An AeroCom assessment of black carbon in Arctic snow and sea ice

Abstract

Though many global aerosols models prognose surface deposition, only a few models have been used to directly simulate the radiative effect from black carbon (BC) deposition to snow and sea-ice. Here, we apply aerosol deposition fields from 25 models contributing to two phases of the Aerosol Comparisons between Observations and Models (AeroCom) project to simulate and evaluate within-snow BC concentrations and radiative effect in the Arctic. We accomplish this by driving the offline land and sea-ice components of the Community Earth System Model with different deposition fields and meteorological conditions from 2004–2009, during which an extensive field campaign of BC measurements in Arctic snow occurred. We find that models generally underestimate BC concentrations in snow in northern Russia and Norway, while overestimating BC amounts elsewhere in the Arctic. Although simulated BC distributions in snow are poorly correlated with measurements, mean values are reasonable. The multi-model mean (range) bias in BC concentrations, sampled over the same grid cells, snow depths, and months of measurements, are –4.4 (–13.2 to +10.7) ng g^−1 for an earlier Phase of AeroCom models (Phase I), and +4.1 (–13.0 to +21.4) ng g^−1 for a more recent Phase of AeroCom models (Phase II), compared to the observational mean of 19.2 ng g^−1. Factors determining model BC concentrations in Arctic snow include Arctic BC emissions, transport of extra-Arctic aerosols, precipitation, deposition efficiency of aerosols within the Arctic, and meltwater removal of particles in snow. Sensitivity studies show that the model–measurement evaluation is only weakly affected by meltwater scavenging efficiency because most measurements were conducted in non-melting snow. The Arctic (60–90° N) atmospheric residence time for BC in Phase II models ranges from 3.7 to 23.2 days, implying large inter-model variation in local BC deposition efficiency. Combined with the fact that most Arctic BC deposition originates from extra-Arctic emissions, these results suggest that aerosol removal processes are a leading source of variation in model performance. The multi-model mean (full range) of Arctic radiative effect from BC in snow is 0.15 (0.07–0.25) W m^−2 and 0.18 (0.06–0.28) W m^−2 in Phase I and Phase II models, respectively. After correcting for model biases relative to observed BC concentrations in different regions of the Arctic, we obtain a multi-model mean Arctic radiative effect of 0.17 W m^−2 for the combined AeroCom ensembles. Finally, there is a high correlation between modeled BC concentrations sampled over the observational sites and the Arctic as a whole, indicating that the field campaign provided a reasonable sample of the Arctic

A Modelling Study of the Impact of On-Road Diesel Emissions on Arctic Black Carbon and Solar Radiation Transfer

“…Available observations show high tropospheric BC concentrations over the Arctic up to 8 km in altitude, with magnitudes comparable to profiles measured over polluted areas at northern mid-latitudes [48], thus demonstrating the role of global atmospheric transport mechanisms. As discussed above, the consequent significant BC deposition over Arctic snow and ice tends to reduce the surface albedo [19], thus indirectly contributing to regional (and global) warming. The direct climate effect of Arctic BC is related to the absorption of solar radiation that is efficiently reflected upwards by the snow/ice covered surface, preventing a significant fraction of the surface reflected solar radiation to reach the top of the atmosphere.…”

“…Details on the treatment of emissions and removal processes (i.e., wet/dry deposition and gravitational settling) can be found in Textor et al [32] and Kinne et al [33]. The ULAQ model has extensively participated in several aerosol evaluation campaigns [34][35][36][37] and in BC-specific studies under the AeroCom project [19,30,38] (Aerosol Comparisons between Observations and Models). Since the first radiative calculations made with the ULAQ model in the framework of AeroCom Phase I [22], a new radiative transfer module has been included.…”

“…Consequently, a meaningful impact study on aerosols located in a remote region requires good knowledge of the anthropogenic emissions in the source regions (1), their potential perturbations related to trends or pollution-control measures (such as a limitation of on-road diesel engines) (2), the efficiency of irreversible loss rates (3) and a proper description of the large-scale transport pathways towards the receptor region (4). The Arctic region is vulnerable to BC transport from Northern Hemisphere mid-latitude source regions; over the Arctic, not only is the radiative efficiency of BC the highest, but its snow-albedo forcing is also potentially important [19].…”

“…Upper level air, in turn, is more efficiently affected by transport from the mid-latitudes due to more intense zonal and southerly winds. BC particles transported to the Arctic polar latitudes and deposited over snow and ice on the surface [19] may have important consequences on the polar surface albedo and on local and global climate forcing [20,21]. In addition, the solar radiation absorbing efficiency of BC particles located above a high albedo surface (i.e., ice and snow) is by far larger than for other types of soils or the ocean [22].…”

Impacts of Aerosol Dry Deposition on Black Carbon Spatial Distributions and Radiative Effects in the Community Atmosphere Model CAM5

“…The effect of dry deposition on the standard deviation of CCN is most prominent on the downstream area of source regions in the tropics. Jiao et al () found that large variations exist in the total BC deposition in the Arctic from AeroCom models and suggested that aerosol removal processes are the leading source of variations for modeled BC‐in‐snow concentrations. Using the GEOS‐Chem model, Qi et al () found that V d on snow and ice surfaces strongly affects surface BC concentrations in the high‐latitudes remote regions.…”