Aerosols in polar regions: A historical overview based on optical depth and in situ observations

Abstract

[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 À1.6% and À2.0% per year over 30 years, depending on location. No significant trend was observed for Antarctica. The time patterns of AOD(500 nm) and Å ngström's parameters a and b 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. Evidence is also shown of marked differences between the aerosol optical parameters measured at coastal and high-altitude sites in Antarctica. In situ optical and chemical composition parameters of aerosol particles measured at Arctic and Antarctic sites are also examined to achieve more complete information on the multimodal size distribution shape parameters and their radiative properties. A characterization of aerosol radiative parameters is also defined by plotting the daily mean values of a as a function of AOD(500 nm), separately for the two polar regions, allowing the identification of different clusters related to fifteen aerosol classes, for which the spectral values of complex refractive index and single scattering albedo were evaluated. Citation: Tomasi, C., et al. (2007), Aerosols in polar regions: A historical overview based on optical depth and in situ observations,

An update on polar aerosol optical properties using POLAR-AOD and other measurements performed during the International Polar Year

“…Before examining the time series of AOD recorded at the Arctic and Antarctic sites and illustrating the novelty of their analysis, it is useful to describe the procedure adopted by Tomasi et al (2007) to analyze the long-term variations of AOD in the Arctic. The AOD measurements were performed at:…”

“…Examining the above sets of daily AOD measurements, Tomasi et al (2007) determined three series of monthly mean values of AOD by first discarding the AOD data recorded during the 1982e1984 and 1992e1994 periods, because they were strongly affected by stratospheric particle extinction effects due to the El Chichón and Pinatubo eruptions, respectively. Subsequently, only the data relative to the four summer months from June to September were considered when calculating the long-term variations of AOD, with the background contribution hereafter referred to as BG, and enhancements attributed to extinction mainly by smoke particles from boreal forest fires (hereafter referred to as FFS) in North America and Siberia (Forster et al, 2001;Damoah et al, 2004;Stohl et al, 2006;Tunved et al, 2006).…”

“…The decreases were attributed mainly to the diminishing anthropogenic emissions of SO 2 , observed especially since 1985 in Europe and Russia, and to the large year-to-year changes in atmospheric transport of anthropogenic aerosols from Europe and Asia (Jaffe et al, 1995), possibly associated with variations in atmospheric circulation patterns that affect cloudiness, temperature and snowfall (Stone, 1997;Stone et al, 2002). The time series of the daily mean values of AOD measured at seven Arctic stations from 1991 to 2006 were analyzed by Tomasi et al (2007) for the purpose of providing an overall evaluation of the variability of Arctic Haze (hereinafter referred to as AH) formation processes during the winterespring period at the various sites. Conversely, no evidence was found of significant AOD variations at Antarctic sites.…”

“…To better define the radiative properties of polar aerosols, Tomasi et al (2007) analyzed a large set of AOD measurements performed over the last 30 years at numerous Arctic and Antarctic sites using filtered actinometers, multi-filter pyrheliometers and multispectral sun-photometers. Three time series of summer AOD measurements obtained for clean-air conditions at Barrow (from 1977Barrow (from to 2002, Ny-Ålesund (from 1991Ny-Ålesund (from to 2006, and five Siberian Arctic sites (from 1981 to 1991) were analyzed to determine the long-term average variations in AOD and Ångström's atmospheric turbidity parameters over the last three decades, finding negative values of the yearly long-term average trend of summer-time AOD, equal to À1.6% at Barrow, À2.2% at Ny-Ålesund, and À7.6% at the Siberian sites.…”

Remote sensing of aerosols over snow using infrared AATSR observations

“…At the same time, satellite retrievals of AOT over snow are necessary to assess and understand the role of aerosol at high latitudes. Aerosols affect energy balance of the Polar Regions in many ways, both on local and global scale (Tomasi et al, 2007). The impact of aerosol particles on climate is considered to be one of the largest uncertainties in climate modeling.…”