Aromatic compounds in a semi-urban site of western India: Seasonal variability and emission ratios

Ambient concentrations of benzene, toluene, and carbon-monoxide (CO) were measured at a semi-urban site in western India during the year 2015. Monthly mixing ratios of benzene, toluene, and CO were in the ranges of 0.58–1.18, 0.62–1.48, and 218–548 ppbv, respectively. In wintertime, the higher concentrations of aromatic volatile organic compounds (VOCs) and CO were due to shallower boundary layer depths and transport from polluted Indo-Gangetic Plain. The ∆toluene/∆benzene slope of 1.18 ± 0.41 ppb ppb⁻¹ characterizes vehicle exhaust emissions. The higher values of ∆toluene/∆benzene in May coincided with the higher ambient temperatures (average ~ 35 °C) indicating emissions from evaporative sources. The emission ratios of ∆benzene/∆CO (1.82 ± 0.47 ppb ppm⁻¹) and ∆toluene/∆CO (2.2 ± 0.72 ppb ppm⁻¹) in the winter season are comparable to those for several urban regions of the world. The ozone formation potential and propylene-equivalent concentration of aromatics were lower than those of alkanes and alkenes measured during the same study period. Ethane (4.4 ± 3 ppbv), propane (3.4 ± 2 ppbv), and n-Butane (3.9 ± 2 ppbv), ethene (5.8 ± 3 ppbv), and propene (1.6 ± 1 ppbv) were the dominant alkane and alkene compounds measured during the study period. The composition of non-methane volatile organic compounds shows small change with the photochemical age suggesting the contributions of several other sources in addition to vehicular exhaust. The positive matrix factorization analysis suggests relatively high loadings of benzene (~30%), toluene (45%), and CO (32%), respectively from vehicle exhaust. The study highlights major contributions of vehicle exhaust, but emissions from other sources such as biomass burning, industries, evaporative loss, and biogenic are also significant.