Evaluating and Improving Light Absorption Retrievals of Black Carbon Using In Situ Polar Nephelometry

Black carbon (BC) is among the major contributors to
global warming,
yet significant uncertainties exist in remote sensing retrievals of
BC light absorption. A key issue is the mismatch between the simplified
spherical morphology assumption commonly used in these retrievals
and the actual fractal-like morphology of BC particles. In situ polar
nephelometry provides a unique opportunity to improve these retrieval
algorithms. Laboratory-based polarimetric measurements allow for a
comparison of retrieved and directly measured properties using independent
instrumentation. In our experiments, bare BC aggregates were generated,
and phase functions were measured using our newly developed polar
nephelometer uNeph. Standard retrievals based on Lorenz-Mie theory
poorly reproduced the phase function and polarized phase function
of BC, leading to significant bias in retrieved properties beyond
the uncertainty of independent measurements. Contrary to previous
studies, we demonstrate a good closure between measured and simulated
phase functions when using the Multi-Sphere T-Matrix (MSTM) method
for BC aggregates in the accumulation size range. BC properties, particularly
absorption coefficient and volume concentration, were accurately and
precisely retrieved by accounting for the fractal-like morphology.
Only two additional parameters were used in MSTM retrieval. This
suggests that considering aggregates in remote sensing retrievals
under real atmospheric conditions could be feasible.