Sink Potential of Canadian Agricultural Soils

Net greenhouse gas (GHG) emissions from Canadian crop and livestock production were estimated for 1990, 1996 and 2001 and projected to 2008. Net emissions were also estimated for three scenarios (low (L), medium (M) and high (H)) of adoption of sink enhancing practices above the projected 2008 level. Carbon sequestration estimates were based on four sink-enhancing activities: conversion from conventional to zero tillage (ZT), reduced frequency of summerfallow (SF), the conversion of cropland to permanent cover crops (PC), and improved grazing land management (GM). GHG emissions were estimated with the Canadian Economic and Emissions Model for Agriculture (CEEMA). CEEMA estimates levels of production activities within the Canadian agriculture sector and calculates the emissions and removals associated with those levels of activities. The estimates indicate a decline in net emissions from 54 Tg CO₂–Eq yr^–1 in1990 to 52 Tg CO₂–Eq yr^–1 in 2008. Adoption of thesink-enhancing practices above the level projected for 2008 resulted in further declines in emissions to 48 Tg CO₂–Eq yr^–1 (L), 42 TgCO₂–Eq yr^–1 (M) or 36 Tg CO₂–Eq yr^–1 (H). Among thesink-enhancing practices, the conversion from conventional tillage to ZT provided the largest C sequestration potential and net reduction in GHG emissions among the scenarios. Although rates of C sequestration were generally higher for conversion of cropland to PC and adoption of improved GM, those scenarios involved smaller areas of land and therefore less C sequestration. Also, increased areas of PC were associated with an increase in livestock numbers and CH₄ and N₂O emissions from enteric fermentation andmanure, which partially offset the carbon sink. The CEEMA estimates indicate that soil C sinks are a viable option for achieving the UNFCCC objective of protecting and enhancing GHG sinks and reservoirs as a means of reducing GHG emissions (UNFCCC, 1992).     

Field study of the emissions of methyl chloride and other halocarbons from biomass burning in Western Africa

A field study of trace gas emissions from biomass burning in Equatorial Africa gave methyl chloride emission ratios of 4.3×10^–5±0.8×10^–5 mol CH₃Cl/mol CO₂. Based on the global emission rates for CO₂ from biomass burning we estimate a range of 226–904×10⁹ g/y as global emission rate with a best estimate of 515×10⁹ g/y. This is somewhat lower than a previous estimate which has been based on laboratory studies. Nevertheless, our emission rate estimates correspond to 10–40% of the global turnover of methyl chloride and thus support the importance of biomass burning as methyl chloride source. The emission ratios for other halocarbons (CH₂Cl₂, CHCl₃, CCl₄, CH₃CCl₃, C₂HCl₃, C₂Cl₄, F-113) are lower. In general there seems to be a substantial decrease with increasing complexity of the compounds and number of halogen atoms. For dichloromethane biomass burning still contributes significantly to the total global budget and in the Southern Hemisphere biomass burning is probably the most important source for atmospheric dichloromethane. For the global budgets of other halocarbons biomass burning is of very limited relevance.     

Estimates of gross and net fluxes of carbon between the biosphere and the atmosphere from biomass burning

In order to estimate the production of charcoal and the atmospheric emissions of trace gases volatilized by burning we have estimated the global amounts of biomass which are affected by fires. We have roughly calculated annual gross burning rates ranging between about 5 Pg and 9 Pg (1 Pg = 10¹⁵ g) of dry matter (2–4 Pg C). In comparison, about 9–17 Pg of above-ground dry matter (4–8 Pg C) is exposed to fires, indicating a worldwide average burning efficiency of about 50%. The production of dead below-ground dry matter varies between 6–9 Pg per year. We have tentatively indicated the possibility of a large production of elemental carbon (0.5–1.7 Pg C/yr) due to the incomplete combustion of biomass to charcoal. This provides a sink for atmospheric CO₂, which would have been particularly important during the past centuries. From meager statistical information and often ill-documented statements in the literature, it is extremely difficult to calculate the net carbon release rates to the atmosphere from the biomass changes which take place, especially in the tropics. All together, we calculate an overall effect lof the biosphere on the atmospheric carbon dioxide budget which may range between the possibilities of a net uptake or a net release of about 2 Pg C/yr. The release of CO₂ to the atmosphere by deforestation projects may well be balanced by reforestation and by the production of charcoal. Better information is needed, however, to make these estimates more reliable.Now at the Max-Planck-Institute for Chemistry, Mainz, FRG.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Potential Soil C Sequestration on U.S. Agricultural Soils

