The future of airborne sulfur-containing particles in the absence of fossil fuel sulfur dioxide emissions

Sulfuric acid (H₂SO₄), formed from oxidation of sulfur dioxide (SO₂) emitted during fossil fuel combustion, is a major precursor of new airborne particles, which have well-documented detrimental effects on health, air quality, and climate. Another precursor is methanesulfonic acid (MSA), produced simultaneously with SO₂ during the atmospheric oxidation of organosulfur compounds (OSCs), such as dimethyl sulfide. In the present work, a multidisciplinary approach is used to examine how contributions of H₂SO₄ and MSA to particle formation will change in a large coastal urban area as anthropogenic fossil fuel emissions of SO₂ decline. The 3-dimensional University of California Irvine–California Institute of Technology airshed model is used to compare atmospheric concentrations of gas phase MSA, H₂SO₄, and SO₂ under current emissions of fossil fuel-associated SO₂ and a best-case futuristic scenario with zero fossil fuel sulfur emissions. Model additions include results from (i) quantum chemical calculations that clarify the previously uncertain gas phase mechanism of formation of MSA and (ii) a combination of published and experimental estimates of OSC emissions, such as those from marine, agricultural, and urban processes, which include pet waste and human breath. Results show that in the zero anthropogenic SO₂ emissions case, particle formation potential from H₂SO₄ will drop by about two orders of magnitude compared with the current situation. However, particles will continue to be generated from the oxidation of natural and anthropogenic sources of OSCs, with contributions from MSA and H₂SO₄ of a similar order of magnitude. This could be particularly important in agricultural areas where there are significant sources of OSCs.