M. A. K. Khalil

Greenhouse gases and other airborne pollutants from household stoves in China: a database for emission factors

Abstract Emissions from household stoves, especially those using solid fuels, can contribute significantly to greenhouse gas (GHG) inventories and have adverse health impacts. Few data are available on emissions from the numerous types of cookstoves used in developing countries. We have systematically measured emissions from 56 fuel/stove combinations in India and China, a large fraction of the combinations in use world-wide. A database was generated containing emission factors of direct and indirect GHGs and other airborne pollutants such as CO2, CO, CH4, TNMHC, N2O, SO2, NOx, TSP, etc. In this paper, we report on the 28 fuel/stove combinations tested in China. Since fuel and stove parameters were measured simultaneously along with the emissions, the database allows construction of complete carbon balances and analyses of the trade-off of emissions per unit fuel mass and emissions per delivered energy. Results from the analyses show that the total emissions per unit delivered energy were substantially greater from burning the solid fuels than from burning the liquid or gaseous fuels, due to lower thermal and combustion efficiencies for solid-fuel/stove combinations. For a given biomass fuel type, increasing overall stove efficiency tends to increase emissions of products of incomplete combustion. Biomass fuels are typically burned with substantial production of non-CO2 GHGs with greater radiative forcing, indicating that biomass fuels have the potential to produce net global warming commitments even when grown renewably.

The Global Distribution of Methane in the Troposphere

Methane has been measured in air samples collected at approximately weekly intervals at 23 globally distributed sites in the NOAA/GMCC cooperative flask sampling network. Sites range in latitude from 90° S to 76° N, and at most of these we report 2 years of data beginning in early 1983. All measurements have been made by gas chromatography with a flame ionization detector at the NOAA/GMCC laboratory in Boulder, Colorado. All air samples have been referenced to a single secondary standard of methane-in-air, ensuring a high degree of internal consistency in the data. The precision of measurements is estimated from replicate determinations on each sample as 0.2%. The latitudinal distribution of methane and the seasonal variation of this distribution in the marine boundary layer has been defined in great detail, including a remarkable uniformity in background levels of methane in the Southern Hemisphere. We report for the first time the observation of a complete seasonal cycle of methane at the South Pole. A significant vertical gradient is observed between a sea level and a high altitude site in Hawaii. Globally averaged background concentrations in the marine boundary layer have been calculated for the 2 year-period May 1983–April 1985 inclusive, from which we find an average increase of 12.8 ppb per year, or 0.78% per year when referenced to the globally averaged concentration (1625 ppb) at the mid-point of this period. We present evidence that there has been a slowing down in the methane growth rate.

The global sources of nitrous oxide

We use atmospheric and ice core data on the concentrations of nitrous oxide to estimate that the present global anthropogenic emissions are 7±1 tg/yr. If the atmospheric lifetime of N2O is a hundred years or more, this estimate is virtually independent of the actual lifetime. The natural sources are estimated to be about 15 tg/yr. We also find that nitrous oxide started increasing rapidly only during the last century. The trends over the last decade are extremely variable; over 3-year periods the trends have ranged from 0.5 ± 0.2 parts per billion by volume (ppbv/yr) to 1.2 ± 0.1 ppbv/yr. The average rate of increase is about 0.80 ± 0.02 ppbv/yr or 0.27 ± 0.01 %/yr (1977–1988). There is an indication that N2O may be increasing faster in recent years than during the middle 1970s by about 0.2 ± 0.1 ppbv/yr. It is likely that several small anthropogenic sources may be causing the present trends, all emitting between 0.1 and 1.5 Tg/yr.

Global sources, lifetimes and mass balances of carbonyl sulfide (OCS) and carbon disulfide (CS2) in the earth's atmosphere

Abstract Emissions of carbonyl sulfide (OCS) and carbon disulfide (CS2) from natural and anthropogenic sources have been estimated consistent with the observed latitudinal and vertical distributions of these trace gases. Anthropogenic emissions appear to be a small part (⩽ 25 %) of the yearly emissions of OCS and CS2. Oceans and soils are the largest sources, representing about 50 % of the yearly emissions of OCS. Biomass burning is perhaps the largest anthropogenic source, while coal-fired power plants and automobiles contribute small amounts. Chemical uses may be the largest anthropogenic source of CS2. The results suggest that the lifetime of OCS is about 2 years and of CS2 about 12 days.

Natural emissions of chlorine‐containing gases: Reactive Chlorine Emissions Inventory

Although there are many chlorine-containing trace gases in the atmosphere, only those with atmospheric lifetimes of 2 years or fewer appear to have significant natural sources. The most abundant of these gases are methyl chloride, chloroform, dichloromethane, perchloroethylene, and trichloroethylene. Methyl chloride represents about 540 parts per trillion by volume (pptv) Cl, while the others together amount to about 120 pptv Cl. For methyl chloride and chloroform, both oceanic and land-based natural emissions have been identified. For the other gases, there is evidence of oceanic emissions, but the roles of the soils and land are not known and have not been studied. The global annual emission rates from the oceans are estimated to be 460 Gg Cl/yr for CH3Cl, 320 Gg Cl/yr for CHCl3, 160 Gg Cl/yr for CH2Cl2, and about 20 Gg Cl/yr for each of C2HCl3, and C2Cl4. Land-based emissions are estimated to be 100 Gg Cl/yr for CH3Cl and 200 Gg Cl/yr for CHCl3. These results suggest that the oceans account for about 12% of the global annual emissions of methyl chloride, although until now oceans were thought to be the major source. For chloroform, natural emissions from the oceans and lands appear to be the major sources. For further research, the complete database compiled for this work is available from the archive, which includes a monthly emissions inventory on a 1° × 1° latitude-longitude grid for oceanic emissions of methyl chloride.

