Measurement report: Three-year characteristics of sulfuric acid in urban Beijing and derivation of daytime sulfuric acid proxies applicable to various sites

Sulfuric acid (H2SO4) is a key precursor in atmospheric new particle formation and cluster early growth. However, long-term measurement of it remains scarce due to technical challenges. Although several proxies for estimating H2SO4 concentration have been proposed, they are always site-specific. Therefore, both reliable H2SO4 measurement and proxies with wider application are highly needed. Here, we conducted a long-term H2SO4 measurement in urban Beijing during 2019–2021, and derived three H2SO4 proxies based entirely on its formation and loss pathways (OH-CS, UVB-CS and UVB-PM2.5 based proxies). Results show that daytime H2SO4 concentration is 2.0–7.4×106 molec cm-3 and shows an overall decline with an average annual decrease of 14 %. This decline is mainly due to the ongoing SO2 emission controls. Daytime H2SO4 shows a clear seasonal variation that tracks UVB. Nighttime H2SO4 concentration is 1.6–6.3×105 molec cm-3, with higher levels in warmer seasons due to stronger sources and lower condensation sink. The diurnal variations of H2SO4 across seasons follow those of photo-oxidation-related parameters, such as UVB, OH radical, and J(NO2). All of the three proxies can reproduce H2SO4 concentration during 10:00–14:00. Importantly, they can estimate H2SO4 concentration at a boreal forest site in Hyytiälä, Finland, suggesting their applicability to sites with diverse environments. Furthermore, the parameters used in UVB-PM2.5 based proxy are available at most observational sites. Further application of this proxy could provide H2SO4 concentrations covering many regions worldwide, which may further facilitate research on atmospheric nucleation and secondary aerosol growth of these sites.