The following are selected highlights of recent research supported by CCSP participating agencies (as reported in the fiscal year 2006 edition of the annual report, Our Changing Planet).
The Aura satellite was launched on 15 July 2004, from Vandenberg Air Force Base, California. Aura is already providing the first-ever daily global measurements of tropospheric ozone and many other trace gases that affect air quality. The spacecraft is delivering these results, in addition to observations of the stratosphere, with unprecedented spatial resolution. Aura's global view of Earth's atmospheric composition provides needed information for research, and for supporting environmental management and policy development related to issues of local, regional, and global air quality, climate change, and stratospheric ozone depletion and recovery. Aura carries four instruments: the Ozone Monitoring Instrument (OMI), the Microwave Limb Sounder (MLS), the High Resolution Dynamics Limb Sounder (HIRDLS), and the Tropospheric Emission Spectrometer (TES). OMI was built by The Netherlands and Finland in collaboration with NASA. HIRDLS was built by the United Kingdom and the United States.
Aura will help scientists globally monitor pollution production and transport from city-to-city, region-to-region, and continent-to continent, on a day-by-day basis, for the first time. Aura's view from space enables us to understand the long-range pathways of pollutants, and early research results using MLS measurements of carbon monoxide have begun to quantify the role of strong convective weather systems and long-range transport of pollution. Aura measurements also offer new insights into the processes that control the distribution of the trace gases important to climate change, and how climate changes influence the recovery of the protective stratospheric ozone layer. Analyses using measurements by the MLS instrument on Aura gave researchers the needed data to diagnose with unprecedented detail and spatial coverage polar ozone loss for the 2004 Antarctic ozone hole.
Aura instruments measure five of the six "criteria pollutants" identified by EPA. The complexity of pollution transport makes it difficult to quantify the extent to which human activities affect local air quality. In addition, the presence of the stratospheric ozone layer between the satellite and the troposphere makes "seeing" tropospheric ozone very difficult. Aura's TES uses new technology to see through the stratospheric ozone layer to measure tropospheric ozone (see Figure 2).
Aura enables new insights into the physical and chemical processes that influence the stratospheric ozone layer and climate. It is producing the most complete suite of chemical measurements ever available to understand the ozone layer and its recovery. These include the first measurements of chemically reactive hydrogen-containing species involved in ozone destruction, and the first simultaneous measurements of key forms of chlorine and bromine, which are also important for ozone destruction.
Smoke Inhibition of Cloud Formation 
Urban air pollution and smoke from fires can have important effects on the climate system. The net effect of aerosols on the atmospheric radiation budget and climate is a key uncertainty in attempts to model and project climate change. Aerosols can counteract regional greenhouse warming by reflecting solar radiation to space or by enhancing cloud reflectance and lifetime. However, some aerosols can add to the warming by absorbing sunlight, which also may have the effect of slowing down the hydrological cycle and reducing cloud cover.
While most greenhouse gases have long lifetimes and a homogeneous distribution in the global atmosphere, aerosols, due to their short lifetimes, have a heterogeneous spatial and temporal distribution. Thus, daily satellite observations and continuous in situ measurements are needed to observe the emission and transport of dense aerosol plumes downwind of populated and polluted regions and regions with vegetation fires.
Remote-sensing observations are providing the first measurements of the effect of aerosols, including sunlight-absorbing black carbon, or soot, on inhibition of cloud formation (see Figure 3). Observations by the Moderate-Resolution Imaging Spectroradiometer (MODIS) instrument aboard the Aqua satellite over the Amazon region during the biomass burning season in 2003 showed that scattered cumulus cloud cover was reduced from 40% in clean conditions down to 0% in heavy smoke conditions. The reduction of clouds due to smoke aerosols leads to less sunlight being reflected and more sunlight being absorbed by the Earth, resulting in warming. This effect may be offsetting some of the cooling attributed to sulfate aerosols.
Multi-Platform Studies of Aerosol Properties and Radiative Effects 
On five occasions spanning the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) field campaign in spring 2001, the Multiangle Imaging Spectroradiometer (MISR) instrument on the NASA Terra satellite took data coincident with high-quality observations by instruments on two or more surface and airborne platforms. The cases capture a range of clean, polluted, and dusty aerosol conditions. A wealth of data was collected through the joint support of NSF, NASA, NOAA, and the Office of Naval Research (ONR). Scientists synthesized the data from over 40 field instruments and satellite observations into layer-by-layer environmental snapshots that summarize what is known about the atmospheric and surface states at key locations during each event. Aerosols within a few kilometers of the surface were composed primarily of Asian dust with mixtures of pollution added from Asian and non-Asian sources. Medium- and coarse-mode particle size distributions varied little among the events; however, the column aerosol optical depth varied considerably depending on the near-surface amounts of absorbing aerosols. The consistency of component particle microphysical properties among the five events, even in this relatively complex aerosol environment, suggests that global, satellite-derived maps of aerosol optical depth and aerosol mixture (air-mass type) extent, combined with targeted in situ component microphysical property measurements, can provide a detailed global picture of aerosol properties and distributions, enabling studies of aerosol impacts on climate.
Intercontinental Tracking of Pollution and Aerosols
An expanse from the western United States to the European continent was the setting in summer 2004 for more than 200 scientists who participated in the largest climate and air quality study to date, the International Consortium for Atmospheric Research on Transport and Transformation (ICARTT). The research was aimed at developing a better understanding of the factors that are involved in the intercontinental transport of pollution and the radiation balance in North America and the North Atlantic. Several U.S. agencies (with scientists in NOAA and NASA as co-leads), academic institutions, and international partners from five other countries used satellite, aircraft, shipboard, and land-based observations to obtain unprecedented information about the composition and transformation of air masses as they crossed the United States, traversed the Atlantic Ocean, and arrived in western Europe (see Figure 4). Natural and anthropogenic emissions react in the atmosphere to produce gases and aerosol particles that affect climate. Tracking the sources, atmospheric transformations, and intercontinental transport of these chemical species by ICARTT is advancing U.S. and international climate research.
