Donald R. Cahoon

Satellite analysis of the severe 1987 forest fires in northern China and southeastern Siberia

Meteorological conditions, extremely conducive to fire development and spread in the spring of 1987, resulted in forest fires burning over extremely large areas in the boreal forest zone in northeastern China and the southeastern region of Siberia. The great China fire, one of the largest and most destructive forest fires in recent history, occurred during this period in the Heilongjiang Province of China. Satellite imagery is used to examine the development and areal distribution of 1987 forest fires in this region. Overall trace gas emissions to the atmosphere from these fires are determined using a satellite-derived estimate of area burned in combination with fuel consumption figures and carbon emission ratios for boreal forest fires.

DETERMINING EFFECTS OF AREA BURNED AND FIRE SEVERITY ON CARBON CYCLING AND EMISSIONS IN SIBERIA

The Russian boreal forest contains about 25% of the global terrestrial biomass, and even a higher percentage of the carbon stored in litter and soils. Fire burns large areas annually, much of it in low-severity surface fires – but data on fire area and impacts or extent of varying fire severity are poor. Changes in land use, cover, and disturbance patterns such as those predicted by global climate change models, have the potential to greatly alter current fire regimes in boreal forests and to significantly impact global carbon budgets. The extent and global importance of fires in the boreal zone have often been greatly underestimated. For the 1998 fire season we estimate from remote sensing data that about 13.3 million ha burned in Siberia. This is about 5 times higher than estimates from the Russian Aerial Forest Protection Service (Avialesookhrana) for the same period. We estimate that fires in the Russian boreal forest in 1998 constituted some 14–20% of average annual global carbon emissions from forest fires. Average annual emissions from boreal zone forests may be equivalent to 23–39% of regional fossil fuel emissions in Canada and Russia, respectively. But the lack of accurate data and models introduces large potential errors into these estimates. Improved monitoring and understanding of the landscape extent and severity of fires and effects of fire on carbon storage, air chemistry, vegetation dynamics and structure, and forest health and productivity are essential to provide inputs into global and regional models of carbon cycling and atmospheric chemistry.

Biogenic soil emissions of nitric oxide (NO) and nitrous oxide (N2O) from savannas in South Africa: The impact of wetting and burning

In this paper we report on the first measurements of microbial soil emissions of nitric oxide (NO) and nitrous oxide (N2O) from the savannas in South Africa. In addition to natural, unperturbed emission measurements, we investigated the impact of natural rainfall, artificial irrigation, and fire on these emissions. Wetting and burning resulted in a significant enhancement in the emissions of NO. Mean background NO emissions from the dry sites ranged from 0.4 to 6.2 ng N m−2 s−1 and from the wetted sites ranged from 4.7 to 34.0 ng N m−2 s−1. After burning, the mean NO emissions from the dry sites increased and ranged from 13.3 to 15.2 ng N m−2 s−1 and from the wetted sites increased, exceeding 60 ng N m−2 s−1. Measurements of biogenic emissions of N2O were attempted, but emissions were not detected throughout the measurement period, indicating emissions below the minimum delectability of the instrumentation (2 ng N m−2 s−1).

Crown fire emissions of CO2, CO, H2, CH4, and TNMHC from a dense Jack pine boreal forest fire

Samples of high-intensity crown fire smoke were collected using a helicopter during the International Crown Fire Modeling Experiment near Fort Providence, Northwest Territories, Canada. The samples were analyzed for carbon dioxide (CO2), carbon monoxide (CO), hydrogen (H2), methane (CH4), and total nonmethane hydrocarbons (TNMHC). CO2− normalized mean emission ratios (ERs) and emission factors (g product/kg fuel burned) were determined for CO2, CO, H2, CH4, and TNMHC. Carbon monoxide production was determined to increase during high-intensity crowning. Unlike CO, a corresponding increase in the production of H2, CH4, and TNMHC during crowning was not detected. This represents the first clear indication that we know of where relative increases in CO production from vegetation fires are not positively correlated with corresponding increases in CH4, H2, and TNMHC production. These results may be important to the atmospheric carbon budget, and to the potential use of CO as a normalizing parameter for boreal forest fire emissions.

Source compositions of trace gases released during African savanna fires

Measurements of biomass burn-produced trace gases were made using low-altitude helicopter penetrations of smoke plumes above burning African savanna during the Southern African Fire-Atmosphere Research Initiative (SAFARI-92). Smoke from two large prescribed fires conducted in the Kruger National Park, South Africa, on September 18 and 24, 1992, was sampled at altitudes ranging from 20 to 100 m above ground level during flaming and smoldering phases of combustion. Carbon dioxide (CO2) normalized emission ratios (dX/dCO2 (vol/vol), where X denotes a trace gas) for carbon monoxide (CO), hydrogen (H2), methane (CH4), total nonmethane hydrocarbons (TNMHC), and nitrous oxide (N2O) were determined. The emission ratios were used in conjunction with fuel consumption estimates to calculate emission factors (grams of product per gram of fuel) for these gases. Emission factors for CO2, CO, CH4, and N2O of 1.61, 0.055, 0.003, and 1.6 × 10−4 g/g fuel, respectively, were determined. The fires advanced rapidly through the savanna (primarily grass) fuels with minimal amounts of smoldering combustion. The relatively low emission ratios determined for these fires indicated excellent combustion efficiency. About 93% of the carbon released into the atmosphere as a result of these fires was in the form of CO2.

