Aerosol Absorption: Progress Towards Global and Regional Constraints

Samset, B. H.; Stjern, Camilla W.; Andrews, Elisabeth; Kahn, Ralph A.; Myhre, Gunnar; Schulz, Mıchael; Schuster, Gregory L.

doi:10.1007/s40641-018-0091-4

Cited by 139 publications

(167 citation statements)

References 160 publications

(233 reference statements)

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“…more cooling particles were over-represented and coarser, more warming particles were underestimated. As a result, observations of dust which include the coarse mode are in demand (Formenti et al, 2011b;Ansmann et al, 2011;Ansmann et al, 2017;Samset et al, 2018) for model validation. There are also implications for satellite optical models and retrievals since these also rely on accurate aerosol optical properties which are affected by PSD.…”

Section: Introductionmentioning

confidence: 99%

Coarse and Giant Particles are Ubiquitous in Saharan Dust Export Regions and are Radiatively Significant over the Sahara

Ryder¹,

Highwood²,

Walser³

et al. 2019

Preprint

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Mineral dust is an important component of the climate system, interacting with radiation, clouds and biogeochemical systems, and impacting atmospheric circulation, air quality, aviation and solar energy generation. These impacts are sensitive 10 to dust particle size distribution (PSD), yet models struggle or even fail to represent coarse (diameter (d) >2.5 µm) and giant (d>20 µm) dust particles and the evolution of the PSD with transport. Here we examine three state-of-the-art airborne observational datasets, all of which measured the full size range of dust (d=0.1 to >100 µm) at different stages during transport, with consistent instrumentation. We quantify the presence and evolution of coarse and giant particles and their contribution to optical properties. Observations are taken from the Fennec fieldwork over the Sahara and in the Saharan Air Layer (SAL) near 15 the Canary Islands, and from the AER-D fieldwork in the vicinity of the Cape Verde Islands in the SAL.Observations show significantly more abundant coarse and giant dust particles over the Sahara compared to the SAL: effective diameters of up to 20 µm were observed over the Sahara, compared to 4 µm in the SAL. Mass profiles show that over the Sahara 40% of dust mass was found in the giant mode, contrasting to 2 to 12% in the SAL. Size-resolved optical property 20 calculations show that in the shortwave (longwave) spectrum excluding the giant mode omits 18% (26%) of extinction over the Sahara, compared to 1-4% (2-6%) in the SAL. Excluding giant particles results in significant underestimation of both shortwave and longwave extinction over the Sahara, as well as of mass concentration, while the effects in the SAL are smaller but non-negligible. Omitting the giant mode results in a greater omission of dust longwave radiative effects compared to the shortwave, suggesting a bias towards a radiative cooling effect of dust when the giant mode is excluded and/or the coarse mode 25 is underestimated. This will be important in dust models, which typically exclude giant particles and underestimate coarse mode concentrations.A compilation of effective diameters against dust age since uplift time suggests that two regimes of dust transport exist. During the initial 1.5 days, both coarse and giant particles are rapidly deposited. During the subsequent 1.5 to 10 days, PSD barely 30 changes with transport, and the coarse mode is retained to a much greater degree than expected from estimates of gravitational

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Section: Introductionmentioning

confidence: 99%

Coarse and Giant Particles are Ubiquitous in Saharan Dust Export Regions and are Radiatively Significant over the Sahara

Ryder¹,

Highwood²,

Walser³

et al. 2019

Preprint

View full text Add to dashboard Cite

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“…The problem is compounded by difficulties in constraining the emissions, transport, and atmospheric lifetime of BC, leading to large uncertainties in the BC mass concentrations in models (Bond et al, 2013;Boucher et al, 2016). A further source of uncertainty is the high sensitivity of BC absorption to particle properties and mixing state (Jacobson, 2001;Matsui et al, 2018;Peng, 2016;Samset et al, 2018). These uncertainties pose a major challenge when using observations to constrain the BC absorption in global models (Chung et al, 2012;Koch et al, 2009;Sato et al, 2003;Wang et al, 2014Wang et al, , 2016.…”

Section: Introductionmentioning

confidence: 99%

Are Changes in Atmospheric Circulation Important for Black Carbon Aerosol Impacts on Clouds, Precipitation, and Radiation?