Nighttime Chemistry More of a Factor than Previously Recognized 
A new technique has enabled measurement of previously hard-to-measure trace gases in the atmosphere. The observations have opened a new frontier area in studying the atmospheric chemistry that occurs at night. The approach uses an advanced spectroscopy technique to measure trace gases in the reactive nitrogen family, some of which occur primarily at night. The gases play important roles in the chemistry that produces ozone, a greenhouse gas and pollutant that has both climate and air quality implications. In atmospheric measurements off the coast of New England, researchers found that the nighttime chemistry involving nitrogen-containing trace gases can effectively remove these gases from the atmosphere, thus "short circuit" the chemistry that would have produced ozone the next day. Related work in 2004 was aimed at developing an aircraft-ready version of the instrument and applying it in the extensive ICARTT summer climate research field campaign.
Identifying which Atmospheric Aerosol Particles are Effective "Seeds" for Cloud Formation 
Among the least-understood but potentially important processes in atmospheric composition science is the relationship between small aerosol particles and ice-cloud formation. The process has important implications for the radiation balance of the atmosphere. Researchers have developed a novel technique to determine the chemical composition of those aerosols capable of forming atmospheric ice clouds, commonly termed "ice nuclei," both on a particle-by-particle basis and in real time. This technique was used in 2003 and 2004 field experiments to address the need for additional and more quantitative information on the chemical composition of atmospheric ice nuclei. Among the important results was the finding that the most efficient ice nuclei are not ubiquitous sulfate aerosols, but are instead rare mineral or fly-ash particles, some of anthropogenic origin. Aerosols rich in organic material were shown to be inefficient ice nuclei. These results have contributed to our understanding of the interaction of aerosol particles with clouds and will provide valuable information for global climate models.
Transport and transformation processes in aerosol plumes were studied in field experiments off the east coasts of Asia (2001) and North America (2002), with the aim of evaluating and refining models of chemical transport and radiative transfer. This research has shown that dust transported out of Asia over the Pacific Ocean includes not only dust, but also anthropogenic gases and particles adsorbed/reacted onto the dust. The transport of dust-pollution plumes out of Asia and across the Pacific has important implications for air quality. In addition, recent satellite data suggest that these pollution plumes are having a regional effect on the Earth's reflectivity. Results from studies in eastern North America show that plumes originating in the United States can be as intense as those downwind of India and northern Asia. The direct radiative effect of the aerosols in these plumes is thus an important factor in climate. Forthcoming data analyses will allow researchers to quantify the radiative forcing due to aerosols and to apportion this forcing based on aerosol composition and, by inference, aerosol source. The information will provide the scientific basis for informing the development of effective strategies to reduce emissions and mitigate climate impacts.
Determining Aerosol Properties from Ground-Based Measurements
Since 1979, satellite retrievals have provided excellent spatial coverage of atmospheric column ozone, certain characteristics of aerosols, and ultraviolet (UV) radiation. Satellite data from Total Ozone Monitoring Spectrometer (TOMS) instruments have been essential for deriving global trends in UV radiation levels and resolving critical questions about the impacts of increased UV radiation due to stratospheric ozone depletion and changes in aerosols and clouds. Continuation and improvement of the TOMS UV data record is a goal of the new Ozone Monitoring Instrument launched on the Earth Observing System (EOS) Aura satellite in 2004.
Currently, ground-based retrievals provide accurate measures of ozone needed for the validation of satellite data, as well as finer temporal resolution of these quantities necessary to determine diurnal variations and understand the observed long-term trends. An excellent example of ground-based support has been provided by the nine years of radiometer data from the CCSP-sponsored UV-B Monitoring and Research Program's (UVMRP) observational network, which has been used to assess the geographic distribution, trends, and year-to-year variability of UV-B radiation in the United States.
Since 2002, CCSP's TOMS, UVMRP, and Aerosol Robotic Network (AERONET) programs have shared equipment, personnel, and analysis tools among member agencies such as USDA, NASA, and NOAA to quantify aerosol absorption using ground-based radiation measurements. Recently, UVMRP data have been used to extend measurements of aerosol properties into UV wavelengths when combined with data obtained from AERONET sun photometers. The new analysis techniques applied to the ground-based radiation measurements provide aerosol scattering and absorption characteristics essential for determining the amount of UV radiation reaching the Earth's surface and for understanding the potential implications for climate change. Determining the spatial and temporal distribution of aerosol properties is critical for projecting climate change, understanding tropospheric chemistry, and making accurate satellite measurements of UV radiation, ozone, and aerosols.
Emissions of Ozone-Depleting Substances in Russia 
The emissions of six ozone-depleting substances (ODS) were estimated from measurements taken by scientists in a 17,000-km journey along the Russian trans-Siberian railway. The measurements are of global interest because Russian sources of ODS are thought to be a significant fraction of the total ODS production worldwide, and because estimates of global emissions are not in good agreement with observed atmospheric abundances over the last ~10 years. The research, based on a 2001 study, has thus far indicated that the modern emissions of ODS in Russia are too small to cause the large, contemporary shortfalls in global emission estimates. A follow-on study in spring 2004 was undertaken to examine the expected reduction of ODS concentrations since the earlier 2001 study. Preliminary results show that the abundances are indeed smaller, suggesting that the international ozone layer protection agreements of the Montreal Protocol are having their intended effect.