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

Abstract

<p><strong>Abstract.</strong> 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 O<sub>3</sub>, NO<sub><i>x</i></sub>, SO<sub>2</sub> 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, [SO<sub>2</sub>], condensation sink (CS), [O<sub>3</sub>], and [HONO] (or [NO<sub><i>x</i></sub>]) as the predictor variables. In this proxy method, we considered the formation of gaseous sulfuric acid from reactions of SO<sub>2</sub> 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 NO<sub><i>x</i></sub>) as a proxy. Our results showed that the UVB intensity and [SO<sub>2</sub>] are dominant factors for the production of gaseous sulfuric acid, and that the simplest proxy could be constructed with the UVB intensity and [SO<sub>2</sub>] alone, resulting in up to 29 % relative deviations when sulfuric acid concentrations were larger than 2.0 × 10<sup>6</sup> molecules cm<sup>−3</sup>. When the OH radical production from both homogenously- and heterogeneously-formed precursors were considered, the relative deviations were lower than 24 %.</p>

Reply to Anonymous Referee #2

“…3) Page 4, Line 3: Equation (2) Please add descriptions on the uncertainty of this equation. Response: The uncertainty of equation 2has been reported by Lu et al, (2019). The correlation coefficient between measured [H2SO4] and the proxy function was 0.83, and the relative error was less than 20%.…”

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

“…The sampling took place from outside the window at the 5 th floor of a university building adjacent to a busy street. For more details, see for example Lu et al (2019); Zhou et al (2020).…”

“…This term is written in the form of sulphuric acid dimer production, which seems to be the first step of cluster formation once stabilized by bases Almeida et al, 2013;Yao et al, 2018). Since measuring the OH concentration is challenging, we first replaced it with the UVB radiation intensity, which has been shown to be a good proxy for the OH concentration (Berresheim et al, 2002;Lu et al, 2019;Rohrer and Berresheim, 2006). Unfortunately, UVB was not measured in all the field studies considered here.…”

“…Proxies developed by Mikkonen et al (2011) suggested that the sulphuric acid concentration depends mostly on the available radiation and SO2 concentration, with little influence by CS. However, Lu et al (2019), who developed a daytime proxy based on measurement in Beijing China, proved the need of taking into account the CS when approximating gaseous H2SO4, especially in areas where the condensational sink can be relatively high. The proxy developed by Lu et al (2019) takes into consideration the formation pathways of H2SO4 via OH radicals from both the conventional photolysis of O3 and from the photolysis of HONO, as well as, the loss of H2SO4 via CS.…”

“…However, Lu et al (2019), who developed a daytime proxy based on measurement in Beijing China, proved the need of taking into account the CS when approximating gaseous H2SO4, especially in areas where the condensational sink can be relatively high. The proxy developed by Lu et al (2019) takes into consideration the formation pathways of H2SO4 via OH radicals from both the conventional photolysis of O3 and from the photolysis of HONO, as well as, the loss of H2SO4 via CS. Here, we derive a new proxy which takes into account the production of gaseous sulphuric acid from SO2 with oxidation by OH and stabilized Criegee Intermediates (Mauldin et al, 2012) reactions, and its losses onto pre-existing aerosol particles (condensation sink) and due to molecular cluster formation.…”