Significance of volatile organic compounds and oxides of nitrogen on surface ozone formation at semi-arid tropical urban site, Hyderabad, India

Abstract

The chemistry and variation in light molecular weight (C2–C5) volatile organic compounds (VOCs) and nitrogen oxides (NO x = NO + NO2) over the formation of tropospheric ozone (O3) 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 O3. 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 O3 vs. propy-equiv. and O3 vs. OFP, respectively. The crossover point relationship between NO x , VOCs, and O3 showed enhancement of O3 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.