Assessing the Impact of Self-Lofting on Increasing the Altitude of Black Carbon in a Global Climate Model

Black carbon (BC) absorbs solar radiation, increasing the buoyancy and vertical ascent of absorbing aerosol in the atmosphere. This self-lofting process has been observed for individual plumes in the troposphere and lower stratosphere but here we show it occurring at broader scales through enhanced large-scale ascent over BC-rich regions. This is demonstrated in a pair of simulation using the UKESM1 Earth-System model where BC aerosols were modeled either with or without the ability to absorb radiation. With absorption included the annual global mean concentration of BC in the upper troposphere and lower stratosphere (8-22 km) rose by up to 50% and the column loading over some remote oceanic regions more than doubled. The increase in aerosol height was particularly notable over the southeast Atlantic where biomass burning aerosol from Africa was elevated up to 1 km higher when their absorption was included. Similar effects were seen over the Arctic where the absorbing haze was transported in at higher levels and surface concentrations were halved. The absorption by BC also increased ascent over southern Asia, which tended to thicken the Asian brown cloud during the dry season but in the wet season enhancing ascent promoted deep convection and had the tendency to deplete the aerosol through wash-out. We conclude that representing aerosol absorption accurately is important in simulating the vertical distribution, transport and abundance of aerosol in the Earth-system that will affect their interactions with climate.