| Summary: | [1] Large sets of filtered actinometer, filtered pyrheliometer and Sun photometer measurements have been carried out over the past 30 years by various groups at different Arctic and Antarctic sites and for different time periods. They were examined to estimate ensemble average, long-term trends of the summer background aerosol optical depth AOD(500 nm) in the polar regions (omitting the data influenced by Arctic haze and volcanic eruptions). The trend for the Arctic was estimated to be between −1.6% and −2.0% per year over 30 years, depending on location. No significant trend was observed for Antarctica. The time patterns of AOD(500 nm) and Ångström's parameters α and β measured with Sun photometers during the last 20 years at various Arctic and Antarctic sites are also presented. They give a measure of the large variations of these parameters due to El Chichon, Pinatubo, and Cerro Hudson volcanic particles, Arctic haze episodes most frequent in winter and spring, and the transport of Asian dust and boreal smokes to the Arctic region (.) This research was supported by the Programma Nazionale di Ricerche in Antartide (PNRA) and developed as a part of Subproject 2006/6.01: ‘‘POLAR-AOD: a network to characterize the means, variability and trends of the climate-forcing properties of aerosols in polar regions. GOA-UVA thanks the Spanish CICyT for supporting project CGL2006-26188-E/CLI and the EU for funding various proposals (the ALOMAR ARI and eARI (Enhanced Access to Research Infrastructure) Projects and the EU’s 5th and 6th Framework Programme (RITA-CT-2003-506208)).
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