USGCRP
Home ![]() ![]() ![]() ![]() |
| Search |
PDF version of this section of the report PDF version of the full report The hardcopy version of this report is available free of charge from the GCRIO Online Catalog
|
![]() |
1. Atmospheric Composition
Great strides have been made in the last two decades concerning our understanding of how atmospheric composition relates to the global environment. The cause of the Antarctic ozone hole, first observed in the 1980s and initially a geophysical mystery, has been conclusively linked to the trace amounts of human-made chlorine- and bromine-containing gases released to the atmosphere starting in the 1960s. The rise in greenhouse gases in the atmosphere has been meticulously documented for several decades. The interactions between natural emissions and emissions caused by human activity have been shown to lie at the heart of the quality of the air we breathe. Increasingly, researchers are taking up the challenge of understanding the complex interactions between issues such as climate change, ozone depletion, and air quality. Better scientific understanding holds the promise of enabling future decisions that will be effective across multiple—and interwoven—societal concerns. CCSP-supported research in atmospheric composition for FY 2007 will have a high-priority focus on improving the predictive understanding of the role of natural and human-influenced aerosols (airborne fine particles) on the climate system. Aerosols interact with radiation directly (by absorbing or scattering radiation) as well as indirectly (e.g., by influencing the formation and properties of clouds). Unlike greenhouse gases, aerosols can either warm or cool the atmosphere through these interactions. Furthermore, the lifetime of aerosols in the atmosphere is shorter than that of many greenhouse gases, therefore the time frame for responding to their influence is correspondingly shorter than for the long-lived greenhouse gases. The understanding of how aerosols influence climate has been identified as one of the most uncertain scientific areas and a high priority for research. The interaction between aerosols, clouds, precipitation, and climate is a specific focus of FY 2007 research in the atmospheric composition component of CCSP. Research also focuses on understanding the role of non-CO2 greenhouse gases (e.g., changes in water vapor distribution due to human influences) in climate, as well as investigating the long-range processes that transport and transform greenhouse gases in the atmosphere. The overall research approach for understanding the role of atmospheric composition is an integrated application of long-term systematic observations, laboratory research, intensive field studies focused on gaining a process-level understanding of atmospheric phenomena, and diagnostic analyses and modeling that advance predictive capabilities. These endeavors are coupled with periodic assessments of understanding and development of products that synthesize findings in ways that are useful to decision makers and planners. Research is highly collaborative and involves partnerships throughout the Nation and the world. Highlights of Recent ResearchThe following are selected highlights of recent research supported by CCSP-participating agencies. Upper Atmospheric Water Vapor Trends and Feedbacks [2, 12, 13]
In other CCSP research, simultaneous observations of water vapor in the upper atmosphere and cloud ice from the Microwave Limb Sounder (MLS) on the NASA Aura satellite have provided new evidence for another kind of positive feedback through convective cloud-induced enhancement of the greenhouse effect in the tropics. The work shows that when sea surface temperature exceeds 27°C (about 300 Kelvin, see Figure 1), water evaporated from the warm surface is carried to the upper atmosphere through the formation of towering cumulus clouds. Ice particles in the upper levels of these clouds eventually evaporate, leaving increased water vapor concentrations in the upper atmosphere. Analyses indicate that this cloud-induced moistening of the tropical upper troposphere leads to an enhanced positive water vapor feedback that is about three times larger than what would be expected in the absence of the clouds. Figure 1 illustrate these relationships. These results indicate that convection is an important, if not dominant, process in the previously identified tropical “super greenhouse effect.”
