn. The diurnal variation of O3 was characterized by day-time maxima/(night-time minima) having concentrations 50.2/(17.2), 46.1/(15.7), and 56.7/(23.6) ppb at JN, CP, and DP, respectively. Distinct differences in concentrations were observed for O3 and its precursors during weekends and weekdays for all the three sites. The analysis revealed that higher/(lower) levels of O3 were observed during weekend/(weekdays). The moderate weekend effect was noticed for all the three sites but highest at rural site; DP.O3 was negatively correlated to RH and NOx, while it was positively correlated to temperature. The observed mean concentrations of O3 and NO2 were found to be below the recommended guideline values established by WHO and the European Union.