Grantee Research Project Results
Final Report: Human Influence on Ozone in the Tropical Troposphere: An Interpretation of Observations Using a Global Three-dimensional Model
EPA Grant Number: R824096Title: Human Influence on Ozone in the Tropical Troposphere: An Interpretation of Observations Using a Global Three-dimensional Model
Investigators: Jacob, Daniel J. , Logan, Jennifer A. , Wofsy, Steven C. , Spivakovsky, Clarisa M.
Institution: Harvard University
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1995 through September 30, 1998 (Extended to March 31, 1999)
Project Amount: $415,798
RFA: Exploratory Research - Chemistry and Physics of Air (1995) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air , Safer Chemicals
Objective:
The purpose of this project was to investigate the effects of human activity on ozone in the tropical troposphere. A major feature of the ozone distribution is a region of high concentrations over Brazil, Africa, and the south tropical Atlantic in austral spring, with much lower values over the western Pacific. Analyses of recent field studies show that most of the ozone is formed photochemically, with emissions from biomass burning playing an important role as a precursor of NO; production of NO from lightning also appears to be an important source of NO at the end of the burning season. Transport mechanisms appear important in "containing" the high ozone values over the region. Increases in biomass burning, as occurred in Brazil during the 1980s, could potentially lead to increases in ozone in the upper troposphere, with consequences for climate.The project involved using a three-dimensional chemical tracer model (CTM) to investigate the mechanisms that are responsible for the observed ozone distribution. As part of this study, an inventory of emissions of NO, CO, and hydrocarbons from biomass burning was developed, with a particular focus on the use of biomass fuels and disposal of agricultural waste, either as a household fuel or by open field burning. In addition to biomass burning, the CTM also allows for production of NO by soils and by lightning, and includes the Global Emissions Inventory Activity (GEIA) for fossil fuel emissions. The CTM includes emissions of hydrocarbons and CO, complete chemistry, surface deposition, and rainout of soluble gases. The model was evaluated with observations of the vertical distribution of ozone, surface data for ozone and CO, and aircraft data for NO, HNO3, PAN, CO, and hydrocarbons. Research objectives were to: (1) assess the magnitude of emissions from biomass fuels and agricultural burning; and (2) investigate the budget of tropospheric ozone in the tropics and the role of transport in maintaining the ozone maximum observed over the tropical south Atlantic in the austral spring.
Summary/Accomplishments (Outputs/Outcomes):
The results of this research project are summarized below.Sources of Trace Gases from Biomass Fuel and Agricultural Waste Burning. Biomass fuels and agricultural burning provide significant sources of trace gases to the atmosphere, particularly in the tropics, yet the magnitudes of these sources are poorly known (Crutzen, et al., 1979; Andreae, 1991). Earlier estimates of quantities burned relied on United Nations Food and Agriculture Organization (FAO) statistics for firewood, which are thought to be low because they account only for wood that is marketed. Prior work also assumed that a uniform fraction of agricultural waste was burned, either in the fields or as fuel. Our approach takes into account regional information on burning practices for biofuels and field residues in the developing world. We relied on energy assessments from the World Bank and other sources, government statistics, and discussions with experts on energy consumption in the developing countries, and in agronomy, forestry, and agroindustries. We have assessed the consumption of the following biomass fuels on a country by country basis: wood, charcoal, dung, and agricultural residues. We also assessed the burning of agricultural wastes in the fields on a crop-by-crop and national basis. Our approach allows the use of emission factors specific to particular fuels and combustion practices, where known. National totals for combustion of biomass fuels and agricultural waste are spatially disaggregated within each country using a 1? x 1? map of rural population that we developed.
Most information on woodfuel use is based on household surveys, and is given in terms of per capita consumption. These data, together with population statistics, were used to estimate use in each country. Our estimate for total fuelwood consumption is 1362 tg dry matter for 1985, distributed among Asia, Africa, and Latin America in the ratio 58:28:14. This is significantly higher than the estimate of Seiler and Crutzen (1980), 743 tg, who relied on FAO statistics for fuelwood.
Andreae (1991) derived an estimate of 1260 tg, similar to ours. However, he averaged estimates based on the FAO statistics and on a single value for per capita fuelwood consumption. Agricultural residues form the other large biofuel. Estimates of residue use as fuel were taken directly from survey data, where available. Otherwise, information on the extent of fuelwood deficit, together with patterns of agricultural residue use as fodder, construction, mulch, household fuel, and open field burning were combined to provide estimates of the amount of residue used as fuel. Other data on consumption of agroindustrial biofuels such as bagasse, coconut husks, and palm by-products from reports on the agroindustries and from World Bank country studies also were used to estimate the amount of residue as agroindustrial fuel. The use of agricultural residues as fuels is most pronounced in India, and China and elsewhere they are mainly used as woodfuel substitutes in wood-deficit regions. Dung is important as a biofuel in northern India, and high altitude regions of western China, Bolivia, and Peru. Our estimate of residue as biofuel for the developing world is 719 tg dry matter/year, with the ratio for Asia, Africa, and Latin America of 80:8:12.