A 350-Year Atmospheric History of a Climate-Related Trace Gas [5]CCSP scientists have published a 350-year atmospheric history of the most abundant sulfur-containing gas in the lower atmosphere, carbonyl sulfide (COS). The research provides the longest atmospheric record of COS to date and shows how the atmospheric abundances of this gas have increased and decreased with industrial sulfur emissions (see Figure 2). Carbonyl sulfide is a relatively long-lived sulfur trace gas with both natural and anthropogenic sources (the latter arising from a variety of industrial processes). Sulfur in COS accounts for about 50% of the sulfate aerosol layer, the atmosphere’s layer of sulfate-containing fine particles, during non-volcanic periods. This layer affects depletion of the ozone layer as well as heating and cooling of the atmosphere and hence climate. CCSP researchers analyzed Antarctic air trapped in ice cores and consolidated snow that provided a record of COS concentrations back to about the year 1650. These data suggest that atmospheric amounts of COS increased substantially during the Industrial Revolution, but decreased by about 10% in recent years as global industrial emissions of sulfur declined. Given that a close relationship exists between the atmospheric history of COS and global industrial sulfur emissions, the CCSP research suggests that human activities may have contributed to long-term changes in the atmosphere’s sulfate aerosol layer.
Assessing and Improving the Inventories of Trace Gas Emissions from North America [7, 8]
Remote Sensing Measurement-based Assessments of Aerosol Direct Radiative Forcing [14]Greenhouse gases and aerosol particles influence the climate system. While increasing greenhouse gases warm the climate system, aerosol particles have a diverse set of both cooling and warming influences on climate. Current scientific assessment finds that the net effect of aerosols is mostly to cool the climate system, temporarily reducing the impact of greenhouse gases on warming. Even though the greenhouse-gas warming is larger than the aerosol cooling, the uncertainty in the aerosol effect is five times larger than the uncertainty in the greenhouse effect. Reducing these uncertainties is critical to understanding of the climate system and its future change.
This review also identifies several issues that require significant future research efforts. Estimates of aerosol forcing over land are less well constrained than over ocean. Uncertainties in estimates of aerosol forcing are also larger on regional scales than on a global scale. The aerosol forcing under cloudy conditions remains relatively unexplored and quite uncertain. In addition, knowledge of the much more complex and probably more important aerosol indirect effects that modify cloud properties and abundance is much less certain and near-term interagency plans reflect the high priority placed on improved characterization of these aerosol-cloud interactions. Improved Estimates of Organic Aerosols in the Lower Atmosphere [4] Atmospheric aerosols are involved in issues ranging from air quality to climate change. Recent airborne measurements indicate that computer model simulations of aerosol composition severely underestimate the actual concentrations of organic aerosols in the free troposphere. The Asia-Pacific Regional Aerosol Characterization Experiment (ACE-Asia) was conducted in April and May of 2001. This large field campaign included two aircraft focused on sampling Asian aerosols transported over the Northwest Pacific Ocean. Three different teams of scientists made measurements of organic aerosols in the troposphere, from the surface to around 6.5 km. Results from the groups were very similar and showed that above 2 km, the concentrations of organic carbon were 10 to 100 times higher than the computer models predicted. However, the models were able to replicate adequately the concentrations of sulfate and elemental carbon, other aerosols that were also measured from the aircraft. The scientists believe that the underestimation of organic carbon by the models is due to secondary organic aerosols, which are formed in the troposphere from the oxidation of volatile organic compounds. They are long-lived and can therefore play a large role in intercontinental pollution transport and radiative forcing of climate. Relative Humidity and the Influence of Atmospheric Particles on Climate [1, 9, 10]CCSP researchers have conducted several studies of how atmospheric humidity affects aerosols and the implications for the climate system. The warming and cooling effects of aerosol particles are among the most uncertain of the influences on climate. Many factors affect how aerosol particles interact directly with light, including the relative humidity of the atmosphere, which can affect the size of the particles. CCSP researchers have conducted several studies that show that the chemical composition of the aerosol is the overriding factor in how size changes with relative humidity. Aerosols that are largely inorganic in composition, such as those containing sulfate or sea salt, tend to take up water and grow larger in more humid atmospheres. Consequently, they scatter more light and hence have a larger climate cooling influence (shown schematically in Figure 3). On the other hand, aerosols that are primarily organic are more hydrophobic, and their interaction with light does not change much as the relative humidity of the atmosphere varies. CCSP research spanned a full range of approaches, including fundamental laboratory studies, analysis of extensive field measurements of aerosols in regions of Asia and North America, and modeling investigations. The work has clearly demonstrated that not all aerosols are alike when it comes to their effects on climate. Because aerosol composition was shown to be such a major factor, one implication is that the climate effects of aerosols will change as the aerosol ages and its chemical composition changes. Climate effects therefore evolve as aerosol particles are transported from their original source. The research has led to more accurate representations of aerosol interactions in climate radiative calculations and will thereby improve the accuracy of estimates of the direct climate forcing by aerosols.
