Simulated aerosol mixing-state response to emission reductions over China

Abstract. The internal mixing state of aerosol particles governs their hygroscopicity, removal and activation to cloud droplets, yet assessments of emission reductions rarely resolve it. When emissions of sulfur dioxide and black carbon (BC) are reduced simultaneously, the remaining sulfate may either concentrate on the surviving BC-containing particles or redistribute to BC-free particles, with different consequences for coating thickness and cloud condensation nuclei. We examine this response with GISS ModelE2.1–MATRIX-VBS in two simulations with identical meteorology differing only in anthropogenic emissions over China, where SO₂, BC and organic carbon emissions are lower by 86, 83 and 66 % over 2031–2050. The sulfate column over China decreases by 24 %, because the region changes from a net exporter to a net importer of sulfate, and the decrease extends across the North Pacific to the Arctic. Sulfate is removed about three times faster from BC-containing than from BC-free particles, at the source and downwind, and the share of column sulfate on BC-containing particles over China decreases from 35 to 28 %. A closed process budget attributes this to the reduced number of BC cores available to receive sulfate: condensation and coagulation onto them weaken by about 70 %. Boundary-layer particle number over China decreases by 26 % and BC-containing particle number by 65 %, whereas the remaining coated particles grow. Cloud droplet number decreases by 29 %, mainly through loss of BC–organic-carbon particles. Particle number, size and mixing state therefore change more than bulk mass, and mass-based assessments do not resolve them.