Arctic Aerosols: Composition, Sources and Transport

Abstract

Arctic aerosols originate from anthropogenic, marine and terrestrial soil sources as well as gas to particle conversion of gaseous sulphur, nitrogen and hydrocarbon compounds. The anthropogenic aerosol commonly known as Arctic haze peaks in concentration (SO4= 1.5 to 4 $μ$g m−3) in the northern lower troposphere (0 to 5 km) from about November to May. It is a minimum in summer ({\textasciitilde} 0.1 $μ$g m−3). Arctic aerosol composition varies with particle size. Below 1 $μ$m diameter, they are predominantly acidic sulphates with black carbon inclusions in about half the particles. Above 1 $μ$m, they consist mainly of sea salt and soil derived particles that have been modified by reaction with acidic sulphur and nitrogen compounds as well as with organic gases to form formate and acetate. Wind blown dust is transported to the Arctic from southern desert sources mainly in the spring months of April and May when dust storms are most common in the source regions and when transport to the north is still strong. Sea salt aerosols peak mostly in the period October to May as a result of stormier conditions in the northern oceans during the winter half of the year and longer aerosol residence times in arctic air in winter than in summer. Anthropogenic primary aerosol constituents (i.e. those released directly into the atmosphere that have no gaseous precursors in the atmosphere), such as black carbon, Pb and V, peak in the high arctic troposphere in December to March while those that have not only a primary component but also a gaseous precursor such as SO4= tend to peak over a longer period from December well into May as gases convert to particles at polar sunrise. Halogens such as Br and I of marine origin have a different seasonal cycle that those above. Both peak at polar sunrise in March and April but only I has a secondary peak in September and October. Br and sulphuric acid aerosols are implicated in lower tropospheric ozone depletion at polar sunrise over the Arctic ocean. The source region of aerosols to the northern region varies markedly with altitude, a phenomenon that is critical in understanding the origin of aerosol constituents deposited to Arctic glaciers. Arctic glaciers range in altitude from 1.6 km on northern Ellesmere Island to 3 km on Greenland to 5.5 km on Mt. Logan in the Canadian Rockies. At lower altitudes during winter, transport to the Arctic of anthropogenic materials favours Eurasian sources. As altitude increases the potential source region in winter expands to include North American and southeast Asian sources as well. This qualitative picture is consistent with aerosol lead isotope observations in the Arctic.