Geochemistry of organic and inorganic ions of late winter arctic aerosols

Abstract

In order to examine possible natural as well as anthropogenic aerosol ionic components in the Arctic troposphere, we have measured the concentrations of 12 organic and inorganic ions in late winter Arctic aerosols at Barrow, Alaska, sampled as separated coarse and fine fractions. Inorganic ion concentrations are similar to previous data reported from the Arctic. The organic anion methanesulfonate (MSA), in total coarse + fine, averages 0.12 ± 0.02 nmol m⁻³. High levels of formate (Fo ⁻) and acetate (Ac⁻) and traces of propionate (Pp⁻) and pyruvate (Py⁻) are found, which altogether account for 20% of the total aerosol mass. Total concentrations, as mean ± S.E. nmol m⁻³, are (Fo⁻) 5.3± 0.7, (Ac⁻) 12.4 ± 2.2, (Pp⁻) 0.3±0.1, and (Py⁻) 0.1 ± 0.04. Internal relationships among the carboxylic acid anions suggest emissions from natural vegetation. Lacking local sources during winter, these organic anions are likely to have come from lower latitudes as acid vapors that condensed with gaseous NH₃ into aerosols in the cold Arctic.

Four aerosol types, evidenced by seven principal components in the coarse and fine aerosol fractions of 69 12-h samples, are found by absolute principal component analysis (APCA). The most prominent type is a contaminated sea salt, apparently transported to the Arctic after scavenging combustion products. The second contains carboxylic acid anions, such as could have resulted from co-condensation with NH₃ of organic acid vapors from natural sources at lower latitudes. The third is a marine aerosol component containing most of the MSA, Br⁻ and NO⁻₃, as well as small amounts of carboxylic acid anions and some sea salt, and may be a collection of products from gas phase oxidation of precursors. Finally, a fine non-sea salt sulfate (nssSO²⁻₄) component is found that may have come from SO₂ conversion in air. Most components have good charge balance of the measured ions as indicated by anion/cation ratios near unity. The ratios reflect approximate acid-base neutralization in the components and indicate aged aerosol systems with long atmospheric residence times.

Viewing similar components in coarse and fine fractions together, about 10% of the carboxylic acid anions are associated with pollutants in aerosol type 1. Type 2 accounts for 80% of Fo⁻and 60% of Ac⁻. Type 3 accounts for 18% of Fo⁻ and 10% of Ac⁻. Thus, the carboxylic acid anions appear to be mostly natural, with more than 90% of Fo⁻ and 70% of Ac⁻ in types 2 and 3. In coarse aerosols viewed separately, 67% of nssSO²⁻₄ is in the contaminated sea salt. In fine aerosols, 52% of nssSO²⁻₄ is in a separate SO²⁻₄ component which may be formed by SO₂ oxidation. Some nssSO²⁻₄ is associated with MSA in both fractions and is attributed to natural marine biogenic S precursors. This SO²⁻₄ is about 20% of total measured nssSO²⁻₄. These results show that natural compounds are measurable constituents of Arctic aerosols and can account for 60% of the total measured aerosol mass. In future Arctic haze studies both natural and anthropogenic substances should be considered.