Interaction of Smoke, Clouds, and Radiation over Brazil (SCAR-B)
--David Herring (herring@ltpmail.gsfc.nasa.gov), Science Systems and Applications, Inc.
Trace gases--such as CO2 and CH4 --contribute to the Earth's greenhouse effect by trapping and containing heat from geothermal and incoming solar radiation. Theoretically, increasing the concentration of these trace gases in the atmosphere will proportionally amplify the greenhouse effect, thereby raising the average global temperature. Scientists estimate that deforestation and corresponding biomass burning are responsible for a significant percentage of the increase in atmospheric CO2 concentration since the beginning of the industrial revolution, around 1850.
Biomass burning releases particulates into the atmosphere that affect its chemistry and physics on local, regional, and global scales. For example, smoke particles can act as cloud condensation nuclei (CCN) interacting with clouds to create smaller, more-numerous cloud droplets, resulting in more dense clouds. Theoretically, increasing CCN should result in an increase in cloud albedo--unless it is compensated with black carbon absorption--having a net cooling effect on climate. However, satellite analysis shows a decrease in cloud albedo as a function of smoke concentration, so further studies are needed to better understand the physics involved.
Paradoxically, biomass burning affects atmospheric dynamics in ways that promote both global warming and regional cooling trends. Which trend will prove greater in the long run? According to SCAR-B Scientist Peter Hobbs, of the University of Washington, the warming trend will prove greater in the long run since the residence time of CO2 in the atmosphere is much longer than that of particles.
Yet, scientists are only just beginning to understand the problem in ways that allow them to construct models of the interactions of smoke, from burning vegetation, with clouds and radiation. Goddard scientists hope to refine these models further so that they can monitor the effects of forest fires, using satellite sensors such as the Moderate-resolution Imaging Spectroradiometer (MODIS), over much greater temporal and spatial scales.
But what about the remaining pieces of the puzzle? Biomass burning accounts for about 23 percent of the CO2 emissions worldwide today. According to Darold Ward, of the U.S. Forest Service, about 80 percent of the biomass burned annually is in tropical countries in Africa and South America. About 40 percent of the total burned comes from deforestation.
Brazil is a logical environment in which to extend the SCAR campaign. Its widespread slash- and burn- agricultural practices account for a significant portion of the tropical burning and deforestation taking place. It is therefore important to characterize the aerosols and trace gases being released into the atmosphere there; as well as to monitor how those emission products are transported to other parts of the world.
More significantly, an infrastructure already exists in Brazil to support a campaign as complex as SCAR. A Memorandum of Understanding was already in place between the U.S. Forest Service and IBAMA (the Brazilian equivalent) for sharing technology and knowledge on natural resource management. Specifically, the U.S. Forest Service has been involved in exchanging information with IBAMA on fire management, prescribed burning, control of fires, air quality considerations, and ways of minimizing adverse effects of smoke from fire management activities. Later, Tim Suttles, previously of NASA Headquarters, and Volker Kirchhoff, SCAR-B co-project scientist from INPE, were able to formulate another Memorandum of Understanding to cover the SCAR experiment.
In addition to the scientific equipment and expertise that Brazil has to offer, state-of-the-art computer networks and communications systems were already in place to facilitate rapid data transmissions and communicating of last-minute mission plans among geographically distributed researchers. During the previous two campaigns, SCAR scientists found that cellular phones were an effective, efficient medium for staying in touch--Brazil's cellular phone system worked nicely.
Flight operations planning requires access to the latest meteorological information, as well as intelligence reports on where the fires are burning. Good communications between scientists--sometimes located out in the field or even a continent away--and aircraft pilots is essential for optimizing flight paths, in response to constantly changing meteorological conditions, to meet the data collection objectives. To address this problem, Elaine Prins set up a World Wide Web server at the University of Wisconsin-Madison on which she continuously posted the latest meteorological data--from GOES --over the entire continent of South America. Several times per day SCAR participants at IBAMA accessed these data via the Internet and relayed them to flight mission planners.
Although SCAR-B data analysis will be ongoing for some time--and conclusive results will take years to pin down--there is one immediate payoff from the campaign. A close, ongoing working relationship between American and Brazilian scientists has been established. Collaboration between our two countries has proven to be successful and greatly beneficial, not only in terms of valuable data exchanged, but also in terms of scientific knowledge and expertise shared. Hobbs summed up his working relationship with the Brazilians in one word--"excellent".
Prior to SCAR-B, Brent Holben, of GSFC, installed a network of sunphotometers throughout Brazil to measure aerosol size distributions and concentrations, as well as their effects on solar radiation. This network permits continuous monitoring of aerosol optical thickness, sky radiance, and ground sampling of aerosols for at least the next several years. This long-term collaboration with Brazilian scientists will yield a spatially wide dataset on the emission of trace gases and particles from biomass burning in Amazonia and cerrado regions.
