Long-term PM trends at boreal forest site in southern Finland from three different measurement techniques

Abstract. Three independent particulate matter (PM) mass concentration measurements and their long-term (2005–2020) trends were compared at the Station for Measuring Ecosystem–Atmosphere Relations (SMEAR II, Hyytiälä, Finland). The different methods (a gravimetric method with a cascade impactor, an online method with a Synchronized Hybrid Ambient Real-time Particulate monitor (SHARP; only PM10), and a calculated PM concentration from a combined particle number size distribution data of a differential mobility particle sizer (DMPS) and an aerodynamic particle sizer (APS)) showed good correlation (Pearson's correlation coefficient of approximately 0.8) in all size classes (PM1, PM2.5, and PM10). The mass concentrations in all PM classes were the highest in summer and the lowest in autumn and winter. Statistically significant (Mann–Kendall test) declining annual trends were observed in DMPS+APS and impactor data in all size classes, ranging from −0.021 to −0.036 µg m−3 yr−1. While the DMPS+APS method also indicated a statistically significant decline in all seasons, the decline in impactor data was statistically significant only in spring and winter. SHARP data could not be used for trend estimation due to the change in the inlet heating temperature, affecting the measured PM10 concentrations. Seasonally, the decline was smallest in summer, which follows the trends also observed in SO2 and NOx concentrations. The results underline both the summertime dominance of biogenic sources for the aerosol mass concentration in the rural boreal forest environment and the reduction of anthropogenic pollution due to the EU-level restrictions for improved air quality.