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Sources and sinks driving sulfuric acid concentrations in contrasting environments: implications on proxy calculations

Abstract. Sulfuric acid has been shown to be a key driver for new particle formation and subsequent growth in various environments, mainly due to its low
volatility. However, direct measurements of gas-phase sulfuric acid are oftentimes not available, and the current sulfuric acid proxies cannot
predict, for example, its nighttime concentrations or result in significant discrepancies with measured values. Here, we define the sources and sinks
of sulfuric acid in different environments and derive a new physical proxy for sulfuric acid to be utilized in locations and during periods when
it is not measured. We used H2SO4 measurements from four different locations: Hyytiala, Finland; Agia Marina, Cyprus; Budapest,
Hungary; and Beijing, China, representing semi-pristine boreal forest, rural environment in the Mediterranean area, urban environment and heavily
polluted megacity, respectively. The new proxy takes into account the formation of sulfuric acid from SO2 via OH oxidation and other
oxidation pathways, specifically via stabilized Criegee intermediates. The sulfuric acid sinks included in the proxy are its condensation
sink (CS) and atmospheric clustering starting from H2SO4 dimer formation. Indeed, we found that the observed sulfuric acid
concentration can be explained by the proposed sources and sinks with similar coefficients in the four contrasting environments where we have tested
it. Thus, the new proxy is a more flexible and an important improvement over previous proxies. Following the recommendations in this paper, a
proxy for a specific location can be derived.