The trend of the oxidants in boreal forest over 2007–2018: comprehensive modelling study with long-term measurements at SMEAR II, Finland

Abstract. Major atmospheric oxidants (OH, O₃ and NO₃) dominate the atmospheric oxidation capacity, while H₂SO₄ is considered as a main driver for new particle formation events. Although numerous studies have investigated the long-term trend of ozone in Europe, the trend of OH, NO₃ and H₂SO₄ at specific sites are to a large extent unknown. In this study, we investigated how the trends in major atmospheric oxidants (OH, O₃ and NO₃) and H₂SO₄ changed in southern Finland during the past 12 years and discuss how these trends relate to decreasing emissions of regulated air pollutants in Europe. The one-dimensional model SOSAA has been applied in several studies at the SMEAR II station, and has been validated by measurements in several projects. Here, we ran the SOSAA model for the years 2007–2018 to simulate the atmospheric chemical components, especially the atmospheric oxidants and H₂SO₄ at SMEAR II. The simulations were evaluated with observations at SMEAR II for several shorter and longer campaigns. Our results show that OH increased by +1.56 (−0.8; +3.17) % yr⁻¹ during daytime and NO₃ decreased by −3.92 (−6.49; −1.79) % yr⁻¹ during nighttime, indicating different trends of the oxidants during day and night. Sulphuric acid decreased during daytime by −5.12 (−11.39; −0.52) % yr⁻¹, which correlated with the observed decreasing concentration of newly formed particles in the size range 3–25 nm by 1.4 % yr⁻¹ at SMEAR II during the years 1997–2012 (Nieminen et al., 2014). Additionally we compared our simulated OH, NO₃ and H₂SO₄ concentrations with proxies, which are commonly applied in case limited amount of parameters are measured and no detailed model simulations are available.