The Pacific Atmospheric Sulfur Experiment (PASE) is a comprehensive study of the chemistry and physics of sulfur in the remote marine troposphere that will take place near Christmas Island (Kiritimati) in August-September 2007. PASE will focus on gas-phase chemistry, and aerosol composition and growth, in the cloud-free convective boundary layer (CBL), as well as the buffer layer, and the formation of new particles in the cloud outflow of marine cumulus. The flux of dimethyl sulfide (DMS) from the ocean is the major source of sulfur to the remote marine atmosphere and DMS oxidation products are important in aerosol formation and growth. In addition to reflecting incoming solar radiation (the direct effect), aerosols also alter the formation and properties of clouds (indirect effects), which are among the most uncertain processes in climate and global change. PASE will provide information essential for developing an understanding of the aerosol indirect effect in this region. State-of-the-art instrumentation on the NSF C-130 aircraft will be used to make accurate and high speed airborne determinations of sulfur dioxide (SO2), DMS, dimethyl sulfoxide, dimethyl sulfone, methane sulfonic acid, hydroxyl radical, ammonia, water vapor (H2O), ozone (O3), hydrogen peroxide, aerosol size distributions and composition, and meteorological parameters. DMS, SO2, H2O and O3 will be measured at 25 Hz, allowing vertical fluxes of these species to be determined by eddy correlation. Chemical budgets for the sulfur species can be calculated from their concentrations and fluxes. These budgets will contain valuable information on chemistry in general, and on chemical formation and loss rates in particular. PASE is a study of the chemistry underlying aerosol forcing in earth's radiation budget and climate change. The PI will attempt to communicate this through public presentations at the Academy of Natural Sciences in Philadelphia, through the news media, and on a speaker tour to high schools. A web page will be maintained at Drexel University that provides access to the goals and results of PASE for the public. Broader impacts also include the participation of undergraduate and graduate students from Drexel, Georgia Institute of Technology, University of California, Davis, Desert Research Institute/University of Nevada, and the University of Hawaii, who will assist with the field phase of the program and participate in data interpretation, data archiving, and reporting of results.
This project has led to a better understanding of the chemical and physical processes in the tropical trade wind regions of the Earth's atmosphere. This area is particularly important to the climate because this is where a large fraction of sunlight is absorbed into the Earth’s climate system. The occurrence of small clouds in this region can change the amount of sunlight absorbed in this region by reflecting sunlight back to space instead of it being absorbed and keeping our planet warm. Previous studies suggested that emissions of sulfur containing gases by algae in the ocean lead to the formation of clouds by chemical reactions in the atmosphere that create particles and those particles then become cloud droplets. These clouds act to cool the Earth's climate by increasing the abundance of low, bright clouds over the relatively dark ocean. It was the emissions of these sulfur compounds and their subsequent chemistry and processing into cloud droplets that was the object of this experiment. The study found that much of the sulfur emitted by the ocean is rapidly re-deposited back to the ocean, possibly short-circuiting or weakening the proposed cooling effects. Furthermore, it was found that agricultural fires from the Amazon, some 6,000 miles upwind, periodically influence the make up of particles in the lower atmosphere over the central Pacific Ocean. Such sooty particles will, in principle, further reduce the proposed cooling effects of oceanic sulfur emissions, but this point needs further study. The results of the experiment also advanced our understanding of the rate at which air is mixed in the lowest layers of the atmosphere adjacent to the ocean, and how the winds and ocean temperatures influence the trade wind clouds. Such detailed observations are fairly rare and should serve to improve climate modeling efforts. Additionally, the work has created some important follow-up questions such as whether or not there is are unrecognized sulfur gases emitted from the ocean, and whether certain halogen compounds (e.g. bromine oxide generated from sea salt spray) are as important as some previous studies have suggested. Prepared by: Ian Faloona, U.C. Davis and Peter DeCarlo Drexel University In memory of Dr. Alan Bandy
