Abstract
Abstract. Despite the potential importance of black carbon (BC) for radiative forcing of the Arctic atmosphere, vertically resolved measurements of the particle light scattering coefficient (σsp) and light absorption coefficient (σap) in the springtime Arctic atmosphere are infrequent, especially measurements at latitudes at or above 80∘ N. Here, relationships among vertically distributed aerosol optical properties (σap, σsp and single scattering albedo or SSA), particle microphysics and particle chemistry are examined for a region of the Canadian archipelago between 79.9 and 83.4∘ N from near the surface to 500 hPa. Airborne data collected during April 2015 are combined with ground-based observations from the observatory at Alert, Nunavut and simulations from the Goddard Earth Observing System (GEOS) model, GEOS-Chem, coupled with the TwO-Moment Aerosol Sectional (TOMAS) model (collectively GEOS-Chem–TOMAS; Kodros et al., 2018) to further our knowledge of the effects of BC on light absorption in the Arctic troposphere. The results are constrained for σsp less than 15 Mm−1, which represent 98 % of the observed σsp, because the single scattering albedo (SSA) has a tendency to be lower at lower σsp, resulting in a larger relative contribution to Arctic warming. At 18.4 m2 g−1, the average BC mass absorption coefficient (MAC) from the combined airborne and Alert observations is substantially higher than the two averaged modelled MAC values (13.6 and 9.1 m2 g−1) for two different internal mixing assumptions, the latter of which is based on previous observations. The higher observed MAC value may be explained by an underestimation of BC, the presence of small amounts of dust and/or possible differences in BC microphysics and morphologies between the observations and model. In comparing the observations and simulations, we present σap and SSA, as measured, and σap∕2 and the corresponding SSA to encompass the lower modelled MAC that is more consistent with accepted MAC values. Median values of the measured σap, rBC and the organic component of particles all increase by a factor of 1.8±0.1, going from near-surface to 750 hPa, and values higher than the surface persist to 600 hPa. Modelled BC, organics and σap agree with the near-surface measurements but do not reproduce the higher values observed between 900 and 600 hPa. The differences between modelled and observed optical properties follow the same trend as the differences between the modelled and observed concentrations of the carbonaceous components (black and organic). Model-observation discrepancies may be mostly due to the modelled ejection of biomass burning particles only into the boundary layer at the sources. For the assumption of the observed MAC value, the SSA range between 0.88 and 0.94, which is significantly lower than other recent estimates for the Arctic, in part reflecting the constraint of σsp<15 Mm−1. The large uncertainties in measuring optical properties and BC, and the large differences between measured and modelled values here and in the literature, argue for improved measurements of BC and light absorption by BC and more vertical profiles of aerosol chemistry, microphysics and other optical properties in the Arctic.
Highlights
Aerosol particles responsible for Arctic haze (Mitchell, 1956) originate from midlatitude pollution sources during winter and spring (e.g. Holmgren et al, 1974; Rahn et al, 1977; Rahn, 1981; Shaw, 1983; Barrie and Hoff, 1985; Radke et al, 1984; Schnell and Raatz, 1984; Barrie, 1986)
Leaitch et al.: Vertical profiles of optical properties in the Arctic associated with black carbon ulations, we present σap and single scatter albedos (SSAs), as measured, and σap/2 and the corresponding SSA to encompass the lower modelled mass absorption coefficients (MACs) that is more consistent with accepted MAC values
The airborne data were collected during April 2015 and combined with ground-based observations from the observatory at Alert, Nunavut, and simulations from the GEOS-Chem–TwOMoment Aerosol Sectional (TOMAS) model (Kodros et al, 2018) in an effort to increase our knowledge of the effects of Black carbon (BC) on absorption
Summary
Aerosol particles responsible for Arctic haze (Mitchell, 1956) originate from midlatitude pollution sources during winter and spring (e.g. Holmgren et al, 1974; Rahn et al, 1977; Rahn, 1981; Shaw, 1983; Barrie and Hoff, 1985; Radke et al, 1984; Schnell and Raatz, 1984; Barrie, 1986). Aerosol particles responsible for Arctic haze (Mitchell, 1956) originate from midlatitude pollution sources during winter and spring Radiative forcing by Arctic haze particles may be an important regional driver of Arctic climate change Leighton, 1983; Valero et al, 1984; Hansen and Rosen, 1984; Blanchet and List, 1987; Valero et al, 1989; Pueschel and Kinne, 1995; Hansen and Nazarenko, 2004; Flanner et al, 2007; McConnell et al, 2007; Law and Stohl, 2007; Quinn et al, 2008; Schindell and Faluvegi, 2009; Brock et al, 2011). Despite profiles of black carbon and optical properties in recent years (e.g. Brock et al, 2011; McNaughton et al, 2011; Schwarz et al, 2010; Spackman et al, 2010; Wofsy et al, 2011; Ferrero et al, 2016; Markowicz et al, 2017), there remains a shortage of such observational data that limit evaluation of models of Arctic BC and light absorption (e.g. Samset et al, 2013) because the Arctic is subject to transport from many pollution sources at southern latitudes during winter and spring, and variability exists in altitude, both in location and from year to year
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