Controls on pristine aerosol over the Southern Ocean (SO) are critical for constraining the strength of global aerosol indirect forcing. Observations of summertime SO clouds and aerosols in synoptically varied conditions during the 2018 SOCRATES aircraft campaign reveal novel mechanisms influencing pristine aerosol‐cloud interactions. The SO free troposphere (3–6 km) is characterized by widespread, frequent new particle formation events contributing to much larger concentrations (≥1,000 mg−1) of condensation nuclei (diameters > 0.01 μm) than in typical sub‐tropical regions. Synoptic‐scale uplift in warm conveyor belts and sub‐polar vortices lifts marine biogenic sulfur‐containing gases to free‐tropospheric environments favorable for generating Aitken‐mode aerosol particles (0.01–0.1 μm). Free‐tropospheric Aitken particles subside into the boundary layer, where they grow in size to dominate the sulfur‐based cloud condensation nuclei (CCN) driving SO cloud droplet number concentrations (
Cloud formation in the Pi Convection–Cloud Chamber is achieved via ionization in humid conditions, without the injection of aerosol particles to serve as cloud condensation nuclei (CCN). Abundant ions, turbulence, and supersaturated water vapor combine to produce new particles, which grow to become CCN sized and eventually are activated to form clouds. Coupling between the new particle formation and cloud droplets causes predator-prey type oscillations in aerosol and droplet concentrations under turbulent conditions. Leading terms are identified in the budgets for Aitken and accumulation mode aerosols and for cloud droplets. The cloud coupling is proposed to be a result of cloud-induced runaway CCN production through aerosol scavenging. The experiments suggest potential applications to marine cloud brightening, in which ions rather than sea-salt aerosols are generated.
more » « less- PAR ID:
- 10545758
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Oxford Open Climate Change
- Volume:
- 4
- Issue:
- 1
- ISSN:
- 2634-4068
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract N d ∼ 60–100 cm−3). Evidence is presented for a hypothesized Aitken‐ buffering mechanism which maintains persistently high summertime SON d against precipitation removal through CCN replenishment from activation and growth of boundary layer Aitken particles. Nudged hindcasts from the Community Atmosphere Model (CAM6) are found to underpredict Aitken and accumulation mode aerosols andN d , impacting summertime cloud brightness and aerosol‐cloud interactions and indicating incomplete representations of aerosol mechanisms associated with ocean biology. -
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.more » « less
-
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.more » « less
-
more » « less
Abstract The impact of giant sea salt aerosols released from breaking waves on rain formation in marine boundary layer clouds is studied using large-eddy simulations (LES). We perform simulations of marine cumuli and stratocumuli for various concentrations of cloud condensation nuclei (CCN) and giant CCN (GCCN). Cloud microphysics are modeled with a Lagrangian method that provides key improvements in comparison to previous LES of GCCN that used Eulerian bin microphysics. We find that GCCN significantly increase precipitation in stratocumuli. This effect is strongest for low and moderate CCN concentrations. GCCN are found to have a smaller impact on precipitation formation in cumuli. These conclusions are in agreement with field measurements. We develop a simple parameterization of the effect of GCCN on precipitation, accretion, and autoconversion rates in marine stratocumuli.
Significance Statement Breaking sea waves release salt particles into the atmosphere. Cloud droplets formed on these salt particles can grow larger than droplets formed on other smaller particles. Therefore, sea salt particles can be important for rain formation over oceans. To investigate this effect, we performed idealized computer simulations of stratocumulus and cumulus clouds. Sea salt particles were modeled with an unprecedented precision thanks to the use of an emerging modeling method. In our simulations sea salt particles significantly enhance rain formation in stratocumuli, but not in cumuli. Our study has implications for climate models, because stratocumuli are important for Earth’s energy budget and for rain enhancement experiments.
-
more » « less
Abstract Recent in situ observations show that haze particles exist in a convection cloud chamber. The microphysics schemes previously used for large‐eddy simulations of the cloud chamber could not fully resolve haze particles and the associated processes, including their activation and deactivation. Specifically, cloud droplet activation was modeled based on Twomey‐type parameterizations, wherein cloud droplets were formed when a critical supersaturation for the available cloud condensation nuclei (CCN) was exceeded and haze particles were not explicitly resolved. Here, we develop and adapt haze‐capable bin and Lagrangian microphysics schemes to properly resolve the activation and deactivation processes. Results are compared with the Twomey‐type CCN‐based bin microphysics scheme in which haze particles are not fully resolved. We find that results from the haze‐capable bin microphysics scheme agree well with those from the Lagrangian microphysics scheme. However, both schemes significantly differ from those from a CCN‐based bin microphysics scheme unless CCN recycling is considered. Haze particles from the recycling of deactivated cloud droplets can strongly enhance cloud droplet number concentration due to a positive feedback in haze‐cloud interactions in the cloud chamber. Haze particle size distributions are more realistic when considering solute and curvature effects that enable representing the complete physics of the activation process. Our study suggests that haze particles and their interactions with cloud droplets may have a strong impact on cloud properties when supersaturation fluctuations are comparable to mean supersaturation, as is the case in the cloud chamber and likely is the case in the atmosphere, especially in polluted conditions.
