An observation of seasonal and diurnal behavior of O 3 –NO x relationships and local/regional oxidant (OX = O 3 + NO 2 ) levels at a semi-arid urban site of western India

The present study summarizes the continuous measurements of ozone (O 3 ) and oxides of nitrogen (NO x = NO + NO 2 ) 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 O 3 as highest/(lowest) during pre-monsoon/(monsoon), respectively. In contrast, NO and NO 2 exhibit the highest and lowest levels during post-monsoon and monsoon, respectively. The diurnal cycle of O 3 and NO x exhibited inverse relationship where surface O 3 showed mid-day peak and lower night-time concentrations. The dataset was used to examine the association of O 3 with the ambient levels of NO, NO 2 and NO x during day-time and night-time, separately. The variation of an oxidant OX (O 3 + NO 2 ) with the levels of NO x was examined to infer the atmospheric sources of OX as sum of NO x -independent regional and NO x -dependent local contributions. Significantly strong positive correlations were observed of O 3 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 O 3 during entire observation period.