Residue that does not have another use (e.g., biofuel, mulch, fodder, etc.) and that does not decompose quickly is frequently burned to clear the fields for planting (or harvesting, in the case of sugar cane). We estimated the fraction of residue burned in the fields by subtracting estimates of residue needed for other purposes from the available residue supply, taking into account information on local field burning customs. Our estimate for open field burning is 440 tg dry matter, distributed among Asia, Africa, and Latin America in the proportions of 68:10:22. The open field burning in Asia is dominated by rice straw, that in Brazil by sugar cane residues, and in Africa by cereal stalks. Combining the residue burned in the field with that used as fuel, we estimate that 1160 tg dry matter is burned annually. This is similar to the total of Andreae (1991), 1360 tg; however, he assumed arbitrarily that 80 percent of available residues are burned in developing countries.
Sources of trace gases from these types of burning were derived using emission factors from experimental studies of residue burning and of combustion of wood, crop residue, and dung as fuel (e.g., Brocard, et al., 1998; Marufu, 1999). Mean factors were derived from data for rice straw, sugar cane, and other cereals, weighted by the amount of burning in each category. We compute emissions from open field burning of 158 tg CO2, 11 tg CO (both as C), and 0.56 tg NOx (as N), and from household fuel of 946 tg CO2, 79 tg CO (both as C) and 2.3 tg NOx (as N). Emissions from biomass fuels are a significant fraction of the sources from combustion of fossil fuels, 170 tg CO (as C) and 21 tg NOx (as N). We are presently revising a draft manuscript describing the inventory for biomass fuel and agricultural waste burning that will be submitted for publication soon.
Analysis of the Global Budget of Ozone With a Focus on the Tropics. We have published three papers describing our model investigations of the factors regulating tropospheric ozone (Wang, et al., 1998a,b,c). This work used meteorological data from a general circulation model (GCM) developed at the Goddard Institute for Space Studies (GISS) (Hansen, et al., 1983) with a grid resolution of 4 in latitude x 5 in longitude and 9 layers in the vertical. It transports 15 tracers to describe the O3-NOx-CO-hydrocarbon chemistry. Details of the model are described in Wang, et al. (1998a).
We evaluated the model extensively with surface, ozonesonde, and aircraft measurements (Wang, et al., 1998b). In particular, we examined seasonal variations and regional distributions of ozone, NO, PAN, CO, ethane, acetone, and H2O2. The model reproduces observed NO and PAN concentrations to within a factor of 2 for a wide range of tropospheric regions including the upper troposphere, but tends to overestimate nitric acid in the remote troposphere (sometimes several fold). This discrepancy implies a missing sink for HNO3 that does not lead to recycling of NOx; only in the upper troposphere over the tropical South Atlantic would a fast conversion of HNO3 to NOx improve the model simulation for NOx. We reproduce observed concentrations of acetone in the model by including a large biogenic source (15 Tg C, Year 1), which accounts for 40 percent of the estimated global source of acetone (37 Tg C, Year 1). Concentrations of H2O2 in various regions of the troposphere are simulated usually to within a factor of 2, providing a test for HOx chemistry in the model. The model reproduces well the observed seasonal variations of ozone in the troposphere, with a few exceptions. We find that the budget of tropospheric ozone in the model is controlled largely by photochemical production and loss within the troposphere, and that NOx emitted in the Southern Hemisphere is twice as efficient at producing ozone compared to NOx emitted in the Northern Hemisphere.
Concentrations of ozone are higher in the model over the south tropical Atlantic than over the western Pacific in the model, as they are in the observations. However, the model underestimates ozone above about 3 km in the tropics, and overestimates it below, at least in the Atlantic. This is thought to be caused in part by excessive vertical mixing across the trade wind inversion. The largest underestimate of ozone is in the southern biomass burning season, July to November. Nitric oxide also is underestimated in the upper troposphere over the south Atlantic, and this contributes to the underestimate of ozone.
