Contributions of anthropogenic and natural sources of sulfur to SO₂, H₂SO₄(g) and nanoparticle formation

Abstract. Atmospheric nanoparticles (NPs) are important intermediates in the transition from gas-phase molecules to new aerosols that can activate into cloud droplets. Through increases in the emissions of sulfur-containing gases, human activities have likely increased the number of NPs produced in the atmosphere. To have significant impacts, however, sulfur pollution must be transported away from the surface, where NP formation is inefficient, to higher altitudes. To characterize this anthropogenic influence, tagged tracers are implemented in a global atmospheric transport model. The tagged tracers are used to track the contributions of sulfur from five sources (anthropogenic, oceanic, volcanic, aircraft, and stratospheric) to the gas-phase burdens of SO₂ and H₂SO₄(g), and the rates of forming atmospheric NPs. Because NPs may be produced by a variety of mechanisms, three different aerosol nucleation schemes (binary, ternary and ion-induced) are used in the model calculations. Of the SO₂ in the global troposphere, the tagged tracers indicate that about 69% originates from anthropogenic surface emissions, 20% from the oceans and 10% from de-gassing volcanoes. The same sources contribute about 56%, 24% and 19%, respectively, to the global tropospheric H₂SO₄(g) burden. The anthropogenic contribution for H₂SO₄(g) is reduced because anthropogenic SO₂ produces H₂SO₄(g) less efficiently than oceanic and volcanic sulfur. Regardless of the underlying nucleation assumptions, the simulations show a pronounced influence of anthropogenic sulfur on atmospheric NP formation, particularly in the Northern Hemisphere. Utilizing the tagged H₂SO₄(g) contributions, anthropogenic sulfur is estimated to account for roughly 69% of the NP formation in the Northern Hemisphere, 31% in the Southern Hemisphere and 56% across the global troposphere. In the key region of the upper troposphere, anthropogenic and oceanic sulfur both make sizeable contributions to NP formation (54% and 37%, respectively). The tagged tracer contributions suggest that human activities have probably more than doubled the NP production rate in the atmosphere from preindustrial to modern times.