Aerosol absorption measurements at Barrow, Mauna Loa and the south pole

Abstract

Aerosol absorption (σ ap) has been measured continuously using aethalometers at Barrow, Alaska (1986 to present); Mauna Loa, Hawaii (1990 to present); and south pole, Antarctica (1987–1990). These three stations are part of a network of baseline monitoring stations operated by the Climate Monitoring and Diagnostics Laboratory (CMDL) of the National Oceanic and Atmospheric Administration (NOAA). Condensation nucleus (CN) concentration and multiwavelength aerosol scattering (σ sp) have also been measured continuously for many years at these stations. Aethalometer measurements are usually reported in terms of atmospheric black carbon aerosol (BC) concentration using the calibration suggested by the manufacturer. Here we deduce the in situ σ ap(550 nm) from aethalometer measurements by assuming that the aerosol absorption on the aethalometer filter is enhanced by a factor of 1.9 over that in the atmosphere. This is consistent with using 19 m 2 g−1 for the specific absorption of BC on the aethalometer filter and 10 m 2 g−1 for the in situ specific absorption of BC in the atmosphere (the ratio of the two specific absorptions is 1.9). Although these values of specific absorption may vary significantly for different environments, the ratio might be expected to be relatively constant. The single‐scattering albedo, defined by ω = σ sp/(σ sp + σ ap), has been estimated from the simultaneous measurements of σ ap and σ sp. Furthermore, assuming a 1/λ dependence for σ ap in the 450 to 700‐nm wavelength region, multiwavelength σ sp measurements allow the estimation of the wavelength dependence of ω. Each station shows a considerable annual cycle in σ ap, σ sp, and ω. The maximum in the Barrow annual cycle is caused primarily by the springtime Arctic haze phenomenon; the maximum in the Mauna Loa annual cycle is caused by the springtime Asian dust transport; and the maximum in the south pole annual cycle is caused by late winter transport from southern midlatitudes. It was found that annual mean values are σ ap = 4.1 × 10−7 m−1 (≈41 ng m−3 BC) and ω = 0.96 for Barrow; σ ap = 5.8 × 10−8 m−1 (≈5.8 ng m−3 BC) and ω = 0.97 for Mauna Loa; and σ ap = 6.5 × 10 −9 m−1 (≈0.65 ng m−3 BC) and ω = 0.97 for south pole. It was also found that the wavelength dependence of ω may be important at Barrow and south pole, but not important at Mauna Loa.