Soil carbon sequestration has been suggested as a means to help mitigate atmospheric CO₂ increases, however there is limited knowledge aboutthe magnitude of the mitigation potential. Field studies across the U.S. provide information on soil C stock changes that result from changes in agricultural management. However, data from such studies are not readily extrapolated to changes at a national scale because soils, climate, and management regimes vary locally and regionally. We used a modified version of the Intergovernmental Panel on Climate Change (IPCC) soil organic C inventory method, together with the National Resources Inventory (NRI) and other data, to estimate agricultural soil C sequestration potential in the conterminous U.S. The IPCC method estimates soil C stock changes associated with changes in land use and/or land management practices. In the U.S., the NRI provides a detailed record of land use and management activities on agricultural land that can be used to implement the IPCC method. We analyzed potential soil C storage from increased adoption of no-till, decreased fallow operations, conversion of highly erodible land to grassland, and increased use of cover crops in annual cropping systems. The results represent potentials that do not explicitly consider the economic feasibility of proposed agricultural production changes, but provide an indication of the biophysical potential of soil C sequestration as a guide to policy makers. Our analysis suggests that U.S. cropland soils have the potential to increase sequestered soil C by an additional 60–70 Tg (10¹²g) C yr^{– 1}, over present rates of 17 Tg C yr^–1(estimated using the IPCC method), with widespread adoption of soil C sequestering management practices. Adoption of no-till on all currently annually cropped area (129Mha) would increase soil C sequestration by 47 Tg C yr^–1. Alternatively, use of no-till on 50% of annual cropland, with reduced tillage practices on the other 50%, would sequester less – about37 Tg C yr^–1. Elimination of summer fallow practices and conversionof highly erodible cropland to perennial grass cover could sequester around 20 and 28Tg C yr^–1, respectively. The soil C sequestration potentialfrom including a winter cover crop on annual cropping systems was estimated at 40Tg C yr^–1. All rates were estimated for a fifteen-yearprojection period, and annual rates of soil C accumulations would be expected to decrease substantially over longer time periods. The total sequestration potential we have estimated for the projection period (83 Tg C yr^–1) represents about 5% of 1999total U.S. CO₂ emissions or nearly double estimated CO₂ emissionsfrom agricultural production (43 Tg C yr^–1). For purposes ofstabilizing or reducing CO₂ emissions, e.g., by 7% of 1990 levels asoriginally called for in the Kyoto Protocol, total potential soil C sequestration would represent 15% of that reduction level from projected 2008 emissions(2008 total greenhouse gas emissions less 93% of 1990 greenhouse gasemissions). Thus, our analysis suggests that agricultural soil C sequestration could play a meaningful, but not predominant, role in helping mitigate greenhouse gas increases.     

Carbonyl sulfide emissions from biomass burning in the tropics

Carbonyl sulfide emissions from biomass burning have been studied during field experiments conducted both in an African savanna area (Ivory Coast) and rice fields, central highland pine forest and savanna areas in Viet-Nam. During these experiments CO₂, CO and C₂H₂ or CH₄ have also been also monitored. COS values range from 0.6 ppbv outside the fires to 73 ppbv in the plumes. Significant correlations have been observed between concentrations of COS and CO (R ²=0.92,n=25) and COS and C₂H₂ (R ²=0.79,n=26) indicating a COS production during the smoldering combustion. COS/CO₂ emission factors (COS/CO₂) during field experiments ranged from 1.2 to 61×10^–6 (11.4×10^–6 mean value). COS emission by biomass burning was estimated to be up to 0.05 Tg S/yr in tropics and up to 0.07 Tg S/yr on a global basis, contributing thus about 10% to the global COS flux. Based on the S/C ratio measured in the dry plant biomass and the COS/CO₂ emission factor, COS can account for only about 7% of the sulfur emitted in the atmosphere by biomass burning.     

