Long-term aerosol mass concentrations in southern Finland: instrument validation, seasonal variation and trends

Abstract. Long-term high-quality aerosol particulate matter (PM) concentration measurements have been conducted in southern Finland at Station for Measuring Ecosystem-Atmosphere Relations (SMEAR II, Hyytiälä) with different, yet comparable measurement equipment since 1995. In this paper, the mass concentrations measured between 2005 and 2017 using three different independent methods: i.e. 1. the cascade impactor, 2. Differential Mobility Particle Sizer (DMPS) and Aerosol Particle Sizer (APS) and 3. Synchronized Hybrid Ambient Real-time Particulate Monitor (SHARP) are compared and analysed. First, the mass concentrations of the different size classes, i.e. PM1 (PM within sub-micrometer particle diameter size range), PM2.5 (sub-2.5 µm) and PM10 (sub-10 µm), are reported. These data were further cross-compared through a bivariate fitting method. The comparison revealed an excellent equivalence among the three methods with slopes approaching unity and reasonable intercepts (≤ 1 µg m⁻³). An analysis of the seasonal variability of PM concentrations revealed that the mass concentrations were generally highest in summer in different size classes. The mean mass concentrations were 5.3, 5.4, and 6.5 µg m⁻³ for PM1, PM2.5 and PM10, respectively. The 2nd highest loadings were attained in spring, which were ca. 80–88 % of those in summer. The lowest loadings were measured in autumn and winter, when the mass concentrations were ca. 74–78 % of those in summer. Temperature had strong influence on the measured concentrations. While the high late spring and summertime temperatures promote secondary organic aerosol (SOA) formation and pollen emissions, the lowest wintertime temperatures enhance the need of residential heating processes yielding anthropogenic aerosol emissions (e.g. from traffic/industry/wood burning). The wintertime concentrations can also be expected to be influenced by boundary layer dynamics, which keep the PM emissions concentrated near Earth surface especially in winter. It is noteworthy that the mass concentrations were lower than those reported prior to 2005 (at SMEAR II). The descending trend (~−0.1–0.2 µg m³ y⁻¹) was clearly visible here for all PM size classes in spring, summer and winter, while the trend in autumn remained statistically insignificant. This might have resulted at least partly from more stringent EU air quality legislation.