Long-term PM trends in southern Finland from three different measurement techniques

Abstract. Different particulate matter (PM) mass concentration measurements and their long-term trends were compared at the Station for Measuring Ecosystem-Atmosphere Relations (SMEAR II, Hyytiälä, Finland). We compare three independent methods: 1) gravimetric method with a cascade impactor, 2) Synchronized Hybrid Ambient Real-time Particulate Monitor (SHARP), and 3) calculated PM concentration from combined Differential Mobility Particle Sizer (DMPS) and Aerosol Particle Sizer (APS) particle number size distribution data. In all size classes (PM₁, PM_2.5 and PM₁₀), the different methods show a good correlation (Pearson’s correlation coefficient approximately 0.8). The mass concentrations in all PM classes were the highest in summer and the lowest in autumn and winter. While all seasons and size classes showed declining trends for PM concentrations (from -0.012 to -0.064 µg m^-3 y^-1) between 2005 and 2020, the decline was smallest in summer, which follows the trends observed also in SO₂ and NO_x concentrations. These 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.