Biomass Burning: The Cycling of Gases and Particulates from the Biosphere to the Atmosphere

5.5 Biomass Burning: The Cycling of Gases and Particulates from the Biosphere to the Atmosphere JS Levine, NASA Langley Research Center, Hampton, VA, USA Published by Elsevier Ltd. This article is reproduced from the previous edition, volume 4, pp. 143–158, Published by Elsevier Ltd.

5.5.1 5.5.2 5.5.3 5.5.4 5.5.5 5.5.6 5.5.7 5.5.8 5.5.9 5.5.9.1 5.5.9.2 5.5.9.2.1 5.5.9.2.2 5.5.9.3 5.5.9.4 5.5.10 5.5.11 5.5.12 5.5.12.1 5.5.12.2 5.5.12.3 5.5.12.4 References

Introduction: Biomass Burning, Geochemical Cycling, and Global Change Global Impacts of Biomass Burning Enhanced Biogenic Soil Emissions of Nitrogen and Carbon Gases: A Postfire Effect The Geographical Distribution of Biomass Burning Biomass Burning in the Boreal Forests Estimates of Global Burning and Global Gaseous and Particulate Emissions Calculation of Gaseous and Particulate Emissions from Fires Biomass Burning and Atmospheric Nitrogen and Oxygen Atmospheric Chemistry Resulting from Gaseous Emissions from the Fires Chemistry of the Hydroxyl Radical (OH) in the Troposphere Production of O3 in the Troposphere CO oxidation chain Methane oxidation chain Chemistry of Nitrogen Oxides in the Troposphere Chemistry of the Stratosphere A Case Study of Biomass Burning: The 1997 Wildfires in Southeast Asia Results of Calculations: Gaseous and Particulate Emissions from the Fires in Kalimantan and Sumatra, Indonesia, August to December 1997 The Impact of the Southeastern Asia Fires on the Composition and Chemistry of the Atmosphere Modeling O3 and CO over Indonesia Measurements over Indonesia Measurements between Singapore and Japan Measurements over Hawaii

5.5.1 Introduction: Biomass Burning, Geochemical Cycling, and Global Change Biomass burning is both a process of geochemical cycling of gases and particulates from the biosphere to the atmosphere and a process of global change. In the preface to the book, One Earth, One Future: Our Changing Global Environment (National Academy of Sciences, 1990), Dr. Frank Press, the President of the National Academy of Sciences, writes: Human activities are transforming the global environment, and these global changes have many faces: ozone depletion, tropical deforestation, acid deposition, and increased atmospheric concentrations of gases that trap heat and may warm the global climate.

It is interesting to note that all four global change ‘faces’ identified by Dr. Press have a common thread – they are all caused by biomass burning. Biomass burning or vegetation burning is the burning of living and dead vegetation and includes human-initiated burning and natural lightning-induced burning. The bulk of the world’s biomass burning occurs in the tropics – in the tropical forests of South America and Southeast Asia and in

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the savannas of Africa and South America. The majority of the biomass burning, primarily in the tropics (perhaps as much as 90%), is believed to be human initiated for land clearing and land-use change. Natural fires triggered by atmospheric lightning only accounts for
10% of all fires (Andreae, 1991). As will be discussed, a significant amount of biomass burning occurs in the boreal forests of Russia, Canada, and Alaska. Biomass burning is a significant source of gases and particulates to the regional and global atmosphere (Crutzen et al., 1979; Seiler and Crutzen, 1980; Crutzen and Andreae, 1990; Levine et al., 1995). Its burning is truly a multidiscipline subject, encompassing the following areas: fire ecology, fire measurements, fire modeling, fire combustion, remote sensing, fire combustion gaseous and particulate emissions, the atmospheric transport of these emissions, and the chemical and climatic impacts of these emissions. Recently, a series of dedicated books have documented much of our understanding of biomass burning in different ecosystems. These volumes include: Goldammer (1990), Levine (1991, 1996a,b), Crutzen and Goldammer (1993), Goldammer and Furyaev (1996), van Wilgen et al. (1997), Kasischke and Stocks (2000), Innes et al. (2000), and Eaton and Radojevic (2001).

http://dx.doi.org/10.1016/B978-0-08-095975-7.00405-8

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5.5.2

Global Impacts of Biomass Burning

On an annual global basis, biomass burning is a significant source of gases and particulates to the atmosphere. The gaseous and particulate emissions produced during biomass burning are dependent on the nature of the biomass matter, which is a function of the ecosystem and the temperature of the fire, which is also ecosystem dependent. In general, biomass is composed mostly of carbon (
45% by weight) and hydrogen and oxygen (
55% by weight), with trace amounts of nitrogen (0.3–3.8% by mass), sulfur (0.1–0.9%), phosphorus (0.01– 0.3%), potassium (0.5–3.4%) and still smaller amounts of chlorine, and bromine (Andreae, 1991). During complete combustion, the burning of biomass matter produces carbon dioxide (CO2) and water vapor as the primary products, according to the reaction CH2 O þ O2 ! CO2 þ H2 O where CH2O represents the appropimate average chemical composition of biomass matter. In the more realistic case of incomplete combustion in cooler and/or oxygen-deficient fires, i.e., the smoldering phase of burning, carbon is released in the forms of carbon monoxide (CO), methane (CH4), nonmethane hydrocarbons (NMHCs), and various partially oxidized organic compounds, including aldehydes, alcohols, ketones, and organic acids and particulate black (soot) carbon. Nitrogen is present in biomass mostly as amino groups (R-NH2) in the amino acids of proteins. During combustion the nitrogen is released by pyrolytic decomposition of the organic matter and partially or completely oxidized to various volatile nitrogen compounds, including molecular nitrogen (N2), nitric oxide (NO), nitrous oxide (N2O), ammonia (NH3), hydrogen cyanide (HCN), cyanogen (NCCN), organic nitriles (acetonitrile (CH3CN), acrylonitrile (CH2CHCN), and propionitrile (CH3CH2CN)), and nitrates. The sulfur in biomass is organically bound in the form of sulfur-containing amino acids in proteins. During burning the sulfur is released mostly in the form of sulfur dioxide (SO2) and smaller amounts of carbonyl sulfide (COS) and nonvolatile sulfate (SO4–). About one-half of the sulfur in the biomass matter is left in the burn ash, whereas very little of the fuel nitrogen is left in the ash. Laboratory biomass burning experiments conducted by Lobert et al. (1991) have identified the carbon (Table 1) and nitrogen (Table 2) compounds released to the atmosphere by burning. The major gases produced during the biomass burning process include CO2, CO, CH4, oxides of nitrogen (NOx ¼ NO þ NO2), and NH3. Carbon dioxide and CH4 are greenhouse gases, which trap Earth-emitted infrared radiation and lead to global warming. Carbon monoxide, methane, and the oxides of nitrogen lead to the photochemical production of ozone (O3) in the troposphere. In the troposphere, O3 is harmful to both vegetation and humans at concentrations not far above the global background levels. Nitric oxide leads to the chemical production of nitric acid (HNO3) in the troposphere. Nitric acid is the fastest growing component of acidic precipitation. Ammonia is the only basic gaseous species that neutralizes the acidic nature of the troposphere. It has been reported that the burning of vegetation results in the complete release of mercury contained in the biomass

