Characteristics, sources and reconstruction of primary & secondary components of PM2.5 in Delhi, India

Atmospheric fine particulate matter (PM_2.5) constitutes a major component of organics, inorganic and heavy & toxic elements which is increasingly recognized as a significant factor of the tropospheric chemistry of planet Earth due to its ability to influence the planet’s radiative balance. In recent years, PM_2.5 have been associated with declining air quality, negatively impacting both human health and the climate. Understanding the sources and behaviour of aerosols, both primary and secondary, as well as their spatial and temporal distribution, it is essential to evaluate their impact on air quality and climate. In the present study, a total 798 PM_2.5 samples were collected and examined for their chemical speciation [carbon contents (OC and EC), inorganic ionic species (NH₄⁺, Cl^-, NO₃^- and SO₄^2-) and elemental contents (Si, Ti, al, Fe, Zn, Cu, Mn, Pb, As, Br, Cr, Mo and P)] at metropolitan site of Delhi over the period of January 2013 to December 2021. On the basis of long-term analysis, the mean concentrations of total carbon contents (OC:15.5 ± 8.5 µg m^-3 and EC: 7.0 ± 3.9 µg m^-3), ionic species (Σ ionic species: 35.6 ± 25.6 µg m^-3) and elements (Σ elements:17.2 ± 8.2 µg m^-3) were estimated to be 18%, 28.5% and 13.7%, respectively of PM_2.5 (126 ± 77 µg m^-3) mass concentrations. Since, oxygen and hydrogen are excluded from the present chemical monitoring process, to estimate the reconstructed gravimetric mass of PM_2.5 and to achieve mass closure, the IMPROVE weighting equations were applied. The IMPROVE equation/model resolved the highest mean contribution of PM_2.5 which comes from particulate organic matter (19.3%), followed by soil/crustal matter (17.2%), aged sea salt (13.9%), ammonium sulphate (12.5%), ammonium nitrate (9.4%) and light absorbing carbon (5.6%) with unidentified mass (22.1%). The seasonal variation in reconstructed PM_2.5 mass was also exercised for winter, summer, monsoon and post-monsoon seasons. In the present analysis, the highest contribution of primary organic aerosol (POA) was estimated to be 18% in winter and lowest in monsoon (13%). Whereas the highest contribution of secondary organic aerosols (SOA) was recorded as 10.4% in post-monsoon and lowest in summer (5.7%). The secondary inorganic components were estimated to be 27% in winter, 21% in summer, 23% in monsoon, and 18% in post-monsoon. Notably, the secondary aerosol formation (inorganic 22% and organic 8%) accounted for significant fractions of PM_2.5 mass (up to 30%) than the primary aerosol formation (16%) (total up to 46% of PM_2.5). Positive Matrix Factorization (PMF) extracted six dominant sources [soil dust (SD: 19%), secondary aerosols (SA: 18%), vehicular emissions (VE: 19%), industrial emissions (IE: 16%), mixed sources (9%), combustion (BB + FFC: 19%)] of PM_2.5 in Delhi, India.