This study will focus on physical and chemical analysis of ambient aerosol and residual cloud droplet nuclei obtained from the NSF C-130 aircraft over the southeast Pacific (SEP) during the 2008 VOCALS-REx (VAMOS Ocean-Cloud-Atmosphere-Land Study Regional Experiment) project. This work addresses the following VOCALS hypotheses: 1) Variability in aerosol properties impacts the formation of drizzle in stratocumulus clouds over the SEP, 2) The small aerosol radii over the SEP are primarily controlled by anthropogenic, rather than natural, aerosol production, and entrainment of polluted air from the lower free troposphere is an important source of cloud condensation nuclei (CCN), and 3) Ocean upwelling affects aerosol precursor gases and the aerosol size distribution in the atmospheric boundary layer over the SEP. To explore these hypotheses, the size and chemical composition of residual nuclei in cloud droplets will be measured following sampling with a counterflow virtual impactor (CVI), which separates droplets from interstitial aerosol and evaporates them. Aerosols outside of clouds will be sampled with an impactor. Filter and impactor samples of residual nuclei and ambient aerosol will be analyzed by electron microscopy with X-ray analysis. These data will be combined with cloud microphysical measurements to determine aerosol effects on cloud optical properties and lifetimes and the characteristics of aerosol particles that nucleate cloud droplets. Nuclei of larger droplets, which initiate drizzle production, will also be explored. These measurements will complement observations being made by other investigators in VOCALS.
This work will lead to insight into how aerosol production due to human activity influences precipitation and the radiative impact of clouds on Earth's climate. The educational plans include an undergraduate student and postdoctoral scholar who will work with the investigators to analyze and interpret the data collected.
This project used aircraft measurements to study how aerosol particles from South American pollution change cloud characteristics in the southeast Pacific Ocean, with the largest persistent stratocumulus deck in the world. Changes in cloud characteristics are important because low clouds reflect sunlight for a net cooling effect on climate. Our major results are given below. Aerosol Particles: In the southeast Pacific, particle number concentration increases near shore due to pollution from continental sources. The effect extends ~800 to 1000 km off the coast of Chile along the 20°S latitude line. Measurements of particles within cloud droplets showed that a variety of particle sizes are scavenged by stratocumulus clouds, down to at least 55 nm in size. The additional particles are mainly composed of sulfate from anthropogenic sources in central Chile. Sulfate may also become internally mixed with sea-salt also in and out of cloud. Based on modeling and electron microscope analysis, copper smelters, while an important source of sulfur dioxide in the region, contributed a relatively small fraction (≤15%) of the particles likely to impact cloud characteristics along 20°S. Cloud Effects: These increased pollutants results in higher droplet number concentrations and smaller droplet sizes in clouds near shore. However, the clouds are thinner near shore, which also results in smaller droplet sizes. About 60% of the decrease in droplet size is estimated to be due to enhanced aerosol particles acting as droplet nuclei, while 40% is due to physically thinner clouds. The amount of sunlight reflected by clouds depends on the vertically integrated liquid water content (the liquid water path), as well as the droplet size. Because the liquid water path is thinner near shore, the cloud reflectance is actually lower near shore than far offshore (>1000 km). However, some increase in cloud reflectance due to more, smaller droplets was seen in an intermediate region, approximately 600-800 km from shore. Broader Impacts: This work directly supported and trained two undergraduate students from Oregon State University, one through NSF’s Research Experience for Undergraduates program. These students helped to analyze and compile aerosol and cloud physics data from the aircraft, and participated in presentation and publication of the work. In addition, two graduate students from other institutions were also involved in collaborative research and publication of this work, which furthers understanding of effects of anthropogenic pollution on climate.