Table 1

Carbon gases produced during biomass burning

Compound

Mean emission factor relative to the fuel C (%)

Carbon dioxide (CO2) Carbon monoxide (CO) Methane (CH4) Ethane (CH3CH3) Ethene (CH2 ¼ CH2) Ethine (CH ¼ CH) Propane (C3H8) Propene (C3H6) n-butane (C4H10) 2-butene (cis) (C4H8) 2-butene (trans) (C4H8) i-butene, i-butene (C4H8 þ C4H8) 1,3-butadiene(C4H6) n-pentane (C3H12) Isoprene (C5H8) Benzene (C6H6) Toluene (C7H8) m-, p-xylene (C8H10) o-xylene (C8H10) Methyl chloride (CH3Cl) NMHC (As C) (C2–C8) Ash (As C)

82.58 5.73 0.424 0.061 0.123 0.056 0.019 0.066 0.005 0.004 0.005 0.033

Total sum C

94.92 (including ash)

0.021 0.007 0.008 0.064 0.037 0.011 0.006 0.010 1.18 5.00

Source: Lobert et al. (1991).

Table 2

Nitrogen gases produced during biomass burning

Compound

Mean emission factor relative to the fuel N (%)

Nitrogen oxides (NOx) Ammonia (NH3) Hydrogen cyanide (HCN) Acetonitrile (CH3CN) Cyanogen (NCCN) (As N) Acrylonitrile (CH2CHCN) Propionitrile (CH3CH2CN) Nitrous oxide (N2O) Methylamine (CH3NH2) Dimethylamine ((CH3)2NH) Ethylamine (CH3CH2NH2) Trimethylamine ((CH3)N) 2-methyl-1-butylamine (C5H11NH2) n-pentylamine (n-C5H11NH2) Nitrates (70% HNO3) Ash (As N)

13.55 4.15 2.64 1.00 0.023 0.135 0.071 0.072 0.047 0.030 0.005 0.02 0.04 0.137 1.10 9.94

Total sum N (As N)

33.66 (Including ash) 21.60 20

Molecular nitrogen (N2) Higher HC and particles Source: Lobert et al. (1991).

(Friedli et al., 2001). About 95% of the mercury is emitted as elemental mercury and the remainder emitted as particulate mercury. Friedli et al. (2001) concluded that the mercury

Biomass Burning: The Cycling of Gases and Particulates from the Biosphere to the Atmosphere

released by burning becomes part of the global mercury reservoir and undergoes chemical transformation in clouds and in the free troposphere and eventually returns to the surface via wet and dry deposition. Particulates, small (usually
10 mm or smaller) solid particles, such as smoke or soot particles, are also produced during the burning process and released into the atmosphere. These solid particulates absorb and scatter incoming sunlight and hence impact the local, regional, and global climate. In addition, these particulates (specifically particulates 2.5 mm or smaller) can lead to various human respiratory and general health problems when inhaled. The gases and particulates produced during biomass burning lead to the formation of ‘smog.’ The word ‘smog’ was coined as a combination of smoke and fog and is now used to describe any smoky or hazy pollution in the atmosphere. Gaseous and particulate emissions produced during biomass burning and released into the atmosphere impact the local, regional, and global atmosphere and climate in several different ways as follows. 1. Biomass burning is a significant global source of CO2 and CH4. Both gases are greenhouse gases that lead to global warming. 2. Biomass burning is a significant global source of CO, CH4, nonmethane hydrocarbons, and oxides of nitrogen. These gases lead to the photochemical production of O3 in the troposphere (this chemistry is outlined in a later section). Tropospheric ozone is a pollutant and irritant and has a negative impact on plant, animal, and human life. 3. Methyl chloride and methyl bromide, while only released in trace amounts during biomass burning, have a negative impact on stratospheric ozone (this chemistry is outlined in a later section). Methyl chloride and methyl bromide produced during biomass burning will become even more important in the future, as human-produced sources of chlorine and bromine are phased out as a result of the Montreal Protocol banning gases containing chlorine and bromine. 4. Particulates produced during biomass burning absorb and scatter incoming solar radiation, which impact climate. In addition, particulates produced by biomass burning lead to reduced atmospheric visibility. Some studies suggest that atmospheric particulates produced during biomass burning may directly enter the stratosphere via strong vertical thermal convective currents produced during the fire. 5. Particulates produced during biomass burning become cloud condensation nuclei (CCN) and impact the formation and distribution of clouds. 6. Particulates produced during biomass burning, particularly particulates of 10 mm or less in diameter, lead to severe respiratory problems when inhaled.

5.5.3 Enhanced Biogenic Soil Emissions of Nitrogen and Carbon Gases: A Postfire Effect Measurements have shown that in addition to the instantaneous production of trace gases and particulates resulting from the combustion of biomass matter, burning also

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enhances the biogenic emissions of NO and NO2 from soil (Anderson et al., 1988; Levine et al., 1988, 1991, 1996a,b) and the biogenic emission of CO from soil (Zepp et al., 1996). It is believed that enhanced biogenic soil emissions of NO and NO2 are related to increased concentrations of ammonium found in soil following burning. Ammonium, a component of the burn ash, is the substrate in nitrification, which is the microbial process believed to be responsible for the production of NO and NO2 (Levine et al., 1988, 1991, 1996a,b). The postfire enhanced biogenic soil emissions of NO and NO2 may be comparable to or even surpass the instantaneous production of these gases during biomass burning (Harris et al., 1996).