Cloud- and Aerosol-Related Fundamentals Elucidated [6]The interactions between water vapor and condensed forms of water in the atmosphere underlie many of the physical processes that govern cloud formation. Cirrus clouds, polar stratospheric clouds, and the large volume of the atmosphere that has a temperature below freezing are key climate-relevant phenomena that depend on the fundamental physical properties of water vapor, liquid water, and ice. Furthermore, the formation of aerosols and the chemical processes within them are affected by heretofore poorly understood processes specific to water that is supercooled below the freezing point. CCSP research has resulted in a comprehensive review of the fundamental physical processes at the vapor/water/ice interface and an advancement of the parameterization of those processes that are most relevant to atmospheric composition and climate. The work provides a first look at the full range of temperature conditions relevant to the atmosphere, yielding new perspectives on processes pertinent to colder regions, such as the Antarctic stratosphere, that had not been previously considered. Various structural forms of liquid water and solid ice are shown to behave differently, thus research has highlighted the need to represent these aspects of real-world behavior in models of atmospheric chemistry and climate. In addition, the work has implications for methodologies used to calculate relative humidity in laboratory applications and in the real atmosphere. Smoke Suppression of Clouds: A Climate-Related Effect of Aerosol Particles [3]
Ozone Layer Shows Signs of Recovery [11]New CCSP research has found that the atmosphere’s protective ozone layer is no longer decreasing over much of the globe. The work is based on several different satellite records and surface monitoring instruments. Previous studies have shown that ozone in the topmost layer of the atmosphere may have stopped declining. However, very little ozone is in these top layers. The latest work focuses on the thickness of the entire ozone column above the Earth’s surface, and therefore has relevance to the amount of harmful ultraviolet radiation reaching the surface of the Earth. Using satellite-derived estimates of ozone levels, as well as ground-station data from North America, Europe, Hawaii, Australia, and New Zealand, researchers found that total column ozone amounts over the southern mid-latitudes have stopped declining and leveled off, while amounts over the mid- and high latitudes of the Northern Hemisphere have increased since 1996. While the study indicates the beginning stages of improvement in the ozone layer, ozone amounts are still lower than those observed 25 years ago and full recovery lies decades in the future. Highlights of Plans for FY 2007
Climate and Ozone Layer AssessmentsCCSP research and leadership are playing a substantial role in international efforts to assess the state of scientific understanding with regard to climate and the ozone layer. Major preparation stages of the IPCC’s Fourth Assessment Report are occurring during 2006, with many CCSP scientists serving as authors and reviewers; the United States provides the co-chair of IPCC’s Working Group I and hosts its Technical Support Unit. In addition, the international assessment of ozone layer depletion was drafted and is being reviewed during 2006. Publication of both the climate and ozone layer assessments will occur in FY 2007. These activities will address Questions 3.1, 3.2, 3.4, and 3.5 of the CCSP Strategic Plan. Climate Field Study in the Texas/Gulf of Mexico RegionIn August and September of 2006, CCSP researchers in several agencies, institutions, and academia led and participated in a field program that investigated important scientific questions that are common to both climate and air quality. The Gulf of Mexico Atmospheric Composition and Climate Study (GoMACCS) component will focus on providing a better understanding of the sources and atmospheric processes