Managing atmospheric CO2

A coupled carbon cycle-climate model is used to compute global atmospheric CO₂ and temperature variation that would result from several future CO₂ emission scenarios. The model includes temperature and CO₂ feedbacks on the terrestrial biosphere, and temperature feedback on the oceanic uptake of CO₂. The scenarios used include cases in which fossil fuel CO₂ emissions are held constant at the 1986 value or increase by 1% yr^–1 until either 2000 or 2020, followed by a gradual transition to a rate of decrease of 1 or 2% yr^–1. The climatic effect of increases in non-CO₂ trace gases is included, and scenarios are considered in which these gases increase until 2075 or are stabilized once CO₂ emission reductions begin. Low and high deforestation scenarios are also considered. In all cases, results are computed for equilibrium climatic sensitivities to CO₂ doubling of 2.0 and 4.0 °C.Peak atmospheric CO₂ concentrations of 400–500 ppmv and global mean warming after 1980 of 0.6–3.2 °C occur, with maximum rates of global mean warming of 0.2–0.3 °C decade^–1. The peak CO₂ concentrations in these scenarios are significantly below that commonly regarded as unavoidable; further sensitivity analyses suggest that limiting atmospheric CO₂ to as little as 400 ppmv is a credible option.Two factors in the model are important in limiting atmospheric CO₂: (1) the airborne fraction falls rapidly once emissions begin to decrease, so that total emissions (fossil fuel + land use-induced) need initially fall to only about half their present value in order to stabilize atmospheric CO₂, and (2) changes in rates of deforestation have an immediate and proportional effect on gross emissions from the biosphere, whereas the CO₂ sink due to regrowth of forests responds more slowly, so that decreases in the rate of deforestation have a disproportionately large effect on net emission.If fossil fuel emissions were to decrease at 1–2% yr^–1 beginning early in the next century, emissions could decrease to the rate of CO₂ uptake by the predominantly oceanic sink within 50–100 yrs. Simulation results suggest that if subsequent emission reductions were tied to the rate of CO₂ uptake by natural CO₂ sinks, these reductions could proceed more slowly than initially while preventing further CO₂ increases, since the natural CO₂ sink strength decreases on time scales of one to several centuries. The model used here does not account for the possible effect on atmospheric CO₂ concentration of possible changes in oceanic circulation. Based on past rates of atmospheric CO₂ variation determined from polar ice cores, it appears that the largest plausible perturbation in ocean-air CO₂ flux due to changes of oceanic circulation is substantially smaller than the permitted fossil fuel CO₂ emissions under the above strategy, so tieing fossil fuel emissions to the total sink strength could provide adequate flexibility for responding to unexpected changes in oceanic CO₂ uptake caused by climatic warming-induced changes of oceanic circulation.     

Global Warming and Tropical Land-Use Change: Greenhouse Gas Emissions from Biomass Burning, Decomposition and Soils in Forest Conversion, Shifting Cultivation and Secondary Vegetation

