Aerosol hygroscopicity influenced by seasonal chemical composition variations in the Arctic region

In this study, we quantified aerosol hygroscopicity parameter using aerosol microphysical observation data (κ_phy), analyzing monthly and seasonal trends in κ_phy by correlating it with aerosol chemical composition over 6 years from April 2007 to March 2013 at the Zeppelin Observatory in Svalbard, Arctic region. The monthly mean κ_phy value exhibited distinct seasonal variations, remaining high from winter to spring, reaching its minimum in summer, followed by an increase in fall, and maintaining elevated levels in winter. To verify the reliability of κ_phy, we employed the hygroscopicity parameter calculated from chemical composition data (κ_chem). The chemical composition and PM_2.5 mass concentration required to calculate κ_chem was obtained through Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2) reanalysis data and the calculation of κ_chem assumed that Arctic aerosols comprise only five species: black carbon (BC), organic matter (OM), ammonium sulfate (AS), sea salt aerosol less than a diameter of 2.5 μm (SSA_2.5), and dust aerosol less than a diameter of 2.5 μm (Dust_2.5). The κ_chem had no distinct correlation but had a similar seasonal trend compared to κ_phy. The κ_chem value followed a trend of SSA_2.5 and was much higher by a factor of 1.6 ± 0.3 than κ_phy on average, due to a large proportion of SSA_2.5 mass concentration in MERRA-2 reanalysis data. This may be due to the overestimation of sea salt aerosols in MERRA-2 reanalysis. The relationship between monthly mean κ_phy and the chemical composition used to calculate κ_chem was also analyzed. The elevated κ_phy from October to February resulted from the dominant influence of SSA_2.5, while the maximum κ_phy in March was concurrently influenced by increasing AS and Dust_2.5 associated with long-range transport from mid-latitude regions during Arctic haze periods and by SSA mass concentration obtained from in-situ sampling, which remained high from the preceding winter. The relatively low κ_phy from April to September can be attributed to low SSA_2.5 and the dominance of organic compounds in the Arctic summer. Either natural sources such as those of marine and terrestrial biogenic origin or long-range-transported aerosols may contribute to the increase in organic aerosols in summer, potentially influencing the reduction in κ_phy of atmospheric aerosols. To our knowledge, this is the first study to analyze the monthly and seasonal variation of aerosol hygroscopicity calculated using long-term microphysical data, and this result provides evidence that changes in monthly and seasonal hygroscopicity variation occur depending on chemical composition.