Small-scale controlled-aerosol field studies to inform models and to quantify key processes in the atmosphere
Many of the uncertainties about how pollution aerosols are affecting the reflection of sunlight by low marine clouds center around how clouds respond at very local scales to the addition of aerosols. These responses have major implications for the potential efficacy of marine cloud brightening as a climate intervention. Studying these very localized processes, and improving them in models, will help us produce more accurate projections of aerosols’ effects on clouds and climate.
The MCB Program’s team of atmospheric scientists have participated in field programs around the globe to observe and study how pollution aerosols are already affecting clouds and the reflection of sunlight by clouds (albedo). In these studies it is often difficult to separate human-produced from natural aerosol, or whether observed changes in clouds were caused by the aerosols, versus shifts in meteorological conditions (like winds) that can affect aerosols and their mixing into the clouds.
A new approach that gets around this problem is to add a single plume of optimized, natural aerosol, such as very small sea salt aerosols — basically, a clean ship-track — to a region of susceptible clouds over the open ocean, and to study clouds with and without the added aerosols in that region. Observations of the atmosphere, aerosols and clouds can then be used to directly contrast the characteristics of clouds in which the only difference is the addition of a known quantity and type of aerosols. This type of controlled study approach was recommended in a 2022 report from the joint NOAA/DOE Marine Cloud Brightening Workshop to help address cloud responses to aerosol perturbations (see also Feingold et al., 2024).
This approach requires the capability to generate a plume of consistently very small sea salt aerosols from ocean water that, once added to the atmosphere, will mix into clouds through natural air motions over the ocean. Studies would be conducted over open ocean regions where the air flow is offshore, so the seawater salt aerosols added to the clouds, which will either fall or be rained out of the clouds within a few days to a week, will be returned back to the ocean surface.
This new research approach requires several interrelated efforts:
- Controlled-aerosol plume studies of aerosol-cloud interactions in the marine environment and, to support this,
- Outdoor studies of sea salt aerosol plume evolution and transport near the source of the plume.
- Development of the ability to generate the required scale and composition of sea salt particles for conducting controlled-aerosol release field studies to support this.
Controlled-aerosol field studies of aerosol-clouds interactions in the ocean environment
Proposed scientific studies of controlled releases of seawater salt aerosols are modeled after similar studies of aerosol-cloud interaction studies that have been conducted for decades in different parts of the globe, observing the effects of pollution aerosols on clouds, with a key difference: they will use a plume of salt aerosol derived from seawater to influence these marine clouds, rather than a broad region of pollution aerosols, and will have the ability to control where, when and how many of these small salt particles are added to the clouds.
As with earlier studies of pollution aerosols’ influences on clouds, we anticipate that the background and generated sea salt aerosol and the clouds will be observed using platforms from federal science agencies, outfitted with instruments and instrumental teams from a range of federal science agencies and academic institutes that have the specialized capabilities to make the necessary detailed measurements. Modeling studies integrated with these observations will allow us to both plan the best strategies for sampling the aerosols and clouds, and to test and improve model simulations of the aerosol transport and evolution and how the aerosols affect the clouds.
Single-plume studies
The first aerosol-cloud interaction studies (see image below) will involve just a single plume of generated sea salt aerosol – the equivalent of a single ship track – applied for short intervals. Studies at this scale will be sufficient to produce measurable changes to local clouds, but due to the scale and duration of the perturbation will not produce a measurable effect on weather or climate.
The focus of these studies will be understanding localized effects, such as changes in the size and concentration of aerosols in the atmosphere in the perturbed vs unperturbed regions, changes in cloud droplet number concentration with aerosol changes, and changes in cloud water amount as a function of atmospheric conditions. These studies will also focus on these changes in the first approximately half day following the aerosol perturbation.
Field studies of sea salt aerosol plume evolution and transport near the source
Before studies of plumes of aerosol interacting with clouds over the ocean, a smaller more localized scale of research promotes understanding of the aerosol plume as the particles leave the source and evolve and move into the atmosphere at very close range. This helps improve plume dispersion models that can inform models at cloud, regional and other scales, and provide important constraints for the design of ocean plume studies. These studies can also help inform models used for regulating these substances in the atmosphere.
