The interplay between assumed morphology and the direct radiative effect of light‐absorbing organic aerosol

Saleh, Rawad; Adams, P. J.; Donahue, Neil M.; Robinson, Allen L.

doi:10.1002/2016gl069786

Cited by 13 publications

(14 citation statements)

References 52 publications

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“…Challenges related to the definition of a chemical species in models add fundamentally to uncertainties in the refractive indices used as inputs to any optical model, particularly in the case of BC and organic carbon. The way the refractive indices of these species are retrieved can lead to large differences in their numerical values (Saleh et al, ). Stier et al () showed that the basic assumption regarding the refractive index of BC plays a key role in the calculation of radiative forcing.…”

Section: Impacts Of Aerosol Mixing State On Climate‐relevant Aerosol mentioning

confidence: 99%

Aerosol Mixing State: Measurements, Modeling, and Impacts

Riemer

Ault

West

et al. 2019

Reviews of Geophysics

271

287

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Atmospheric aerosols are complex mixtures of different chemical species, and individual particles exist in many different shapes and morphologies. Together, these characteristics contribute to the aerosol mixing state. This review provides an overview of measurement techniques to probe aerosol mixing state, discusses how aerosol mixing state is represented in atmospheric models at different scales, and synthesizes our knowledge of aerosol mixing state's impact on climate‐relevant properties, such as cloud condensation and ice nucleating particle concentrations, and aerosol optical properties. We present these findings within a framework that defines aerosol mixing state along with appropriate mixing state metrics to quantify it. Future research directions are identified, with a focus on the need for integrating mixing state measurements and modeling.

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Section: Impacts Of Aerosol Mixing State On Climate‐relevant Aerosol mentioning

confidence: 99%

Aerosol Mixing State: Measurements, Modeling, and Impacts

Riemer

Ault

West

et al. 2019

Reviews of Geophysics

271

287

View full text Add to dashboard Cite

show abstract

“…However, even if this was the case, our two-species (BC + brC) approach would still provide the optical parameters required for climate models to correctly represent the wavelength-dependent light absorption of HFO PM which we have observed; the inaccuracy would be in the relative attribution of absorption to the different light-absorbing species. Moreover, as demonstrated by Saleh et al (2016), climate models which employ Mie theory in radiative transfer calculations produce more accurate estimates of radiative forcing when Mie-theory-retrieved parameters are used, compared to more-complex retrievals.…”

Section: Introductionmentioning

confidence: 99%

Brown and Black Carbon Emitted by a Marine Engine Operated on Heavy Fuel Oil and Distillate Fuels: Optical Properties, Size Distributions, and Emission Factors

et al. 2018

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We characterized the chemical composition and optical properties of particulate matter (PM) emitted by a marine diesel engine operated on heavy fuel oil (HFO), marine gas oil (MGO), and diesel fuel (DF). For all three fuels, ∼80% of submicron PM was organic (and sulfate, for HFO at higher engine loads). Emission factors varied only slightly with engine load. Refractory black carbon (rBC) particles were not thickly coated for any fuel; rBC was therefore externally mixed from organic and sulfate PM. For MGO and DF PM, rBC particles were lognormally distributed in size (mode at d rBC ≈120 nm). For HFO, much larger rBC particles were present. Combining the rBC mass concentrations with in situ absorption measurements yielded an rBC mass absorption coefficient MAC BC,780 nm of 7.8 ± 1.8 m 2 /g at 780 nm for all three fuels. Using positive deviations of the absorption Ångström exponent (AAE) from unity to define brown carbon (brC), we found that brC absorption was negligible for MGO or DF PM (AAE(370,880 nm) ≈ 1.0 ± 0.1) but typically 50% of total 370-nm absorption for HFO PM. Even at 590 nm, ∼20 of the total absorption was due to brC. Using absorption at 880 nm as a reference for BC absorption and normalizing to organic PM mass, we obtained a MAC OM,370 nm of 0.4 m 2 /g at typical operating conditions. Furthermore, we calculated an imaginary refractive index of (0.045 ± 0.025)( ∕370 nm) −3 for HFO PM at 370 nm> > 660 nm, more than twofold greater than previous recommendations. Climate models should account for this substantial brC absorption in HFO PM. Plain Language SummaryWe characterized the fundamental properties of marine engine exhaust that are relevant to its aerosol-radiation interactions in climate models. In particular, we focussed on "brown carbon" light absorption (i.e., absorption in excess of that expected for the black carbon in canonical soot). We found that brown carbon can increase the direct radiative forcing of heavy-fuel-oil ship exhaust by 18% over snow.

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“…Aerosols in the atmosphere affect the climate both directly by interacting with sunlight (i.e., absorption and scattering) and indirectly by altering cloud properties. The magnitude and even the sign of the impact of particle scattering and absorption, in particular, is not very well constrained, in part because they depend on many factors including particle size, shape, composition, and mixing state (Cazorla et al 2013;Laskin et al 2015;Fierce et al 2016;Saleh et al 2016). It is known that black carbon (BC) particles, for example, efficiently absorb solar radiation throughout the UV-visible region of the spectrum with an estimated anthropogenic contribution to positive radiative forcing second only to carbon dioxide (Bond et al 2013).…”

Section: Introductionmentioning

confidence: 99%

Spectroscopic comparison of water- and methanol-soluble brown carbon particulate matter

Phillips

Smith

2017

Aerosol Science and Technology

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It is now recognized that some organic components of ambient aerosols absorb light with a spectrum distinct from that of other absorbers such as black carbon and mineral components. The most common method for isolating this light-absorbing organic fraction, or "brown carbon," is to collect particulate matter on filters and extract in a solvent, usually water or methanol. Here, we compare the absorption spectra of water-soluble (WS) and methanol-soluble (MS) extracts from ambient samples collected in Athens, Georgia. We find that despite syringe filtering the MS extracts, extinction by suspended particles is evident in the spectra leading to an overestimation of absorption by a factor of two on average. No such particle extinction is evident in the WS extracts. We demonstrate that it is possible to subtract the extinction contribution in the MS extracts by fitting the spectrum to the sum of two power-law functions, one describing the absorption spectrum and the other describing the extinction spectrum. With extinction thus removed, we find that integrated absorption (300-800 nm) by the MS brown carbon extract is highly correlated with the WS extract and is on average 1.55£ larger. The wavelength dependence of the WS and MS spectra are also correlated and very similar with average absorption A ngstr€ om exponents of 6.1 (§0.7) and 6.7 (§1.1), respectively. This study demonstrates that for the samples collected: (1) brown carbon absorption can be overestimated if scattering in MS spectra is not accounted for, (2) there is no spectral evidence that the WS and MS chromophores are different, and (3) it may be possible to use WS spectra to represent total brown carbon absorption using a simple scaling factor. These findings may differ for other types of aerosol samples and analytical methods.

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The interplay between assumed morphology and the direct radiative effect of light‐absorbing organic aerosol

Cited by 13 publications

References 52 publications

Aerosol Mixing State: Measurements, Modeling, and Impacts

Aerosol Mixing State: Measurements, Modeling, and Impacts

Brown and Black Carbon Emitted by a Marine Engine Operated on Heavy Fuel Oil and Distillate Fuels: Optical Properties, Size Distributions, and Emission Factors

Spectroscopic comparison of water- and methanol-soluble brown carbon particulate matter

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