Tropical forest conversion, shiftingcultivation and clearing of secondary vegetation makesignificant contributions to global emissions ofgreenhouse gases today, and have the potential forlarge additional emissions in future decades. Globally, an estimated 3.1×10⁹ t of biomasscarbon of these types is exposed to burning annually,of which 1.1×10⁹ t is emitted to the atmospherethrough combustion and 49×10⁶ t is converted tocharcoal (including 26–31×10⁶ t C of blackcarbon). The amount of biomass exposed to burningincludes aboveground remains that failed to burn ordecompose from clearing in previous years, andtherefore exceeds the 1.9×10⁹ t of abovegroundbiomass carbon cleared on average each year. Above-and belowground carbon emitted annually throughdecomposition processes totals 2.1×10⁹ t C. Atotal gross emission (including decomposition ofunburned aboveground biomass and of belowgroundbiomass) of 3.41×10⁹ t C year^-1 resultsfrom clearing primary (nonfallow) and secondary(fallow) vegetation in the tropics. Adjustment fortrace gas emissions using IPCC Second AssessmentReport 100-year integration global warming potentialsmakes this equivalent to 3.39×10⁹ t ofCO₂-equivalent carbon under a low trace gasscenario and 3.83×10⁹ t under a high trace gasscenario. Of these totals, 1.06×10⁹ t (31%)is the result of biomass burning under the low tracegas scenario and 1.50×10⁹ t (39%) under thehigh trace gas scenario. The net emissions from allclearing of natural vegetation and of secondaryforests (including both biomass and soil fluxes) is2.0×10⁹ t C, equivalent to 2.0–2.4×10⁹ t of CO₂-equivalent carbon. Adding emissions of0.4×10⁹ t C from land-use category changesother than deforestation brings the total for land-usechange (not considering uptake of intact forest,recurrent burning of savannas or fires in intactforests) to 2.4×10⁹ t C, equivalent to 2.4–2.9×10⁹ t of CO₂-equivalent carbon. The totalnet emission of carbon from the tropical land usesconsidered here (2.4×10⁹ t C year^-1)calculated for the 1981–1990 period is 50% higherthan the 1.6×10⁹ t C year^-1 value used by the Intergovernmental Panel on Climate Change. The inferred (= `missing') sink in the global carbonbudget is larger than previously thought. However,about half of the additional source suggested here maybe offset by a possible sink in uptake by Amazonianforests. Both alterations indicate that continueddeforestation would produce greater impact on globalcarbon emissions. The total net emission of carboncalculated here indicates a major global warmingimpact from tropical land uses, equivalent toapproximately 29% of the total anthropogenic emissionfrom fossil fuels and land-use change.     

A Local-Scale Study of the Trace Gas Emissions from Vegetation Burning around the Village of Dalun, Ghana, with Respect to Seasonal Vegetation Changes and Burning Practices

The annual trace gas emissions from a West African rural region were calculated using direct observations of gas emissions and burning practices, and the findings compared to the guidelines published by the IPCC. This local-scale study was conducted around the village of Dalun in the Northern Region of Ghana, near the regional capital of Tamale. Two types of fires were found in the region – agricultural fires andwildfires. Agricultural fires are intentionally set in order to remove shrub and crop residues; wildfires are mostly ignited by herders to remove inedible grasses and to promote the growth of fresh grass. An agricultural fire is ignited with a fire front moving against the wind (backfire), whereas a wildfire moves with the wind (headfire). Gas emissions (CO₂, CO and NO) weremeasured by burning eight experimental plots, simulating both headfires and backfires. A common method of evaluating burning conditions is to calculate modified combustion efficiency (MCE), which expresses the percentage of the trace gases released as CO₂. Modified combustion efficiency was95% in the wildfires burned as headfires, but only 90% in the backfires.The burned area in the study region was determined by classifying a SPOT HRV satellite image taken about two months into the dry season. Fires were classified as either old burned areas or new burned areas as determined by the gradient in moisture content in the vegetation from the onset of the dry season. Classified burned areas were subsequently divided into two classes depending on whether the location was in the cultivated area or in the rangeland area, this sub-classification thus indicating whether the fire had been burned as a backfire or headfire. Findings showed that the burned area was 48% of the total region, and that the ratio of lowland wildfiresto agricultural fires was 3:1. The net trace gas release from the classified vegetation burnings were extrapolated to 26–46×10⁸ gCO₂, 78–302×10⁶ g CO,17–156×10⁵ g CH₄,16–168×10⁵ g NMHC and 11–72×10³ NO_x. Calculation of the emissionsusing proposed IPCC default values on burned area and average biomass resulted in a net emission 5 to 10 times higher than the measured emission values. It was found that the main reason for this discrepancy was not the emission factorsused by the IPCC, but an exaggerated fuel load estimate.     

