Sources of non-methane hydrocarbons in surface air in Delhi, India

Rapid economic growth and development have exacerbated air quality problems across India, driven by many poorly understood pollution sources and understanding their relative importance remains critical to characterising the key drivers of air pollution. A comprehensive suite of measurements of 90 non-methane hydrocarbons (NMHCs)(C2-C14), including monoterpenes and higher molecular weight aromatics, were made at an urban site in Old Delhi during the pre-monsoon (28-May to 05-Jun 2018) and post-monsoon (11 to 27-Oct 2018) seasons. Significantly higher mixing ratios of NMHCs were measured during the post-monsoon campaign, with a mean night-time enhancement of around 6. Like with NOx and CO, strong diurnal profiles were observed for all NHMCs, except isoprene, with very high NMHC mixing ratios between 35 – 1485 ppbv. Monoaromatic species peaked at 370 ppbv and monoterpenes, which correlated strongly with other anthropogenic NMHCs, at 6 ppbv. A detailed source apportionment study was conducted that suggested the primary source of anthropogenic NMHCs in Delhi was from traffic emissions (petrol and diesel), with average mixing ratio contributions of 38 % from petrol, 14 % from diesel and 32 % from liquified petroleum gas (LPG) with a smaller contribution (16 %) from solid fuel combustion. Detailed consideration of the underlying meteorology during the campaigns showed that the extreme night-time mixing ratios of NMHCs were the result of emissions into a very shallow and stagnant boundary layer. The results of this study suggest that despite widespread open burning in India, traffic related petrol and diesel emissions remain the key drivers of gas-phase urban air pollution in Delhi.