Aerosol Decadal Trends: In-situ Measurements of Number Concentration and Optical Properties

摘要

Decadal trends of climate-relevant aerosol variables were evaluated within the framework of the WMO-GAW program, using the latest, quality-controlled information provided by NOAA, IMPROVE and EUSAAR/ACTRIS networks. Few in-situ measurement records of aerosol optical and physical properties exceed 10 years; this paper considers 10-year trends in light scattering, light absorption, and number concentration from 24, 10, and 16 stations, respectively, located in Europe, North America, Antarctica, and on Pacific Ocean islands. Three different statistical methods were used to identify significant trends and calculate their magnitudes, both for the overall record and for each month of the year; the three statistical methods generally agreed on the sign and significance of the trends, but differed on the magnitudes. High natural variability on time scales ranging from hours to years, as well as uncertainties caused by changes and biases in measurement methodology, are a major limitation for deriving statistically relevant trends. Close collaboration with station operators allowed identification of instrument problems and ruptures in the records, such as gaps, instrumental changes, or sampling protocol changes. No significant trends in optical properties were found for the three continental European sites. Statistically significant trends were found for the two European marine sites but the signs of the trends varied with aerosol property and location. Statistically significant decreasing trends for both scattering and absorption coefficient were found for most North American stations, although positive trends were found for a few desert and high-altitude sites. The decrease in scattering was greatest at the IMPROVE stations in the eastern US. No significant trends in scattering coefficient were found for the Arctic or Antarctic stations, whereas the Arctic station had a negative trend in absorption coefficient. The majority of the sites showed clear decreasing trends in number concentration, especially during the winter months. We could not find a consistent agreement between the trends of number concentration and particle optical properties in the few stations with long records of all of these properties. Regardless of the overall tendency of the 10-year trends, analysis of trends for specific seasons reveals additional complexity of aerosol variability.