Emissions of Polycyclic aromatic hydrocarbons by savanna fires

Although Polycyclic aromatic hydrocarbons (PAH) are known as anthropogenic compounds arising from the combustion or the pyrolysis of fossil fuels, they may be also emitted by the combustion of vegetation. A field study was carried out in January 1991 at Lamto (Ivory Coast) as part of the FOS DECAFE experiment (Fire Of Savanna). Some ground samplings were devoted to the qualitative and quantitative characterization of atmospheric emissions by savanna fires during prescribed burns and under background conditions. Specific collections for gaseous and particulate PAHs have shown that the African practice of burning the savanna biomass during the winter months is an important source of PAHs. These compounds are emitted mainly in gaseous form but a significant fraction, essentially heavy PAHs, is associated with fine carbonaceous particles and can therefore represent a hazard for human health, since some of these compounds are mutagenic and carcinogenic. Twelve compounds were identified during the fire episodes and in the atmospheric background. The total concentration in the fires is of the order of 10 ng m^–3 for the gas phase and from 0.1 to 1 ng m^–3 in the aerosols. In the atmospheric background the mean concentrations are regular, 0.15 ng m^–3 and 2 pg m^–3, respectively. These concentrations are comparable with what is observed in European rural zones. The particulate emissions of PAHs by the savanna fires are distinguished by the abundance of some compounds which can be considered as tracers, although they are also slightly emitted by fossil fuel sources. These compounds are essentially pyrene, chrysene and coronene. In the gas phase, although no individual PAH may be considered as specific of the biomass combustion emissions, the relative abundances of the main PAHs are characteristic of the biomass burning. The concentrations of pyrene and fluorene are always predominant; these compounds could be considered as characteristic emission products of smoldering and flaming episodes, respectively. In the background the PAH composition shows that in a tropical region the air consists of a mixture coming from the various sources, but the biomass combustion is by far the most important source.The fluxes of total PAH emitted by savanna biomass burning in Africa were estimated to be of the order of 17 and 600 ton yr^–1, respectively, for the particulate PAHs and the gaseous PAHs, respectively.     

Dynamics of Russian Forests and the Carbon Budget in 1961–1998: An Assessment Based on Long-Term Forest Inventory Data

Development trends of Russian forests and their impact on the global carbon budget were assessed at the national level on the basis of long-term forest inventory data (1961–1998). Over this period, vegetation of Russian forest lands are estimated as a carbon sink, with an annual average level of carbon sequestration in vegetational organic matter of 210 ± 30 Tg C · yr^–1 (soil carbon is not considered in this study), of which 153 Tg C · yr^–1 were accumulated in live biomass and 57 Tg C · yr^–1 in dead wood. The temporal variability of the sink is very large; for the five-year averages used in the analysis, the C sequestration varies from about 60 to above 300 Tg C· yr^–1. It is shown that long-term forest inventory data could serve as an important information base for assessing crucial indicators of full carbon accounting of forests.     

Decadal Methane Emission Trend Inferred from Proxy GOSAT XCH4 Retrievals: Impacts of Transport Model Spatial Resolution

