Investigating Carbonaceous Aerosol and Its Absorption Properties From Fires in the Western United States (WE‐CAN) and Southern Africa (ORACLES and CLARIFY)

Abstract

Biomass burning (BB) produces large quantities of carbonaceous aerosol (black carbon and organic aerosol, BC and OA, respectively), which significantly degrade air quality and impact climate. BC absorbs radiation, warming the atmosphere, while OA typically scatters radiation, leading to cooling. However, some OA, termed brown carbon (BrC), also absorbs visible and near UV radiation; although, its properties are not well constrained. We explore three aircraft campaigns from important BB regions with different dominant fuel and fire types (Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen [WE‐CAN] in the western United States and ObseRvations of Aerosols above CLouds and their intEractionS and Cloud‐Aerosol‐Radiation Interactions and Forcing for Year downwind of southern Africa) and compare them with simulations from the global chemical transport model, GEOS‐Chem using GFED4s. The model generally captures the observed vertical profiles of carbonaceous BB aerosol concentrations; however, we find that BB BC emissions are underestimated in southern Africa. Our comparisons suggest that BC and/or BrC absorption is substantially higher downwind of Africa than in the western United States and, while the Saleh et al. (2014, https://doi.org/10.1038/ngeo2220) and FIREX parameterizations based on the BC:OA ratio improve model‐observation agreement in some regions, they do not sufficiently differentiate absorption characteristics at short wavelengths. We find that photochemical whitening substantially decreases the burden and direct radiative effect of BrC (annual mean of +0.29 W m−2 without whitening and +0.08 W m−2 with). Our comparisons suggest that whitening is required to explain WE‐CAN observations; however, the importance of whitening for African fires cannot be confirmed. Qualitative comparisons with the OMI UV aerosol index suggest our standard BrC whitening scheme may be too fast over Africa.

Hygroscopicity of Secondary Brown Carbon Aerosol from Aqueous Photo-Oxidation of Phenolic Precursors

“…Light-absorbing organic aerosol, or brown carbon (BrC), impacts climate directly by scattering and absorbing sunlight , and indirectly by influencing cloud formation and properties. , Together, these effects are predicted to result in a significant overall warming effect for BrC, , but there are uncertainties regarding both the direct and indirect radiative effects. − Primary BrC, including nitrophenols emitted from biomass burning, − may react with light or oxidants and ultimately whiten during its atmospheric lifetime. − Initially nonabsorbing emissions may react to give secondary BrC, − potentially counteracting the whitening of primary BrC to some extent. − If primary BrC from a particular source were not highly absorbing or if it were to whiten rapidly, the warming effect of BrC would be lessened. Furthermore, if BrC were to become more hygroscopic during its atmospheric lifetime, generally consistent with oxidation, the cooling effect of enhanced cloud activation would be increased.…”

Accurate Measurement of the Optical Properties of Single Aerosol Particles Using Cavity Ring-Down Spectroscopy

“…Numerous organic species absorb in the visible window, complicating the tractability of this problem. Moreover, these species undergo photooxidation chemistry that bleaches the chromophores and therefore drives temporal evolution of the light absorption over the atmospheric lifetimes of aerosol particles. , Measuring the complex refractive indices for droplets containing various BrC chromophores is a challenge well-suited to our single-particle CRDS technique. Deriving the changes in n and k as levitated organic particles are exposed to reactive gases (e.g., NO x , O 3 ) and actinic flux will deepen current understanding of the kinetics of chromophore formation and loss in the natural environment.…”

Identifying the Fraction of Core–Shell Black Carbon Particles in a Complex Mixture to Constrain the Absorption Enhancement by Coatings

“…The evolution of BC at the level of single particles however raises more complexity as ambient BC contains a range of morphologies, and this could have large spatiotemporal variation. Recent studies using combined field work still found large discrepancies even at the same coating/rBC ratio. , This may be explained by the heterogenicity of particles when the coatings are distributed to a range of individual BC particles in bulk. − To identify the population of single BC in bulk for which the core–shell microstructure should be applied (hereby E abs exists) or not is important to estimate the overall absorbing properties. Previous studies identified a transition regime of BC where hybrid optical models (Mie core–shell approach or without E abs ) should be applied, which corresponded with a coating/rBC mass ratio between 1 and 3 or at other ratio ranges depending on environments .…”

“…Recent studies using combined field work still found large discrepancies even at the same coating/rBC ratio. 11,12 This may be explained by the heterogenicity of particles when the coatings are distributed to a range of individual BC particles in bulk. 13−15 To identify the population of single BC in bulk for which the core−shell microstructure should be applied (hereby E abs exists) or not is important to estimate the overall absorbing properties.…”