Intra- and inter-urban variability in chemical characteristics of residential outdoor PM2.5 in Indian metropolitan cities.

Particulate matter (PM_2.5) and its detailed chemical characteristics have not been adequately explored in urban residential communities, and to our best knowledge, there are no such studies in India. This study examined the chemical constituents (carbonaceous components, trace elements, ions) of PM_2.5 at residential outdoor locations in three cities--Mumbai, Bangalore, and Delhi--during the winter period. The average ± (standard deviation) PM_2.5 levels were 131.3 ± (71.8) for Mumbai, 75.2 ± (33.4) for Bangalore, and 192.2 ± (75.5) µg/m³ for Delhi, with 75% of the days exceeding the NAAQS of India (60 μg/m³). Coefficient of divergence (COD) analysis showed significant heterogeneity between cities but not within cities. The major portion of the resolved mass constituted organic matter (39, 41, and 32%) followed by elemental carbon (13, 12, 11%), secondary ions (28, 21, and 22%), and sea salt (6, 12, and 18%) for Mumbai, Bangalore, and Delhi, respectively. Anthropogenic sources associated with industry, brake wear, vehicular exhausts, resuspended road dust, biomass and solid waste burning, and secondary photochemical processes are likely the important sources. The backward trajectories suggest long-range transport of pollutants from higher-polluted regions. Proximity-based correlations between ambient stations and homes underscored the influence of local outdoor sources on residential exposures. An accumulated carcinogenic (non-carcinogenic) risk of 7 × 10^-8 (2.2 × 10^-5) and 6.5 × 10^-8 (5.8 × 10^-6) was estimated for Bangalore and Delhi, respectively. These findings emphasize the need for localized air quality interventions and source-specific mitigation strategies to protect urban residential populations.