Evolution of Aerosol Size and Composition in the Indo-Gangetic Plain: Size-Resolved Analysis of High-Resolution Aerosol Mass Spectra

This work highlights the first multiweek size-resolved analysis of high-resolution (HR) mass spectra from the Aerodyne aerosol mass spectrometer (AMS). High-resolution analysis allows for separation of isobaric interferences in the mass spectra and provides a more detailed view of the dynamic process by which both aerosol composition and size distribution change in the atmosphere. The data used in this analysis were obtained in the Indo-Gangetic Plain (IGP) during the winter of 2016 from 1 to 31 January. We report the temporal variations of particulate matter < 1 μm (PM₁) composition and link them to observed trends in chemically speciated particle size distributions. Organic and inorganic species are found to contribute on average to 54% and 46% of the aerosol mass, respectively, at the site. Positive matrix factorization (PMF) analysis of the organic HR-AMS spectra indicates that the primary organic aerosol sources are dominated by hydrocarbon-like organic aerosol (HOA) and biomass burning organic aerosols (BBOA 1 and BBOA 2). Chloride-containing aerosol events that correlate with increases in BBOAs are also observed. Significant organic aerosol (OA) mass contributions are attributed to low volatile oxygenated organic aerosol, LV-OOA (37%), and biomass-influenced oxygenated organic aerosol, OOA-BBOA (21%). Average time trends in binned, speciated size distributions are estimated using individual high-resolution proxy ions or sums of ions that are most representative of the various aerosol species. The mean modal aerodynamic diameters (MMDs) of NO₃^-, SO₄^2-, NH₄₊, and LV-OOA species are found to be greater than 600 nm and do not change significantly throughout the day. In contrast, primary organic aerosol (POA) particles such as HOA and BBOA show sharp diurnal trends in their MMDs. The average MMD of POA increases from ∼375 nm in the morning to ∼500 nm at midday, while a corresponding increase in MMD from ∼450 to ∼650 nm is observed for BBOA. Primary and secondary aerosol species have distinctive size distributions during the mornings and evenings, indicating that they are more externally mixed when primary sources are dominant. The size distributions of these species become more similar at midday within the time span of ∼4 to 6 h when condensation of secondary oxidized organic aerosol likely results in rapid internal mixing and increasing degree of oxidation of aerosol species. The diurnal increases in MMDs correlate well with the diurnal increases in mass concentrations of LV-OOA. Taken together, the trends in size distributions and mass concentrations suggest that rapid particle growth due to condensation of photochemically produced secondary organic species is important in IGP even in the winter.