5.5.4 The Geographical Distribution of Biomass Burning The locations of biomass burning are varied and include tropical savannas (Figure 1), tropical, temperate and boreal forests (Figure 2) and agricultural lands after the harvest. The burning of fuelwood for domestic use is another source of biomass burning. Global estimates of the annual amounts of biomass burning from these sources are estimated in Table 3 (Andreae, 1991). In Table 3, the unit of biomass burned is Tg dm year1 (1 Tg ¼ 1012 g ¼ 106 metric tons; dm ¼ dry matter (biomass matter)). As already noted, biomass matter is
45% by weight composed of carbon. Table 3 also gives estimates of the carbon released (Tg C year1) by the burning of this biomass (the total biomass burned is multiplied by 45% to determine the amount of carbon released into the atmosphere during burning). Combining estimates of the total amount of biomass matter burned per year (Table 3) with measurements of the gaseous and particulate emissions from biomass burning (Tables 1 and 2) permits estimates of the global production and release into the atmosphere of gases and particulates from burning. Estimates of the global contribution of biomass burning are summarized in Table 4 (Andreae, 1991). The data in Tables 3 and 4 clearly indicate

Figure 1 A savanna fire with its characteristic long fire front as it traverses across the savanna in South Africa. The low-intensity fire consumes grass and shrubs and consumes on an annual and global basis more total biomass than any other kind of fire (see Table 3) (photograph by J. S. Levine, NASA).

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Biomass Burning: The Cycling of Gases and Particulates from the Biosphere to the Atmosphere

Figure 2 A boreal forest fire in Canada. The boreal forest fire is the most intense and energetic fire. The gases and particulates produced during boreal forest fires can be directly transported into the stratosphere by the strong vertical thermal convection currents produced by the fire (photograph by B. J. Stocks, Canadian Forest Service). Table 3 Global estimates of annual amounts of biomass burning and of the resulting release of carbon to the atmosphere Source

Biomass burned (Tg dm year1)

Carbon released (Tg C year1)

Savanna Agricultural waste Fuelwood Tropical forests Temperate/boreal forests World totals

3690 2020 1430 1260 280 8680

1660 910 640 570 130 3910

Source: Andreae (1991).

that biomass burning is a truly global process of major importance in the global budgets of atmospheric gases and particulates.

5.5.5

Biomass Burning in the Boreal Forests

In the past, it was generally assumed that biomass burning was primarily a tropical phenomenon. This is because most of the information that we have on the geographical and temporal distribution of biomass burning is largely based on tropical burning. Very little information is available on the geographical and temporal distribution on biomass burning in the boreal forests, which cover
25% of the world’s forests. To illustrate how our knowledge of the geographical extent of burning in the world’s boreal forests has increased in recent years, consider the following: early estimates based on surface fire records and statistics suggested that 1.5 Mha (1 ha ¼ 2.47 acres) of boreal forests burn annually (Seiler and Crutzen, 1980). Later studies, based on more comprehensive surface fire records and statistics, indicated that earlier values underestimated burning in the world’s boreal forests and that an

average of 8 Mha burned annually during the 1980s, with great year-to-year fluctuations (Stocks et al., 1991). One of the largest fires ever measured occurred in the boreal forests of the Heilongjiang province of Northeastern China in May 1987. In less than four weeks, more than 1.3 Mha of boreal forest were burned (Levine et al., 1991; Cahoon et al., 1994). At the same time, extensive fire activity occurred across the Chinese border in Russia, particularly in the area east of Lake Baikal between the Amur and Lena rivers. Estimates based on NOAA AVHRR imagery indicate that 14.4 Mha in China and Siberia were burned in 1987 (Cahoon et al., 1994), dwarfing earlier estimates of boreal forest fire burned area. While 1987 was an extreme fire year in Eastern Asia, the sparse database may suggest a fire trend. Is burning in the boreal forests increasing with time, or are satellite measurements providing more accurate data? Satellite measurements are certainly providing a more accurate assessment of the extent and frequency of burning in the world’s boreal forests. As global warming continues, warmer and dryer conditions in the world’s boreal forests are predicted to result in more frequent and larger fires and greater production of CO2 by these fires. The increased burning will have an amplifying effect on global warming! Calculations using the satellite-derived burn area and measured emission ratios of gases for boreal forest fires indicate that the Chinese and Siberian fires of the 1987 contributed
20% of the total CO2 produced by savanna burning, 36% of the total CO produced by savanna burning, and 69% of the total CH4 produced by savanna burning (Cahoon et al., 1994). Since savanna burning represents the largest component of tropical burning in terms of the vegetation consumed by fire (Table 3), it is apparent that the atmospheric emissions from boreal forest burning must be included in global species budgets. There are several reasons that burning in the world’s boreal forests is very important: 1. The boreal forests are very susceptible to global warming. Small changes in the surface temperature can significantly influence the ice/snow/albedo feedback. Thus, infrared absorption processes by fire-produced greenhouse gases, as well as fire-induced changes in surface albedo and infrared emissivity in the boreal forest regions are more environmentally significant than in the tropics. 2. In the world’s boreal forests, global warming will result in warmer and drier conditions. This, in turn, may result in enhanced frequency of fire and the accompanying enhanced production of greenhouse gases that will amplify the greenhouse effect. 3. Fires in the boreal forests are the most energetic in nature. The average fuel consumption per unit area in the boreal forest is
2.5104 kg ha–1, which is about an order of magnitude greater than in the tropics. Large boreal forest fires typically spread very quickly, most often as ‘crown fires,’ causing the burning of the entire tree up to and including the crown (Figure 3). Large boreal forest fires release enough energy to generate convective smoke columns that routinely reach well into the upper troposphere, and on occasion, may directly penetrate across the tropopause into the stratosphere. The tropopause is at a minimum height over the world’s boreal forests. As an example, a 1986 forest fire in northwestern Ontario

Biomass Burning: The Cycling of Gases and Particulates from the Biosphere to the Atmosphere

Table 4

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Comparison of global emissions from biomass burning with emissions from all sources (including biomass burning)

Species b

CO2 (gross) CO2 (Net)c CO CH4 NMHCd N2O NOx NH3 Sulfur COS CH3Cl H2 Tropospheric O3 TPMe POCf ECg

Biomass burning (Tg elementa year1)

All sources (Tg elementa year1)

% due to biomass burning

3500 1800 350 38 24 0.8 8.5 5.3 2.8 0.09 0.51 19 420 104 69 19

8700 7000 1100 380 100 13 40 44 150 1.4 2.3 75 1100 1530 180 <22

40 26 32 10 24 6 21 12 2 6 22 25 38 7 39 >86

Tg element year1 where C, N, S, and Cl are the elements. Biomass burning plus fossil fuel burning. c Deforestation plus fossil fuel burning. d Nonmethane hydrocarbons (excluding isoprene and terpenes). e Total particulate matter (Tg year1). f Particulate organic matter (including elemental carbon). g (black-soot) carbon. Source: Andreae (1991). a

b

and increased stratospheric aerosols during the same period. 4. The cold temperature of the troposphere over the world’s boreal forests results in low levels of tropospheric water vapor. The deficiency of tropospheric water vapor and the scarcity of incoming solar radiation over most of the year results in very low photochemical production of the hydroxyl (OH) radical over the boreal forests. The OH radical is the overwhelming chemical scavenger in the troposphere and controls the atmospheric lifetime of many tropospheric gases. The very low concentrations of the OH radical over the boreal forests result in enhanced atmospheric lifetimes for most tropospheric gases, including the gases produced by biomass burning. Hence, gases produced by burning, such as CO, CH4, and the oxides of nitrogen, will have enhanced atmospheric lifetimes over the boreal forest. New information about burning in the world’s boreal forests based on satellite measurements was reported by Kasischke et al. (1999). Some of the findings reported in this study are summarized here. Figure 3 A crown boreal forest fire in Canada. The boreal forest crown fire consumes the entire tree up to and including the crown (photograph by B. J. Stocks, Canadian Forest Service).

