Effect of different sources and meteorological processes on the variability of VOC composition in a metropolitan city of Western India during summer season

Abstract

High time- and mass-resolution measurements of volatile organic compounds (VOCs) using proton-transfer-reaction-time of flight-mass spectrometry were conducted in Ahmedabad metropolitan city in India during the summer (April–May) of 2014. The concentrations of aromatic VOCs were lower than in winter (January 2014), whereas acetaldehyde and acetone levels were almost the same during both seasons. Higher isoprene (2.14 ± 0.66 ppb(v)) and monoterpenes (0.34 ± 0.13 ppb) concentrations in summer than in winter indicate enhanced biogenic emissions. Automotive emissions were the major sources of anthropogenic VOCs, with higher emission ratios of oxygenated and aromatic compounds for heavy-duty and light-duty vehicles, respectively. Daytime oxygenated VOC levels in summer were 2–3 times higher than in winter, indicating greater contributions from the secondary sources. The daytime relative contributions of acetone, acetaldehyde, and isoprene in summer increased by ∼35%–45% compared to winter. The OH-reactivity of isoprene was highest among the measured VOCs, with much higher summer contributions than winter. Oxygenated VOCs contribute significantly to the ozone formation potential in both seasons, while biogenic VOCs only in summer. The box model results at different NOx-VOC combinations, using observed and simulated VOC data, show that the reductions of VOC levels can effectively reduce the daytime ozone formation rates at higher NOx concentrations. The study reveals that with the reduction of anthropogenic VOC emissions, biogenic/secondary sources become increasingly important in atmospheric processes in tropical urban regions. Our analysis will be valuable in developing mitigation policies to control primary and secondary pollutants in urban areas in India.