responsible for the formation and distribution of ozone and aerosols in the atmosphere and the influence that these species have on regional and global climate. Analyses will continue in FY 2007. These activities will address Questions 3.1 and 3.3 of the CCSP Strategic Plan. Evaluating the North American Contributions to “Greenhouse” OzoneCCSP research will analyze ozone profiles measured across mid-latitude North America during a 2004 climate field study, with the aim of estimating how much ozone forms over the continent in the summer. Further studies will elucidate the export of ozone from North America to the North Atlantic Ocean and Europe. Ozone in the lower atmosphere is a greenhouse gas that affects climate because it absorbs infrared radiation. These activities will address Question 3.3 of the CCSP Strategic Plan. Analysis of Ozone and Ozone-Depleting Substances Data Sets for Trends and Ozone-Layer Recovery DetectionDecisions over the last 2 decades with regard to the protection of the stratospheric ozone layer have resulted in declining use of ozone-depleting substances (ODS). An important focus now is documenting and understanding how the atmosphere is responding to those decisions, both with regard to ODS abundances (which in general are in decline) and the ozone layer itself. Satellite and ground-based data sets have established a multi-decadal record that will be increasingly scrutinized for signs of recovery of the ozone layer via analyses of ozone and ODS. These activities will address Question 3.4 of the CCSP Strategic Plan. Oklahoma City Field StudyA field campaign to study North American aerosols involving CCSP scientists from multiple agencies is planned for the summer of 2007. Anthropogenic aerosols are relatively short-lived in the atmosphere and therefore have a high degree of variability over space and time. For this reason, a large contrast in aerosol properties is expected to be found between the upwind and downwind regions of their urban source area. The campaign will investigate the role of such differences for a mid-size, mid-latitude metropolitan area in North America and the consequences for the warming and cooling effects of aerosols. In addition, the campaign will evaluate changes to aerosols as they move through relatively simple fair-weather cumulus systems found over much of mid-latitude North America during the summer months. This activity will address Question 3.1 of the CCSP Strategic Plan. Megacity Initiative: Local And Global Research Observations (MILAGRO)The goal of the MILAGRO project is to characterize emissions, chemical transformations, and aerosol and pollutant outflow from Mexico City, the world’s second largest megacity. In 2007, CCSP scientists supported by NSF, DOE, and NASA will analyze and evaluate data collected during the 2006 campaign. Field measurements involved numerous ground sites and research aircraft, which along with satellite observations provide information at multiple spatial scales, ranging from the urban Mexico City “super site” to larger areas extending downwind into the Gulf of Mexico. U.S. scientists together with university and government scientists from Mexico will review MILAGRO data and analysis plans at a workshop in Boulder, Colorado, in FY 2007. The data will enable characterization of the evolution of the Mexico City plume, including changes in reactive trace gas concentrations; the evolution of chemical, physical, optical, and cloud nucleating properties of aerosols; and the interactions between gases and aerosols. These measurements and analyses should prove valuable for developing and testing models of aerosol evolution. These activities will address Question 3.3 of the CCSP Strategic Plan. Improve Estimates of Aerosol Direct Radiative Forcing via Satellite Data AnalysisMuch progress has been made in using global observations from space to assess aerosol radiative forcing. The lidar instrument aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite, launched 28 