Changes in background aerosol composition in Finland during polluted and clean periods studied by TEM/EDX individual particle analysis
1Department of Biological and Environmental Sciences, University of Helsinki, P.O. Box 27, 00014 Helsinki, Finland
2Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland
3Nordic Envicon Ltd., Koetilantie 3, 00790 Helsinki, Finland
4Department of Physical Sciences, University of Helsinki, P.O. Box 64, 00014 Helsinki, Finland
Abstract. Aerosol samples were collected at a rural background site in southern Finland in May 2004. The elemental composition, morphology and mixing state of individual aerosol particles in three size fractions were studied using transmission electron microscopy (TEM) coupled with energy dispersive X-ray (EDX) microanalyses. The TEM/EDX results were complemented with the size-segregated bulk chemical measurements of selected ions and organic and elemental carbon. Many of the particles in PM0.2−1 and PM1−3.3 size fractions were strongly internally mixed with S, C and/or N. The major particle types in all PM0.2−1 samples were ammonium sulphates with/without carbon and dark inclusion(s) (number proportion of particles 54–86%), carbon/sulphate mixture with dark inclusions(s) (4–10%), internally mixed soot/sulphate particles (2–22%), soot (0–12%) and K-rich particles with S and/or C (0–9%). During the pollution episode (PM1.6~18 μg m−3, the proportion of Ca-rich particles was very high (26–48%) in the PM1−3.3 and PM3.3−11 samples, while the PM0.2−1 and PM1−3.3 samples contained elevated proportions of silicates (22–33%, also fly ash particles), metal oxides/hydroxides (1–9%) and tar balls (1–4%). These aerosols originated mainly from polluted areas of Eastern Europe, and some open biomass burning smoke was also brought by long-range transport. During the clean period (PM1.6~2 μg m−3, when air masses arrived from the Arctic Ocean, PM1−3.3 samples contained mainly sea salt particles (67–89%) with a variable rate of Cl substitution (mainly by NO3−. During the intermediate period (PM1.6~5 μg m−3, the PM1−3.3 sample contained porous (sponge-like) Na-rich particles (35%) with abundant S, K and O. They might originate from the burning of wood pulp wastes of paper industry. The proportion of biological particles and C-rich fragments (probably also biological origin) were highest in the PM3.3−11 samples (0–81% and 0–22%, respectively). The origin of different particle types and the effect of aging processes on particle composition and their hygroscopic and optical properties are discussed.