Factors controlling black carbon distribution in the Arctic

Abstract

Abstract. We investigate the sensitivity of black carbon (BC) in the Arctic, including BC concentration in snow (BC snow , ng g −1 ) and surface air (BC air , ng m −3 ), as well as emissions, dry deposition, and wet scavenging using the global threedimensional (3-D) chemical transport model (CTM) GEOSChem. We find that the model underestimates BC snow in the Arctic by 40 % on average (median = 11.8 ng g −1 ). Natural gas flaring substantially increases total BC emissions in the Arctic (by ∼ 70 %). The flaring emissions lead to up to 49 % increases (0.1-8.5 ng g −1 ) in Arctic BC snow , dramatically improving model comparison with observations (50 % reduction in discrepancy) near flaring source regions (the western side of the extreme north of Russia). Ample observations suggest that BC dry deposition velocities over snow and ice in current CTMs (0.03 cm s −1 in the GEOS-Chem) are too small. We apply the resistance-in-series method to compute a dry deposition velocity (v d ) that varies with local meteorological and surface conditions. The resulting velocity is significantly larger and varies by a factor of 8 in the Arctic (0.03-0.24 cm s −1 ), which increases the fraction of dry to total BC deposition (16 to 25 %) yet leaves the total BC deposition and BC snow in the Arctic unchanged. This is largely explained by the offsetting higher dry and lower wet deposition fluxes. Additionally, we account for the effect of the Wegener-Bergeron-Findeisen (WBF) process in mixed-phase clouds, which releases BC particles from condensed phases (water drops and ice crystals) back to the interstitial air and thereby substantially reduces the scavenging efficiency of clouds for BC (by 43-76 % in the Arctic). The resulting BC snow is up to 80 % higher, BC loading is considerably larger (from 0.25 to 0.43 mg m −2 ), and BC lifetime is markedly prolonged (from 9 to 16 days) in the Arctic. Overall, flaring emissions increase BC air in the Arctic (by ∼ 20 ng m −3 ), the updated v d more than halves BC air (by ∼ 20 ng m −3 ), and the WBF effect increases BC air by 25-70 % during winter and early spring. The resulting model simulation of BC snow is substantially improved (within 10 % of the observations) and the discrepancies of BC air are much smaller during the snow season at Barrow, Alert, and Summit (from −67-−47 % to −46-3 %). Our results point toward an urgent need for better characterization of flaring emissions of BC (e.g., the emission factors, temporal, and spatial distribution), extensive measurements of both the dry deposition of BC over snow and ice, and the scavenging efficiency of BC in mixed-phase clouds. In addition, we find that the poorly constrained precipitation in the Arctic may introduce large uncertainties in estimating BC snow . Doubling precipitation introduces a positive bias approximately as large as the overall effects of flaring emissions and the WBF effect; halving precipitation produces a similarly large negative bias.

Source attribution of Arctic black carbon constrained by aircraft and surface measurements

“…Eckhardt et al (2015) similarly observed enhanced concentrations in July at Barrow in three models (DEHM, CESM1-CAM5 and ECHAM6-HAM2) driven with the GFED3 inventory for biomass burning emissions. At NyÅlesund, all simulations overestimate measured concentrations for most of the year, potentially indicating insufficient wet deposition from riming in mixed phase clouds that occurs more frequently at this site (Qi et al, 2017a). .…”

Long-term measurements (2010–2014) of carbonaceous aerosol and carbon monoxide at the Zotino Tall Tower Observatory (ZOTTO) in central Siberia

“…In addition, during the transition from winter to summer, differential scavenging of the aerosol species can occur in mixed-phase clouds due to the Wegener-Bergeron-Findeisen effect (Wegener, 1911;Bergeron, 1935;Findeisen, 1938). In this type of clouds, the scavenging efficiency of EC decreased by more than a factor of 5 (Cozic et al, 2007;Qi et al, 2017).…”

Impacts of large-scale atmospheric circulation changes in winter on black carbon transport and deposition to the Arctic

“…The large bias of simulated BC and EBC concentrations in the Arctic is a known issue, shared with several global climate and chemical transport models (Eckhardt et al, 2015;Sand et al, 2017). Qi et al (2017) estimated that the Wegener-Bergeron-Findeisen process in mixed-phase clouds increases BC in the atmosphere by 25 to 70 % by reducing wet scavenging efficiency. Other factors which may improve the simulated BC distribution in the Arctic are dry deposition velocities calculated with resistancein-series method over all surfaces (ocean, snow/ice) and improved BC flaring emissions (Qi et al, 2017;Stohl et al, 2013).…”

“…Qi et al (2017) estimated that the Wegener-Bergeron-Findeisen process in mixed-phase clouds increases BC in the atmosphere by 25 to 70 % by reducing wet scavenging efficiency. Other factors which may improve the simulated BC distribution in the Arctic are dry deposition velocities calculated with resistancein-series method over all surfaces (ocean, snow/ice) and improved BC flaring emissions (Qi et al, 2017;Stohl et al, 2013). Jiao et al (2014) show that BC concentrations in snow are poorly correlated with measurements, and a large spread is found among 25 model simulations, with BC lifetime in the Arctic ranging from about 4 to 23 days, implying large differences in local BC deposition efficiency.…”