Alberto W. Setzer

AERONET-a federated instrument network and data archive for aerosol Characterization

Abstract The concept and description of a remote sensing aerosol monitoring network initiated by NASA, developed to support NASA, CNES, and NASDA’s Earth satellite systems under the name AERONET and expanded by national and international collaboration, is described. Recent development of weather-resistant automatic sun and sky scanning spectral radiometers enable frequent measurements of atmospheric aerosol optical properties and precipitable water at remote sites. Transmission of automatic measurements via the geostationary satellites GOES and METEOSATS’ Data Collection Systems allows reception and processing in near real-time from approximately 75% of the Earth’s surface and with the expected addition of GMS, the coverage will increase to 90% in 1998. NASA developed a UNIX-based near real-time processing, display and analysis system providing internet access to the emerging global database. Information on the system is available on the project homepage, http://spamer.gsfc.nasa.gov . The philosophy of an open access database, centralized processing and a user-friendly graphical interface has contributed to the growth of international cooperation for ground-based aerosol monitoring and imposes a standardization for these measurements. The system’s automatic data acquisition, transmission, and processing facilitates aerosol characterization on local, regional, and global scales with applications to transport and radiation budget studies, radiative transfer-modeling and validation of satellite aerosol retrievals. This article discusses the operation and philosophy of the monitoring system, the precision and accuracy of the measuring radiometers, a brief description of the processing system, and access to the database.

An emerging ground‐based aerosol climatology: Aerosol optical depth from AERONET

Long-term measurements by the AERONET program of spectral aerosol optical depth, precipitable water, and derived Angstrom exponent were analyzed and compiled into an aerosol optical properties climatology. Quality assured monthly means are presented and described for 9 primary sites and 21 additional multiyear sites with distinct aerosol regimes representing tropical biomass burning, boreal forests, midlatitude humid climates, midlatitude dry climates, oceanic sites, desert sites, and background sites. Seasonal trends for each of these nine sites are discussed and climatic averages presented.

Potential global fire monitoring from EOS‐MODIS

The National Aeronautic and Space Administration (NASA) plans to launch the moderate resolution imaging spectroradiometer (MODIS) on the polarorbiting Earth Observation System (EOS) providing morning and evening global observations in 1999 and afternoon and night observations in 2000. These four MODIS daily fire observations will advance global fire monitoring with special 1 km resolution fire channels at 4 and 11 μm, with high saturation of about 450 and 400 K, respectively. MODIS data will also be used to monitor burn scars, vegetation type and condition, smoke aerosols, water vapor, and clouds for overall monitoring of the fire process and its effects on ecosystems, the atmosphere, and the climate. The MODIS fire science team is preparing algorithms that use the thermal signature to separate the fire signal from the background signal. A database of active fire products will be generated and archived at a 1 km resolution and summarized on a grid of 10 km and 0.5°, daily, 8 days, and monthly. It includes the fire occurrence and location, the rate of emission of thermal energy from the fire, and a rough estimate of the smoldering/flaming ratio. This information will be used in monitoring the spatial and temporal distribution of fires in different ecosystems, detecting changes in fire distribution and identifying new fire frontiers, wildfires, and changes in the frequency of the fires or their relative strength. We plan to combine the MODIS fire measurements with a detailed diurnal cycle of the fires from geostationary satellites. Sensitivity studies and analyses of aircraft and satellite data from the Yellowstone wildfire of 1988 and prescribed fires in the Smoke, Clouds, and Radiation (SCAR) aircraft field experiments are used to evaluate and validate the fire algorithms and to establish the relationship between the fire thermal properties, the rate of biomass consumption, and the emissions of aerosol and trace gases from fires.

Biomass Burning Airborne and Spaceborne Experiment in the Amazonas (BASE‐A)

