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1. Aerosol Hygroscopicity and Surface-Active Coverage for the Droplet Growth of Aerosol Mixtures

   https://doi.org/10.1021/acsestair.4c00303  

   Ferdousi-Rokib, Nahin; A_Malek, Kotiba; Mitchell, Ian; M_Fierce, Laura; Asa-Awuku, Akua_A (July 2025, ACS ES&T Air)
2. Remote Aerosol Simulated During the Atmospheric Tomography (ATom) Campaign and Implications for Aerosol Lifetime

   https://doi.org/10.1029/2022JD036524  

   Gao, Chloe Yuchao; Heald, Colette L.; Katich, Joseph M.; Luo, Gan; Yu, Fangqun (November 2022, Journal of Geophysical Research: Atmospheres)

   Abstract We investigate and assess how well a global chemical transport model (GEOS‐Chem) simulates submicron aerosol mass concentrations in the remote troposphere. The simulated speciated aerosol (organic aerosol (OA), black carbon, sulfate, nitrate, and ammonium) mass concentrations are evaluated against airborne observations made during all four seasons of the NASA Atmospheric Tomography Mission (ATom) deployments over the remote Pacific and Atlantic Oceans. Such measurements over pristine environments offer fresh insights into the spatial (Northern [NH] and Southern Hemispheres [SH], Atlantic, and Pacific Oceans) and temporal (all seasons) variability in aerosol composition and lifetime, away from continental sources. The model captures the dominance of fine OA and sulfate aerosol mass concentrations in all seasons. There is a high bias across all species in the ATom‐2 (NH winter) simulations; implementing recent updates to the wet scavenging parameterization improves our simulations, eliminating the large ATom‐2 (NH winter) bias, improving the ATom‐1 (NH summer) and ATom‐3 (NH fall) simulations, but producing a model underestimate in aerosol mass concentrations for the ATom‐4 (NH spring) simulations. Following the wet scavenging updates, simulated global annual mean aerosol lifetimes vary from 1.9 to 4.0 days, depending on species. Aerosol lifetimes in each hemisphere vary by season, and are longest for carbonaceous aerosol during the southern hemispheric fire season. The updated wet scavenging parameterization brings simulated concentrations closer to observations and reduces global aerosol lifetime for all species, indicating the sensitivity of global aerosol lifetime and burden to wet removal processes. 

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3. Aerosol properties and their influences on low warm clouds during the Two-Column Aerosol Project

   https://doi.org/10.5194/acp-19-9515-2019  

   Liu, Jianjun; Li, Zhanqing (January 2019, Atmospheric Chemistry and Physics)

   Abstract. Twelve months of measurements collected during the Two-ColumnAerosol Project field campaign at Cape Cod, Massachusetts, which started inthe summer of 2012, were used to investigate aerosol physical, optical, andchemical properties and their influences on the dependence of clouddevelopment on thermodynamic (i.e., lower tropospheric stability, LTS)conditions. Relationships between aerosol loading and cloud properties underdifferent dominant air-mass conditions and the magnitude of the firstindirect effect (FIE), as well as the sensitivity of the FIE to differentaerosol compositions, are examined. The seasonal variation in aerosol numberconcentration (Na) was not consistent with variations in aerosoloptical properties (i.e., scattering coefficient, σs, andcolumnar aerosol optical depth). Organics were found to have a largecontribution to small particle sizes. This contribution decreased during theparticle growth period. Under low-aerosol-loading conditions, the liquidwater path (LWP) and droplet effective radius (DER) significantly increasedwith increasing LTS, but, under high-aerosol-loading conditions, LWP and DERchanged little, indicating that aerosols significantly weakened thedependence of cloud development on LTS. The reduction in LWP and DER fromlow- to high-aerosol-loading conditions was greater in stable environments,suggesting that clouds under stable conditions are more susceptible toaerosol perturbations than those under more unstable conditions. Highaerosol loading weakened the increase in DER as LWP increased andstrengthened the increase in cloud optical depth (COD) with increasing LWP,resulting in changes in the interdependence of cloud properties. Under bothcontinental and marine air-mass conditions, high aerosol loading cansignificantly increase COD and decrease LWP and DER, narrowing theirdistributions. Magnitudes of the FIE estimated under continental air-massconditions ranged from 0.07±0.03 to 0.26±0.09 with a meanvalue of 0.16±0.03 and showed an increasing trend as LWP increased.The calculated FIE values for aerosols with a low fraction of organics aregreater than those for aerosols with a high fraction of organics. Thisimplies that clouds over regions dominated by aerosol particles containingmostly inorganics are more susceptible to aerosol perturbations, resultingin larger climate forcing, than clouds over regions dominated by organicaerosol particles. 

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4. Multiphase Guaiacol Photooxidation: Fenton Reactions, Brown Carbon, and Secondary Organic Aerosol Formation in Suspended Aerosol Particles

   https://doi.org/10.1021/acsestair.3c00057  

   De_Haan, David O; Hawkins, Lelia N; Weber, Jacob A; Moul, Benjamin T; Hui, Samson; Cox, Santeh A; Esse, Jennifer U; Skochdopole, Nathan R; Lynch, Carys P; Le, Chen; et al (May 2024, ACS ES&T Air)

   Guaiacol, present in wood smoke, readily forms secondary organic aerosol (SOA), and, in the aqueous phase, brown carbon (BrC) species. Here, BrC is produced in an illuminated chamber containing guaiacol(g), HOOH(g) as an OH radical source, and either deliquesced salt particles or guaiacol SOA at 50% relative humidity. BrC production slows without an OH source (HOOH), likely due to low levels of radical generation by photosensitization, perhaps involving surface-adsorbed guaiacol and dissolved oxygen. With or without HOOH, BrC mass absorption coefficients at 365 nm generated by the guaiacol + OH reaction reach a maximum at ~6 h of atmospheric OH exposure, after which photobleaching becomes dominant. In the presence of soluble iron but no HOOH, more BrC is produced, likely due to insoluble polymer production observed in previous studies. However, with both soluble iron and HOOH (enabling Fenton chemistry), significantly less SOA and BrC are produced due to very high oxidation rates, and the average SOA carbon oxidation state reaches 2, indicating carboxylate products like oxalate. These results indicate that SOA and BrC formation by guaiacol photooxidation can take place over a wider range of atmospheric conditions than previously thought, and that the effects of iron(II) depend on HOOH. Multiphase guaiacol photooxidation likely makes a significant contribution to producing highly oxidized SOA material in smoke plumes. 

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5. Aerosol Mixing State: Measurements, Modeling, and Impacts

   https://doi.org/10.1029/2018RG000615  

   Riemer, N.; Ault, A. P.; West, M.; Craig, R. L.; Curtis, J. H. (May 2019, Reviews of Geophysics)

   Abstract 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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