24 Apr 2023
 | 24 Apr 2023
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Measurements and Calculations of Enhanced Side/Back Scattering of Visible Radiation by Black Carbon Aggregates

Carynelisa Haspel, Cuiqi Zhang, Martin Johann Wolf, Daniel James Cziczo, and Maor Sela

Abstract. Aerosol particles have both natural and anthropogenic origins and are ubiquitous in the atmosphere. One particularly important type is carbonaceous aerosol, including a specific subset, often termed ‘elemental carbon’ chemically or ‘black carbon’ (BC) radiatively. Carbonaceous aerosol particles have implications for atmospheric chemistry, human health, and climate both directly and via their ability to act as site of cloud droplet or ice crystal formation. Laboratory experiments and theory are needed to better understand these particles, specifically their radiative impact. We present here laboratory measurements of side/back scattering of visible radiation by analogues of atmospheric BC aggregates obtained using a depolarizing optical particle counter and accompanying theoretical calculations of scattering by compact and fractal theoretical BC aggregates. We show that with random-orientation, the theoretical calculations reproduce the qualitative behavior of the measurements but are unable to reproduce the highest values of the linear depolarization ratio; we are only able to obtain high values of the linear depolarization ratio using fixed orientation. Thus, we suggest that it is possible that models of scattering by BC aggregates that employ the random orientation assumption/option may underpredict the linear depolarization ratio of actual BC aggregates. Both our measurements and our theoretical calculations point to the possibility that bare (uncoated) BC aggregates, as opposed to the aged/coated BC or soot that was investigated in previous studies, can exhibit higher backscattering linear depolarization than previously assumed.

Carynelisa Haspel et al.

Status: open (extended)

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  • RC1: 'Comment on egusphere-2023-556', Anonymous Referee #1, 15 May 2023 reply

Carynelisa Haspel et al.

Carynelisa Haspel et al.


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Short summary
Small particles, commonly termed aerosols, can be found throughout the atmosphere and come from both natural and anthropogenic sources. One important type of aerosol is black carbon (BC). In this study, we conducted laboratory measurements of light scattering by particles meant to mimic atmospheric BC and compared them to calculations of scattering. We find that it is likely that calculations underpredict the scattering by BC particles of certain polarizations of light in certain directions.