Measurements and calculations of enhanced side- and back-scattering of visible radiation by black carbon aggregates

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 sites of cloud droplet or ice crystal formation. Laboratory experiments and theory are needed to better understand these particles, specifically their radiative impact. Here we present laboratory measurements of scattering of visible radiation by analogs of atmospheric BC aggregates at scattering angles of 135 +/- 20 circle 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. Both our measurements and our theoretical calculations point to the possibility that fresh, unaged, bare, or uncoated BC aggregates, as opposed to the aged or coated BC or soot that was investigated in previous studies, can exhibit a higher back-scattering linear depolarization than previously assumed.