Seasonal variation of the atmospheric aerosol near the top of the marine boundary layer over Spitsbergen related to the Arctic sulphur cycle^*

* Contribution No. 722.

Abstract

With an emphasis on marine biogenic sulphur the first 26 months of fine particle (< 1 μm radius), aerosol data from the new air chemistry station on Zeppelinfjellet, Spitsbergen (79°N, 12°E, 474 m asl), were evaluated to elucidate source- and transformation processes of the Arctic aerosol. Results from 2 particle counters, an integrating nephelometer, and filter samples were available for our interpretation. On the filters we had analysed soot, (EC), sodium (Na⁺), methansulphonate, MSA^-, and sulphate (SO₄^2-). Fine particle composition revealed a strong regional marine biological source of MSA^-x and SO₄^2- which we estimated to contribute 26% to the non-sea salt sulphur in summer. In winter, no more than 2% of the non-sea-salt sulphur could be attributed to the marine biological source. Rigorous air mass analyses combined with the EC data as a tracer for regional anthropogenic combustion sources showed that this regional biological source became active already in March over the Barents Sea and over the North Atlantic. In summer, the levels of the biogenic sulphur components were very similar to those measured at the southern hemispheric marine site of Cape Grim (1.5 and 0.92 nmol m^-3 for MSA^- and nss-SO₄^2-, respectively). For samples with minimum anthropogenic influence we found a constant MSA^-/nss-SO₄^2- ratio in the fine particle size range. This ratio had a value of 28% and was temperature-independent. Our results comprise the first long-term record of Arctic aerosol data taken in the upper part of the planetary boundary layer which often is influenced by persistent Arctic stratus. With a cloud-segregation scheme we segregated the aerosol data into a group measured interstitially, i.e., inside boundary layer clouds (INT), and an out-of cloud group, (OOC). Average INT/OOC-ratios of fine particle mass, nss-SO₄^2-, and soot were 0.19, 0.21, and 0.21, respectively. While exhibiting similar INT/OOC-ratios in winter, MSA^- had an average INT/OOC-ratio of 0.63 in summer implying that it was less scavenged than the other components. Complementary physical aerosol data corroborated our interpretation of MSA1 on average residing on smaller particles than nss-SO₄^2-. Together with the scavenging results our data support the concept that MSA^- most likely is formed by condensation onto existing particles while SO₄^2- predominantly is formed by in-cloud oxidation of SO₂.

Notes

* Contribution No. 722.