Sulfate trends in a Col du Dôme (French Alps) ice core: A record of anthropogenic sulfate levels in the European midtroposphere over the twentieth century

A high-resolution sulfate record from a Col du Dome (CDD, 4250 m above sea level, French Alps) ice core was used to investigate the impact of growing SO2 emissions on the midtroposphere sulfate levels over Europe since 1925. The large annual snow accumulation rate at the CDD site permits examination of the summer and winter sulfate trends separately. Being close to 80±10 ng g−1 in preindustrial summer ice, sulfate CDD summer levels then increase at a mean rate of 6 ng g−1 per year from 1925 to 1960. From 1960 to 1980 the increase continued at a rate of 24 ng g−1 per year. Concentrations reach a maximum of 860 ng g−1 in 1980 and subsequently decrease to 600 ng g−1 in the 1990s. These summer sulfate changes closely follow the course of growing SO2 emissions from source regions located within 700–1000 km around the Alps (France, Italy, Spain, and to a lesser extent, former West Germany). In winter the CDD sulfate levels are 3 to 8 times lower than in summer because of more limited upward transport of air masses from the boundary layer at that season. Being close to 20 ng g−1 in the preindustrial ice, winter levels were regularly enhanced at a mean annual rate of 1.2 ng g−1 from 1925 to 1980. The weak winter change from the preindustrial era to 1980 (a factor of 4 instead of 10 in summer) reflects a limited contamination of the free troposphere which, in contrast to summer, occurs at a larger scale (total Europe/former USSR). Intimately connected to Europe, these long-term changes in the Alps clearly differ in time and amplitude with the ones revealed by Greenland ice cores which indicate an increase by a factor of 3 between 1880 and 1970 in relation with long-range transport of pollutants from Eurasia as well as from North America. Furthermore, because of a lower natural contribution to the total sulfate level the anthropogenic changes can be more accurately derived in the Alps than in Greenland. Using the observed relationship between present-day concentrations in air and snowpack, the CDD ice core record permits reconstruction of present and past atmospheric sulfate concentrations at 4300 m above sea level over Europe in summer and winter. These data are compared with the sulfate levels simulated by current global sulfur models at 600 hPa for which uncertainties still range within a factor of 2. Together with observations made at lower elevation in the early 1990s the atmospheric levels derived for the CDD site (∼20 and 400 ng m−3 STP in winter and summer, respectively) document the vertical sulfate distribution between the ground and 4300 m elevation over western Europe at that time. In this way, data gained at high-elevation Alpine sites are powerful in evaluating the recent role of sulfate aerosol in forcing the climate over Europe.