Understanding how emission products are transported by wind currents in South America is also key. "There are very nice results from trajectory analyses that clearly show strong transport of aerosols from biomass burning from Brazil into [the atmosphere over] the South Atlantic and entering into the global circulation," Artaxo states. "We also observed some emission products being transported from the Amazon Basin to the Pacific through the Andes Mountains."
Another goal during SCAR-B was to characterize the physical and radiative properties of smoke particles from biomass burning. Explains Lorraine Remer, of Science Systems and Applications, Inc., "We had an interesting picture emerge from the SCAR-A campaign of urban industrial aerosols. But we knew from Brent's [Holben] sun photometer data from earlier years that [our characterization of aerosols in America] wouldn't be applicable to Brazil because the aerosols are different there."
Indeed, there is even variation in Brazilian aerosols found over the cerrado, or dry savannah, region versus those found over the rainforest of Amazonia. "My interest was to obtain data to see how aerosols differed and varied regionally," Remer states. "I wanted to know if different types of smoke in Brazil could be modeled by one model, or would I need several?"
As mentioned previously, collecting data on the interaction of smoke particles with clouds was another important goal in SCAR-B. Interestingly, Kaufman and Robert Fraser (GSFC) noticed that smoke affects clouds over the northern rain forest regions of Brazil, but does not seem to affect the clouds over the cerrado. They don't yet fully understand why, but speculate that cloud drop radii in clouds over the cerrado region may already be too small to be significantly affected by smoke. In Africa, they note, the cerrado type of aerosols from biomass burning is more dominant, and this finding may be important to understanding their capability to affect clouds.
In addition to its impact on the atmosphere, biomass burning affects the health of surrounding vegetation and the ecosystem it supports. Using SCAR-B remote sensing data of different land surface covers in the presence and absence of smoke, Alfredo Huete (University of Arizona) and the MODIS Land Discipline Group hope to develop improved vegetation indices enabling them to monitor the conditions of tropical vegetation using MODIS data once that sensor is launched.
In addition to the ground sunphotometer network, aircraft and satellite platforms were employed to gather remote sensing data during SCAR-B. The NASA ER-2 flew 80 research flight hours carrying a payload consisting of the MODIS Airborne Simulator (MAS), the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS), the Cloud Lidar System (CLS), a Spectral Electro-Optic Radiometer (SEOR), and solar and infrared flux radiometers. On average, the ER-2 was flown at an operating altitude of 20 km. Primarily through the efforts of Michael King, SCAR-B co-project mission scientist, the MAS data system was upgraded prior to the campaign from a 12-channel, 8-bit digitizer to a 50-channel, 12-bit digitizer.
Aircraft in situ measurements, as well as sampling of aerosol particulates and trace gases, were made by instruments aboard the University of Washington's C-131A and the Brazilian INPE Bandeirante aircraft, flying about 75 and 90 research hours, respectively. These aircraft were flown at multiple altitudes to help gather data on how smoke particles and trace gas properties evolve over time and as a function of height. Satellite remote sensing data were obtained from the Geostationary Operational Environmental Satellite (GOES), Meteosat, Landsat Thematic Mapper (TM), and Advanced Very High Resolution Radiometer (AVHRR) to complement the aircraft and ground measurements.
For example, as a result of the SCAR campaigns, Kaufman and a group of MODIS researchers developed a new method for remote sensing of aerosols over land. The method involves identifying dark pixels using the 2.1-um channel and then predicting the reflectance in the blue and red channels using the measured reflectance at 2.1 um. Kaufman states that this algorithm is now a major part of his present MODIS aerosol retrieval algorithm. Kaufman adds that the image data from MAS and AVIRIS, covering 2 million square kilometers, will give us detailed spectral information on thousands of fires and the related smoke emitted from them. These remote sensing data will be complemented with spectral data from Holben's sunphotometer network. Additionally, Hobbs will have detailed data on the chemistry and optical properties of emission products.
Kaufman expects the SCAR scientists to spend several years retrieving valuable science from the data set, so it will be a while before they are ready to draw substantive conclusions. Data from the campaign are currently being processed by the respective data collecting agencies. Eventually, these data will be stored at the NASA Langley Research Center Distributed Active Archive Center (DAAC) and made available to the science user community from there.
For more details on how to access SCAR-B data, contact Sue Sorlie, SCAR-B data manager, at sorlie@magician. larc.nasa.gov; or call her at (804) 864-8660.
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