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

Abstract

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.