In recent studies, proxy XCH₄ retrievals from the Japanese Greenhouse gases Observing SATellite (GOSAT) have been used to constrain top-down estimation of CH₄ emissions. Still, the resulting interannual variations often show significant discrepancies over some of the most important CH₄ source regions, such as China and Tropical South America, by causes yet to be determined. This study compares monthly CH₄ flux estimates from two parallel assimilations of GOSAT XCH₄ retrievals from 2010 to 2019 based on the same Ensemble Kalman Filter (EnKF) framework but with the global chemistry transport model (GEOS-Chem v12.5) being run at two different spatial resolutions of 4° × 5° (R4, lon × lat) and 2° × 2.5° (R2, lon × lat) to investigate the effects of resolution-related model errors on the derived long-term global and regional CH₄ emission trends. We found that the mean annual global methane emission for the 2010s is 573.04 Tg yr ^–1 for the inversion using the R4 model, which becomes about 4.4 Tg yr ^–1 less (568.63 Tg yr ^–1) when a finer R2 model is used, though both are well within the ensemble range of the 22 top-down results (2008–17) included in the current Global Carbon Project (from 550 Tg yr ^–1 to 594 Tg yr ^–1). Compared to the R2 model, the inversion based on the R4 tends to overestimate tropical emissions (by 13.3 Tg yr ^–1), which is accompanied by a general underestimation (by 8.9 Tg yr ^–1) in the extratropics. Such a dipole reflects differences in tropical–mid-latitude air exchange in relation to the model’s convective and advective schemes at different resolutions. The two inversions show a rather consistent long-term CH₄ emission trend at the global scale and over most of the continents, suggesting that the observed rapid increase in atmospheric methane can largely be attributed to the emission growth from North Africa (1.79 Tg yr ^–2 for R4 and 1.29 Tg yr ^–2 for R2) and South America Temperate (1.08 Tg yr ^–2 for R4 and 1.21 Tg yr ^–2 for R2) during the first half of the 2010s, and from Eurasia Boreal (1.46 Tg yr ^–2 for R4 and 1.63 Tg yr ^–2 for R2) and Tropical South America (1.72 Tg yr^–2 for R4 and 1.43 Tg yr ^–2 for R2) over 2015–19. In the meantime, emissions in Europe have shown a consistent decrease over the past decade. However, the growth rates by the two parallel inversions show significant discrepancies over Eurasia Temperate, South America Temperate, and South Africa, which are also the places where recent GOSAT inversions usually disagree with one other.     

Global NOx Production by Lightning

Lightning is thought to represent an important source of tropospheric reactive nitrogen species NO_x (NO + NO₂),but estimates of global production of NO_x by lightning varyconsiderably. We evaluate the production of NO_x by lightning using a global chemical/transport model, satellite lightning observations, and airborne NO_x measurements. Various model calculations are conducted toassess the global NO_x production rate of lightning by comparing the model calculations with airborne measurements. The results show that the simulated NO_x in the tropical middle and upper troposphere are very sensitiveto the amount and altitude of the lightning NO_x used in the model. A global lightning NO_x production of 7 Tg N yr^–1uniformly distributed in convective clouds or 3.5 Tg N yr^–1 distributedin the upper cloud regions produces good agreement between calculated and measured NO_x concentrations in the tropics.     

Automobile Emissions of Acetonitrile: Assessment of its Contribution to the Global Source

Based on an estimated global fuel consumption of 2.57 × 10¹⁵g(C) y^–1 and the assumption thatthe fossil fuel burned in Austria is globallyrepresentative, an upper limit of 0.021 (+150%, –50%)Tg y^–1 for global CH₃CN emission dueto fossil fuel burning was obtained from the relativeenhancement of the concentrations of toluene, benzene,and acetonitrile (methyl cyanide) during strong,short-term traffic pollution. This is less than 6% ofthe total global budget of CH₃CN, which is dominatedby an emission rate of 0.8 Tg y^–1 from biomassburning.     

Global distribution of natural freshwater wetlands and rice paddies,their net primary productivity,seasonality and possible methane emissions

A global data set on the geographic distribution and seasonality of freshwater wetlands and rice paddies has been compiled, comprising information at a spatial resolution of 2.5° by latitude and 5° by longitude. Global coverage of these wetlands total 5.7×10⁶ km² and 1.3×10⁶ km², respectively. Natural wetlands have been grouped into six categories following common terminology, i.e. bog, fen, swamp, marsh, floodplain, and shallow lake. Net primary productivity (NPP) of natural wetlands is estimated to be in the range of 4–9×10¹⁵ g dry matter per year. Rice paddies have an NPP of about 1.4×10¹⁵ g y^–1. Extrapolation of measured CH₄ emissions in individual ecosystems lead to global methane emission estimates of 40–160 Teragram (1 Tg=10¹² g) from natural wetlands and 60–140 Tg from rice paddies per year. The mean emission of 170–200 Tg may come in about equal proportions from natural wetlands and paddies. Major source regions are located in the subtropics between 20 and 30° N, the tropics between 0 and 10° S, and the temperate-boreal region between 50 and 70° N. Emissions are highly seasonal, maximizing during summer in both hemispheres. The wide range of possible CH₄ emissions shows the large uncertainties associated with the extrapolation of measured flux rates to global scale. More investigations into ecophysiological principals of methane emissions is warranted to arrive at better source estimates.     

