Non methane hydrocarbon (NMHC) fingerprints of major urban and agricultural emission sources active in South Asia for use in source apportionment studies
Authors: Kumar Ashish , Sinha Vinayak , Shabin Muhammed , Hakkim Haseeb , Bonsang Bernard , Gros Valerie 
Published: January 2020

Abstract. In complex atmospheric emission environments such as urban agglomerates, multiple sources control the ambient chemical composition driving air quality and regional climate. In contrast to pristine sites, where reliance on single or few chemical tracers is often adequate to resolve pollution plumes and source influences, comprehensive chemical fingerprinting of sources using non-methane hydrocarbons and identification of suitable tracer molecule/molecules and emission ratios is necessary. Here, we characterize and present chemical fingerprints of some major urban and agricultural emission sources active in South Asia such as paddy stubble burning, garbage burning, idling vehicular exhaust and evaporative fuel emissions. Whole air samples were collected actively from the emission sources in passivated air sampling steel canisters and then analyzed for 49 NMHCs (22 alkanes, 16 aromatics, 10 alkene and 1 alkyne) using thermal desorption gas chromatography flame ionisation detection (TD-GC-FID). Based on the measured source profiles, chemical tracers were identified for distinguishing varied emission sources and also for use in PMF source apportionment models. Thus, we were able to identify chemical tracers such as i-pentane for petrol vehicular exhaust and evaporative emissions, propane for LPG evaporative and LPG vehicular exhaust emissions, and acetylene for the biomass fires during the flaming stage. Furthermore, we observed propane to be a major NMHC emission (8 %) from paddy stubble fires and therefore in an emission environment impacted by crop residue fires, use of propane as a fugitive LPG emission tracer requires caution. Isoprene was identified as a potential tracer for distinguishing paddy stubble and garbage burning in the absence of isoprene emissions at night from biogenic sources. Diesel vehicular exhaust comprised of > 50 % alkenes and alkyne by mass composition while diesel evaporative emissions were enriched in C5–C8 alkanes and aromatics. The secondary pollutant formation potential and human health impact of the sources was also assessed in terms of their OH reactivity (s−1), ozone formation potential (OFP, gO3/gNMHC) and fractional BTEX content. Petrol vehicular exhaust emissions, paddy stubble fires and garbage fires were identified as the most polluting among the sources studied in this work. Source specific inter-NMHC molar ratios which are often employed for identifying ambient air pollution emission plumes and assessing photochemical ageing were also examined. Toluene / benzene (T / B) ratios were a good tracer for distinguishing the paddy stubble fire emissions in flaming (0.42) and smoldering stages (1.39), garbage burning emissions (0.21–0.32) and traffic emissions (3.54). While i-butane / n-butane ratios were found to be similar (0.20–0.30) for many sources, i-pentane / n-pentane ratios were useful for distinguishing biomass burning emissions (0.09–0.70) from the traffic/fossil fuel emissions (1.55–8.77). The results of this study provide a new foundational framework for quantitative source apportionment studies in complex emission environments such as South Asia.