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1. Less atmospheric radiative heating by dust due to the synergy of coarser size and aspherical shape

   https://doi.org/10.5194/acp-21-16869-2021  

   Ito, Akinori; Adebiyi, Adeyemi A.; Huang, Yue; Kok, Jasper F. (January 2021, Atmospheric Chemistry and Physics)

   Abstract. Mineral dust aerosols cool and warm the atmosphere byscattering and absorbing solar (shortwave: SW) and thermal (longwave: LW)radiation. However, significant uncertainties remain in dust radiativeeffects, largely due to differences in the dust size distribution andspectral optical properties simulated in Earth system models. Dust modelstypically underestimate the coarse dust load (more than 2.5 µm indiameter) and assume a spherical shape, which leads to an overestimate ofthe fine dust load (less than 2.5 µm) after the dust emissions in themodels are scaled to match observed dust aerosol optical depth at 550 nm(DAOD550). Here, we improve the simulated dust properties with data setsthat leverage measurements of size-resolved dust concentration, asphericityfactor, and refractive index in a coupled global chemical transport modelwith a radiative transfer module. After the adjustment of size-resolved dustconcentration and spectral optical properties, the global and annual averageof DAOD550 from the simulation increases from 0.023 to 0.029 and fallswithin the range of a semi-observationally based estimate (0.030 ± 0.005). The reduction of fine dust load after the adjustment leads to areduction of the SW cooling at the top of the atmosphere (TOA). To improveagreement against a semi-observationally based estimate of the radiativeeffect efficiency at TOA, we find that a less absorptive SW dust refractiveindex is required for coarser aspherical dust. Thus, only a minor differenceis estimated for the net global dust radiative effect at TOA (−0.08 vs.−0.00 W m−2 on a global scale). Conversely, our sensitivitysimulations reveal that the surface warming is substantially enhanced nearthe strong dust source regions (less cooling to −0.23 from −0.60 W m−2 on a global scale). Thus, less atmospheric radiativeheating is estimated near the major source regions (less heating to 0.15from 0.59 W m−2 on a global scale), because of enhanced LWwarming at the surface by the synergy of coarser size and aspherical shape. 

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2. Underestimated Passive Volcanic Sulfur Degassing Implies Overestimated Anthropogenic Aerosol Forcing

   https://doi.org/10.1029/2022GL102061  

   Jongebloed, U. A.; Schauer, A. J.; Cole‐Dai, J.; Larrick, C. G.; Wood, R.; Fischer, T. P.; Carn, S. A.; Salimi, S.; Edouard, S. R.; Zhai, S.; et al (January 2023, Geophysical Research Letters)

   Abstract The Arctic is warming at almost four times the global rate. An estimated sixty percent of greenhouse‐gas‐induced Arctic warming has been offset by anthropogenic aerosols, but the contribution of aerosols to radiative forcing (RF) represents the largest uncertainty in estimating total RF, largely due to unknown preindustrial aerosol abundance. Here, sulfur isotope measurements in a Greenland ice core show that passive volcanic degassing contributes up to 66 ± 10% of preindustrial ice core sulfate in years without major eruptions. A state‐of‐the‐art model indicates passive volcanic sulfur emissions influencing the Arctic are underestimated by up to a factor of three, possibly because many volcanic inventories do not include hydrogen sulfide emissions. Higher preindustrial volcanic sulfur emissions reduce modeled anthropogenic Arctic aerosol cooling by up to a factor of two (+0.11 to +0.29 W m⁻²), suggesting that underestimating passive volcanic sulfur emissions has significant implications for anthropogenic‐induced Arctic climate change. 

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3. Sulfur isotopes of sulfate measurements from a Greenland ice core (1200-1850) and volcanic gas measurements from various volcanoes and hot springs in Iceland

   https://doi.org/10.18739/A2N873162  

   Jongebloed, Ursula; Alexander, Becky; Cole-Dai, Jihong; Schauer, Andrew; Larrick, Carleigh; Wood, Robert; Fischer, Tobias; Carn, Simon; Salimi, Sara; Edouard, Shana; et al (January 2022, NSF Arctic Data Center)

