Dominant contribution of fossil fuel combustion to carbonaceous aerosol pollution in Delhi: Insights from radiocarbon and organic tracers

Delhi experiences some of the highest levels of fine particulate matter (PM2.5) pollution among megacities worldwide. Here, we integrated radiocarbon (¹⁴C) analysis with organic molecular tracers to quantify the sources of carbonaceous aerosols in Delhi. Through time-resolved seasonal and diurnal PM2.5 sampling at two representative urban sites and using ¹⁴C as an unambiguous tracer, we provide robust quantitative constraints on source contributions. We found that fossil fuel combustion is the dominant contributor, accounting for 62-65 % of organic carbon and 64-66 % of elemental carbon in PM2.5. Crucially, primary organic carbon from fossil fuels (POCFF) constituted the largest fraction of PM2.5 organic carbon (31-44 %). Its contribution peaked in the post-monsoon season, driven mainly by traffic emissions and coal combustion. Secondary organic carbon from fossil sources (SOCFF), biomass burning (OCBB), and cooking emissions (OCCK) contributed 21-29 %, 10-18 % and 3-7 % of PM2.5 organic carbon, respectively. Furthermore, comparisons with Positive Matrix Factorization (PMF) results suggest that conventional methods may overestimate the biomass burning contribution, underscoring the value of the ¹⁴C-based approach for accurate apportionment in this complex environment. This study underscores the critical need to reduce fossil fuel reliance and accelerate the shift toward clean energy infrastructure to effectively combat carbonaceous aerosol pollution in Delhi.