The surface level ozone (O 3 ), nitrogen oxides (NO x = NO 2 + NO), volatile organic compounds (VOCs), black carbon (BC) and meteorological parameters were continuously monitored for the period of one year (2010) at an urban site in Hyderabad using different trace gas monitors. The local emissions and meteorology have been found to play a significant role in diurnal variations of O 3 . The average peak time mixing ratios of O 3 were observed in the range of 23 ± 8 to 67 ± 13 ppbv. Diurnalseasonal variation of O 3 and its precursors were also reported. The observed O 3 concentrations were higher during the day than in the night. Maximum O 3 concentrations in the three seasons i.e., summer, winter and monsoon were 56 ± 14 ppbv, 50 ± 9 ppbv, and 28 ± 10 ppbv, respectively. Weekday/weekend variation of O 3 and its precursors were studied during the three seasons. High O 3 concentrations were observed on weekends compared to weekdays, however, NO x and BC levels were found low during weekends. The weekend effect of O 3 was high during winter. The annual average afternoon peak time O 3 levels on weekends were 11% higher than the weekday concentrations. Weekday/weekend variations of NO x at morning traffic rush hour were 14%, 9% and 8% in winter, summer and monsoon, respectively. The annual mean of NO x and BC concentrations at weekend were observed to be lower than weekday about 10% and 9% respectively. VOC/NO x ratio is more (5) during weekend than weekday (4) which resulted in enhanced O 3 formation.

Increasing concentration of tropospheric ozone (O 3 ) is a serious air pollution problem faced commonly by the urban people. The present study emphasizes on variations of air pollutant concentrations viz., O 3 , nitrogen oxides (NO x ), carbon monoxide (CO), sulfur dioxide (SO 2 ) and black carbon (BC) at a tropical urban site located in the Deccan plateau region with semi-arid climate. The air monitoring site revealed typical diurnal/seasonal trends attributing to the complex chemistry of surface O 3 formation from its precursors. Role of SO 2 in the formation of free radical (HO Á 2 ) and its impact on O 3 concentration is distinguished part of the study. The results showed the highest mean O 3 in summer (57.5 ± 15.2 ppbv) followed by winter and monsoon. Observations of BC aerosols showed the highest mean value during winter (8.2 ± 2 lg m -3 ) and the lowest in monsoon (4.2 ± 1 lg m -3 ). Besides local influences, long-range transport of air masses were also studied by simulating back trajectories at different elevations during the study period. Furthermore, statistical analysis and modeling was performed with both linear (regression) and nonlinear (neural network) methods.

The influence of lightning over surface-level trace gases was examined for pre-monsoon and monsoon seasons in the year 2012. Lightning events were measured using ground-based electric field monitor (EFM) and space-based lightning imaging sensor (LIS). The results showed that lightning frequency was higher during pre-monsoon period compared to monsoon, which is in good agreement with the satellite retrievals. The increase in concentration of NO x on lightning event led to a subsequent decrease in surface O 3 due to the titration reaction. Source apportionment study of SO 2 /NO x (S/N) and CO/NO x (C/N) ratios and poor correlation of NO x vs CO and NO x vs SO 2 on the lightning day confirmed the emission of NO x from dissimilar sources.

The chemistry and variation in light molecular weight (C 2 -C 5 ) volatile organic compounds (VOCs) and nitrogen oxides (NO x =NO+NO 2 ) over the formation of tropospheric ozone (O 3 ) was studied for a time period of 1 year (2013) at a tropical urban site located in Deccan plateau region of Hyderabad, India, with semi-arid climate. Diel pattern of hydrocarbons showed maxima in the morning and night and minima in the afternoon. Ethylene and propylene showed relatively larger diurnal amplitude than other hydrocarbons. Among the analyzed hydrocarbons, acetylene was the most abundant with an annual mean of 5.5±1.3 ppbv. All the VOCs exhibited a seasonal variation with monsoon and summer minimum and winter maximum. Ozone formation potentials (OFP) and propylene-equivalents (propy-equiv.) were calculated to account the contribution of individual hydrocarbons towards formation of O 3 . Propylene had the highest contribution of propy-equiv. (34 %) and OFP (28.4 %) among the VOCs observed. The concentrations of VOCs and their reactivity with hydroxyl radicals played a significant role on the levels of propy-equiv. and OFP. Strong correlations 0.65 and 0.77 were observed between O 3 vs. propy-equiv. and O 3 vs. OFP, respectively. The crossover point relationship between NO x , VOCs, and O 3 showed enhancement of O 3 at lower levels and decreased at higher levels of NO x in the range of VOCs concentrations studied. Among hydrocarbons, propylene (10) and ethane (6.5) showed the highest and lowest crossover points, respectively.

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