Chemical Characteristics and Source Apportionment of Non-refractory PM1 from a Marine Urban Location

Atmospheric aerosol particles, known for their direct interaction with incoming solar radiations (direct effect) and for perturbation of the cloud properties (indirect effect) by acting as cloud condensation nuclei (CCN), represents largest uncertainty in the current and future understanding of the climate change. In part, this uncertainty is attributed to the lack of accurate measurements of aerosol physical and chemical properties for the improvement of various schemes in prognostic modelling useful for the effective prediction of cloud and precipitation formation. The Indian tropical region, constitutes ~18% of the world’s total population spread heterogeneously over diverse land cover, experiences a distinctive meteorological phenomenon by means of Indian Summer Monsoon (ISM). Thus, the sources, chemical properties and characteristics of aerosols are also expected to have significant variations over the Indian subcontinent depending upon the location and seasons. Online continuous measurements of NR-PM₁ (Non refractory particulate matter ≤1 µm) have been carried out in near real-time using ACSM (Aerosol Chemical Speciation Monitor) at a marine urban location of Chennai, from 4^th January to 2^nd February, 2019, complimented by simultaneous measurements of meteorological parameters. Average NR-PM₁ mass concentration for the duration of the measurements was 30.37±28.31 µg/m³ with organics constituting major fraction of ~47.43% followed by sulphate (~33.34%), ammonium (~11.89%), nitrate (~4.57%) and chloride (~2.74%). Back trajectory analysis using HYSPLIT model enabled the classification of air samples measured in to three periods: “Continental polluted”, “Marine polluted” and “Clean marine”. The polluted periods were distinguished by the potential biomass burning event, which occurs during the regional festival Bhogi, celebrated on 14^th of January in this part of the country. During this period the organics had a peak concentration of 211 µg/m³ followed by chloride ~ 42 µg/m³. During the clean marine period, low mass concentration of PM₁ is attributed to change in meteorological conditions accompanied by airmass originating from the Bay of Bengal. The average mass concentration of NR-PM₁ during this period was observed to be 7.14±2.78 µg/m³, which is ~5 times lesser than the polluted period.

            A comprehensive source apportionment study was carried out using Positive Matrix Factorization (PMF) model implemented through the multilinear engine tool (ME-2) in Source Finder (SoFi) graphical user interface, to understand the contribution of primary and secondary sources to the organic aerosols. Primary anthropogenic emissions contributed on average ~45% (~19% from traffic, ~16.7% from cooking, ~10% from biomass burning) to the total organic mass for entire measurement period, while the major contribution was associated with secondary formation ~55%. On the other hand, for clean marine period, the fractional contribution of secondary formation to PM₁ increased to ~75% to 85%, while that of primary emissions decreased to less than ~15%.

In brief, these findings indicate the influence of oceanic air masses on aerosol mass concentration and composition. Further details will be presented.