Organic aerosol sources and their water-solubility in Delhi NCR: Insights from offline Aerosol mass spectrometric technique.

Abstract

A major fraction of organic aerosol (OA) is water-soluble. The water-soluble organic aerosol (WSOA) significantly impacts aerosol hygroscopicity and cloud condensation nuclei (CCN) formation, and adversely affects human health. In this study, characterization of WSOA in PM2.5 was done for the samples collected at two sites (Hauz Khas and Pusa) in Delhi NCR, one of the most polluted cities in the world, during the agricultural crop-residue burning period (October-November, 2019) and winter (December, 2019) using offline aerosol mass spectrometry (AMS) technique and compared with co-located real-time AMS measurements. Offline AMS provides quantitative separation of OA factors that can be primary or secondary. Offline AMS analysis showed that approximately 68% and 64% of OA are water-soluble in Hauz Khas and Pusa, respectively, which was corroborated by the filter-based measurements of water-soluble organic carbon (WSOC) and organic carbon (OC) using a TOC-L analyzer and OCEC analyzer, respectively. Three primary factors, including traffic, biomass burning, and solid fuel combustion, and two secondary factors (or sources) were resolved with Positive Matrix Factorization (PMF) analysis on the WSOA data from offline AMS. The results showed that secondary factors dominated the WSOA (~41%), followed by biomass-burning organic aerosol (BBOA) (30-34%).  In addition, the recoveries of the organic factors from several sources, including traffic, biomass burning, solid fuel combustion, and secondary organic aerosol are discussed. More oxidized organic aerosol (MO-OOA) is highly water soluble (88-92%), representing highly oxidized compounds generated from aqueous-phase reactions. The relatively small contribution of hydrocarbon-like organic aerosol (HOA) to WSOA was most likely due to their low water solubility. Overall, this study improves the understanding of the OA sources and their water solubility over the study region.