A proxy for atmospheric daytime gaseous sulfuric acid concentration in urban Beijing

Abstract

Abstract. Gaseous sulfuric acid is known as one of the key precursors for atmospheric new particle formation processes, but its measurement remains a major challenge. A proxy method that is able to derive gaseous sulfuric acid concentrations from parameters that can be measured relatively easily and accurately is therefore highly desirable among the atmospheric chemistry community. Although such methods are available for clean atmospheric environments, a proxy that works well in a polluted atmosphere, such as those in Chinese megacities, is yet to be developed. In this study, the gaseous sulfuric acid concentration was measured in February–March, 2018, in urban Beijing by a nitrate based – Long Time-of-Flight Chemical Ionization Mass Spectrometer (LToF-CIMS). A number of atmospheric parameters were recorded concurrently including the ultraviolet radiation B (UVB) intensity, concentrations of O3, NOx, SO2 and HONO, and aerosol particle number size distributions. A proxy for atmospheric daytime gaseous sulfuric acid concentration was derived using a statistical analysis method by using the UVB intensity, [SO2], condensation sink (CS), [O3], and [HONO] (or [NOx]) as the predictor variables. In this proxy method, we considered the formation of gaseous sulfuric acid from reactions of SO2 and OH radicals during the daytime, and loss of gaseous sulfuric acid due to its condensation onto the pre-existing particles. In addition, we explored formation of OH radicals from the conventional gas-phase photochemistry using ozone as a proxy and from the photolysis of heterogeneously-formed HONO using HONO (and subsequently NOx) as a proxy. Our results showed that the UVB intensity and [SO2] are dominant factors for the production of gaseous sulfuric acid, and that the simplest proxy could be constructed with the UVB intensity and [SO2] alone, resulting in up to 29 % relative deviations when sulfuric acid concentrations were larger than 2.0 × 106 molecules cm−3. When the OH radical production from both homogenously- and heterogeneously-formed precursors were considered, the relative deviations were lower than 24 %.