   The Arctic is warming at almost four times the global rate. Cooling caused by anthropogenic aerosols has been estimated to offset sixty percent of greenhouse-gas-induced Arctic warming, but the contribution of aerosols to radiative forcing (RF) represents the largest uncertainty in estimating total RF, largely due to unknown preindustrial aerosol abundance. Here, sulfur isotope measurements in a Greenland ice core show that passive volcanic degassing contributes up to 66 ± 10% of preindustrial ice core sulfate in years without major eruptions. A state-of-the-art model indicates passive volcanic sulfur emissions influencing the Arctic are underestimated by up to a factor of three, possibly because many volcanic inventories do not include hydrogen sulfide emissions. Higher preindustrial volcanic sulfur emissions reduce modeled anthropogenic Arctic aerosol cooling by up to a factor of two (+0.11 to +0.29 W m-2 (watts per square meter)), suggesting that underestimating passive volcanic sulfur emissions has significant implications for anthropogenic-induced Arctic climate change. These data include sulfur isotopes of sulfate measurements from a Greenland ice core and volcanic gas measurements (CO2:S (carbon dioxide:sulfur) ratios) from various volcanoes and hot springs in Iceland. 

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4. Investigation of aerosol–cloud interactions under different absorptive aerosol regimes using Atmospheric Radiation Measurement (ARM) southern Great Plains (SGP) ground-based measurements

   https://doi.org/10.5194/acp-20-3483-2020  

   Zheng, Xiaojian; Xi, Baike; Dong, Xiquan; Logan, Timothy; Wang, Yuan; Wu, Peng (January 2020, Atmospheric Chemistry and Physics)

   null (Ed.)

   Abstract. The aerosol indirect effect on cloud microphysical and radiative propertiesis one of the largest uncertainties in climate simulations. In order toinvestigate the aerosol–cloud interactions, a total of 16 low-level stratuscloud cases under daytime coupled boundary-layer conditions are selectedover the southern Great Plains (SGP) region of the United States. Thephysicochemical properties of aerosols and their impacts on cloudmicrophysical properties are examined using data collected from theDepartment of Energy Atmospheric Radiation Measurement (ARM) facility at the SGP site. The aerosol–cloud interaction index (ACIr) is used to quantify the aerosol impacts with respect to cloud-droplet effective radius. The mean value of ACIr calculated from all selected samples is0.145±0.05 and ranges from 0.09 to 0.24 at a range of cloudliquid water paths (LWPs; LWP=20–300 g m−2). The magnitude of ACIr decreases with an increasing LWP, which suggests a diminished cloud microphysical response to aerosol loading, presumably due to enhanced condensational growth processes and enlarged particle sizes. The impact of aerosols with different light-absorbing abilities on the sensitivity of cloud microphysical responses is also investigated. In the presence of weak light-absorbing aerosols, the low-level clouds feature a higher number concentration of cloud condensation nuclei (NCCN) and smaller effective radii (re), while the opposite is true for strong light-absorbing aerosols. Furthermore, the mean activation ratio of aerosols to CCN (NCCN∕Na) for weakly (strongly) absorbing aerosols is 0.54 (0.45), owing to the aerosol microphysical effects, particularly the different aerosol compositions inferred by their absorptive properties. In terms of the sensitivity of cloud-droplet number concentration (Nd) to NCCN, the fraction of CCN that converted to cloud droplets (Nd∕NCCN) for the weakly (strongly) absorptive regime is 0.69 (0.54). The measured ACIr values in the weakly absorptive regime arerelatively higher, indicating that clouds have greater microphysicalresponses to aerosols, owing to the favorable thermodynamic condition. Thereduced ACIr values in the strongly absorptive regime are due to the cloud-layer heating effect induced by strong light-absorbing aerosols. Consequently, we expect larger shortwave radiative cooling effects from clouds in the weakly absorptive regime than those in the strongly absorptive regime. 

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5. Divergent global-scale temperature effects from identical aerosols emitted in different regions

   https://doi.org/10.1038/s41467-018-05838-6  

   Persad, Geeta G.; Caldeira, Ken (August 2018, Nature Communications)

   Abstract The distribution of anthropogenic aerosols’ climate effects depends on the geographic distribution of the aerosols themselves. Yet many scientific and policy discussions ignore the role of emission location when evaluating aerosols’ climate impacts. Here, we present new climate model results demonstrating divergent climate responses to a fixed amount and composition of aerosol—emulating China’s present-day emissions—emitted from 8 key geopolitical regions. The aerosols’ global-mean cooling effect is fourteen times greater when emitted from the highest impact emitting region (Western Europe) than from the lowest (India). Further, radiative forcing, a widely used climate response proxy, fails as an effective predictor of global-mean cooling for national-scale aerosol emissions in our simulations; global-mean forcing-to-cooling efficacy differs fivefold depending on emitting region. This suggests that climate accounting should differentiate between aerosols emitted from different countries and that aerosol emissions’ evolving geographic distribution will impact the global-scale magnitude and spatial distribution of climate change. 

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