(Red Lake) generated a convective smoke column 12–13 km in height, penetrating across the tropopause into the stratosphere (Stocks and Flannigan, 1987). Fromm et al. (2000) have found a strong link between boreal forest fires in Canada and eastern Russia in 1998

1. Fires in the boreal forest covering at least 105 ha are not uncommon. 2. In the boreal forests of North America, most fires (>90%) are crown fires. The remainder are surface fires. Crown fires consume much more fuel (30–40 t of biomass material per hectare burned) than surface fires (8–12 t of biomass material per hectare burned). 3. The fire record for North America since the early 1970s clearly shows the episodic nature of fire in the boreal forests. Large fire years occur during extended periods of

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drought, which allow naturally ignited fires (i.e., lightning ignited fires) to burn large areas. Since 1970, the area burned during six episodic fire years in the North American boreal forest was 6.2 Mha year1, while 1.5 Mha burned per year in the remaining years. There is evidence that a similar episodic pattern of fire may also exist in the Russia boreal forest. 4. The fire data in the North American boreal forest show a significant increase in the annual area burned since the early 1970s, with an average 1.5 Mha year1 burning during the 1970s and 3.2 Mha year1 burning during the 1990s. This increase in burning corresponds to rises of 1.0–1.6  C over the same period (Hansen et al., 1996). The projected 2–4  C increase in temperature due to projected increases in greenhouse gases should result in high levels of fire activity throughout the world’s foreal forests in the future (Stocks et al., 1991). 5. During typical years in the boreal forests, the amounts of biomass consumed during fire ranges between 10 t and 20 t ha1. During the drought years with episodic fires, the amounts of biomass consumed during biomass burning may be as high as 50–60 t ha1. Assuming that biomass is
50% carbon by mass, such amounts would release 450–600 Tg C globally. These amounts are considerably higher than the often-quoted value for total carbon released by biomass burning in the world’s boreal and temperate forests of 130 Tg C globally (Andreae, 1991: see Table 3).

5.5.6 Estimates of Global Burning and Global Gaseous and Particulate Emissions Global estimates of the annual amounts of biomass burning from these sources are estimated in Table 3 (Andreae, 1991). In Table 3, the unit of biomass burned is Tg dm year1. As already noted, biomass matter is composed of
45% by weight of carbon. Table 3 also gives estimates of the carbon released (Tg C year1) by the burning of this biomass. Combining estimates of the total amount of biomass matter burned per year (Table 3) with measurements of the gaseous and particulate emissions from biomass burning (Tables 1 and 2) permits estimates of the global production and release into the atmosphere of gases and particulates from burning. Estimates of the global contribution of biomass burning are summarized in Table 4 (Andreae, 1991). The data in Tables 3 and 4 clearly indicate that biomass burning is a truly global process of major importance in the global budgets of atmospheric gases and particulates.

5.5.7 Calculation of Gaseous and Particulate Emissions from Fires To assess both the environmental and health impacts of biomass burning, information is needed on the gaseous and particulate emissions produced during the fire and released into the atmosphere. The calculation of gaseous emissions from vegetation and peat fires can be calculated using a form of an

expression from Seiler and Crutzen (1980) for each burning ecosystem/ terrain: M ¼ ABE

[1]

where M is total mass of vegetation or peat consumed by burning (tons), A the area burned (km2), B the biomass loading (t km2), and E the burning efficiency (dimensionless). The total mass of carbon, M(C), released to the atmosphere during burning is related to M by the following expression: MðCÞ ¼ CMðtons of carbonÞ

[2]

C is the mass percentage of carbon in the biomass. For tropical vegetation, C ¼ 0.45 (Andreae, 1991); for peat, C ¼ 0.50 (Yokelson et al., 1996). The mass of CO2, M(CO2), released during the fire is related to M(C) by the following expression: MðCO2 Þ ¼ CE MðCÞ

[3]

The combustion efficiency (CE) is the fraction of carbon emitted as CO2 relative to the total carbon compounds released during the fire. For tropical vegetation fires, CE ¼ 0.90 (Andreae, 1991); for peat fires, CE ¼ 0.77 (Yokelson et al., 1997). Once the mass of CO2 produced by burning is known, the mass of any other species, Xi (M(Xi)), produced by burning and released to the atmosphere can be calculated with knowledge of the CO2-normalized species emission ratio (ER(Xi)). The emission ratio is the ratio of the production of species Xi to the production of CO2 in the fire. The mass of species, Xi, is related to the mass of CO2 by the following expression: MðXi Þ ¼ ER ðXi Þ MðCO2 Þðtons of Xi Þ

[4]

where Xi ¼ CO, CH4, NOx, NH3, and O3. It is important to re-emphasize that O3 is not a direct product of biomass burning. However, O3 is produced via photochemical reactions of CO, CH4, and NOx, all of which are produced directly by biomass burning. Hence, the mass of ozone resulting from biomass burning may be calculated by considering the ozone precursor gases produced by biomass burning. Values for emission ratios for tropical forest fires and peat fires are summarized in Table 5. To calculate the total particulate matter (TPM) released from tropical forest fires and peat fires, we use the following expression (Ward, 1990) TPM ¼ MP ðtons of carbonÞ

[5]

where P is the conversion of biomass matter or peat matter to particulate matter during burning. For the burning of tropical vegetation, P ¼ 20 t of TPM per kiloton of biomass consumed by fire; for peat burning, we assume P ¼ 35 t of TPM per kiloton of organic soil or peat consumed by fire (Ward, 1990). Perhaps the major uncertainties in the calculation of gaseous and particulate emissions resulting from fires involve poor or incomplete information about four fire and ecosystem parameters: (1) the area burned (A), (2) the ecosystem or terrain that burned, i.e., forests, grasslands, agricultural lands, peat lands, etc., and (3) the biomass loading (B), i.e., the

