Arctic lower tropospheric aerosol trends and composition at Alert, Canada: 1980–1995

Abstract

Fifteen years (1980 to 1995) of observations of weekly mean concentrations of 18 constituents in the aerosol of the lower Arctic troposphere at Alert, Canada, have been analyzed by time series analysis for seasonal and long‐term trends and by positive matrix factorization for aerosol components. The dominant anthropogenic constituent measured is SO 42− which is composed of two major components: a neutralized ammonium sulphate salt and a highly acidic sulphate salt. The latter component becomes increasingly prominent as the Arctic atmosphere goes from complete darkness in January to complete sunlight in April and SO 2 is oxidized to sulphuric acid in the northern troposphere. In winter/spring the 18 observed constituents are associated with 10 distinct aerosol factors (components). They are as follows: PHOTO‐S (i.e., anthropogenic sulphur from oxidation of SO 2 in the Arctic), ANTHRO‐S, MIXED PHOTO‐S/SEA‐SALT, MSA (i.e., biogenic sulphur), SEA‐SALT, SMELTER, SOIL, NITRATE, BROMINE, and IODINE. Most winter/spring pollution comes to Alert from Eurasia. In the 1980s, trends in the winter/spring levels of anthropogenic constituents are evident for some (Pb, Zn, Cu, xV, and xMn) but not for SO 42−. The lack of trend in SO 42− is consistent with no change in emissions in the former Soviet Union between 1985 and 1990. However, since 1991, all of these constituents show a marked decrease suggesting that the collapse of industry in the early years of the new Eurasian republics has left its mark in the Arctic. The exception to this explanation is Pb which has clearly seen a major decline (70%) due to decreased use of Pb additives in automobile fuel in western Europe. Natural aerosol components are SEA‐SALT, SOIL, MSA, BROMINE, and IODINE. The first two are related to mechanical generation of aerosols and long‐range transport. SEA‐SALT peaks in late autumn and winter due to highest winds in the North Atlantic and North Pacific Ocean source regions. SOIL is composed of a local component and a long‐range Asian dust component that peaks in spring. MSA is a product of marine biogenic production and shows two peaks: one in May due to long‐range transport from the North Atlantic and one in summer due to more local sources in the Arctic Ocean. BROMINE and IODINE aerosol components are associated with lower tropospheric ozone depletion at polar sunrise. In autumn, aerosol iodine but not bromine has a secondary peak.