A modelling study of OH, NO and HSO in 2007-2018 at SMEAR II, Finland: analysis of long-term trends.

Major atmospheric oxidants (OH, O and NO) dominate the atmospheric oxidation capacity, while HSO is considered as a main driver for new particle formation. Although numerous studies have investigated the long-term trend of ozone in Europe, the trends of OH, NO and HSO at specific sites are to a large extent unknown. 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 applied the SOSAA model for the years 2007-2018 to simulate the atmospheric chemical components, especially the atmospheric oxidants OH and NO, as well as HSO at SMEAR II. The simulations were evaluated with observations from several shorter and longer campaigns at SMEAR II. Our results show that daily OH increased by 2.39% per year and NO decreased by 3.41% per year, with different trends of these oxidants during day and night. On the contrary, daytime sulfuric acid concentrations decreased by 2.78% per year, which correlated with the observed decreasing concentration of newly formed particles in the size range of 3-25 nm with 1.4% per year at SMEAR II during the years 1997-2012. Additionally, we compared our simulated OH, NO and HSO concentrations with proxies, which are commonly applied in case a limited number of parameters are measured and no detailed model simulations are available.