Biomass Burning: The Cycling of Gases and Particulates from the Biosphere to the Atmosphere

Table 5

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Emission ratios for tropical forest fires and peat fires

Species

Tropical forest fires

References

Peat fires

References

CO2 CO CH4 NOx NH3 O3 TPMa

90.00% 8.5% 0.32% 0.21% 0.09% 0.48% 20 t kt1

Andreae (1991) Andreae et al. (1988) Blake et al. (1996) Andreae et al. (1988) Andreae et al. (1998) Andreae et al. (1988) Ward (1990)

77.05% 18.15% 1.04% 0.46% 1.28% 1.04% 35 t kt1

Yokelson et al. (1997) Yokelson et al. (1997) Yokelson et al. (1997) Derived from Yokelson et al. (1997) (see text) Yokelson et al. (1997) Derived from Yokelson et al. (1997) (see text) Ward (1990)

a

Total particulate matter emission ratios are in units of tons/kiloton (tons of total particulate matter/kiloton of biomass or peat material consumed by fire).

amount of biomass per unit area of the ecosystem prior to burning, and (4) the fire efficiency (C), i.e., the amount of biomass in the burned ecosystem that was actually consumed by burning.

5.5.8 Biomass Burning and Atmospheric Nitrogen and Oxygen Biomass burning is both an instantaneous source (combustion of biomass) and a long-term source (enhanced biogenic soil emissions via increased nitrification and denitrification in soil) of gases to the atmosphere. These gases impact the chemistry of the troposphere and stratosphere, as outlined in this chapter. In addition, biomass burning may also impact atmospheric concentrations and the biogeochemical cycling of nitrogen and oxygen, the two major constituents of the atmosphere. Through the process of nitrogen fixation, molecular nitrogen (N2) is transformed to the surface in the form of ‘fixed’ nitrogen, i.e., ammonium (NH4þ) and nitrate (NO3). Nitrogen fixation results from both natural processes (biological fixation in root modules in certain agricultural crops and atmospheric lightning) and human processes (the production of nitrogen fertilizer and high-temperature combustion). The world’s use of industrially fixed nitrogen fertilizer has increased from
3 Tg N year1 in 1940 to
75 Tg N year1 in 1990 (Levine et al., 1996b). The ‘fixed’ nitrogen in the forms of NH4þ and NO3 is returned to the atmosphere mainly in the form of N2, with smaller amounts of N2O, and still smaller amounts of NO by denitrification and in the form of NO by nitrification. Burning or ‘pyrodenitrification’ may also be an important source of nitrogen, mostly in the form of N2, from the biosphere to the atmosphere (Lobert et al., 1991; Levine et al., 1996b). The problem is that it is difficult to quantify the amount of N2 released during burning (Lobert et al., 1991); however, biomass burning or pyrodenitrification may prove to be an important process in the recycling of nitrogen compounds from the biosphere to the atmosphere. Burning impacts the concentration of atmospheric oxygen in two ways. Carbon released during the burning of biomass combines with atmospheric oxygen to form CO2. Hence, burning is a sink for atmospheric oxygen. In addition, biomass burning destroys the very source of atmospheric oxygen – its production in the biosphere via the process of photosynthesis

in the world’s forests. In addition, the burial of fire-produced charcoal is a long-term source of oxygen.

5.5.9 Atmospheric Chemistry Resulting from Gaseous Emissions from the Fires 5.5.9.1 Chemistry of the Hydroxyl Radical (OH) in the Troposphere The hydroxyl radical (OH) is the major chemical scavenger in the troposphere and it controls the atmospheric lifetime of most gases in the troposphere. The atmospheric lifetime, t, of any gas, xi, that reacts with the OH radical is given by the following expression t ¼ 1/k[OH] where k is the kinetic reaction rate for the reaction between OH and xi and [OH] is the concentration of the OH radical (molecules cm3). The concentration of the OH radical is controlled by the balance between its chemical production and destruction. The OH radical is formed by the reaction of excited atomic oxygen (O(1D)) with water vapor k1

Oð1 DÞ þ H2 O ! 2OH Tropospheric excited atomic oxygen (O(1D)) is produced by the photolysis of O3. In the troposphere, this photolysis reaction occurs over a very narrow spectral interval, between 290 nm and 310 nm. The production of excited atomic oxygen decreases as the latitude increases, i.e., less incoming solar radiation for photolysis is available. The bulk of the water vapor in the atmosphere resides in the troposphere. The amount of H2O in the atmosphere is controlled by the saturation vapor pressure, which decreases with decreasing atmospheric temperature, i.e., as altitude or latitude increases. The OH radical is destroyed via its reactions with CO and CH4, both important products of biomass burning: k2

OH þ CO ! H þ CO2 and k3

OH þ CH4 ! CH3 þ H2 O Assuming that the production of OH is controlled by the reaction of water vapor with excited atomic oxygen and its loss is controlled by its reactions with CO and CH4, the concentration of the OH radical (molecules cm3) is determined by dividing the OH production term by its destruction terms: 

½OH ¼ 2k1 O 1 D ½H2 O =ðk2 ½CO þ k3 ½CH4 Þ

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5.5.9.2

Biomass Burning: The Cycling of Gases and Particulates from the Biosphere to the Atmosphere

Production of O3 in the Troposphere

In addition to controlling the chemical destruction of OH, the oxidation of CO and CH4 by OH outlined above initiates the CO and CH4 oxidation schemes, which lead to the photochemical production of O3 in the troposphere.

5.5.9.2.1 CO oxidation chain CO þ OH ! CO2 þ H H þ O2 þ M ! HO2 þ M HO2 þ NO ! NO2 þ OH NO2 þ hv ! NO þ O O þ O2 þ M ! O3 þ M Net reaction : CO þ 2O2 ! CO2 þ O3 Note that the key gases in the CO oxidation chain leading to the photochemical production of tropospheric ozone, CO and NO are both produced by biomass burning.

5.5.9.2.2 Methane oxidation chain CH4 þ OH ! CH3 þ H2 O CH3 þ O2 þ M ! CH3 O2 þ M CH3 O2 þ NO ! CH3 O þ NO2 CH3 O þ O2 ! CH2 O þ HO2 HO2 þ NO ! NO2 þ OH 2ðNO2 þ hv ! NO þ OÞ 2ðO þ O2 þ M ! O3 þ MÞ Net reaction : CH4 þ 4O2 ! CH2 O þ H2 O þ 2O3 Note that the key gases in the CH4 oxidation chain leading to the photochemical production of tropospheric ozone, CH4 and NO are both produced by biomass burning.

