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

[摘要] Aerosol samples were collected at a rural background site in southernFinland in May 2004 during pollution episode (PM₁~16 µg m⁻³,backward air mass trajectories from south-east), intermediateperiod (PM₁~5 µg m⁻³, backtrajectories fromnorth-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 sizefractions were studied using transmission electron microscopy (TEM) coupledwith energy dispersive X-ray (EDX) microanalyses. The TEM/EDX results werecomplemented with the size-segregated bulk chemical measurements of selectedions and organic and elemental carbon. Many of the particles in PM_0.2–1and PM_1–3.3 size fractions were strongly internally mixed with S, Cand/or N. The major particle types in PM_0.2–1 samples were 1) soot and2) (ammonium)sulphates and their mixtures with variable amounts of C, K,soot and/or other inclusions. Number proportions of those two particle groupsin PM_0.2–1 samples were 0–12% and 83–97%, respectively. Duringthe 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 thePM_0.2–1 and PM_1–3.3 samples contained elevated proportions ofsilicates (22–33%), metal oxides/hydroxides (1–9%) and tar balls(1–4%). These aerosols originated mainly from polluted areas of EasternEurope, and some open biomass burning smoke was also brought by long-rangetransport. During the clean period, when air masses arrived from the ArcticOcean, PM_1–3.3 samples contained mainly sea salt particles (67–89%)with a variable rate of Cl substitution (mainly by NO₃⁻). Duringthe intermediate period, the PM_1–3.3 sample contained porous(sponge-like) Na-rich particles (35%) with abundant S, K and O. Theymight originate from the burning of wood pulp wastes of paper industry. Theproportion of biological particles and C-rich fragments (probably alsobiological origin) were highest in the PM_3.3–11 samples (0–81% and0–22%, respectively). The origin of different particle types and theeffect of aging processes on particle composition and their hygroscopic andoptical properties are discussed.

[学科分类] 大气科学