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[1] It is observed that side by side plots in rice fields, managed by the same practices, produce methane emissions that are often different by factors of 2 to 4. Similarly on a given day when many plots are sampled, the emissions from one plot may differ from another by factors of 3–4 on average. These large variations must be taken into account if these data are extrapolated to larger scales such as countries or the world. In this paper we analyze and delineate the nature of this variability. We distinguish temporal and spatial variability and examine the effects of both on the observed seasonal average emission rates. While temporal variability is managed well in most experiments because frequent measurements are taken, the spatial variability remains a potential major uncertainty in many experiments. Using the characteristics of the observed variability we use simulations to show that with a proper and realistic sampling strategy, it is possible to get to within 15%–20% of the true flux for most cases of interest. We calculate how many plots are needed and how often they should be sampled during the growing season to obtain a seasonal average flux within prescribed limits relative to the true flux.

[1] Emissions of methane and nitrous oxide, two significant greenhouse gases, were measured from rice fields at Qingyuan in Guangdong Province, China. The region has a subtropical climate which allows two crops of rice to be grown every year. The prevailing agricultural practices create a complex interaction between factors known to have a major effect on methane and nitrous oxide emissions from rice fields, namely, intermittent flooding and use of organic fertilizers. In this region, the farmers depend on nitrogen fertilizers and, at least in recent years, have only intermittently flooded their fields during the growing seasons. These factors tend to reduce methane emissions. But the rice straw and crop residues from the first crop of the year are plowed into the fields, providing a large addition of organic material under hot weather conditions favorable to quick decomposition during the second crop period. This, and the addition of farmyard manure, increases emissions of methane emissions from these fields. The results of the present study show that the effect of these competing factors and their timing lead to an average rate of emissions of 5 ± 2 and 6 ± 2 mg/m2/h from the first crops for the 2 years when measurements were taken (2003 and 2004), and 12 ± 8 and 13 ± 8 mg/m2/h from the second crop. Further, production measurements showed that during the 2 years of these experiments, the average production rates were about 27 mg/m2/h for the first crop and 22–34 mg/m2/h for the second crop, resulting in estimated oxidation rates of about 80% for the first crop and 50–60% for the second crop. The higher fluxes in the second crop therefore appear to be caused more by reduced oxidation than higher production. Nitrous oxide emissions, when they were detected, usually occurred within a few days after the application of nitrogen fertilizers. The seasonally averaged emissions were between 0.01 and 0.02 mg/m2/h except in the first year when large emissions over one short period pushed the average upward.

[1] Detailed field studies of methane emissions from rice fields show that when nitrogen fertilizers are used with intermittent irrigation the seasonal average flux is about 3 mg/m2/hr. When continuous flooding and organic material is added to the same fields, the emissions rise to nearly 30 mg/m2/hr. Production rates measured during the two years when both organic material and water were applied to these fields were found to be 60 and 90 mg/m2/hr in consecutive years. The fraction oxidized is calculated to be about 80% one year and 43% the next year. The results suggest that high organic fertilizer increases production, but may affect the emissions more by reducing oxidation. The data show that seasonally averaged emissions vary by up to factor of 2 among adjacent fields but the variability of production is only about 20%. Information obtained from the farmers suggests that there is a trend towards less use of organic manure as nitrogen fertilizers have become available and the occurrence of intermittent flooding has increased for various reasons. These conditions have likely led to decreasing emissions of methane from rice fields in China in the recent decades.

The atmospheric concentration of fluorocarbon 11 (F-11) has increased steadily since it was first put into commercial use in the late 1930s and early 1940s. The observed trends, however, have two periods of dramatic declines. The first occurred around 1974, when trends started falling from their all time high of 13.8 pptv/yr down to around 7.8 pptv/yr (1 pptv = 10−12 parts by volume) in 1982. This decline occurred at first, probably, because of market conditions and later because of the ban in the United States on inessential uses of F–11, particularly in aerosol spray cans. In the meantime, other uses of F–11, such as blowing foams, increased, causing an increasing trend once again until around 1987, when the atmospheric trends reached 11 pptv/yr (measured 1986–1988, inclusive). After this time, however, the trends have fallen dramatically and are now only 4.6 pptv/yr (measured 1990–1992) and even lower in the middle northern latitudes (about 2 pptv/yr). The recent decline of trend is attributed to the effect of the Montreal Protocol and subsequent agreements that are designed to ban worldwide production before the turn of this century. The atmospheric trends reflect estimated emissions, which reached highest values in 1974 (340–355 Gg/yr; 1 Gg = 109g) and again in 1988 (314–380 Gg/yr). The observed concentrations and trends agree extremely well with those calculated from independent estimates of emissions from the various sources. Atmospheric concentrations of F–11 are calculated from a mass balance theory involving seven reservoirs and their interactions. Three of the reservoirs are at the Earth's surface arising from the uses of F–11 (quick-release applications such as open cell foams and aerosols, nonhermetically sealed refrigeration, and rigid polyurethane foams). We estimate that of the 9150 Gg of F–11 that have been produced throughout its history, at present about 1040 Gg are tied up in rigid foams (90 Gg and 24 Gg are tied up in refrigeration and quick-release applications, respectively). There are four environmental reservoirs (deep oceans, the ocean mixed layer, the troposphere, and the stratosphere). F–11 is dynamically exchanged between these reservoirs and is destroyed principally by photolysis in the stratosphere and by soils at the Earth's surface. At present there are about 5360 Gg in the troposphere, 740 Gg in the stratosphere, and small amounts in the oceans (27 Gg and 6 Gg in the mixed layer and deep oceans, respectively). About 81% of the 9 Tg of F–11 produced is still in the surface or environmental reservoirs (1 Tg = 1012g). The future concentrations of F–11 depend on leakage rates from the surface reservoirs and the atmospheric lifetime. It seems that concentrations are not likely to reach the peaks expected earlier and are likely to decline faster than previously thought. The peak concentration is expected to be about 275 pptv and may occur within the next 2 to 3 years. This abstract, the figures, and their captions form a self-contained condensed description of our results.

