Regional Power Emissions and Heterogeneous Chemistry Drive Extreme Sulfate Levels in Delhi

Severe wintertime particulate pollution in Delhi endangers millions of lives as well as the economy, yet the processes driving its chemical composition remain poorly understood. Sulfate is a major constituent of fine particulate matter, but its sources and formation mechanisms have not been properly explored in India. We combine numerical simulations with in situ observations in Delhi to show that most sulfate is transported from nearby regions, especially coal-based power plants. We demonstrate that conventional sulfate chemistry is inadequate and screen several recently discovered chemical pathways to establish that metal-catalyzed oxidation of gas-phase SO2 on the aerosol surface plays a dominant role in sulfate formation during transport, contributing around 43% of the sulfate. The mechanism is highly pH-dependent, resulting in enhanced sulfate levels when the pH is between 4 and 5. Our results suggest that sulfate pollution in Delhi is predominantly regional, driven by a key heterogeneous chemical pathway with implications for urban air quality across South Asia.