Abstract. The Station for Measuring Ecosystem–Atmosphere Relations (SMEAR) II is well
known among atmospheric scientists due to the immense amount of
observational data it provides of the Earth–atmosphere interface. Moreover,
SMEAR II plays an important role for the large European research infrastructure,
enabling the large scientific community to tackle climate- and air-pollution-related questions, utilizing the high-quality long-term data sets recorded
at the site. So far, this well-documented site was missing the description of
the seasonal variation in aerosol chemical composition, which helps
understanding the complex biogeochemical and physical processes governing
the forest ecosystem. Here, we report the sub-micrometer aerosol chemical
composition and its variability, employing data measured between 2012 and
2018 using an Aerosol Chemical Speciation Monitor (ACSM). We observed a
bimodal seasonal trend in the sub-micrometer aerosol concentration culminating
in February (2.7, 1.6, and 5.1 µg m−3 for the median, 25th,
and 75th percentiles, respectively) and July (4.2, 2.2, and 5.7 µg m−3 for the median, 25th, and 75th percentiles, respectively). The
wintertime maximum was linked to an enhanced presence of inorganic aerosol
species (ca. 50 %), whereas the summertime maximum (ca. 80 % organics) was linked to
biogenic secondary organic aerosol (SOA) formation. During the exceptionally
hot months of July of 2014 and 2018, the organic aerosol concentrations were up to
70 % higher than the 7-year July mean. The projected increase in heat
wave frequency over Finland will most likely influence the loading and
chemical composition of aerosol particles in the future. Our findings
suggest strong influence of meteorological conditions such as radiation,
ambient temperature, and wind speed and direction on aerosol chemical
composition. To our understanding, this is the longest time series reported
describing the aerosol chemical composition measured online in the boreal
region, but the continuous monitoring will also be maintained in the future.
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