Pollution by cereal waste burning in Spain

In this paper, the amount of cereal waste burned in Spain, which represents the most important source of biomass burning in this country, is estimated. During the period between 1980 and 1998, an average mass of 8 Tg of cereal waste was burned annually, with remaining 1 Tg of ash on the cereal fields after combustion. By using emission factors previously calculated by Ortiz de Zárate et al. [Ortiz de Zárate, I., Ezcurra, A., Lacaux, J.P., Van Dihn, P., 2000. Emission factor estimates of cereal waste burning in Spain. Atmos. Environ. 34, 3183–3193.], it is deduced that pollutant emissions linked to cereal waste-burning process reach values of 11 Tg CO₂, 80 Gg of TPM and 23 Gg of NO_x year⁻¹ during the cereal-burning period. These emissions represent 46% of total CO₂ and 23% NO_x emitted in Spain during the burning period that lasts 1 month after harvesting. Therefore, the relative importance of cereal waste burning as pollutant source in Spain almost during fire period becomes evident.Finally, our study allows to deduce that the production of 1 kg of cereal crop implies that 410 g of carbon and 3.3 g of nitrogen are going to be introduced into the atmosphere by this pollutant process. We estimate a total gaseous emission of 3.3 Tg of C and 25 Gg N as different pollutants by cereal waste burning.     

Measurements of trace gases emitted by Australian savanna fires during the 1990 dry season

During 18–23 July 1990, 31 smoke samples were collected from an aircraft flying at low altitudes through the plumes of tropical savanna fires in the Northern Territory, Australia. The excess (above background) mixing ratios of 17 different trace gases including CO₂, CO, CH₄, several non-methane hydrocarbons (NMHC), CH₃CHO, NO _x (– NO + NO₂), NH₃, N₂O, HCN and total unspeciated NMHC and sulphur were measured. Emissionratios relative to excess CO₂ and CO, and emissionfactors relative to the fuel carbon, nitrogen or sulphur content are determined for each measured species. The emission ratios and factors determined here for carbon-based gases, NO _x , and N₂O are in good agreement with those reported from other biomass burning studies. The ammonia data represent the first such measurements from savanna fires, and indicate that NH₃ emissions are more than half the strength of NO _x emissions. The emissions of NO _x , NH₃, N₂O and HCN together represent only 27% of the volatilised fuel N, and are primarily NO _x (16%) and NH₃ (9%). Similarly, only 56% of the volatilised fuel S is accounted for by our measurements of total unspeciated sulphur.     

Rainwater Chemistry and Wet Deposition over the Equatorial Forested Ecosystem of Zoétélé (Cameroon)