5.5.9.3

Chemistry of Nitrogen Oxides in the Troposphere

In addition to being a key player in the CO and CH4 oxidation chains leading to the chemical production of O3 in the troposphere, NO also leads to the chemical production of HNO3, the fastest growing component of acidic precipitation. NO is chemically transformed to nitrogen dioxide (NO2) and then to NO via the following reactions: NO þ O3 ! NO þ HO2 ! NO þ CH3 O2 ! NO2 þ OH þ M

5.5.9.4

NO2 þ O2 NO2 þ OH NO2 þ CH3 O ! HNO3 þ M

Chemistry of the Stratosphere

Approximately 90% of the ozone in the atmosphere is found in the stratosphere (15–50 km), with only
10% in the troposphere (0–15 km). Stratospheric ozone is very important because it absorbs ultraviolet radiation (200–300 nm) from the Sun and shields the surface from this biologically lethal radiation. Stratospheric ozone is destroyed via a series of chemical reactions involving NO, OH, Cl, and Br. These species destroy stratospheric ozone through the following catalytic cycle where X may be any of the following: NO, OH, Cl, or Br (Wayne, 1991)

X þ O3 ! XO þ O2 XO þ O ! X þ O2 Net reaction : O þ O3 ! 2O2 Another competing O3 stratospheric catalytic cycle where X ¼ OH, Cl, or Br and Y ¼ OH, Cl, or Br is represented by (Wayne, 1991) X þ O3 ! XO þ O2 Y þ O3 ! YO þ O2 XO þ YO ! XY þ O2 XY þ hv ! X þ Y Net reaction : O þ O3 ! 2O2

5.5.10 A Case Study of Biomass Burning: The 1997 Wildfires in Southeast Asia Extensive and widespread tropical forest and peat fires swept throughout Kalimantan and Sumatra, Indonesia, between August and December 1997 (Brauer and Hisham-Hishman, 1988; Hamilton et al., 2000). The fires resulted from burning for land clearing and land-use change. However, the severe drought conditions resulting from El Nino caused small landclearing fires to become large uncontrolled wildfires. Based on satellite imagery, it has been estimated that a total of 4.56  104 km2 burned on Kalimantan and Sumatra between August and December 1997 (Liew et al., 1998). The gaseous and particulate emissions produced in these fires and released into the atmosphere reduced the atmospheric visibility, impacted the composition and chemistry of the atmosphere, and affected human health. Some of the consequences of the fires in Southeast Asia were: (1) more than 200 million people were exposed to high levels of air pollution and particulates produced during the fires, (2) more than 20 million smokerelated health problems were recorded, (3) fire-related damage cost in excess of $4 billion, (4) on September 26, 1997, a commercial airliner (Garuda Airlines Airbus 300-B4) crashed in Sumatra owing to very poor visibility due to smoke from the fires on landing with 234 passengers killed, and (5) on September 27, 1997, two ships collided at sea due to poor visibility in the Strait of Malacca, off the coast of Malaysia, with 29 crew members killed. International concern about the environmental and health impacts of these fires was great. Three different agencies of the United Nations organized workshops and reports on the environmental and health impacts of these fires: The World Meteorological Organization (WMO) Workshop on Regional Transboundary Smoke and Haze in Southeast Asia, Singapore, June 2–5, 1998, The World Health Organization (WHO) Health Guidelines for Forest Fires Episodic Events, Lima, Peru, October 6–9, 1998, and the United Nations Environmental Program (UNEP) Report on Wildland Fires and the Environment: A Global Synthesis, published in February, 1999 (Levine et al., 1999). The Indonesian fires formed the basis of an article in National Geographic magazine, entitled, Indonesia’s Plague of Fire (Simons, 1998). Indonesia ranks third, after Brazil and the Democratic Republic of the Congo (formerly Zaire), in its area of tropical forest. Of Indonesia’s total land area of 1.9 Mkm2, current forest cover estimates range from 0.9 Mkm2 to 1.2 Mkm2, or

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Biomass Burning: The Cycling of Gases and Particulates from the Biosphere to the Atmosphere

48% to 69% of the total. Forests dominate the landscape of Indonesia (Makarim et al., 1998). Large areas of Indonesian forests burned in 1982 and 1983. In Kalimantan alone, the fires burned from 2.4 Mha to 3.6 Mha of forests (Makarim et al., 1998). It is interesting to note that there is an uncertainty of 1.2 Mha or an uncertainty of 50% in our knowledge of the burned area of fires that occurred 16 years ago! Liew et al. (1998) analyzed 766 SPOT ‘quick-look’ images with almost complete coverage of Kalimantan and Sumatra from August to December 1997. Liew et al. (1998) estimate the burned area in Kalimantan to be 3.06 X 104 km2 and the burned area in Sumatra to be 1.5 X 104 km2, for a total burned area of 4.56 X 104 km2. (This is equivalent to the combined areas of the states of Rhode Island, Delaware, Connecticut, and New Jersey, in the United States.) The estimate of Liew et al. (1998) represents only a lower limit estimate of the area burned in Southeast Asia in 1997, since the SPOT data only covered Kalimantan and Sumatra and did not include fires on the other Indonesian islands of Irian Jaya, Sulawesi, Java, Sumbawa, Komodo, Flores, Sumba, Timor, and Wetar or the fires in the neighboring countries of Malaysia and Brunei. What is the nature of the ecosystem/terrain that burned in Kalimantan and Sumatra? In October 1997, NOAA satellite monitoring produced the following distribution of fire hot spots in Indonesia (UNDAC, 1998): agricultural and plantation areas – 45.95%; bush and peat soil areas – 24.27%; productive forests – 15.49%; timber estate areas – 8.51%; protected areas – 4.58%; and transmigration sites – 1.20% (the three forest/timber areas add up to a total of 28.58% of the area burned). While the distribution of fire hot spots is not an actual index for area burned, the NOAA satellite-derived hot spot distribution is quite similar to the ecosystem/terrain distribution of burned area deduced by Liew et al. (1998) based on SPOT images of the actual burned areas: agricultural and plantation areas – 50%; forests and bushes – 30%; and peat swamp forests – 20%. Since the estimates of burned ecosystem/terrain of Liew et al. (1998) are based on actual SPOT images of the burned area, their estimates were adopted in our calculations. What is the biomass loading for the three terrain classifications identified by Liew et al. (1998)? Values for biomass loading or fuel load for various tropical ecosystems are summarized in Table 6. The biomass loading for tropical forests in Southeast Asia ranges from 5000 t km2 to 5.5 X 104 t km2, with a mean value of 2.3 X 104 t km2 (Brown and Gaston, 1996). However, in our calculations we have used a value of 104 t km2 to be conservative. The biomass loading for agricultural and plantation areas (mainly rubber trees and oil palms) of 5000 t km2 is also a conservative value (Liew et al., 1998). Nichol (1997) has investigated the peat deposits of Kalimantan and Sumatra and used a biomass loading value of 9.75  104 t km2 (Supardi and Subekty, 1993) for the dry peat deposits 1.5 m thick as representative of the Indonesian peat in her study. Brunig (1997) gives a similar value for peat biomass loading. The combustion efficiency for forests is estimated at 0.20 and for peat is estimated at 0.50 (Levine and Cofer, 2000). Based on the discussions presented in this section, the values for burned area, biomass loading, and combustion efficiency used in the calculations are summarized in Table 7.

