Aerosol decadal trends – Part 1: In-situ optical measurements at GAW and IMPROVE stations

摘要

Currently many ground-based atmospheric stations include in-situ measurementsof aerosol physical and optical properties, resulting in more than 20long-term (> 10 yr) aerosol measurement sites in the Northern Hemisphereand Antarctica. Most of these sites are located at remote locations andmonitor the aerosol particle number concentration, wavelength-dependent lightscattering, backscattering, and absorption coefficients. The existence ofthese multi-year datasets enables the analysis of long-term trends of theseaerosol parameters, and of the derived light scattering ?ngstr?mexponent and backscatter fraction. Since the aerosol variables are notnormally distributed, three different methods (the seasonal Mann-Kendall testassociated with the Sen's slope, the generalized least squares fit associatedwith an autoregressive bootstrap algorithm for confidence intervals, and theleast-mean square fit applied to logarithms of the data) were applied todetect the long-term trends and their magnitudes. To allow a comparison amongmeasurement sites, trends on the most recent 10 and 15 yr periods werecalculated. No significant trends were found for the three continentalEuropean sites. Statistically significant trends were found for the twoEuropean marine sites but the signs of the trends varied with aerosolproperty and location. Statistically significant decreasing trends for bothscattering and absorption coefficients (mean slope of−2.0% yr?1) were found for most North American stations,although positive trends were found for a few desert and high-altitude sites.The difference in the timing of emission reduction policy for the Europe andUS continents is a likely explanation for the decreasing trends in aerosoloptical parameters found for most American sites compared to the lack oftrends observed in Europe. No significant trends in scattering coefficientwere found for the Arctic or Antarctic stations, whereas the Arctic stationhad a negative trend in absorption coefficient. The high altitude Pacificisland station of Mauna Loa presents positive trends for both scattering andabsorption coefficients.