Ronald E. Babbitt

Smoke and fire characteristics for cerrado and deforestation burns in Brazil: BASE‐B Experiment

Fires of the tropical forests and savannas are a major source of particulate matter and trace gases affecting the atmosphere globally. A paucity of quantitative information exists for these ecosystems with respect to fuel biomass, smoke emissions, and fire behavior conditions affecting the release of emissions. Five test fires were performed during August and September 1990 in the cerrado (savannalike region) in central Brazil (three fires) and tropical moist forest (two fires) in the eastern Amazon. This paper details the gases released, the ratios of the gases to each other and to particulate matter, fuel loads and the fraction consumed (combustion factors), and the fire behavior associated with biomass consumption. Models are presented for evaluating emission factors for CH4, CO2, CO, H2, and particles less than 2.5 μm diameter (PM2.5) as a function of combustion efficiency. The ratio of carbon released as CO2 (combustion efficiency) for the cerrado fires averaged 0.94 and for the deforestation fires it decreased from 0.88 for the flaming phase to <0.80 during the smoldering phase of combustion. For tropical ecosystems, emissions of most products of incomplete combustion are projected to be lower than previous estimates for savanna ecosystems and somewhat higher for fires used for deforestation purposes.

Emissions of formaldehyde, acetic acid, methanol, and other trace gases from biomass fires in North Carolina measured by airborne Fourier transform infrared spectroscopy

Biomass burning is an important source of many trace gases in the global troposphere. We have constructed an airborne trace gas measurement system consisting of a Fourier transform infrared spectrometer (FTIR) coupled to a “flow-through” multipass cell (AFTIR) and installed it on a U.S. Department of Agriculture Forest Service King Air B-90. The first measurements with the new system were conducted in North Carolina during April 1997 on large, isolated biomass fire plumes. Simultaneous measurements included Global Positioning System (GPS); airborne sonde; particle light scattering, CO, and CO2; and integrated filter and canister samples. AFTIR spectra acquired within a few kilometers of the fires yielded excess mixing ratios for 10 of the most common trace gases in the smoke: water, carbon dioxide, carbon monoxide, methane, formaldehyde, acetic acid, formic acid, methanol, ethylene, and ammonia. Emission ratios to carbon monoxide for formaldehyde, acetic acid, and methanol were each 2.5±1%. This is in excellent agreement with (and confirms the relevance of) our results from laboratory fires. However, these ratios are significantly higher than the emission ratios reported for these compounds in some previous studies of “fresh” smoke. We present a simple photochemical model calculation that suggests that oxygenated organic compounds should be included in the assessment of ozone formation in smoke plumes. Our measured emission factors indicate that biomass fires could account for a significant portion of the oxygenated organic compounds and HOx present in the tropical troposphere during the dry season. Our fire measurements, along with recent measurements of oxygenated biogenic emissions and oxygenated organic compounds in the free troposphere, indicate that these rarely measured compounds play a major, but poorly understood, role in the HOx, NOx, and O3 chemistry of the troposphere.

Measurements of excess O3, CO2, CO, CH4, C2H4, C2H2, HCN, NO, NH3, HCOOH, CH3COOH, HCHO, and CH3OH in 1997 Alaskan biomass burning plumes by airborne Fourier transform infrared spectroscopy (AFTIR).

We used an airborne Fourier transform infrared spectrometer (AFTIR), coupled to a flow-through, air-sampling cell, on a King Air B-90 to make in situ trace gas measurements in isolated smoke plumes from four, large, boreal zone wildfires in interior Alaska during June 1997. AFTIR spectra acquired near the source of the smoke plumes yielded excess mixing ratios for 13 of the most common trace gases: water, carbon dioxide, carbon monoxide, methane, nitric oxide, formaldehyde, acetic acid, formic acid, methanol, ethylene, acetylene, ammonia and hydrogen cyanide. Emission ratios to carbon monoxide for formaldehyde, acetic acid, and methanol were 2.2±0.4%, 1.3±0.4%, and 1.4±0.1%, respectively. For each oxygenated organic compound, a single linear equation fits our emission factors from Alaska, North Carolina, and laboratory fires as a function of modified combustion efficiency (MCE). A linear equation for predicting the NH3/NOx emission ratio as a function of MCE fits our Alaskan AFTIR results and those from many other studies. AFTIR spectra collected in downwind smoke that had aged 2.2±1 hours in the upper, early plume yielded ΔO3/ΔCO ratios of 7.9±2.4% resulting from O3 production rates of ∼50 ppbv h−1. The ΔNH3/ΔCO ratio in another plume decreased to 1/e of its initial value in ∼2.5 hours. A set of average emission ratios and emission factors for fires in Alaskan boreal forests is derived. We estimate that the 1997 Alaskan fires emitted 46±11 Tg of CO2.

