Investigating seasonality in sulfate presence and radiative forcing in the North Pacific

Abstract

Sulfate aerosols, formed from the atmospheric oxidation of sulfur dioxide (SO2), are well-established contributors to global cooling through both direct and indirect radiative effects. Despite this, substantial uncertainties remain regarding their spatiotemporal distribution and resulting radiative effects, especially in relation to anthropogenic emissions and downstream burden. One underexplored aspect is the seasonal evolution of sulfate burden in response to changes in SO2 emissions from East Asia. In this study, we employ the Community Earth System Model Version 2 (CESM2) to simulate the impact of varying East Asian anthropogenic SO2 emissions on downwind sulfate aerosol burden over the North Pacific Ocean. Model experiments are run using unchanging meteorological conditions but with SO2 emissions set at climatological levels from 1850, 1970, 2000, and 2006. While the SO2 emissions themselves exhibit limited seasonality, North Pacific modeled sulfate burden shows a consistent seasonal cycle with minimums in boreal winter and peaks in late boreal spring. Through differencing experiments, we find that East Asian anthropogenic emissions account for up to 50% of the springtime sulfate burden in the downstream North Pacific. Enhanced springtime oxidation and chemical processing over East Asia largely explains the modeled sulfate seasonality. Further, we quantify the radiative effects of these aerosols, decomposing them into direct, indirect (cloud-mediated), and surface albedo components. All components contribute to regional cooling, with indirect radiative effects dominating the total radiative forcing. On average, indirect radiative forcing is an order of magnitude stronger than the direct effect and accounts for over 95% of the total aerosol-induced cooling in the North Pacific.