Abstract
Delhi is one of the world’s most polluted cities, and hence pollution control is a very high priority. A key aspect of a cost-effective strategy is to differentiate emissions from local sources from pollution arising from regional transport. Detailed chemical characterisation of atmospheric fine particulate matter (PM2.5) has been made at two sites in three seasons with a view to source attribution. Chloride ion (Cl-) concentrations make an exceptional contribution at both sites in winter, reaching a maximum of 76 and 97 μg m-3 during the winter campaign at the central and suburban sites, respectively. Differing diurnal patterns of Cl- were observed between the seasons, indicating contrasts in sources and secondary chemical processes between seasons. Inter-site comparisons show high correlation between secondary inorganics (NO3-, SO42-, and NH4+) in the winter and pre-monsoon, indicating the influence of regional processes for their formation at both sites, while for Cl- and other measured ions (Na+, Ca2+, Mg2+, PO42- and K+), local influences were found to be dominant through inter-site comparisons and ventilation coefficient analyses. A significant contribution of local sources to secondary inorganic compounds and Organic Matter mass within PM2.5 in the post-monsoon period was observed, implying that the regional sources (such as crop residue burning, often indicted as the cause of poor air quality) are not the sole driver for high PM2.5 loadings in Delhi in autumn, contrary to previous studies.