Causes of increasing atmospheric methane - Depletion of hydroxyl radicals and the rise of emissions

Abstract The observed rapid rise of atmospheric methane at rates of about 1.3% per year may be caused by the increase of emissions effected by anthropogenic activities and by a possible decline in the global concentrations of hydroxyl radicals (OH) which remove methane from the atmosphere. Moreover, the measurements of methane in air bubbles buried deep in polar ice suggest that the concentrations of methane several hundred years ago may have been only about 45% of present levels. We extrapolated the present emissions of methane from human activities (250 Tg y−1) backwards in time proportional to the estimated change of human population. Our calculations show that much of the increase of methane over the past 200 years is probably due to the increase of emissions (70%) and a lesser amount is due to a possible depletion of OH (30%). Projections indicate that in another 20 years the average tropospheric concentrations of methane may reach 1900–1950 ppbv or about 25% greater than present (1980) levels. Our findings, based on two complementary approaches, support the idea that the present abundance of hydroxyl radicals may be about 20% less than several hundred years ago.

Carbon monoxide in the earth's atmosphere - Increasing trend

The results of an analysis of more than 60,000 atmospheric measurements of carbon monoxide taken over 3� years at Cape Meares, Oregon (45�N, 125�W), indicate that the background concentration of this gas is increasing. The rate of increase, although uncertain, is about 6 percent per year on average. Human activities are the likely cause of a substantial portion of this observed increase; however, because of the short atmospheric lifetime of carbon monoxide and the relatively few years of observations, fluctuations of sources and sinks related to the natural variability of climate may have affected the observed trend. Increased carbon monoxide may deplete tropospheric hydroxyl radicals, slowing down the removal of dozens of man-made and anthropogenic trace gases and thus indirectly affecting the earths climate and possibly the stratospheric ozone layer.

The global cycle of carbon monoxide: trends and mass balance.

Abstract The annual global emissions of CO are estimated to be about 2,600 ± 600 Tg, of which about 60% are from human activities including combustion of fossil fuels and oxidation of hydrocarbons including methane. The remaining 40% of the emissions are from natural processes, mostly from the oxidation of hydrocarbons but also from plants and the oceans. Almost all the CO emitted into the atmosphere each year is removed by reactions with OH radicals (85%), by soils (10%), and by diffusion into the stratosphere. There is a small imbalance between annual emissions and removal, causing an increase of about 1% per year. It is very likely that the imbalance is due to increasing emissions from anthropogenic activities. The average concentration of CO is about 90 ppbv, which amounts to about 400 Tg in the atmosphere, and the average lifetime is about 2 months. This view of the global cycle of CO is consistent with the present estimates of average OH concentrations and the budgets of other trace gases including methane and methylchloroform. There are large remaining uncertainties that may in the future upset the apparently cohesive present budget of CO. If the present view of the global cycle of CO is correct, then it is likely that, in time, increasing levels of CO will contribute to widespread changes in atmospheric chemistry.

Methane, Carbon Monoxide and Methylchloroform in the Southern Hemisphere

New observational data on CH4, CO and CH3CCl3 in the southern hemisphere are reported. The data are analysed for long term trends and seasonal cycles. CH3CCl3 data are used to scale the OH fields incorporated in a two dimensional model, which in turn, is used to constrain the magnitude of a global CH4 source function. The possible causes of observed seasonality of CH3CCl3, CH4 and CO are identified, and several other aspects of observed CH4 variability are discussed.Possible future research directions are also given.

Atmospheric methyl chloride

There are about 5 Tg of methyl chloride in the Earth’s atmosphere making it one of the largest reservoirs of gas-phase chlorine. We discuss the time series of global measurements taken over the last 16 yr at seven locations distributed among the polar, middle, and tropical latitudes of both hemispheres (1981—1997). Measurements were also taken at 20 more sites between 1987 and 1989. The vertical distribution was measured during campaign experiments in the Arctic, Western Atlantic, and over Brazil. Small, mostly decreasing trends are observed, showing that on average, there was 4% less methyl chloride during the last three years (1994—1996) than there was in the first three years (1985—1987) of the experiment. The latitudinal variation is marked by highest concentrations in the tropics and lowest in the polar regions. Sites representing inland locations show higher concentrations, suggesting continental sources, mostly confined to the tropics. There are seasonal variations at various latitudes that can be explained mostly by the cycles of OH radicals, which are the dominant removal process for methyl chloride in the atmosphere. Based on these data, the expected emissions can be calculated at the polar, middle, and tropical latitudes represented by the six long-term primary sites. Using a photochemical model of OH, we estimate that a global source of about 3.7 Tg yr~1 of methyl chloride is needed to explain the observed concentrations. Other removal processes have been identified that may add to this estimate of the global annual emissions. The results further establish that some 85% of the emissions must come from the half of the earth’s surface between 30iS and 30iN, representing tropical and sub-tropical latitudes. Small emissions are estimated for the middle latitudes, and no emissions are expected from the polar regions. ( 1999 Elsevier Science Ltd. All rights reserved.