Heterogeneous chemistry and regional coal power emissions drive Delhi’s sulfate pollution

The Indo-Gangetic Plain (IGP) experiences persistent and severe air pollution, with wintertime conditions particularly extreme. Cities across the IGP, including Delhi, consistently rank among the world’s most polluted. Fine particulate matter (PM2.5) dominates Delhi’s pollution burden, with sulfate contributing about 9-12% of non-refractory PM2.5. Yet atmospheric models consistently underestimate sulfate concentrations, both globally and over in India, largely because key physical and chemical processes governing sulfate formation under local conditions remain insufficiently represented. Moreover, India does not have publicly available emission inventory. As a result, most modelling studies rely on global inventories that do not fully capture region-specific emission characteristics or the impact of recent policy measures.

Sulfate primarily forms through gas-phase oxidation of SO₂ by OH radicals and through aqueous-phase oxidation of S(IV) by O₃, H₂O₂, NO₂, and transition-metal-ion (TMI)-catalyzed reactions with O₂. In extreme pollution episodes over Delhi during winter, suppressed sunlight limits OH production, weakening gas-phase oxidation. Furthermore, aqueous-phase pathways mainly occur in cloud water, whereas haze liquid water content is substantially lower, reducing their effectiveness. Conversely, the large aerosol surface area during haze episodes suggests an enhanced role for heterogeneous reactions.

To better represent regional emissions, we updated the global emissions inventory by integrating local policy interventions and revised regional energy-sector activity profiles. Numerical simulations using this modified inventory were evaluated using comprehensive winter observations at IIT Delhi. While the updated inventory substantially improves representation of total sulfur (NMB of 1.34%), the model continues to underestimate sulfate. After evaluating several recently proposed sulfate formation mechanisms for haze conditions (e.g., H₂O₂ and NO₂ oxidation pathways), we find that metal-catalyzed heterogeneous oxidation of SO₂ by O₂ on aerosol surfaces is the dominant contributor, accounting for ~43% of the observed sulfate. Implementation of this mechanism significantly improves model agreement with observations. Lagrangian analysis indicates that this pathway is highly pH-dependent, with elevated sulfate production occurring at pH values between 4 and 5. Additionally, a substantial fraction of sulfate is formed during regional transport from nearby states power plants surrounding Delhi.

Our findings highlight that Delhi’s elevated sulfate concentrations are primarily driven by regional transport from nearby coal power plants and by metal-catalyzed heterogeneous oxidation on aerosol surfaces under severe winter haze conditions.