Black Carbon Reflects Extremely Efficient Aerosol Wet Removal in
Monsoonal Convective Transport
Abstract
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 (ACCLIP) 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 while 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 non-zero transport of rBC number that is not
relevant to concentration.