Evolution of size and composition of fine particulate matter in the Delhi megacity during later winter

This study presents the first multi-site evolution analysis of fine Organic Aerosols (OA) and inorganics in the Delhi megacity (National Capital Region (NCR)) using high-resolution-particle time of flight (HR-PToF) size distribution obtained from Aerodyne HR-ToF-AMS. It provides a comprehensive view of the atmospheric dynamic processes responsible for the aerosol size and composition transformation. The measurements were performed at two different sampling sites (1) Indian Institute of Technology, Delhi (IITD), and (2) Manav Rachna International University, Faridabad (MRIU) during a late winter period (February to March 2018). Among organics, primary OA (POA) showed particle growth, whereas it was absent in secondary OA proxies. IITD was observed to have a distinct growth pattern as compared to MRIU because of high vehicular density and heterogeneity, and the presence of growth-promoting factors such as acidity and RH conditions. IITD aerosols were observed to be more acidic (ANR∼ 0.75) compared to MRIU (ANR∼1) and showed a distinct diurnal bin-wise increase during the photochemically active period (PAP). Such high acidic conditions are responsible for promoting acid-catalyzed SOA productions over the broader size bins, especially during the PAPs. Primary OA (POA) such as HOA and BBOA proxies show a diurnal growth from ∼440 to 970 nm and from ∼370 to 550 nm, at IITD and from ∼270 to 400 nm and ∼430–470 nm, at MRIU respectively. Further, OA families (amines and hydrocarbons), and inorganics such as chloride and nitrate also showed distinct concurrent diurnal growth patterns at IITD (from an MMDs of ∼380–520 nm to 650–960 nm). We have also observed a positive correlation between OA and inorganics growth and the mass fraction (MF = SOA/OA) of SOA at both sites of NCR. For 0.1 (MF) increase in SOA, the sizes of HOA, chloride, primary CH family and amines (CHN family), and secondary amines (CHON family) are increased by ∼93, 80, 64, 66, and 47 nm at IITD and ∼14, 35, 8, 11, and 16 nm at MRIU respectively. The concurrent growth of these species with the increase in the SOA concentration indicates the existence of a similar size evolution mechanism at both NCR sites, further promoting the formation of internally mixed aerosols during some phase of their evolution. This suggests that condensation is significantly governing the growth mechanism in NCR. Our study indicates the inclusion of such dynamic growth patterns arising from local prevailing conditions into the models for a better understanding of atmospheric aerosol evolution and estimation of fine particulate matter budget.