2019

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Black carbon (BC) aerosols strongly absorb solar radiation, but their effective radiative forcing and impacts on regional climate remain highly uncertain owing to strong feedbacks of BC heating on clouds, convection, and precipitation. This study investigates the role of large‐scale circulation changes in governing such feedbacks. In the HadGEM3 climate model BC emissions were increased to 10 times present‐day values while keeping sea surface temperatures fixed, to assess the rapid adjustments to increased BC absorption. The BC perturbation led to an effective radiative forcing of 2.7 W/m2 and a 0.13‐mm/day reduction in global precipitation. There were also large shifts in the spatial distribution of tropical convection, increased low cloud over oceans, and a weakening and poleward shift of midlatitude storm tracks, especially in the Northern Hemisphere. In a parallel experiment, horizontal winds were nudged toward meteorological reanalyses to deliberately suppress circulation responses while allowing changes to the thermodynamic structure of the atmosphere. Surprisingly, BC had approximately the same impact on global‐mean radiation and global precipitation in the nudged experiment, even though regional changes in clouds and convection were not fully captured. The results show that large‐scale dynamical responses to BC are important for regional impacts but have a limited role in determining the effective radiative forcing and global‐mean climate response. The rapid adjustments of clouds, radiation, and global precipitation were primarily a response to increased radiative absorption and atmospheric stability. This implies that short nudged simulations may be sufficient to assess absorbing aerosol impacts on global‐mean radiation and precipitation.

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“…Larger particles might experience preferential gravitational settling, leading to an aerosol size distribution that is both narrower and, on average, smaller. Coatings deposited on particles increase their size, which, in turn, can increase scattering efficiency and enhance SSA as the plume ages [14,15]. Coatings deposited on BC particles can sometimes enhance light absorption through a lensing effect.…”

mentioning

confidence: 99%

“…They do, however, provide considerable detail about aerosol microphysical properties, such as SSA and size distribution, which, in the past, were comparatively less well represented in satellite retrievals and models.Due to the observational limitations mentioned above, much is unknown about the temporal and spatial distribution of fire-generated particle types and their aging. To date, most climate models do not discriminate between BC and BrC even though these particles display distinctly different physical, chemical, and optical properties (e.g., References [14,23]). Models are also uncertain about the role wildfire smoke plays in the aerosol-cloud interaction.…”

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confidence: 99%

Wildfire Smoke Particle Properties and Evolution, from Space-Based Multi-Angle Imaging

et al. 2020

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Emitted smoke composition is determined by properties of the biomass burning source and ambient ecosystem. However, conditions that mediate the partitioning of black carbon (BC) and brown carbon (BrC) formation, as well as the spatial and temporal factors that drive particle evolution, are not understood adequately for many climate and air-quality related modeling applications. In situ observations provide considerable detail about aerosol microphysical and chemical properties, although sampling is extremely limited. Satellites offer the frequent global coverage that would allow for statistical characterization of emitted and evolved smoke, but generally lack microphysical detail. However, once properly validated, data from the National Aeronautics and Space Administration (NASA) Earth Observing System's Multi-Angle Imaging Spectroradiometer (MISR) instrument can create at least a partial picture of smoke particle properties and plume evolution. We use in situ data from the Department of Energy's Biomass Burning Observation Project (BBOP) field campaign to assess the strengths and limitations of smoke particle retrieval results from the MISR Research Aerosol (RA) retrieval algorithm. We then use MISR to characterize wildfire smoke particle properties and to identify the relevant aging factors in several cases, to the extent possible. The RA successfully maps qualitative changes in effective particle size, light absorption, and its spectral dependence, when compared to in situ observations. By observing the entire plume uniformly, the satellite data can be interpreted in terms of smoke plume evolution, including size-selective deposition, new-particle formation, and locations within the plume where BC or BrC dominates.as well as purely scattering aerosols, all of which have wide-ranging environmental impacts. Smoke aerosols that escape the planetary boundary layer (PBL) have the potential to stay aloft for several days or more, altering the regional radiative budget on time scales that can extend beyond the age of the fire itself and can affect air quality hundreds of kilometers downwind [1,2]. These particles also have the potential to act as cloud-condensation nuclei (CCN), resulting in aerosol-cloud interactions that can alter cloud reflectivity, cloud lifetime, and the frequency of precipitation [3][4][5][6].The fuel type and amount, fire regime, and meteorology are known to affect smoke plume aerosol composition, with evidence indicating systematic differences in particle size distribution, particle light absorption, and the spectral dependence of absorption (e.g., References [7-10]). Although the dominant absorbing aerosol in biomass burning smoke is often BC, which is highly absorbing across all visible wavelengths, there is a variable fraction of BrC as well, which is usually less absorbing overall and exhibits stronger absorption at shorter wavelengths. For example, studies suggest higher BrC fractions in smoke plumes from smoldering than from flaming fires [11][12][13]. However, we do not yet have much u...

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Aerosol Absorption: Progress Towards Global and Regional Constraints

Cited by 139 publications

References 160 publications

Coarse and Giant Particles are Ubiquitous in Saharan Dust Export Regions and are Radiatively Significant over the Sahara

Coarse and Giant Particles are Ubiquitous in Saharan Dust Export Regions and are Radiatively Significant over the Sahara

Are Changes in Atmospheric Circulation Important for Black Carbon Aerosol Impacts on Clouds, Precipitation, and Radiation?

Wildfire Smoke Particle Properties and Evolution, from Space-Based Multi-Angle Imaging

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