Table 6 Biomass load range and burning efficiency in tropical ecosystems Vegetation type

Biomass load range (t km2)

Burning efficiency

Peata Tropical rainforestsb Evergreen forests Plantations Dry forests Fynbos Wetlands Fertile grasslands Forest/savanna mosaic Infertile savannas Fertile savannas Infertile grasslands Shrublands

97 500 500055 000 500010 000 50010 000 30007000 20004500 3401000 150550 150500 150500 150500 150350 50200

0.50 0.20 0.30 0.40 0.40 0.50 0.70 0.96 0.45 0.95 0.95 0.96 0.95

a

Brunig (1997) and Supardi et al. (1993). Brown and Gaston (1996). Source: Scholes et al. (1996) (except as noted). b

Table 7

Parameters used in calculations

(i) Total area burned in Kalimantan and Sumatra, Indonesia in 1997: 45600 km2 (ii) Distribution of burned areas, biomass loading, and combustion efficiency (a) Agricultural and plantation areas 50%, 5000 t km2, 0.20 (b) Forests and bushes 30%, 10 000 t km2, 0.20 (c) Peat swamp forests 20%, 97 500 t km2, 0.50

5.5.11 Results of Calculations: Gaseous and Particulate Emissions from the Fires in Kalimantan and Sumatra, Indonesia, August to December 1997 The calculated gaseous and particulate emissions from the fires in Kalimantan and Sumatra, from August to December 1997, are summarized in Table 8 (Levine, 1999). (It is important to keep in mind that wildfires continued throughout Southeast Asia from January to April 1998 and that the fires covered much more of the region than Kalimantan and Sumatra.) For each of the seven species listed, the emissions due to agricultural/plantation burning (A), forest burning (F), and peat burning (P) are given. The total (T) of all three components (A þ F þ P) is also given. The ‘best estimate’ total emissions are: CO2 – 191.485 million metric tons of C (Mt C); CO – 32.794 Mt C; CH4 – 1.845 Mt C; NOx – 5.898 Mt N; NH3 – 2.585 Mt N; O3 – 7.100 Mt O3; and total particulate matter – 16.154 Mt C. The CO2 emissions from these fires is
2.2% of the global annual production of CO2 from all sources (see Table 4 for global annual production of CO2, which is 8700 Mt C and 191.485 Mt C/8700 Mt C ¼ 2.2%). The percentage for other gases produced by these fires compared to the global annual production from all sources is: CO – 2.98%; CH4 – 0.48%; oxides of nitrogen – 2.43%; NH3 – 5.87%; and total particulate matter – 1.08%. Scholes et al. (1996) calculated the biomass consumed by burning in 11 different ecosystems in another tropical ecosystem, southern Africa, and performed a detailed statistical

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Table 8 Gaseous and particulate emissions from the fires in Kalimantan and Sumatra in 1997 (for total burned area ¼ 4.56  104 km2) (Levine, 1999)

CO2 CO CH4 NOx NH3 O3 TPM

Agricultural/plantation fire emissions

Forest fire emissions

Peat fire emissions

Total fire emissions

9.234 (4.61713.851) 0.785 (0.3921.177) 0.030 (0.0150.045) 0.023 (0.0110.034) 0.010 (0.0050.015) 0.177 (0.0880.265) 0.460 (0.230.69)

11.080 (5.5416.62) 0.942 (0.4711.413) 0.035 (0.0170.052) 0.027 (0.0130.040) 0.012 (0.0060.018) 0.213 (0.1060.319) 0.547 (0.2730.820)

171.170 (85.585256.755) 31.067 (15.53346.600) 1.780 (0.892.67) 0.921 (0.4601.381) 2.563 (1.2813.844) 6.710 (3.3510.06) 15.561 (7.78023.341)

191.485 (95.742287.226) 32.794 (16.39749.191) 1.845 (0.9222.767) 0.971 (0.4851.456) 2.585 (1.2923.877) 7.100 (3.5510.65) 16.568 (8.28424.852)

For each species, the best estimate emission value is on the first line and the range of emission values in parentheses to the right (see text for discussion of emission estimate range and uncertainty calculations). Units of emissions: Million metric tons of C (Mt C) for CO2, CO, and CH4; Mt N for NOx and NH3; Mt O3 for O3; and Mt C for particulates (1 Mt ¼ 1012 g ¼ 1 Tg).

analysis of the errors associated with the calculated values of biomass consumed by fire using a statistical procedure of Nelson (1992), which assumes that all error terms (e) are independent. In the error analysis of Scholes et al. (1996), the total error (Etotal), which corresponds to the three-sigma (99%) confidence level, is estimated using the following expression:  1=2 Etotal ¼ e2burned area þ e2fuel load þ e2combustion completeness [6] Scholes et al. (1996) assumed the following uncertainties for each calculation parameter: eburned area ¼ 30%, efuel load ¼ 30%, and ecombustion completeness ¼ 25%. In the error analysis of the calculations presented in this chapter, the error associated with uncertainties in the emission ratio (eemission ratio) (30%) has also been included. The uncertainties in the calculation parameters in the detailed error of analysis of Scholes et al. (1996) were adopted in this study, with the exception of the uncertainty in the burned area of 30%. The Scholes et al. (1996) determination of burned area was based on satellite measurements of active fires that were converted to such area, which is not a simple one-to-one transformation and introduces errors. The burned area determination used in this chapter was more straightforward since it was based on direct satellite photography of burned areas using SPOT images (Liew et al., 1998). Hence, there is less uncertainty in this burned area determination and an uncertainty of 10% was assumed (Liew et al. (1998) did not give a burned area uncertainty in their paper). Using eqn [6], the calculated uncertainty in the emission calculations presented in this chapter is 50.2%. The uncertainty range for each species emission is shown in parentheses under the ‘best estimate’ value in Table 8. However, it is important to re-emphasize that these emission calculations represent lower limit values since the calculations are only based on burning in Kalimantan and Sumatra in 1997. The calculations do not include burning in Java, Sulawesi, Irian Jaya, Sumbawa, Komodo, Flores, Sumba, Timor, and Wetar in Indonesia or in neighboring Malaysia and Brunei. It is interesting to compare the gaseous and particulate emissions from the 1997 fires in Kalimantan and Sumatra with those from the Kuwait oil fires of 1991, described as a major environmental catastrophe. Laursen et al. (1992) have calculated the emissions of CO2, CO, CH4, NOx, and particulates from the Kuwait oil fires in units of metric tons per day.