April 2006, promises to revolutionize knowledge of aerosol vertical distribution and greatly expand knowledge of the effects of aerosols in the lowermost atmosphere. Efforts are being made to develop methods of integrating global satellite observations with in situ and surface data to fill in major gaps in observations of aerosol particle properties (e.g., how they scatter versus absorb light), and to help separate anthropogenic from natural components. Further advances in the near term are also expected from multi-angle,ultraviolet, and polarization-based techniques for measuring the amount of aerosols in the atmosphere over land surfaces. These activities will address Question 3.1 of the CCSP Strategic Plan. ![]() References1) Baynard, T., R.M. Garland, A.R. Ravishankara, M.A. Tolbert, and E.R. Lovejoy, 2006: Key factors influencing the relative humidity dependence of aerosol light scattering. Geophysical Research Letters, 33, L06813, doi:10.1029/2005GL024898. 2) Cess, R.D., 2005: Water vapor feedback in climate models. Science, 310, 795-796. 3) Feingold, G., H. Jiang, and J.Y. Harrington, 2005: On smoke suppression of clouds in Amazonia. Geophysical Research Letters, 32(2), L02804, doi:10.1029/2004GL021369. 4) Heald, C.L., D.J. Jacob, R.J. Park, L.M. Russell, B.J. Huebert, J.H. Seinfeld, H. Liao, and R.J. Weber, 2005: A large organic aerosol source in the free troposphere missing from current models. Geophysical Research Letters, 32, L18809, doi:10.1029/2005GL023831. 5) Montzka, S.A., M. Aydin, M. Battle, J.H. Butler, E.S. Saltzman, B.D. Hall, A.D. Clarke, D. Mondeel, and J.W. Elkins, 2004: A 350-year atmospheric history for carbonyl sulfide inferred from Antarctic firn air and air trapped in ice. Journal of Geophysical Research, 109, D22303, doi:10.1029/2004JD004686. 6) Murphy, D.M. and T. Koop, 2005: Review of the vapour pressures of ice and supercooled water for atmospheric applications. Quarterly Journal of the Royal Meteorological Society, 131, 1539-1565, doi:10.1256/qj.04.94. 7) NARSTO Emission Inventory Assessment Team, 2005: Improving Emission Inventories for Effective Air Quality Management Across North America: A NARSTO Assessment. NARSTO 05-001, NARSTO, Pasco, WA, 310 pp. Available at <www.narsto.com>. 8) Parrish, D.D., 2006: Critical evaluation of U.S. on-road vehicle emission inventories. Atmospheric Environment, 40(13), 2288-2300. 9) Quinn, P.K., T.S. Bates, T. Baynard, A.D. Clarke, T.B. Onasch, W. Want, M.J. Rood, E. Andrews, J. Allan, C.M. Carrico, D. Coffman, and D. Worsnop, 2005: Impact of particulate organic matter on the relative humidity dependence of light scattering: A simplified parameterization. Geophysical Research Letters, 32, L22809, doi:10.1029/2005GL024322. 10) Randels, C.A., L.M. Russell, and V. Ramaswamy, 2004: Hygroscopic and optical properties of organic sea salt aerosol and consequences for climate forcing. Geophysical Research Letters, 31, L16108, doi:10.1029/2004GL020628. 11) Reinsel, G.C., A.J. Miller, E.C. Weatherhead, L.E. Flynn, R.M. Nagatani, G.C. Tiao, and D.J. Wuebbles, 2005: Trend analysis of total ozone data for turnaround and dynamical contributions. Journal of Geophysical Research, 101, D16306, doi:10.1029/2004JD004662. 12) Soden, B.J., D.L. Jackson, V. Ramaswamy, M.D. Schwarzkopf, and Z. Hunag, 2005: The radiative signature of upper tropospheric moistening. Science, 310, 841-844. 13) Su, H., W.G. Read, J.H. Jiang, J.W. Waters, D.L. Wu, and E.J. Fetzer, 2006: Enhanced positive water vapor feedback associated with tropical deep convection: New evidence from Aura MLS. Geophysical Research Letters, 33, L05709, doi:10.1029/2005GL025505. 14) Yu, H., Y.J. Kaufman, M. Chin, G. Feingold, L.A. Remer, T.L. Anderson, Y. Balkanski, N. Bellouin, O. Boucher, S. Christopher, P. DeCola, R. Kahn, D. Koch, N. Loeb, M.S. Reddy, M. Schultz, T. Takemura, and M. Zhou, 2006: A review of measurement-based assessments of the aerosol direct radiative effect and forcing. Atmospheric Chemistry and Physics, 6, 613-666. |
|