Importance of deposition processes in simulating the seasonality of the Arctic black carbon aerosol

Abstract

[1] Anthropogenic aerosol components in the Arctic troposphere, such as black carbon (BC), show a strong seasonal variation characterized by a peak in later winter and early spring. The seasonality, however, is not properly simulated by most existing global aerosol models. Using the Canadian global air quality model with an online aerosol algorithmGlobal Environmental Multiscale model with Air Quality processes (GEM-AQ), this work investigates the mechanisms of the seasonal variation of the Arctic BC. Through enhancements to parameterizations of wet and dry depositions in the Canadian Aerosol Module (CAM), the GEM-AQ model is able to simulate the observed seasonality of BC over the Arctic. The observed seasonality of Arctic BC is mainly attributed to the seasonal changes in aerosol wet scavenging. Seasonal injection of aerosols (e.g., BC from the European and the former USSR sectors and to a less extent from the North Atlantic sector) also contributes to the seasonality of Arctic aerosols in the lower troposphere. Although dry deposition has little effect on the seasonal pattern of BC in the Arctic lower troposphere, it significantly changes BC surface concentration in the Arctic. The enhanced model suggests an annual budget of BC deposition to the Arctic of 0.11 Tg, a 10% increase over the original estimation. The enhanced GEM-AQ model also suggests that the below-cloud scavenging dominates the contribution of BC removal over the Arctic with an estimation of 48% for 2001, whereas the contributions of in-cloud scavenging and dry deposition contribute about 27% and 25%, respectively. The estimated global BC burden is 0.28 Tg, which implies a global average BC lifetime of 9.2 days, whereas the AeroCom project suggests a range of 4.9-11.4 days.

Sources of carbonaceous aerosols and deposited black carbon in the Arctic in winter-spring: implications for radiative forcing

“…Wet processes in our model account for 85-91 % of total BC deposition to the Arctic in winter-spring. This is higher than in the previous model studies of Huang et al (2010) and Liu et al (2011), but consistent with the studies of . Spackman et al (2010) inferred a dry deposition flux for BC of 100-5300 ng m −2 day −1 over snow/ice during ARCPAC on the basis of observed BC depletion in the boundary layer.…”

“…Dry deposition in GEOS-Chem follows a standard resistance-in-series scheme (Wesely, 1989) as implemented by Wang et al (1998), with deposition velocities calculated locally using GEOS-5 data for surface values of momentum and sensible heat fluxes, temperature, and solar radiation. The global annual mean dry deposition velocity is 0.1 cm s −1 for BC and OA, typical of current models (Reddy and Boucher, 2004;Huang et al, 2010). Over snow/ice the Wesely (1989) parameterization yields a mean dry deposition velocity of 0.08 cm s −1 .…”

“…The order-of-magnitude differences between models may be explained by inadequate representations of wet scavenging, which is particularly important for modeling BC in the Arctic because of multiple e-folding loss during transport from northern mid-latitudes (Liu et al, 2011). Individual model studies targeting the Arctic demonstrate much better comparisons to BC observations (Koch and Hansen, 2005;Koch et al, 2009b;Huang et al, 2010;Liu et al, 2011), with our evaluation being the most extensive by encompassing surface air, aircraft, and snow observations. Huang et al (2010) show good comparisons to observed BC concentrations in surface air while Koch and Hansen (2005) and Liu et al (2011) reproduce the seasonality but still underestimate the winter-spring maximum by a factor of 2-3.…”

“…Individual model studies targeting the Arctic demonstrate much better comparisons to BC observations (Koch and Hansen, 2005;Koch et al, 2009b;Huang et al, 2010;Liu et al, 2011), with our evaluation being the most extensive by encompassing surface air, aircraft, and snow observations. Huang et al (2010) show good comparisons to observed BC concentrations in surface air while Koch and Hansen (2005) and Liu et al (2011) reproduce the seasonality but still underestimate the winter-spring maximum by a factor of 2-3. Comparisons to ARCTAS vertical profiles show slight underestimation in Koch et al (2009b) and no significant bias in Liu et al (2011).…”

Sensitivity of remote aerosol distributions to representation of cloud–aerosol interactions in a global climate model

“…Other studies with somewhat-higher emissions produced BC burdens of 0.2-0.3 Tg C. In the first AeroCom intercomparison , the median emissions, burden, and lifetime were 11.3 Tg C yr −1 , 0.21 Tg C, and 6.54 days, respectively. With BC emission of 10.9 Tg C yr −1 , Huang et al (2010) estimate an annual average BC burden of 0.28 Tg C and lifetime of 9.2 days. Actual BC burdens may be even higher, as Koch et al (2009b) showed that for the AeroCom models the simulated column BC burden over six regions is about half of that estimated from AERONET retrievals.…”