Within the framework of IDAF (IGAC DEBITS AFRICA: International GlobalAtmospheric Chemistry/DEposition of Biogeochemically Important TraceSpecies/Africa) network, data analysis is realised on precipitation chemical composition collected in Zoétélé, in Southern Cameroon. This station, located atabout 200 km from the Atlantic Ocean, is representative of a so-called `Evergreen Equatorial Forest' ecosystem. An automatic wet-only precipitation collector was operated at the station from 1996 to 2000. The rainfall regime, associated with eastward advection of moist and cool monsoon air masses, amounts to an average of 1700 mm/year. Inorganic and organic content of the precipitation were determined by IC in 234 rainfall events, representing a total 4,583 mm of rainfall from an overall of 7,100 mm.The mean annual precipitation chemistry and wet deposition fluxes characteristic of an African equatorial forest are quantified. Typical atmospheric gases and particles sources influence the precipitation chemical content and the associated deposition of chemical species. Indeed, hydrogen concentration is the highest (12.0 eq.L^–1) of the IDAF measurements, leading to acid rains with a low mean pH 4.92. The mineral species are dominated by nitrogenous compounds (NH₄ ⁺:10.5 and NO₃ ^–: 6.9 eq.L^–1), Ca²⁺ (8.9 eq.L^–1) and SO₄ ^{2 –} 5.1 eq.L^–1. Relationship between Ca^{2 +} and SO₄ ^{2 –} indicated aterrigeneous particulate source and an additional SO₄ ^{2 –} contributionprobably due to swamps and volcano emissions. Na⁺ and Cl^–concentrations, around 4.0 eq.L^–1, seem very low for this site,accounting for the marine source. Besides, strong correlations between NH₄ ⁺/K⁺/Cl^– indicate the biomass burning originof these species. Accordingly, precipitation chemistry in Zoétéléis influenced by three major sources: biogenic emissions from soil and forest ecosystems, biomass burning from savannah, and terrigenous signature from particles emissions of arid zones; and three minor sources: marine, volcano and anthropogenic. In spite of the relatively low concentration of all these elements, the wet deposition is quite significant due to the high precipitation levels, with for example a nitrogenous compounds deposition of 34 mmol.m^–2.yr^–1.     

Arctic hazes in summer over Greenland and the North American Arctic. III: A contribution from the natural burning of carbonaceous materials and pyrites

Airborne measurements of the emissions from natural fires, fueled by pyrites and organic materials, at the Smoking Hills in the Northwest Territories, show that they are a regionally significant source of SO₂ (0.3 kg s^–1 or 10⁴ T yr^–1) and particles (0.3 kg s^–1). It appears likely that the Smoking Hills are a source for some of the dense, lower-level, haze layers that occur in the North American Arctic.     

Airborne measurements of savanna fire emissions and the regional distributio of pyrogenic pollutants over Western Africa

We measured CO₂, CO, CH₄, H₂, and NO₂ in air masses polluted by savanna fires over Côte d'Ivoire, western Africa. Elevated concentrations of these trace gases were found in fire plumes and also in extensive haze layers. Trace gas mixing ratios ranged as high as 605 ppmv for CO₂, 14.8 ppmv for CO, 2.7 ppmv for CH₄, 4.2 ppmv for H₂, and 25 ppbv for NO₂. We compare our emission ratios to those obtained in previous field and laboratory studies. The emission ratios, expressed as an average and as a range or as an average only, were: dCO/dCO₂ 5.3×10^–2 (3–18×10^–2); dCH₄/dCO 5.3×10^–2; dH₂/dCO 2.4×10^–1 and dNO₂/dCO₂ 1.8×10^–4 (1.5–2.2×10^–4). The values found match those found during similar measurements, though our results point to rather vigorous burning in the savanna of western Africa.     

Emission of aliphatic amines from animal husbandry and their reactions: Potential source of N2O and HCN

We measured the emissions of volatile aliphatic amines and ammonia produced by the manure of beef cattle, dairy cows, swine, laying hens and horses in livestock buildings. The amine emissions consisted almost exclusively of the three methylamines and correlated with those of ammonia. The molar emission ratios of the methylamines to ammonia, and data on NH₃ emissions from animal husbandry in Europe, together with global statistics on domestic animals, were used to estimate the global emissions of amines. Annual global methylamine-N input to the atmosphere from animal husbandry in 1988 was 0.15±0.06 TgN (Tg=10¹² g). Almost 3/4 of these emissions consisted of trimethylamine-N. This represents about half of all methylamine emissions to the atmosphere. Other sources are marine coastal waters and biomass burning.Possible reaction pathways for atmospheric methylamines are shown. Among various speculative but possible products N₂O and HCN are of interest because the emission of methylamines could contribute to the global budgets of these compounds. Maximum atmospheric N₂O production from methylamines are below 0.4 Tg N/year, which is less than 10% of the annual N₂O growth rate. Although we do not expect the methylamine emissions to contribute in a major way to the atmospheric N₂O budget, more studies are needed to establish this conclusion beyond doubt. Similar conclusions hold for HCN.