Influence of Sunlight on Chemical Composition of Oceanic-Derived Sea-Salt Particles in the Lower AtmosphereESF in Bermuda
Although the atmosphere is mostly comprised of gases, it also contains many particles (better known as aerosols). Individual aerosols are too small to be seen with the naked eye. However, their presence is clearly evident in the darker layer of air near the Earth's surface one often observes from the window of an airplane or by the slimy film that accumulates on the windshield of a car parked near the beach. Although small, these aerosols are quite numerous (often exceeding 1000 per square centimeter) and consequently they can have a profound affect on the Earth’s radiation balance influencing life and our climate. The oceans are the major source of aerosols mass in the lower atmosphere and one of the primary sources worldwide. Breaking waves on the ocean surface produce bubbles that, upon bursting, inject seawater constituents into the overlying atmosphere. The familiar “fizz” that one hears following a breaking wave at the beach is the sound that accompanies aerosol production.
In terms of mass, most of the sea-salt aerosols are in the 1 -10 micron diameter size-range, which persist in the atmosphere for only a few hours to a few days. Many smaller submicron-sized aerosols are also generated and, depending on meteorological conditions, these aerosols persist in the atmosphere for a week or more. Depending on their size and associated atmospheric lifetime, oceanic aerosols can be transported great distances and undergo significant chemical changes in their composition. For example, although sea salt aerosols start off at a slightly alkaline pH of 8 (which is roughly the same pH as a solution of baking soda), within minutes they pick up gas phase acids (primarily nitric and sulfuric) and become very acidic (pH 3-5, lemon juice is approximately pH 4). Because they accumulate acids over longer lifetimes, the pH of submicron aerosols is typically even lower. The low pH and interaction with sunlight cause additional chemical changes in the aerosols, which are thought to significantly influence atmospheric ozone concentrations, sulfur cycling, radiation balance and climate, and trace-metal fertilization of the surface ocean. While the scientific community is well aware that chemical changes will occur in sea-salt aerosols during their atmospheric lifetime, the details of these chemical changes are poorly known.
My colleagues at the University of Virginia (William Keene, firstname.lastname@example.org), Wadsworth Center, New York State Department of Health and School of Public Health of SUNY at Albany (Xianliang Zhou), Max Planck Institute for Chemistry (Rolf Sander), and University of Miami Rosenstiel School of Marine and Atmospheric Science (Hal Maring) and I (David Kieber) proposed a three-year field and modeling study to better understand the chemical changes that occur in sea-salt aerosols as they are exposed to sunlight. This project was comprised of two phases. The first phase was conducted at the Bermuda Institute for Ocean Sciences over the past two years. At this facility, we were able to test a large, sea salt aerosol generator (Images 1, 2 & 3), fabricated at the University of Virginia, to determine the physical and chemical properties of the sea-salt aerosols that were freshly generated from bursting bubbles. The aerosol samples that were collected were diluted in water and exposed to sunlight to quantify the production of reactive oxygen species (ROS) and to assess their impact on the processing of the organic matter in the aerosols. The amount of ROS that are produced in our studies will be compared to other known sources and chemical processes in the lower atmosphere employing a Model of Chemistry Considering Aerosols (MOCCA).
The second phase of this study is being conducted now at the Bermuda Institute for Ocean Science’s Tudor Hill Atmospheric Observatory, which consists of an atmospheric sampling tower and two trailers (Image 4) to house equipment, computers, and facilities for visiting scientists such our group (Image 5).
This site is influenced by chemically distinct air masses originating from different source regions. When under the influence of the Bermuda high during late spring through early autumn, the site is often enveloped by relatively clean marine air (with lots of sea salt aerosols) that has not been substantially modified by recent continental emissions. This site is also periodically influenced by air originating from Africa, Canada, Europe or the eastern United States all of which give rise to very different air mass types thereby giving us an opportunity to collect and examine distinct atmospheric aerosols. Many past investigations (both short- and long-term) have characterized aerosol properties at Bermuda thereby providing context for temporally extrapolating results of our study.
To collect aerosol samples, we used two types of samplers at the Tudor Hill site; both of these samplers are secured to the top of the tower (Image 6). The first sampler is called a MOUDI cascade impactor (Multi Orifice Uniform Deposit Impactor), which has ten stages to collect aerosols in different size ranges all the way from the largest aerosols greater than 10 microns to the very smallest aerosols less than approximately 0.2 microns in diameter (Image 7). Basically, the cascade impactor works by air moving around a series of plates stacked on top of each other. If an aerosol is too big, it cannot keep up with the air flow and hits (or impacts) one of the plates. As the air flows from one end of the impactor to the other end, the air velocity increases such that smaller and smaller aerosols are impacted on the plates. In Image 8, one of the stages of the disassembled impactor is shown revealing the aerosols that were impacted on the aluminum plate. The diameters of the nozzles above each impactor plate are designed in such a manner that each sampling plate collects particles of predominantly one size range. The second type of aerosol collector that we are using is a dichotomous sampler, which works on the same principle as the cascade impactor except that only two size fractions are collecteda supermicron fraction with particles above approximately 1 micron in diameter and a submicron fraction with all aerosols less than one micron. Although the dichotomous samples provides less information about the size of the aerosols, it has one distinct advantage over the MOUDI sampler in that it can collect a lot more sample (aerosols) because larger air flows through the sampler (i.e. 100 liters of air per minute vs 30 liters per minute for the MOUDI) can be achieved and the sample is partitioned into only 2 as opposed to 10 size fractions. Once the sample is collected, it is extracted from the sampler plate with water for subsequent analysis or exposure to sunlight.
So far we have obtained some very exciting results. In particular, we have shown that the oceans can be a much more important source of very small, submicron aerosols than previously thought. These are very important in the atmosphere because they can act as cloud condensation nuclei or the “seeds” for cloud formation thereby affecting both the brightness and duration of clouds and consequently our Earth’s radiation balance and ultimately climate. Second, we have shown that sea-salt aerosols can be an important source of the hydroxyl radical, which is the ultimate cleaning agent in the atmosphere, breaking down and removing organic matter in the air. We are developing a mathematical model to determine if this radical plays the same role in the processing of organic matter in sea-salt aerosols.
Results from our study will improve understanding of the fundamental processes that control the chemistry of the lower atmosphere and related influence on Earth systems such as climate. Our results will also directly benefit from and build on a number of interrelated national and international programs investigating the chemical and physical evolution of marine aerosols and their related climatic implications. The National Science Foundation (NSF) recently funded BBSR to refurbish the Tudor Hill in support of anticipated new investigations under these programs, which include the International Geosphere-Biosphere Programme’s Surface Ocean Lower Atmosphere Study (IGBP-SOLAS), U.S. SOLAS, the National Aerosol-Climate Interaction Program (NACIP), and other emerging initiatives investigating influences of the lower atmosphere on climate.
Our research has established and fostered research and educational collaborations between the faculty, staff and students at five academic institutions. A number of peer-reviewed publications are in preparation stemming from our work. Some of these future manuscripts are summarized in abstracts that were submitted to the American Geophysical Union for presentation at the upcoming fall meeting in San Francisco, CA.