Table 9 Comparison of gaseous and particulate emissions: the Indonesian fires and the Kuwait oil fires Species

Indonesian fires

Kuwait oil firesa

CO2 CO CH4 NOx Particulates

1.28  106 2.19  105 1.23  104 6.19  103 1.08  105

5.0  105 4.4  103 1.5  103 2.0  102 1.2  104

Units of emissions: Metric tons per day of C for CO2, CO, and CH4; metric tons per day of N for NOx; metric tons per day for particulates (1 Mt ¼ 1012 g ¼ 1 Tg). a Laursen et al. (1992).

The Laursen et al. (1992) calculations are summarized in Table 9. To compare these calculations with the calculations for Kalimantan and Sumatra (Table 8), we have normalized our calculations by the total number of days of burning. The SPOT images (Liew et al., 1998) covered a period of 5 months (August-December 1997) or
150 days. For comparison with the Kuwait fire emissions, we divided our calculated emissions by 150 days. These values are summarized in Table 9. The gaseous and particulate emissions from the fires in Kalimantan and Sumatra significantly exceeded the emissions from the Kuwait oil fires. The 1997 fires in Kalimantan and Sumatra were a significant source of gaseous and particulate emissions to the local, regional, and global atmosphere.

5.5.12 The Impact of the Southeastern Asia Fires on the Composition and Chemistry of the Atmosphere A series of papers have discussed the impact of the Southeastern Asia fires on the composition and chemistry of the atmosphere (Fujiwara et al., 1999; Hauglustaine et al., 1999; Matsueda and Inoue, 1999; Nakajima et al., 1999; Rinsland et al., 1999; Sawa et al., 1999; Tsutsumi et al., 1999). The results of these studies are summarized below.

5.5.12.1

Modeling O3 and CO over Indonesia

Hauglustaine et al. (1999) used MOZART, a global chemical transport model to investigate the impact of fire emissions calculated by Levine (1999) (presented in Table 8) on the

Biomass Burning: The Cycling of Gases and Particulates from the Biosphere to the Atmosphere

photochemical production of O3 over Indonesia. The calculations indicate that the tropospheric O3 column density increased by 20–25 Dobson Units (DU) and the O3 mixing ratio reached 50 ppbv in the mid-troposphere in November. (These model calculations are consistent with in situ O3 measurements obtained by Fujiwara et al. (1999) (described in the next section), and satellite measurements of O3 obtained with the Earth Probe/TOMS (Chandra et al., 1998) and the ERS-2/ GOME (Burrows et al., 1999)). South of the source region, low O3 mixing ratios of 20–25 ppbv were calculated in the boundary layer due to marine air influence and reduced photochemical activity in the presence of biomass burning aerosols. Normal, nonfire surface concentrations of CO over Indonesia are usually less than
100 ppbv. The calculations of Hauglustaine et al. (1999) indicate increases of CO by up 2900 ppbv at the surface over Kalimantan and Suimatra. A perturbation of 50–1000 ppbv was calculated over the entire region of Indonesia. In the free troposphere, CO increased by 1000 ppbv over Indonesia due to the upward transport prevailing in this region. The calculated high levels of CO are consistent with actual measurements of CO obtained over Indonesia during the fires by Sawa et al. (1999).

5.5.12.2

Measurements over Indonesia

Aircraft measurements over Kalimantan, Indonesia, on October 13, 1997, indicated high concentrations of O3, NOx, CO, and aerosols (Tsutsumi et al., 1999). The maximum concentration of O3 (80.5 ppbv) was found in the middle layer of the smoke haze and very low concentrations (
20 ppbv) were measured in the lower smoke layer. The authors concluded that the low O3 concentrations near the surface may be caused by the reduction in solar flux due to aerosols high in the haze layer and the loss of O3 due to large aerosol surface area in the lower haze layer. Pronounced enhancements of total and tropospheric O3 were observed with a Brewer spectrophotometer and ozonesondes at Watukosek, Indonesia in October 1997 (Fujiwara et al., 1999). The integrated tropospheric O3 increased from 20 DU to 55 DU in October 1997. On October 22, 1997, the O3 concentrations were more than 50 ppbv throughout the troposphere and exceeded 100ppbv at several altitudes. The authors conclude that the enhanced levels of O3 measured in October 1997, resulted from O3 precursors, produce during the fires and released into the atmosphere. Sawa et al. (1999) measured CO and hydrogen over Kalimantan during the fire. They reported CO concentrations in the range of 3000–9000 ppbv below an altitude of 2.6 km. Above 4.4 km altitude, the CO concentration was 500–1200 ppbv.

5.5.12.3

Measurements between Singapore and Japan

Aircraft measurements of CO2, CO, and CH4 were obtained between Japan and Singapore in October 1993, 1996, and 1997 (Matsueda and Inoue, 1999). The mixing ratios of all three gases at 9–12 km were enhanced over the South China Sea in 1997 compared with measurements in 1993 and 1996.

5.5.12.4

149

Measurements over Hawaii

Infrared solar spectral measurements of CO, ethane (C2H6), and hydrogen cyanide (HCN) were obtained for over 250 days between August 1995 and February 1998 above Mauna Loa, Hawaii (Rinsland et al., 1999). Correlated variations of CO, C2H6, and HCN and unusual seasonal cycles observed above Mauna Loa, Hawaii, during the second half of 1997 suggest a common origin for these emissions. Back-trajectory model calculations indicate that the source of these anomalous concentrations over Hawaii was the fires in Southeast Asia. From August 1997 through early 1998, the entire region of Southeast Asia, but particularly Indonesia and Malaysia, experienced extensive and widespread wildfires. These fires were initiated for land clearing and land-use change, as is the usual custom in this region of the world. However, in 1997, the severe drought conditions resulting from a very severe El Nino Southern Oscillation resulted in very widespread, uncontrolled wildfires that lasted for months, well into 1998. These emissions significantly reduced atmospheric visibility and produced record high levels of atmospheric pollution. These gaseous and particulate emissions also impacted human health. Three different agencies of the United Nations (UNEP, WMO, and WHO) conducted studies and workshops and issued reports on the environmental and health impacts of these fires. Many lessons were learned by much work needs to be done to avoid such catastrophes in the future.

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