Investigation of major emission sources of non-methane hydrocarbons and their role in ozone formation in New Delhi, India

Non-methane hydrocarbons (NMHCs) play a critical role in the atmospheric chemistry and air quality in the urban regions. They are the major precursors of ozone, secondary organic aerosols. New Delhi is one of the most polluted cities in the world where emission sources of NMHCs are poorly understood. The estimation of the contributions of different sources of NMHCs in New Delhi is important for effective to control the levels of ozone and other secondary pollutants. In additions, the mixing ratio of some NMHCs can be used as a marker for particular emission source such as vehicular exhaust, solvent usage, and vegetation. A comprehensive suite of measurements of C2-C6 NMHCs, higher molecular weight NMHCs including toluene, benzene, etc. were performed at an urban site in New Delhi during winter and summer seasons using C2-C6 VOCs analyzer and proton transfer reaction- time of flight- mass spectrometer (PTR-TOF-MS). The mean mixing ratios (MRs) of NMHCs except isoprene were significantly higher in winter than those during summer. The daily mean MRs of ethane and propane were measured 24±12 and 34±23 ppbv during the winter and 18±10 and 23±12 ppbv, respectively during the summer. Such higher MRs of NMHCs were mainly influenced by local sources and favorable meteorological conditions. The strong diurnal profiles were observed for all measured NMHCs. The significantly higher level of isoprene during daytime (~4 ppbv) in the summer and poor correlation with ethane and acetylene (r2 <.01) suggest a strong biogenic emission of isoprene in Delhi. The ratios of two NMHC species were used to characterized NMHCs sources. During winter, the emission ratio (ER) of toluene/benzene (T/B, 2.07 ppb ppb-1) and i-pentane/n-pentane (I/N, 2.95 ppb ppb-1) suggest NMHCs are mainly emitted from vehicular exhaust. While the higher ER of T/B (2.35 ppb ppb-1) and I/N (3.27 ppb ppb-1) suggesting the combined effect of vehicular exhaust and gasoline evaporation (due to higher temperature) during summer. Even though the MRs of alkane were higher than alkenes and isoprene, the ozone formation potential and OH reactivity of alkenes and isoprene were substantially higher than those of aromatics and alkanes. The results presented in this study suggest that both light and heavy duty vehicles are the main emission sources of gas-phase urban air pollution in Delhi.