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1. Enhancements in Cloud Condensation Nuclei Activity From Turbulent Fluctuations in Supersaturation

   https://doi.org/10.1029/2022GL102635  

   Anderson, Jesse C; Beeler, Payton; Ovchinnikov, Mikhail; Cantrell, Will; Krueger, Steven; Shaw, Raymond A; Yang, Fan; Fierce, Laura (September 2023, Geophysical Research Letters)

   The effect of aerosols on the properties of clouds is a large source of uncertainty in predictions of weather and climate. These aerosol‐cloud interactions depend critically on the ability of aerosol particles to form cloud droplets. A challenge in modeling aerosol‐cloud interactions is the representation of interactions between turbulence and cloud microphysics. Turbulent mixing leads to small‐scale fluctuations in water vapor and temperature that are unresolved in large‐scale atmospheric models. To quantify the impact of turbulent fluctuations on cloud condensation nuclei (CCN) activation, we used a high‐resolution Large Eddy Simulation of a convective cloud chamber to drive particle‐based cloud microphysics simulations. We show small‐scale fluctuations strongly impact CCN activity. Once activated, the relatively long timescales of evaporation compared to fluctuations causes droplets to persist in subsaturated regions, which further increases droplet concentrations. 

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2. Development of a single-particle technique to study insoluble residues from isoprene-derived secondary organic aerosol in droplets

   Parham, Rebecca; Fankhauser, Alison; Shi, Jia; Cooke, Madeline; YAN, JIN; Waters, Cara; Xiao, Yao; Kolozsvari, Katherine; Armstrong, N Cazimir; Zhang, Zhenfa; et al (March 2023, 2023 Spring ACS Meet ing)

   Atmospheric aerosols are key contributors to cloud condensation nuclei (CCN) and ice nucleating particle (INP) formation, which can offset positive radiative forcing. Aerosol particles can undergo many cycles of droplet activation and subsequent drying before their removal from the atmosphere through dry or wet deposition. Cloud-aerosol-precipitation interactions are affected by cloud droplet or ice crystal formation, which is related to the physicochemical properties of aerosol particles. Isoprene-derived secondary organic aerosol (iSOA) is an abundant component aerosol and has been previously found in INPs and cloud water residues, and it includes both soluble and insoluble residues in its particle matrix. Currently, most of our understanding of iSOA is derived from studying the soluble residues, but there has been a measurement gap for characterizing the insoluble residues. These measurements are needed as previous research has suggested that insoluble components could be important with respect to CCN and INP formation. Herein, a unique approach is utilized to collect the insoluble residues of SOA in ~3 µm droplets collected from a Spot Sampler from Aerosol Devices, Inc. iSOA is generated by reactive uptake of IEPOX onto acidic seed particles (ammonium sulfate + sulfuric acid) in a humidified atmospheric chamber under dark conditions. Droplets are impacted directly on a substrate or in a liquid medium to study the roles of insoluble residues from both single-particle and bulk perspectives. A suite of microspectroscopy techniques, including Raman and optical photothermal infrared (O-PTIR) spectroscopy, are used to probe the chemical composition of the residues. Atomic force microscopy – photothermal infrared (AFM-PTIR) spectroscopy and Nanoparticle Tracking Analysis (NTA) are used to measure the size distributions of the residues. These insights may help understand the properties of residues from cloud droplet evaporation and subsequent cloud-aerosol-precipitation interactions in the atmosphere. 

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3. Development of a Single-Particle Technique to Study Insoluble Residues from Isoprene-Derived Secondary Organic Aerosol in Droplets

   Rebecca L. Parham, Alison M. (March 2023, Spring ACS Meeting 2023)

   Atmospheric aerosols are key contributors to cloud condensation nuclei (CCN) and ice nucleating particle (INP) formation, which can offset positive radiative forcing. Aerosol particles can undergo many cycles of droplet activation and subsequent drying before their removal from the atmosphere through dry or wet deposition. Cloud-aerosol-precipitation interactions are affected by cloud droplet or ice crystal formation, which is related to the physicochemical properties of aerosol particles. Isoprene-derived secondary organic aerosol (iSOA) is an abundant component aerosol and has been previously found in INPs and cloud water residues, and it includes both soluble and insoluble residues in its particle matrix. Currently, most of our understanding of iSOA is derived from studying the soluble residues, but there has been a measurement gap for characterizing the insoluble residues. These measurements are needed as previous research has suggested that insoluble components could be important with respect to CCN and INP formation. Herein, a unique approach is utilized to collect the insoluble residues of SOA in ~3 μm droplets collected from a Spot Sampler from Aerosol Devices, Inc. iSOA is generated by reactive uptake of IEPOX onto acidic seed particles (ammonium sulfate + sulfuric acid) in a humidified atmospheric chamber under dark conditions. Droplets are impacted directly on a substrate or in a liquid medium to study the roles of insoluble residues from both single-particle and bulk perspectives. A suite of microspectroscopy techniques, including Raman and optical photothermal infrared (O-PTIR) spectroscopy, are used to probe the chemical composition of the residues. Atomic force microscopy – photothermal infrared (AFM-PTIR) spectroscopy and Nanoparticle Tracking Analysis (NTA) are used to measure the size distributions of the residues. These insights may help understand the properties of residues from cloud droplet evaporation and subsequent cloud-aerosol-precipitation interactions in the atmosphere. 

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4. Decrease in Nucleated Particles and Cloud Condensation Nuclei Observed across a Range of Environments

   https://doi.org/10.1021/acs.est.5c18035  

   Park, Do-Hyeon; Laj, Paolo; Andrews, Elisabeth; Rose, Clémence; Benedetti, Angela; Kulmala, Markku; Zabala, Inés; Ahlberg, Erik; Alastuey, Andrés; Asmi, Eija; et al (May 2026, Environmental Science & Technology)

   Understanding new particle formation (NPF) and the fate of nanoparticles is crucial because of their close links to air quality, cloud formation, and climate. These effects vary spatially and temporally owing to diverse aerosol sources and their relatively short atmospheric lifetime. Here, we present a comprehensive analysis of long-term trends in NPF-associated nucleation-mode particles and cloud condensation nuclei (CCN) concentrations across diverse observation environments using quality-controlled particle number size distribution (PNSD) and CCN data from 37 sites, primarily from Global Atmosphere Watch (GAW) stations. We identify declining decadal trends in both NPF occurrences and nucleated particle concentrations across most site types, with the strongest declines in urban areas. We observe simultaneous reductions in both CCN concentrations and nucleation-mode particles, suggesting that newly formed particles are a potential source of CCN. This, in turn, suggests that cloud microphysical properties and radiative effects can be indirectly influenced through aerosol−cloud interactions that modify cloud droplet formation. These findings indicate that decreasing anthropogenic emissions could influence the climate forcing potential of aerosol−cloud interactions, with important implications for future climate projections. 

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5. 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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