In the Biomass Burning Airborne and Spaceborne Experiment in the Amazonas (BASE-A), conducted in September 1989, trace gas and particulate matter emissions were measured from biomass burning due to deforestation and grassland fires in South America. This information is required for a better understanding of the environmental impacts of biomass burning in the tropics and to improve algorithms for remote sensing of biomass burning from satellite platforms. The field experiment utilized the twin-engine Embraer Bandeirante EMB-1Ol instrumented aircraft of the Brazilian Institute for Space Research (INPE). Concentrations of ozone, CO2, CO, CH4, and particulate matter were measured from the aircraft. Fires were observed from satellite imagery, and the smoke optical thickness, particle size, and profiles of the extinction coefficient were measured using sunphotometers in the aircraft and from the ground. Four smoke plumes were sampled, three vertical profiles were measured, and extensive ground measurements were conducted of smoke optical characteristics for different smoke types. The collected data were analyzed for determining the emission ratios and combustion efficiency (the efficiency of a fire to convert the total burned carbon to carbon dioxide) and were compared with the results from fires in North America. Combustion efficiency was found to be higher in the tropics (97% for the cerrado and 90% for the deforestation fires) with emission factors similar to those of North American fires, for a given combustion efficiency. A strong relation was found between the spatial distribution of fires (up to 9000 per day in one state) and ozone concentration (up to 80 ppbv) and between biomass burning and concentrations of trace gases, particulate matter, and ozone. These relations strongly suggest a correlation between biomass burning in the tropics and ozone formation. An optical model of the smoke aerosol was derived and applied to radiance measurements. The smoke single scattering albedo was computed from the graphitic carbon concentration (assuming external mode mixture) as 0.90 ± 0.01. The particles effective radii were 0.1 to 0.2 μm, except for 1-day aged smoke with values up to 0.4 μm. Radiance measurements indicate that the width of the particle size distribution may be smaller in the tropics than for North American fires. The measured optical properties of smoke and the high correlation between emitted trace gases and particles form a basis for remote sensing of radiatively important trace gases and particulate matter from biomass burning using AVHRR imagery.

Measurements of irradiance attenuation and estimation of aerosol single scattering albedo for biomass burning aerosols in Amazonia

Investigation of the effects of biomass burning aerosols on the surface irradiance were conducted as a part of the Smoke, Clouds, and Radiation - Brazil (SCAR-B) experiment during August–September 1995. Measurements of broadband and spectral irradiance, in conjuction with measurements of aerosol physical and optical properties (optical depth, phase function, and size distribution) were made under varying conditions of aerosol loading during the SCAR-B field campaign. Estimates of aerosol single scattering albedo (ω0) were made from matching of the measured irradiance values to the model computed irradiances by varying ω0, for observations made under cloudless conditions. Values of ω0, at approximately 550 nm, estimated from this technique using broadband 400–700 nm irradiance measurements, ranged from approximately 0.82 to 0.94 for the dates and times of these SCAR-B measurements. Utilizing spectral irradiance data, the model retrieved values of ω0 decreased with increasing wavelength, with the change of ω0 as a function of wavelength differing on different days. Reductions in photosynthetically active radiation (PAR; 400-700 nm) incident at the surface were computed to range from about 20 to 45% compared to background aerosol conditions for the 2 month biomass burning season at several locations in the southern Amazon Basin. These large reductions in incident PAR at the surface due to the heavy aerosol loadings could have implications for primary production of sensitive ecosystems. In addition, reductions of total incident solar radiation from aerosol direct radiative effects may have significant impact on reducing surface heating and increasing aerosol layer heating from absorption.

Effect of dry-season biomass burning on Amazon basin aerosol concentrations and optical properties, 1992–1994

Aerosol concentrations and properties have been derived from a network of ground-based Sun-sky radiometer measurements in Brazils Amazon basin region since 1992. The measurements characterize the background aerosol environment and aerosol emissions from biomass burning at eight selected sites. The duration and frequency of the measurements provide the foundation of an aerosol climatology based on direct sun measurements of aerosol optical thickness and retrievals of size distribution from solar aureole measurements. The aerosol optical thickness measurements clearly illustrate that for sites located within regions of biomass burning the duration of smoke above background levels often exceeds 2 months and frequently at levels an order of magnitude above background. The aerosol optical thickness range during preburning conditions was 0.11 to 0.27 at 440 nm. Under these conditions, stratospheric aerosols from Pinatubo constituted a significant part of the signal in 1993 but were about 50% less in 1994. During the burning season, smoke elevated the aerosol optical thickness above 1.0 for seasonally averaged values measured at 440 nm at sites located in active source regions in Mato Grosso, Rondonia, and Tocantins states. The measurement sites are located in the cerrado and forest conversion areas. Analysis of the size distribution of the particles indicated that the increase in aerosol optical thickness was associated with an increase of an accumulation and coarse particle modes. The asymmetry factor “g”, computed from the phase function, showed considerable spectral dependence between the preburning and burning seasonal phases. The 1020-nm channel was reduced from 0.66 to ∼0.53, while at 440 nm little seasonal phase variation was noted. Conditions of burning were sufficiently strong that the atmospheric conditions associated with the climatological definition of a dry season was subdivided into (1) preburning, (2) transition to burning, (3) burning, and (4) transition to wet season phases for most sites. Averages and frequency distributions were used to characterize each seasonal phase by site. Changes in total column water vapor amount, also retrieved from direct sun measurements, did not have an apparent effect on the optical properties of the aerosols.