Between 1979 and 1992 we took some 120,000 measurements of atmospheric methane at Cape Meares on the Oregon coast. The site is representative of methane concentrations in the northern latitudes (from 30 deg N to 90 deg N). The average concentration during the experiment was 1698 parts per billion by volume (ppbv). Methane concentration increased by 190 ppbv (or 11.9 percent) during the 13-year span of the experiment. The rate of increase was about 20 +/- 4 ppbv/yr in the first 2 yr and 10 +/- 2 ppbv/yr in the last 2 yr of the experiment, suggesting a substantial decline in the trend at northern middle and high latitudes. Prominent seasonal cycles were observed. During the year, the concentration stays more or less constant until May and then starts falling, reaching lowest levels in July and August, then rises rapidly to nearly maximum concentrations in October. Interannual variations with small amplitudes of 2-3 ppbv occur with periods of 1.4 and 6.5 yr.

We report studies of N2O concentrations in the ambient plumes of power plants. The results show that there is very little excess N2O within 100 m of the stack. There is some evidence that N2O is formed in the plumes farther downwind. The magnitude of the effect cannot be quantitatively evaluated with the present data. Based on our results and published studies, we estimate that the global direct emissions of N2O from power plants are about 0.05 Tg/yr out of a total anthropogenic source of about 7 Tg/yr.

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.

Based on field studies of mounds of Australian termites we estimate that on a global scale termites emit about 12 × 1012 g/yr of methane (< 20 tg/yr) and about 4 ×1015 g CO2/yr (< 8 pg/yr). Most of the detailed results are based on studies of the species Coptotermes lacteus. We found that in mid-latitudes the emissions vary seasonally. As much methane is emitted in the summers as in all other seasons combined. The soils a few meters from the mounds consumed methane at an average rate of 40 μg/m2/h. We found no evidence of net emissions of CO and found that H2 is consistently consumed by the mounds and the soils near the mounds. All six species studied produced chloroform. The concentrations of chloroform inside the mounds of C. lacteus were a thousand times greater than ambient levels, but our calculations show that termites are not likely to be a significant global source of chloroform. Finally, we used the results of our study, and others before us, to construct a view of the role of termites in the global carbon cycle.

During the second Amazon Boundary Layer Experiment (ABLE 2B), meteorological observations, chemical measurements, and model simulations are utilized in order to interpret convective cloud draft structure and to analyze its role in transport and vertical distribution of trace gases. One-dimensional photochemical model results suggest that the observed poststorm changes in ozone concentration can be attributed to convective transports rather than photochemical production and the results of a two-dimensional time-dependent cloud model simulation are presented for the May 6, 1987 squall system. The mesoscale convective system exhibited evidence of significant midlevel detrainment in addition to transports to anvil heights. Chemical measurements of O3 and CO obtained in the convective environment are used to predict photochemical production within the troposphere and to corroborate the cloud model results.

Sporadic measurements of atmospheric methane, using gas chromatography, have been taken since the early 1960s. We found 188 measurements between 1962 and 1979 reported in various publications with data in 70 of the 216 months. We have analyzed these measurements to estimate the trends of methane between 1960 and 1980. The data were adjusted for differences of absolute calibration and for latitudinal variations, and analyzed by oridinary regression analysis, weighted regression analysis, and nonparametric methods. Our best estimate of the increasing trend during this period is 13{plus minus}3 ppbv/yr which is similar to, but somewhat less than, the trend of about 17 ppbv/yr over the last decade. {copyright} American Geophysical Union 1989

Rasmussen and Khalil (1981) have shown that the concentration of methane is increasing in the earth's atmosphere. A continuing increase of methane may perturb the global environment in the future by warming the earth and leading to more ozone and carbon monoxide in the atmosphere. It appears that the present concentration of methane may be more than twice as high as the natural levels of 150 years ago. An analysis of air bubbles buried long ago in polar ice makes it possible to deduce the concentrations of methane in the old and ancient atmospheres. The present investigation is concerned with the results of an analysis of more than 80 ice core samples, taking into account both polar regions of the earth. The samples range in age from about 100 to nearly 3000 years old. It is found that the concentration of methane started changing significantly about 150 years ago. These findings suggest that the increase of methane is probably indirectly caused by the rapid increase of human population.