Effect of fuel composition on combustion efficiency and emission factors for African savanna ecosystems

Savanna burning in Africa occurs over a wide range of environmental, vegetation, and land use conditions. The emission factors for trace emissions from these fires can vary by a factor of 6 to 8, depending on whether the fires burn in miombo woodlands or in ecosystems where grass vegetation dominates. Ground-based measurements of smoke emissions and aboveground biomass were made for fires in grassland and woodland savanna ecosystems in South Africa and Zambia. A high combustion efficiency ( η⌢) was measured for the pure grassland; i.e., a high proportion of carbon was released as CO2. The η⌢ was lower for woodland savanna ecosystems with variable amounts of grass and with a more compact layer of leaf material and litter lying near the ground. The η⌢ was found to be dependent on the ratio of grass to the sum of grass and litter. Models developed for estimating emissions were integrated in a nomogram for estimating total emissions of CO2, CO, CH4, nonmethane hydrocarbons, and particles of less than 2.5 μm diameter per unit area.

Emissions from the laboratory combustion of wildland fuels : Particle morphology and size

[1] The morphology of particles emitted by wildland fires contributes to their physical and chemical properties but is rarely determined. As part of a study at the USFS Fire Sciences Laboratory (FSL) investigating properties of particulate matter emitted by fires, we studied the size, morphology, and microstructure of particles emitted from the combustion of eight different wildland fuels (i.e., sagebrush, poplar wood, ponderosa pine wood, ponderosa pine needles, white pine needles, tundra cores, and two grasses) by scanning electron microscopy. Six of these fuels were dry, while two fuels, namely the tundra cores and one of the grasses, had high fuel moisture content. The particle images were analyzed for their density and textural fractal dimensions, their monomer and agglomerate number size distributions, and three different shape descriptors, namely aspect ratio, root form factor, and roundness. The particles were also probed with energy dispersive X-ray spectroscopy confirming their carbonaceous nature. The density fractal dimension of the agglomerates was determined using two different techniques, one taking into account the three-dimensional nature of the particles, yielding values between 1.67 and 1.83, the other taking into account only the two-dimensional orientation, yielding values between 1.68 and 1.74. The textural fractal dimension that describes the roughness of the boundary of the two-dimensional projection of the particle was between 1.10 and 1.19. The maximum length of agglomerates was proportional to a power a of their diameter and the proportionality constant and the three shape descriptors were parameterized as function of the exponent a.

Comparison of aerosol optical thickness measurements by MODIS, AERONET sun photometers, and Forest Service handheld sun photometers in southern Africa during the SAFARI 2000 campaign

The Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on the NASA Terra satellite has been used to monitor aerosol optical thickness (AOT, τ) daily at 10 km×10 km resolution worldwide since August 2000. This information, together with the locations of active fires detected by the MODIS instrument, is essential for understanding the seasonal trends and interannual variability of fires and their impacts on air pollution, atmospheric chemistry, and global climate. We compared aerosol optical thickness derived from MODIS, five automated sun photometers of the Aerosol Robotic Network (AERONET), and 38 Forest Service (FS) handheld sun photometers in western Zambia from 20 August to 20 September 2000. Aerosol optical thicknesses derived from AERONET sun photometers and FS sun photometers were also compared in the same region between mid‐June and late September 2000. Our objectives were to validate the AOT measurements by MODIS and to investigate the factors that affect AOT measurements. We demonstrated that in the regions of intense biomass burning, MODIS aerosol optical thickness was consistently 40–50% lower at 470, 550, and 660 nm compared with ground‐based AOT measurements by automated and handheld sun photometers and airborne measurements by NASA Ames Airborne Tracking 14‐channel Sunphotometers (AATS‐14). The satellite look angles can influence the MODIS AOT values, with the actual MODIS AOT values being as much as 0.06 higher than model‐calculated MODIS AOT values on the right edge of the MODIS scene. This phenomenon may be due to error in the assumed aerosol scattering phase function or surface directional properties. Density of vegetation cover can also affect MODIS measurements of aerosol optical thickness.