. Organic aerosols (OA) that strongly absorb solar radiation in the near-UV are referred to as brown carbon (BrC). The sources, evolution, and optical properties of BrC remain highly uncertain and contribute significantly to uncertainty in the estimate of the global direct radiative effect (DRE) of aerosols. Previous modeling studies of BrC optical properties and DRE have been unable to fully evaluate model performance due to the lack of direct measurements of BrC absorption. In this study, we develop a global model simulation (GEOS-Chem) of BrC and test it against BrC absorption measurements from two aircraft campaigns in the continental US (SEAC4RS and DC3). To the best of our knowledge, this is the first study to compare simulated BrC absorption with direct aircraft measurements. We show that BrC absorption properties estimated based on previous laboratory measurements agree with the aircraft measurements of freshly emitted BrC absorption but overestimate aged BrC absorption. In addition, applying a photochemical scheme to simulate bleaching/degradation of BrC improves model skill. The airborne observations are therefore consistent with a mass absorption coefficient (MAC) of freshly emitted biomass burning OA of 1.33 m2 g−1 at 365 nm coupled with a 1-day whitening e-folding time. Using the GEOS-Chem chemical transport model integrated with the RRTMG radiative transfer model, we estimate that the top-of-the-atmosphere all-sky direct radiative effect (DRE) of OA is −0.344 Wm−2, 10 % higher than that without consideration of BrC absorption. Therefore, our best estimate of the absorption DRE of BrC is +0.048 Wm−2. We suggest that the DRE of BrC has been overestimated previously due to the lack of observational constraints from direct measurements and omission of the effects of photochemical whitening.

/pdf/exploring-the-observational-constraints-on-the-simulation-of-dhz6koe5pg.pdf

Exploring the observational constraints on the simulation of brown carbon

Observations from June to October 2016, from a surface-based ARM Mobile Facility deployment on Ascension Island (8∘S, 14.5∘W) indicate that refractory black carbon (rBC) is almost always present within the boundary layer. The rBC mass concentrations, light absorption coefficients, and cloud condensation nuclei concentrations vary in concert and synoptically, peaking in August. Light absorption coefficients at three visible wavelengths as a function of rBC mass are approximately double that calculated from black carbon in lab studies. A spectrally-flat absorption angstrom exponent suggests most of the light absorption is from lens-coated black carbon. The single-scattering-albedo increases systematically from August to October in both 2016 and 2017, with monthly means of 0.78 ± 0.02 (August), 0.81 ± 0.03 (September), and 0.83 ± 0.03 (October) at the green wavelength. Boundary layer aerosol loadings are only loosely correlated with total aerosol optical depth, with smoke more likely to be present in the boundary layer earlier in the biomass burning season, evolving to smoke predominantly present above the cloud layers in September–October, typically resting upon the cloud top inversion. The time period with the campaign-maximum near-surface light absorption and column aerosol optical depth, on 13–16 August 2016, is investigated further. Backtrajectories that indicate more direct boundary layer transport westward from the African continent is central to explaining the elevated surface aerosol loadings. Plain Language Summary First findings from the remote Ascension Island midway between Africa and South America in the Atlantic Ocean indicate that smoke is present much more often near the surface than has been previously thought. The new measurements from a 17-month-long campaign suggest that August is the smokiest month near the surface. The smoke includes other aerosols besides black carbon, and is most absorptive of sunlight in June and least in October. The smoke is more present near the surface earlier in the biomass burning season, or June, while later on toward September and October, more of the smoke resides above the cloud layer. This has implications for which aerosol-cloud microphysical and radiative interactions are dominant when. The campaign-maximum aerosol loading event is investigated further and attributed to an unusual direct westward flow from the continental African fire sources at low altitudes.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2017GL076926

The Ascension Island Boundary Layer in the Remote Southeast Atlantic is Often Smoky

. Hygroscopicity is one of the most important physicochemical properties of
aerosol particles and also plays indispensable roles in many other
scientific and technical fields. A myriad of experimental techniques, which
differ in principles, configurations and cost, are available for
investigating aerosol hygroscopicity under subsaturated conditions (i.e.,
relative humidity below 100 %). A comprehensive review of these techniques
is provided in this paper, in which experimental techniques are broadly
classified into four categories, according to the way samples under
investigation are prepared. For each technique, we describe its operation
principle and typical configuration, use representative examples reported in
previous work to illustrate how this technique can help better understand
aerosol hygroscopicity, and discuss its advantages and disadvantages. In
addition, future directions are outlined and discussed for further technical
improvement and instrumental development.

/pdf/a-review-of-experimental-techniques-for-aerosol-b2lcsknd5x.pdf

A review of experimental techniques for aerosol hygroscopicity studies

It has been hypothesized that black carbon (BC) influences mixed‐phase clouds by acting as an ice‐nucleating particle (INP). However, the literature data for ice nucleation by BC immersed in supercooled water are extremely varied, with some studies reporting that BC is very effective at nucleating ice, whereas others report no ice‐nucleating ability. Here we present new experimental results for immersion mode ice nucleation by BC from two contrasting fuels (n‐decane and eugenol). We observe no significant heterogeneous nucleation by either sample. Using a global aerosol model, we quantify the maximum relative importance of BC for ice nucleation when compared with K‐feldspar and marine organic aerosol acting as INP. Based on the upper limit from our laboratory data, we show that BC contributes at least several orders of magnitude less INP than feldspar and marine organic aerosol. Representations of its atmospheric ice‐nucleating ability based on older laboratory data produce unrealistic results when compared against ambient observations of INP. Since BC is a complex material, it cannot be unambiguously ruled out as an important INP species in all locations at all times. Therefore, we use our model to estimate a range of values for the density of active sites that BC particles must have to be relevant for ice nucleation in the atmosphere. The estimated values will guide future work on BC, defining the required sensitivity of future experimental studies.

/pdf/is-black-carbon-an-unimportant-ice-nucleating-particle-in-32o443upu2.pdf

Is Black Carbon an Unimportant Ice-Nucleating Particle in Mixed-Phase Clouds?

National Aeronautics and Space Administration [NNX12AC10G, NNX14AP75G]; United States Navy Office of Naval Research [N00014-10-1-0811]; NASA Earth and Space Science Fellowship [NNX14AK79H]; NASA [NNX12AC03G, NNX15AT96G]; Visiting Scientist Program at the National Institute of Aerospace (NIA)

/pdf/characterization-of-the-real-part-of-dry-aerosol-refractive-uxsdmm1wsc.pdf

Characterization of the Real Part of Dry Aerosol Refractive Index Over North America From the Surface to 12 km

NOAA Atmospheric Composition and Climate Program; NASA Radiation Sciences Program; NASA Upper Atmosphere Research Program; NASA [NNX12AC10G, NNX14AP75G, NNX14AK79H, NNX12AC03G, NNX15AT96G]; NASA Earth Science Division [NNX12AC20G, NNX14AP45G]

/pdf/in-situ-measurements-of-water-uptake-by-black-carbon-c4vu76mefg.pdf

Brett D. Palm

Papers

In situ measurements of water uptake by black carbon-containing aerosol in wildfire plumes