Ground-based urban sulfate aerosol transported from the boundary layer aloft

Despite significant reductions of sulfur dioxide emissions in urban atmosphere globally, sulfate aerosol levels have not decreased proportionally with their precursors. Critical gaps persist in understanding sulfate sources and formation mechanisms, impairing the precision of air pollution control strategies and sulfate chemistry parameterization in atmospheric models. Here we show, using a unique 528-meter observation platform in Beijing coupled with large eddy simulations, that higher concentrations of sulfate aerosol than at the ground occur in the boundary layer aloft, and the high sulfate contributes to the ground by thermal-sheared turbulent mixing triggered by solar radiation. This transport mechanism better explains the characteristic forenoon sulfate peaks observed in 16 global urban measurement campaigns. Our results reveal a ubiquitous sulfate source from the boundary layer aloft in densely populated areas, which was a previously overlooked contributor to ground-level sulfate. We emphasize the need to incorporate this critical source in air quality mitigation policies.