Monitoring fires in southwestern Amazonia Rain Forests

this disaster and defining emergency actions and policies. By mid-July, hot spot counts (fire pixels in satellite images) for Acre began to climb. In mid-August, counts from the Advanced Very High Resolution Radiometer (AVHRR) U.S. National Oceanic and Atmospheric Administration’s (NOAA)-12 sensor had exceeded the number for all of 2004, and anecdotal reports of fires escaping control and invading forests became frequent. Smoke began to affect urban life in Rio Branco, the capital of Acre. At a meeting of Acre’s State Fire Committee, satellite imagery from MODIS sensors combined with hot spot data and meteorological information convinced the committee to recommend the prohibition of fires. The governor of Acre proclaimed the prohibition, but hot spot counts continued to climb until the total for 2005 was more than twofold that of 2004.

Spectral characteristics of fire scars in Landsat-5 TM images of Amazonia

Abstract This work describes spectral characteristics of fire scars in an area of intense deforestation in the Amazon tropical forest as recorded in the seven Landsat-5 TM spectral channels of an image in 1985. Ground covers were divided into four themes: natural forests, ‘capoeiras’, pastures in general and fire scars. Single-cell and maximum likelihood classification algorithms were used in the analysis. TM channel 4 was the best to identify fire scars, with digital counts differences in relation to other themes about two times higher than in other channels. Fire scars were also detected in channel 5, with the limitation that recent scars could be mistaken with water. Channels 3 and 7 also contained information about the scars. Results indicated that TM images can be used in automatic detection and assessment of biomass burning in tropical forests, contributing to the understanding of biogeochemical cycles related to the use of fire in these areas. Comparison of the results for channels 3, 4 and 5 in tw...

Airborne measurements of aerosols from burning biomass in Brazil related to the TRACE A experiment

Results are reported from an airborne campaign to investigate the impacts of burning biomass upon the loading of lower-tropospheric aerosols and its composition over the Brazilian tropics. The flights, conducted as part of the NASA/Transport and Atmospheric Chemistry Near the Equator-Atlantic (TRACE A) mission, started on September 1, 1992, when the dry (fire) season still prevailed in the central part of Brazil, and ended on September 29. Of the total number of burnings detected in Brazil by the advanced very high resolution radiometer (AVHRR)/NOAA satellite sensor, 74% were concentrated in the states of Amazonas, Maranhao, Mato Grosso, Para, Roraima, and Tocantins during this period. Aerosol particles were sampled from a twin-engine aircraft in transit and vertical profile flights were made up to 4,000 m altitude. Black carbon measurements made in real time and in areas of burning biomass peaked at ∼2,500 m above the ground, increasing to ∼12,000 ng/m3. In other areas these values were lower by 1 order of magnitude. A condensation nuclei counter measuring small particles (>0.014 μm) produced values ranging from 2,000 to 16,000/cm3 for areas with low and high burning biomass, respectively. Deposition filters in a two-stage cascade impactor, and Nuclepore filters collected aerosols for analysis of 13 elements through particle-induced X ray emissions (PIXE). Primary elements associated with soil dust (Al, Si, Mn, Fe, Ni) prevailed in the aerosol coarse mode (>1 μm) while the fine mode aerosols were enriched in S, K, Br, and Rb, which are tracers normally associated with burning of biomass. The good correlation between fire spot counts, obtained via AVHRR aboard NOAA satellites, and black carbon, counts of small particles and total aerosol mass, suggests the determining of local concentrations of fire-derived aerosol fire emissions by satellite to be a new and useful approach.

Spectral characteristics of deforestation fires in NOAA/AVHRR images

Abstract This work presents optical-spectral and radiometric characteristics of fires associated to tropical deforestation as recorded by full resolution AVHRR/NOAA-9 images in the Amazon region during a dry season. Results showed that fires and smoke clouds were spectrally distinct and easily separated from surrounding ground covers by automatic digital processing. Channel 3 (3–55 to 3–93 /*m) was the most appropriate to identify active fires whose pixels had digital counts about one order of magnitude higher than common ground covers.