Both black carbon (BC) and brown carbon (BrC) absorb sunlight, leading to warming of the atmosphere, yet understanding of the absolute and relative contributions of both is incomplete. While lab studies and theory demonstrate that coating of secondary material on black carbon particles can enhance absorption by black carbon, field observations have yielded conflicting results regarding the extent to which this enhancement actually occurs in the atmosphere.
Making measurements in two distinct environments—wintertime Fresno in the San Joaquin Valley and summertime Fontana in the South Coast Air Basin of California—Cappa et al.  show that coatings have little influence on the absorption by ambient black carbon. Most likely, this is owing to the coating material being distributed non-evenly across the entire population of particles and to individual black carbon-containing particles having complex shapes.
Citation: Cappa, C. D., Zhang, X., Russell, L. M., Collier, S., Lee, A. K. Y., Chen, C.‐L., et al . Light absorption by ambient black and brown carbon and its dependence on black carbon coating state for two California, USA cities in winter and summer. Journal of Geophysical Research: Atmospheres, 124. https://doi.org/10.1029/2018JD029501
—Minghua Zhang, Editor in Chief, JGR: Atmospheres