Rights Reserved. Black Carbon Reflects Extremely Efficient Aerosol Wet Removal in Monsoonal Convective Transport

Refractory black carbon (rBC) is a primary aerosol species, produced through incomplete combustion, that absorbs sunlight and contributes to positive radiative forcing. The overall climate effect of rBC depends on its spatial distribution and atmospheric lifetime, both of which are impacted by the efficiency with which rBC is transported or removed by convective systems. These processes are poorly constrained by observations. It is especially interesting to investigate rBC transport efficiency through the Asian Summer Monsoon (ASM) since this meteorological pattern delivers vast quantities of boundary layer air from Asia, where rBC emissions are high to the upper troposphere/lower stratosphere (UT/LS) where the lifetime of rBC is expected to be long. Here, we present in situ observations of rBC made during the Asian Summer Monsoon Chemistry and Climate Impact Project of summer, 2022. We use observed relationships between rBC and CO in ASM outflow to show that rBC is removed nearly completely (>98%) from uplifted air and that rBC concentrations in ASM outflow are statistically indistinguishable from the UT/LS background. We compare observed rBC and CO concentrations to those expected based on two chemical transport models and find that the models reproduce CO to within a factor of 2 at all altitudes whereas rBC is overpredicted by a factor of 20–100 at altitudes associated with ASM outflow. We find that the rBC particles in recently convected air have thinner coatings than those found in the UTLS background, suggesting transport of a small number of rBC particles that are negligible for concentration. Plain Language Summary Black carbon is a kind of airborne particle that absorbs sunlight and contributes to global warming. Calculations of the amount of global warming that comes from black carbon depend on our ability to predict how long it stays in the atmosphere and where it travels. Although we have reasonable information about where black carbon comes from (any kind of combustion), it has been harder to measure how fast it is removed from the air. Many models assume that airborne black carbon is not as easily removed by convection as it actually is and therefore predict that it will travel long distances and hang around in the air for a long time. This leads to overpredictions in black carbon's warming effect. We measured black carbon in the air that comes out the top of the Asian Summer Monsoon (a huge circulation pattern that moves vast amounts of air from the surface to high altitudes) and found that almost all of it had been removed during transport. This is valuable information for models aimed at predicting global black carbon transport and calculating global radiative forcing.