Atmospheric Composition
USGCRP Fiscal Year 2006 Accomplishments

USGCRP
Program Elements

Atmospheric Composition

Ecosystems

Global Carbon Cycle

Decision-Support Resources Development and Related Research on Human Contributions and Responses

Climate Variability and Change

The Global
Water Cycle

Observing and Monitoring the Climate System

Communications

International Research and Cooperation

Upper Atmospheric Water Vapor Trends and Feedbacks [2, 12, 13]

Climate models predict that as ocean surfaces warm due to a warming climate, the increased evaporation will moisten all levels of the atmosphere, especially the upper atmosphere. Since water vapor is itself a greenhouse gas, this moistening further increases the warming. Models of Earth’s climate suggest that this serves as a powerful positive feedback, more than doubling the sensitivity of the surface temperature to anthropogenic greenhouse gas forcing. Regional moistening trends in the lower atmosphere have been observed since the mid-1970s from balloon-borne measurements and are strongly linked to changes in surface temperature. Upper atmospheric trends are, however, difficult to assess from conventional observing systems, such as balloon-borne sensors, due to fundamental limitations of observing capabilities. New CCSP research has used data from the High Resolution Infrared Radiometer Sounder (HIRS) and Microwave Sounding Unit (MSU) satellite instruments to evaluate upper atmospheric water vapor and temperature trends since 1979. The observed trends are consistent with those obtained from global climate model simulations; both indicate moistening over this period. The observed results are starkly different from what would be expected in a “no moistening” scenario, which would have greatly reduced the magnitude of model estimates of future global warming. This study provides strong evidence supporting the capability of current climate models to simulate this important climate process, and adds to the credibility of climate model projections of future global warming.

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]

The CCSP community has made substantial progress in appraising the understanding of North American emissions of trace gases that are precursors for the formation of aerosols and ozone, both of which have implications for climate as well as for air quality in the lower atmosphere. The North American Research Strategy for Tropospheric Ozone (NARSTO) Emission Inventory Assessment, published in FY 2005, is a detailed examination of current approaches to measuring, inventorying, and modeling emissions of trace gases and an assessment of potential improvements to those approaches. Findings of particular note regarding the U.S. National Emission Inventory for mobile sources included: (i) carbon monoxide emissions are about half as large as earlier estimates suggested; and (ii) nitrogen oxide emissions increased throughout the 1990s, contrary to previous assumptions. Accurate estimates of emissions are needed to understand the quantity of aerosols, ozone, and their precursors that are exported from North America to the global atmosphere. Related CCSP research has examined the chemistry of the atmosphere in the New England region, especially with respect to pollutant emissions that affect ozone and aerosols. Findings show a different mix of volatile organic compounds than had been expected; the improved information will lead to a more accurate picture of the chemistry of continental air masses. The New England field research also documented the transport and transformation of aerosols and their associated radiative heating effects as they were carried downwind from their North American sources. This work will enable scientists to reduce the uncertainty in estimates of the effects of aerosols on climate.

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.

Aerosols have both direct and indirect cooling effects on climate. The direct cooling effect is due to the aerosol particles’ reflection of sunlight to space. Indirect effects include aerosol-induced changes in cloud properties, cloud distribution, and precipitation that can cool the Earth by enhancing cloud reflectivity. Past assessments of the direct effect of aerosols on climate were conducted through modeling efforts. Now,CCSP research has assessed the direct impact of aerosols on climate through global measurements. New satellite sensors introduced in the last decade, as well as the Aerosol Robotic Network (AERONET), a global network of ground-based remote-sensing instruments, have provided the opportunity to make a measurement-based assessment of the aerosol direct effect on climate. A major scientific synthesis and assessment of the aerosol direct effect on climate has been completed, published, and served as a major resource for Working Group I of the Intergovernmental Panel on Climate Change (IPCC) in the preparation of its Fourth Assessment Report. The paper reviews recent progress in characterizing aerosols and assessing the aerosol direct effect, focusing on measurements. High-accuracy satellite measurements of aerosol properties have made it feasible to obtain observational constraints for the aerosol direct forcing, especially over the global ocean. Measurements show that aerosols in total reflect about 10% of incoming sunlight to space, much more than the models had indicated. The human contribution to the global aerosol concentrations suggested by the models agrees with the measurement assessment to within measurement accuracy and is on order of approximately 20%. 

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]

Improved modeling capabilities have enabled CCSP researchers to simulate smoke-cloud-surface interactions in biomass-burning regions and to study the mechanisms by which smoke suppresses cloud formation. The model includes unprecedented treatment of coupled smoke aerosol-cloud-radiative feedbacks in three-dimensional simulations of biomass-burning regions of Amazonia. Absorbing aerosol particles, such as smoke particles, can affect cloudiness by modifying heating rates and atmospheric stability, by modifying how much cloud droplets are heated, or by modifying heat flows from the land surface. CCSP research has evaluated the relative importance of these processes in continental areas, demonstrating that the primary reason for smoke suppression of clouds is the reduction of surface heat flow caused by the presence of absorbing smoke particles (shown schematically in Figure 4). Further, the change in surface heat flow may cause changes in climate-relevant cloud properties, such as cloud amounts. However, the ultimate effect on cloudiness is case-specific and is highly dependent upon cloud type, smoke quantity and properties, location of the smoke within the atmosphere and relative to the clouds, and the degree to which surface heat flows are reduced. The research has helped to quantify a poorly understood aspect of the influence of atmospheric aerosol particles on the climate-related phenomena of cloudiness and surface heating.

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.

References

1)   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.