Changes in background aerosol composition in Finland during polluted and clean periods studied by TEM/EDX individual particle analysis

J. V. Niemi (1) , S. Saarikoski (2) , H. Tervahattu (3) , T. Mäkelä (2) , R. Hillamo (2) , H. Vehkamäki (4) , L. Sogacheva (4) , M. Kulmala (4)

1 Division of Population Biology
2 FMI - Finnish Meteorological Institute
3 Nordic Envicon Ltd.
4 Department of Physics [Helsinki]

Aerosol samples were collected at a rural background site in southern Finland in May 2004 during pollution episode (PM₁~16 µg m^?3, backward air mass trajectories from south-east), intermediate period (PM₁~5 µg m^?3, backtrajectories from north-east) and clean period (PM₁~2 µg m^?3, backtrajectories from north-west/north). 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 PM_0.2?1 and PM_1?3.3 size fractions were strongly internally mixed with S, C and/or N. The major particle types in PM_0.2?1 samples were 1) soot and 2) (ammonium)sulphates and their mixtures with variable amounts of C, K, soot and/or other inclusions. Number proportions of those two particle groups in PM_0.2?1 samples were 0?12% and 83?97%, respectively. During the pollution episode, the proportion of Ca-rich particles was very high (26?48%) in the PM_1?3.3 and PM_3.3?11 samples, while the PM_0.2?1 and PM_1?3.3 samples contained elevated proportions of silicates (22?33%), 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, when air masses arrived from the Arctic Ocean, PM_1?3.3 samples contained mainly sea salt particles (67?89%) with a variable rate of Cl substitution (mainly by NO₃^?). During the intermediate period, the PM_1?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 PM_3.3?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.

Domaines: Océan, Atmosphère

J. V. Niemi, S. Saarikoski, H. Tervahattu, T. Mäkelä, R. Hillamo, et al.. Changes in background aerosol composition in Finland during polluted and clean periods studied by TEM/EDX individual particle analysis. Atmospheric Chemistry and Physics, 2006, 6 (12), pp.5049-5066. ⟨hal-00296072⟩