We used the model to investigate the factors regulating tropospheric ozone (Wang, et al., 1998c). Model results indicate a close balance between chemical production and loss in the tropospheric column at all latitudes except high latitudes in winter. Using separate tracers for ozone produced in the stratosphere and in different regions in the troposphere, we find that the contribution of transport from the stratosphere to ozone concentrations is about 30 percent at mid latitudes in winter, and 10 percent in summer. In the tropics, photochemical production and loss dominate the budget of ozone, with the stratosphere contributing only about 5 percent; deposition represents a minor sink for ozone. Production of ozone in the upper, middle, and continental lower troposphere all make significant contributions (10 to 50 percent) to ozone concentrations throughout the troposphere, including the tropics. The middle troposphere is a major global source region for ozone even though it is not a region of net ozone production. We investigated the causes of the springtime maximum of ozone at remote sites in the northern hemisphere, and found that it is caused by a phase overlap between ozone transported from the stratosphere that peaks in late winter to early spring, and ozone produced in the continental lower troposphere that peaks in late spring and early summer. Our results do not support previous explanations of the springtime maximum based on wintertime accumulation of ozone or its precursors in the Arctic. The strong spring maximum at Hawaii is attributed to long-range transport of Asian pollution over the North Pacific in spring.
In the later part of the project period, we started using meteorological data from NASA Goddard's Data Assimilation Office (DAO), instead of GCM winds; the model uses the same emissions fields as in Wang, et al. (1998a). The model has been tested extensively with the same data described in Wang, et al. (1998b) using a model run for July 1993, to December 1994. Model results are usually within 10-20 ppb of the observations, and the model captures well the seasonal cycle as well as the hemispheric gradient. There is excellent agreement between model and observations at Hilo, Hawaii, and below 4 km at Samoa and Natal, except for an underestimate at Natal in September to December. There is an underestimate of ozone above 4 km of about 10 ppb at Natal, smaller than the underestimate found using the GCM winds. Again, the NO values are somewhat low above 6 km in the south tropical Atlantic, likely contributing to the underestimate of ozone. We are presently using the model with DAO winds for other projects, and will be examining the ozone budget in the tropics in more detail in these studies.
References:
Andreae MO. Biomass burning: its history, use, and distribution and its impact on environmental quality and global climate. In: Levine JS, ed. Global Biomass Burning: Atmospheric, Climatic and Biospheric Implications. MIT Press, Cambridge, MA, 1991.
Brocard D, Lacaux JP, Eva H. Domestic biomass combustion and associated emissions in West Africa. Global Biogeochemistry Cycles 12:127-139.
Marufu L. Photochemistry of the African troposphere—the influence of biomass burning. Thesis, Universiteit Utrecht, Faculteit Natuuren Sterrenkunde, Utrecht, Nederlands, 1999, p. 145.
Seiler W, Crutzen P. Estimates of the gross and net flux of carbon between the biosphere and the atmosphere from biomass burning. Climate Change 1980;2:207-247.
Journal Articles on this Report : 4 Displayed | Download in RIS Format
Other project views: | All 4 publications | 4 publications in selected types | All 4 journal articles |
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Lin G, Qiao J, Steier P, Danielsen M, Guonason K, Joensen H, Stedmon C. Tracing Atlantic water transit time in the subarctic and Arctic Atlantic using Tc-99-U-233-U-236. SCIENCE OF THE TOTAL ENVIRONMENT 2022;851(2):158276 |
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Wang YH, Jacob DJ, Logan JA. Global simulation of tropospheric O-3-NOx-hydrocarbon chemistry: 3. Origin of tropospheric ozone and effects of nonmethane hydrocarbons. Journal of Geophysical Research-Atmospheres 1998;103(D9):10757-10767. |
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Wang YH, Jacob DJ, Logan JA. Global simulation of tropospheric O3-NOx-hydrocarbon chemistry: 1. Model formulation. Journal of Geophysical Research-Atmospheres 1998;103(D9):10713-10725. |
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Wang YH, Logan JA, Jacob DJ. Global simulation of tropospheric O3-NOx-hydrocarbon chemistry: 2. Model evaluation and global ozone budget. Journal of Geophysical Research-Atmospheres 1998;103(D9):10727-10755. |
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Supplemental Keywords:
tropospheric ozone, tropics, biomass burning, agricultural waste, wood fuel, emissions., RFA, Scientific Discipline, Air, Geographic Area, Physics, Environmental Chemistry, climate change, Air Pollution Effects, tropospheric ozone, Engineering, Chemistry, & Physics, International, Atmosphere, ambient aerosol, transport model, Brazil, environmental monitoring, atmospheric particles, hydrocarbon, air modeling, ozone, climate variations, biomass, chemical detection techniques, chemical transport modeling, chemical kinetics, human exposure, global three dimensional model, Africa, troposphere, fossil fuel emissions, atmospheric modelsRelevant Websites:
http://www-as.harvard.eduProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.