An observation of seasonal and diurnal behavior of O3–NOx relationships and local/regional oxidant (OX = O3 + 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