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

Abstract. 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-PM_2.5 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-PM_2.5 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.