Evaluation of a technique for satellite-derived area estimation of forest fires

Satellite data have been used increasingly during the past few years to examine burning around the globe. One such satellite instrument, the advanced very high resolution radiometer (AVHRR), has been found useful for the location and monitoring of both smoke and fires because of the daily observations, the large geographical coverage of the imagery, the spectral characteristics of the instrument, and the spatial resolution of the instrument. Earlier studies using AVHRR imagery have focused on locating and monitoring fires and studying the characteristics of smoke. This paper will discuss the application of AVHRR data to assess the geographical extent of burning. Methods have been developed to estimate the surface area of burning by analyzing the surface area effected by fire with AVHRR imagery. Characteristics of the AVHRR instrument, its orbit, field of view, and archived data sets are discussed relative to the unique surface area of each pixel. The errors associated with this surface area estimation technique are determined using AVHRR-derived area estimates of target regions with known sizes. This technique is used to evaluate the area burned during the Yellowstone fires of 1988.

Frequency and distribution of forest, savanna, and crop fires over tropical regions during PEM-Tropics A

Advanced very high resolution radiometer 1.1 km resolution satellite radiance data were used to locate active fires throughout much of the tropical region during NASAs Global Tropospheric Experiment (GTE) Pacific Exploratory Mission-Tropics (PEM-Tropics A) aircraft campaign, held in September and October 1996. The spatial and temporal distributions of the fires in Australia, southern Africa, and South America are presented here. The number of fires over northern Australia, central Africa, and South America appeared to decrease toward the end of the mission period. Fire over eastern Australia was widespread, and temporal patterns showed a somewhat consistent amount of burning with periodic episodes of enhanced fire counts observed. At least one episode of enhanced fire counts corresponded to the passage of a frontal system which brought conditions conducive to fire to the region, with strong westerlies originating over the hot, dry interior continent. Regions that were affected by lower than normal rainfall during the previous wet season (e.g., northern Australia and southwestern Africa) showed relatively few fires during this period. This is consistent with a drought-induced decrease in vegetation and therefore a decreased availability of fuel for burning. Alternatively, a heavier than normal previous wet season along the southeastern coast of South Africa may have contributed to high fuel loading and an associated relatively heavy amount of burning compared to data from previous years.

A meteorological interpretation of the Arctic Boundary Layer Expedition (ABLE) 3B flight series

The Arctic Boundary Layer Expedition (ABLE) 3B was conducted to determine the summertime tropospheric distribution, sources, and sinks of important trace gas and aerosol species over the wetlands and boreal forests of central and eastern Canada. Isentropic trajectories and analyzed midtropospheric circulation patterns were used to group flights according to the transport histories of polar, midlatitude, or tropical air masses which were sampled. These data were then divided into bands of potential temperature levels representing the low, middle, and maximum aircraft altitudes to assess the effects of both local and long distance transport and natural and man-made pollutants to the measured chemical species. Detailed case studies are provided to depict the complex three-dimensional airflow regimes that transported air with differing chemical signatures to the study area. Mission 6 details the large-scale movement of smoke in the generally prevailing west to northwesterly airflow that was observed on the majority of flights. Mission 1 analyzes the horizontal and vertical motions of maritime Pacific air in the upper troposphere that was routinely mixed downward to the aircraft altitude. Finally, mission 14 tracks the far northward excursion of tropical air that had been associated with a Pacific typhoon. The following three factors all had important influences on the collected chemical data sets: (1) local and distant stratospheric in puts into the upper and middle troposphere; (2) biomass-burning plumes from active fires in Alaska and Canada; (3) a band of low ozone upper tropospheric air that was observed by airborne differential absorption lidar (DIAL) above the aircraft maximum altitude. Other modification factors observed on some flights included urban pollution from U.S. and Canadian cities, tropical air that had been associated with a Pacific typhoon, and precipitation scavenging by clouds and rain. Many flights were affected by several of the above factors which led to complex chemical signatures that will be discussed in other companion papers.

Meteorological overview of the Arctic Boundary Layer Expedition (ABLE 3A) flight series

A meteorological overview of the Arctic Boundary Layer Expedition (ABLE 3A) flight series is presented. Synoptic analyses of mid-tropospheric circulation patterns are combined with isentropic back trajectory calculations to describe the long-range (400–3000 km) atmospheric transport mechanisms and pathways of air masses to the Arctic and sub-Arctic regions of North America during July and August 1988. Siberia and the northern Pacific Ocean were found to be the two most likely source areas for 3-day transport to the study areas in Alaska. Transport to the Barrow region was frequently influenced by polar vortices and associated short-wave troughs over the Arctic Ocean, while the Bethel area was most often affected by lows migrating across the Bering Sea and the Gulf of Alaska, as well as ridges of high pressure which built into interior Alaska. July 1988 was warmer and dryer than normal over much of Alaska. As a result, the 1988 Alaska fire season was one of the most active of the past decade. Airborne lidar measurements verified the presence of biomass burning plumes on many flights, often trapped in thin subsidence layer temperature inversions. Several cases of stratosphere/troposphere exchange were noted, based upon potential vorticity analyses and aircraft lidar data, especially in the Barrow region and during transit flights to and from Alaska.

Impact of biomass burning in equatorial Africa on the downward surface shortwave irradiance: Observations versus calculations

Long-term ground-based measurements of downward surface shortwave irradiance are compared with the satellite-derived downward surface shortwave irradiance for equatorial Africa. Good agreement between satellite-derived and measured values are shown in the spring and fall (seasons relative to the northern hemisphere). Large differences between satellite-derived and measured values are found in regions where extensive savanna fires take place during the dry season. Discrepancies are found in regions north of the equator during the winter months, and the differences shift to the southern hemisphere during summer. Significant differences of 40–80 W m−2 are found for extended areas, corresponding to 25–40% of the measured monthly mean downward surface shortwave irradiance. Savanna fires release large amounts of particles into the atmosphere, increasing the aerosol optical depth. The increased optical depth leads to an overestimation of the downward surface shortwave irradiance by the satellite reduction algorithms which do not account for the fire-produced aerosol.