Abstract
Abstract. Gaseous sulfuric acid (H2SO4) is known as one of the key precursors for atmospheric new particle formation (NPF) processes, but its measurement remains challenging. Therefore, a proxy method that is able to derive gaseous sulfuric acid concentrations from parameters that can be measured relatively easily and accurately is highly desirable for the atmospheric chemistry community. Although such methods are available for clean atmospheric environments, a proxy that works well in a polluted atmosphere, such as that found 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 using 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, the concentrations of O3, NOx (sum of NO and NO2), SO2, and HONO, and aerosol particle number size distributions. A proxy for atmospheric daytime gaseous sulfuric acid concentration was derived via a statistical analysis method using the UVB intensity, [SO2], the condensation sink (CS), [O3], and [HONO] (or [NOx]) as the predictor variables, where square brackets denote the concentrations of the corresponding species. In this proxy method, we considered the formation of gaseous sulfuric acid from reactions of SO2 and OH radicals during the daytime, and the loss of gaseous sulfuric acid due to its condensation onto the preexisting particles. In addition, we explored the formation of OH radicals from the conventional gas-phase photochemistry using O3 as a proxy and from the photolysis of HONO using HONO (and subsequently NOx) as a proxy. Our results showed that the UVB intensity and [SO2] are dominant factors in the production of gaseous sulfuric acid, and that the simplest proxy could be constructed with the UVB intensity and [SO2] alone. When the OH radical production from both homogenously and heterogeneously formed precursors were considered, the relative errors were reduced by up to 20 %.
Highlights
Gaseous sulfuric acid (H2SO4) is a key precursor for atmospheric new particle formation (NPF) processes (Kerminen, 2018; Kirkby et al, 2011; Kuang et al, 2008; Kulmala and Kerminen, 2008; Sipilä et al, 2010)
Reported real-time measurements of gaseous sulfuric acid are currently based on chemical ionization mass spectrometry with NO−3 and its ligands as reagent ions because nitrate CIMS with an atmospheric pressure interface (API) has a low detection limit for the atmospheric concentration range of gaseous sulfuric acid (Jokinen et al, 2012); a constant fraction of sulfuric acid present in the air sample will be ionized by excessive nitrate ions in CIMS under constant instrumental conditions (Kürten et al, 2012; Zheng et al, 2010), which makes the quantification of gaseous sulfuric acid feasible
The 5 %–95 % percentile ranges of the ultraviolet radiation B (UVB) intensity, [SO2], [NOx], and [O3] were 0–0.45 W m−2, 0.9–11.4, 3.3–61.4, and 3.5–23.3 ppbv, respectively
Summary
Gaseous sulfuric acid (H2SO4) is a key precursor for atmospheric new particle formation (NPF) processes (Kerminen, 2018; Kirkby et al, 2011; Kuang et al, 2008; Kulmala and Kerminen, 2008; Sipilä et al, 2010). The condensation of gaseous sulfuric acid onto newly formed particles contributes to their initial growth (Kuang et al, 2012; Kulmala et al, 2013). Measurements of gaseous sulfuric acid in the lower troposphere are challenging because its ambient concentration is typically quite low (106–107 molecule cm−3) (Kerminen et al, 2010; Mikkonen et al, 2011). Reported real-time measurements of gaseous sulfuric acid are currently based on chemical ionization mass spectrometry with NO−3 and its ligands as reagent ions (nitrate CIMS) because nitrate CIMS with an atmospheric pressure interface (API) has a low detection limit for the atmospheric concentration range of gaseous sulfuric acid (Jokinen et al, 2012); a constant fraction of sulfuric acid present in the air sample will be ionized by excessive nitrate ions in CIMS under constant instrumental conditions (Kürten et al, 2012; Zheng et al, 2010), which makes the quantification of gaseous sulfuric acid feasible
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