Particles in the atmosphere play a key role in cloud formation, acting as nuclei for water droplets. Clouds play an important role in absorbing and reflecting heat, hence they can potentially mitigate or exacerbate global warming. Aerosol-cloud radiative interactions are widely held to be the largest single source of uncertainty in climate model projections of future climate change due to increasing anthropogenic emissions (IPCC, 2007). The underlying causes of this uncertainty among modeled predictions of climate are the gaps in our fundamental understanding of cloud processes. There has been significant progress with both observations and models on these important questions. However, the quantitative representation of these processes is nontrivial and limits our ability to represent them in global climate models (GCMs), resulting in the largest uncertainties in predictions of future climate. Given the timeliness of these questions for advancing GCMs, it is essential to address the unanswered questions in cloud dynamical response to aerosol perturbations.

Intellectual merit. This research is a targeted aircraft campaign with embedded modeling studies to inform the experiment planning and to facilitate the interpretation of the results. The study will use the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft in July 2011 off the coast of Monterey, California, with a full payload of instruments to measure particle and cloud number, mass and composition distributions. The research is composed of three novel and important additional, climate-focused studies: 1. Controlled release and atmospheric distribution of three different size ranges of particles in flight and on or by a dedicated ship; 2. Large Eddy Simulations and Aerosol-Cloud Parcel modeling studies constrained by the observations to test our ability to quantitatively predict the dynamical response to increases in particle concentrations in the natural atmosphere; 3. Satellite analyses of marine stratocumulus to constrain the radiative properties of the natural, perturbed, and regional cloud systems.

Broader impacts. The broader scientific impacts of the research will be the improved understanding of fundamental aerosol-cloud processes that can be incorporated in global climate models to better inform decision makers. The broader educational impacts of the research will be realized through: (1) Promotion of teaching, training and learning through development and piloting of an informal science education program targeting an underserved audience; (2) Broadened participation of underrepresented groups - in this case, retired and elderly people - in research as well as in outreach; (3) Enhancement of infrastructure for teaching through partnerships with an established educational organization (Osher Lifelong Learning Institute); (4) Broad dissemination of results through presentations, peer-reviewed publications and via the web; and (5) Societal benefits in terms of improved understanding of climate science and the related ethical issues.

Project Report

The project aim was to study aerosol-cloud-radiation interactions through the controlled emission of known aerosols into the California marine stratocumulus deck and the measurement of their effects from ship, aircraft and satellite observational platforms. The Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) was successfully executed between July and August 2011 off the central coast of California. The PI’s research group led the characterization and deployment of a new counterflow virtual impactor inlet on a Twin Otter to characterize the physicochemical properties of droplet residual particles during thirty research flights. An aerosol hygroscopic growth measurement probe was developed and deployed during E-PEACE on the Research Vessel (R/V) Point Sur during a two week cruise to characterize aerosol physicochemical properties. E-PEACE measurements revealed that both incidental smoke and ship emissions are effective at modifying cloud albedo, and that giant salt nuclei can increase droplet coalescence rates. Other selected scientific advancements as a result of this project include the following: (i) A two-year satellite remote sensing dataset from the NASA A-Train was used to examine conversion rates of cloud water to rain water for warm maritime clouds. Using satellite-retrieved data from MODIS and CloudSat, conversion rates were calculated to generally be higher at reduced values of lower tropospheric static stability (LTSS). When examining data in two selected ranges for LTSS, higher conversion rates are coincident with higher cloud liquid water path (LWP) and factors co-varying or rooted in the presence of aerosol types exhibiting lower aerosol index values, where aerosol index is considered as a proxy for columnar cloud condensation nuclei (CCN) number concentration. In conditions of high pollution and generally smaller "fine-mode" aerosol, the slowest conversion rates and longest conversion times are observed most likely owing to higher LTSS, relatively low wind speeds, and more numerous and smaller drops in clouds that hamper the conversion process. (ii) An analysis was conducted for stratocumulus cloud albedo responses in ship tracks, based on in situ aircraft measurements from E-PEACE and three years of satellite observations of nearly 600 individual ship tracks. The sign (increase or decrease) and magnitude of the albedo response in ship tracks was found to depend on factors such as free tropospheric moisture, cloud top height, and mesoscale cloud structure. In a closed cell structure (cloud cells ringed by a perimeter of clear air), approximately a third of ship tracks exhibited a reduction in cloud albedo. (iii) During E-PEACE, a plume of organic aerosol was produced by a smoke generator and emitted into the marine atmosphere from aboard the R/V Point Sur. We examined the hygroscopic properties and the chemical composition of the plume at plume ages between 0 and 4 hours in different meteorological conditions. In sunny conditions, the plume particles had very low hygroscopic growth factors (GFs = ratio of wet particle diameter to dry particle diameter): between 1.05 and 1.09 for 30 nm and 1.02 - 1.1 for 150 nm dry size at a relative humidity (RH) of 92%, contrasted by an average marine background GF of 1.6. New particles were produced in large quantities (several 10,000 cm-3), which lead to substantially increased CCN concentrations at supersaturations between 0.07 – 0.88%. In the marine background aerosol, GFs for 150 nm particles at 40% RH were found to be enhanced at higher organic mass fractions. The results add to the knowledge base to improve understanding of fundamental aerosol-cloud-radiation interactions. Results have been disseminated in the form of nine peer-reviewed publications and five oral presentations at major scientific conferences. Five of the PI’s graduate students (four female, 1 male) have been trained and educated via this project by conducting data analysis, participating in group meetings, presenting their results orally, and drafting both manuscripts and thesis dissertation chapters.

National Science Foundation (NSF)
Division of Atmospheric and Geospace Sciences (AGS)
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A. Gannet Hallar
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University of Arizona
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