On the basis of 22 months of almost continuous automated, GC/FID measurements of atmospheric CH4 and Cape Meares (45 deg N), it is shown that the concentration of CH4 is increasing at about 2% per yr (+ or - 0.5%/yr). The data also revealed stable seasonal cycles with peak concentrations in October and minimum concentrations in July. The magnitude of the seasonal variations during these months is about + or - 20 ppby from the average (+ or - 1.2%). If the current trend continues, the increased CH4 concentration may result in a 0.2 K to 0.4 K average increase in earth's surface temperature over the next 40 years or so (based on calculations of Wang et al. (1976)). The coupling of CH4 to tropospheric and stratospheric chemical processes is discussed.

Data obtained during the 1985 ABLE expedition to the Amazon are used to describe the diurnal and vertical variations of isoprene. Isoprene is a natural hydrocarbon emitted by many species of trees, particularly those in tropical forests. The concentrations of isoprene at lower levels in the atmosphere undergo large diurnal variations, with the highest concentrations during midday and the lowest during the night. At ground level, outside the forest, peak concentrations of about 3-parts per billion by volume (ppbv) of isoprene were observed around midday. Concentrations were nearly zero before sunrise, increased to their maximum values during the day, and declined after sunset. Concentrations of 1-2 ppbv of isoprene were observed up to 300 m. Near the canopy level, up to 8 ppbv of isoprene were observed. In the forest, concentrations are generally quite low below the canopy and are highest at the level of the canopy. Since the reaction of isoprene with OH radicals is extremely fast, its concentrations fall off rapidly with altitude, so that practically none of it was seen above the boundary layer. During nighttime, however, concentrations comparable to daytime values were observed at altitudes of 300 m and above.

The tropical forests of the world are hypothesized to be an important source and/or sink for a number of atmospheric gas and aerosol species. As part of the National Aeronautics and Space Administration's Global Tropospheric Experiment, an aircraft flight (June 27, 1984) was conducted over the wet tropical forest of Guyana for purposes of characterizing the forest boundary layer. Instrumentation on board the NASA Electra aircraft included measurement systems for a variety of chemical species, including ozone, carbon monoxide, dimethylsulfide, nonmethane hydrocarbons, and aerosols. The data reported represent the first comprehensive characterization of the wet tropical forest boundary layer from an aircraft platform. The data indicate that the boundary layer is a source of carbon monoxide, isoprene, and dimethylsulfide as well as being a sink for ozone. Reported aerosol data, including number density, mass concentration, and composition, indicate an increased aerosol presence in the boundary layer relative to the overriding troposphere. From an analysis of the aerosol composition this increase is the result of direct emissions from the forest as well as from various photochemical processes. The data are discussed in terms of their significance in the understanding of the wet tropical forest boundary layer as a source or sink for various species important to tropospheric chemistry.

Careful measurements by GC/MS techniques on atmospheric carbon tetrafluoride confirm that its concentration in 1979 was constant at about 69.9 pptv (standard deviation is 7.2 pptv). It is almost certainly produced entirely by emissions from the aluminum industry. Other compounds in the atmosphere containing the CF/sub 3/ group have been identified by GC/MS as C/sub 2/F/sub 6/, CF/sub 3/CL, CF/sub 3/Br,and C/sub 2/F/sub 5/CL. The concentrations of the first pair are about 4 pptv, and the latter two are about 0.7 pptv and 4 pptv, respectively. The C/sub 2/F/sub 6/ is produced simultaneously with CF/sub 4/ in the manufacture of aluminum. CF/sub 3/Cl, CF/sub 3/Br, and C/sub 2/F/sub 5/Cl are probably produced by the chemical industry. The environmental importance of these compounds is discussed in the context of two papers on CF/sub 4/ recently published by Cicerone (1979) and by Wang et al. (1980).

It is found that atmospheric concentrations of CH3I are not greater than about 3 pptv over most of the earth's surface and drop to less than half this value above the boundary layer, suggesting that the compound is not likely to play such important roles, on a global scale, as the destruction of tropospheric O3 and free radicals and the increasing of the NO2/NO ratio and hydroxyl radical densities. It is hypothesized that a large portion of the global CH3I comes from oceanic regions of high biomass productivity, where the compound may play a key role in local atmospheric chemistry. Attention is given to CH3I measurement uncertainties with regard to global distribution, sources, and sinks.