Cross-polar transport and scavenging of Siberian aerosols containing black carbon during the 2012 ACCESS summer campaign

During the ACCESS airborne campaign in July 2012, extensive boreal forest fires resulted in significant aerosol transport to the Arctic. A 10 day episode combining intense biomass burning over Siberia and low-pressure systems over the Arctic Ocean resulted in efficient transport of plumes containing black carbon (BC) towards the Arctic, mostly in the upper troposphere. A combination of in situ airborne observations, satellite analysis and WRF-Chem simulations are used to understand the vertical and horizontal transport mechanisms of BC with a focus on the role of wet removal. During transport to the Arctic region, a large fraction of BC particles are scavenged by two wet deposition processes, namely wet removal by large-scale precipitation and removal in wet convective updrafts, with both processes contributing almost equally to the total accumulated deposition of BC. Our results underline that applying a finer horizontal resolution (40 vs 100km) improves the model performance, as it significantly reduces the overestimation of BC levels observed at a coarser resolution in the mid-troposphere. According to the simulations at 40km, the transport efficiency of BC (TEBC) in biomass burning plumes is about 60%, which is impacted by small accumulated precipitation along trajectory (APT) (1mm). In contrast TEBC is very small (<30%) and APT is larger (5−10 mm) in plumes influenced by urban anthropogenic sources and flaring activities in Northern Russia, resulting in transport to lower altitudes. TEBC due to grid scale precipitation is responsible for a sharp meridional gradient in the distribution of BC concentrations. Wet removal in subgrid parameterized clouds (cumuli) is the cause of modeled vertical gradient of TEBC , especially in the mid-latitudes, reflecting the distribution of convective precipitation, but is dominated in the Arctic region by the grid-scale wet removal associated with the formation of stratocumulus clouds in the PBL that produced frequent drizzle.