An observation of seasonal and diurnal behavior of O3-NOx relationships and local/regional oxidant (OXÂ =Â O3Â +Â NO2) levels at a semi-arid urban site of western India

The present study summarizes the continuous measurements of ozone (O3) and oxides of nitrogen (NOx = NO + NO2) along with meteorological conditions in the ambient atmosphere of semi-arid urban site of western India during the year 2012-13. Seasonal and diurnal variations of gaseous pollutants were investigated and compared using the results of time series analysis. The marked seasonal difference was observed for O3 as highest/(lowest) during pre-monsoon/(monsoon), respectively. In contrast, NO and NO2 exhibit the highest and lowest levels during post-monsoon and monsoon, respectively. The diurnal cycle of O3 and NOx exhibited inverse relationship where surface O3 showed mid-day peak and lower night-time concentrations. The dataset was used to examine the association of O3 with the ambient levels of NO, NO2 and NOx during day-time and night-time, separately. The variation of an oxidant OX (O3 + NO2) with the levels of NOx was examined to infer the atmospheric sources of OX as sum of NOx-independent regional and NOx-dependent local contributions. Significantly strong positive correlations were observed of O3 with temperature and solar radiation with strong negative correlation with relative humidity during the studied seasons. Inconsiderable difference (< 10 μg m−3) was observed between weekends and weekdays for the levels of O3 during entire observation period.