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.

Keywords: Multi-Sphere T-Matrix; aerosol property retrieval; black carbon aggregate; light absorption; light scattering; phase function; polarimetry.

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Experimental setup. uNeph: polar nephelometer; AAC: Aerodynamic Aerosol Classifier; MFC: Mass Flow Controller; CPC: Condensation Particle Counter; SMPS: Scanning Mobility Particle Sizer; APM: Aerosol Particle Mass analyzer; PAAS: Photo-Acoustic Absorption Spectrometer.

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Retrieved CRI compared with literature data for monodisperse PSL and nigrosin aerosol samples of different sizes − as indicated in symbol size ranging from 150 to 1000 nm. Both the real and imaginary parts of the refractive index were free parameters in either case.

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(a–d) Phase function and polarized phase function of aggregates: example measurement and fit. Gray shading indicates the estimated measurement error taken from a previous study.

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Performance of different optical kernels: MSTM versus Mie for the aerosol parameter a) absorption coefficient, b) volume concentration, c) volume equivalent diameter, and d) particle number concentration.

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(a–f) Precision of retrieved and derived aerosol parameters (statistics of the top 50 ranked state parameter combinations relative to their mean value). Results for different aerodynamic diameters are shown in separate panels. Boxes show the interquartile range (IQR), and the whiskers extend to the farthest data point lying within the 10th or 90th percentile (d _pp: primary monomer diameter; c _vol: volume concentration; b _abs: absorption coefficient).

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Benchmarking uNeph-MSTM retrieval results (red) for size-selected BC aggregates against independent measurements (black) of a) absorption coefficient, b) volume concentration, c) volume equivalent diameter, and d) single scattering albedo. Box and whiskers indicate retrieval precision; the gray shading indicates uncertainty of the independent data. Orange markers indicate uNeph-MSTM retrieval results, enforcing identical complex refractive index for all sizes.