These initial small-scale studies require the development of a system capable of producing a single plume of sea salt aerosols optimized for marine cloud brightening. For this, the MCB Program team has developed a new Cloud-Aerosol Research Instrument (CARI). We are using CARI system at new research facility, the Coastal Atmospheric Aerosol Research and Engagement facility (CAARE), to conduct these initial studies of aerosol and plume evolution and transport. The CAARE facility is located at a fixed coastal facility in the pacific marine environment with many similar conditions to the open ocean that will operate as an ongoing science, education and engagement facility.
The essential science goals of these studies are to:
1. quantify how the size distribution of the generated sea salt aerosol from the CARI spray system evolves,
2. quantify the plume dispersion, mixing and aerosol concentrations under a range of atmospheric boundary layer conditions, particularly those similar to that in marine stratocumulus regions, at distances from immediately (<10 m) to ~200 m (or ~1 minute, depending on wind speed) downstream of the system, and
3. constrain and improve simulations of aerosol transport and evolution over this same range.
These goals can be realized in a study that focuses on aerosol characterization within 200 m of the source because it is expected that the dry aerosol size distribution at this point will be representative of the size when the plume reaches cloud base. Simulating the evolution of a plume within the first minute after emission from a point source is extremely challenging. This study will allow us to better quantify and test models’ ability to simulate this evolution, up to the point where the scale of the plume is large enough (of order 50 m in diameter) that models have better fidelity in simulating subsequent plume evolution.
Simulation of three realizations of how the sea-salt aerosol plume generated by CARI could evolve downwind of the CARI system under atmospheric conditions representative of the marine environment
Combined, these goals will allow us to determine the sensitivity of the generated aerosol plume characteristics (size, concentration, plume dispersion and vertical transport) to ambient meteorological conditions and aerosol source operational conditions.
To perform these studies, the CARI system is being used to generate a plume of sea salt aerosols that flow down wind across a series of instrument risers containing sophisticated and sensitive instruments for measuring aerosols over a wide range of sizes and concentrations (see image below). On these and at other stations, meteorology measurements are also taken. Plume generation occurs in brief intervals (generally 5-30 min) during periods of favorable meteorological conditions. Background aerosols are captured in other periods. These measurements will be taken periodically over time to capture findings in different seasons and meteorological conditions.
A detailed description of the coastal studies of aerosol near-field evolution and transport is available here.
A Scientific Review Board of senior scientists reviewed the study design with reference to its scientific value, the suitability and feasibility of planned observations, modeling and analyses, identification of any measurable physical risks from aerosol output, and any additional concerns. They found the study design to be valuable, well-structured, feasible and without identifiable physical risks. A report describing their findings is available here.
The study plans also included extensive efforts in support of open-science, effective governance, regulatory compliance and input, and public and stakeholder engagement. Learn more here.
Learn more:
Marine Cloud Brightening Program (main page)
Small-Scale Field Studies to Quantify Key Processes & Inform Models
Generating Aerosols for Cloud-Aerosol Research
Governance, Engagement and CAARE
Our Team, Partners and Funders
References specific to this section
Feingold, G., Ghate, V. P., Russell, L. M., Blossey, P., Cantrell, W., Christensen, M. W., Diamond, M. S., Gettelman, A., Glassmeier, F., Gryspeerdt, E., Haywood, J., Hoffmann, F., Kaul, C. M., Lebsock, M., McComiskey, A. C., McCoy, D. T., Ming, Y., Mülmenstädt, J., Possner, A., … Zheng, X. (2024). Physical science research needed to evaluate the viability and risks of marine cloud brightening. Science Advances, 10(12), eadi8594. https://doi.org/10.1126/sciadv.adi8594
Feingold G., V. P. Ghate, L. M. Russell, et al. 2022: DOE-NOAA Marine Cloud Brightening Workshop. U.S. Department of Energy and U.S. Department of Commerce NOAA; DOE/SC-0207; NOAA Technical Report OAR ESRL/CSL-1.