Net Emission Rates Research Articles

Assessment of emissions and potential occupational exposure to carbon monoxide during biowaste composting.

To date, only a few studies focused on the carbon monoxide (CO) production during waste composting; all targeted on CO inside piles. Here, the CO net emissions from compost piles and the assessment of worker's occupational risk of exposure to CO at large-scale composting plants are shown for the first time. CO net emissions were measured at two plants processing green waste, sewage sludge, or undersize fraction of municipal solid waste. Effects of the location of piles (hermetised hall vs. open yard) and turning (before vs. after) were studied. Higher CO net emission rates were observed from piles located in a closed hall. The average CO flux before turning was 23.25 and 0.60 mg‧m-2‧h-1 for hermetised and open piles, respectively, while after- 69.38 and 5.11 mg‧m-2‧h-1. The maximum CO net emissions occurred after the compost was turned (1.7x to 13.7x higher than before turning). The top sections of hermetised piles had greater CO emissions compared to sides. Additionally, 5% of measurement points of hermetised piles switched to 'CO sinks'. The 1-h concentration in hermetised composting hall can reach max. ~50 mg CO∙m-3 before turning, and >115 mg CO∙m-3 after, exceeding the WHO thresholds for a 1-h and 15-min exposures, respectively.

Soil VOC emissions of a Mediterranean woodland are sensitive to shrub invasion.

Many belowground processes, such as soil respiration and soil-atmosphere VOC (volatile organic compounds) exchange, are closely linked to soil microbiological processes. However, little is known about how changes in plant species cover, i.e. after plant invasion, alter these soil processes. In particular, the response of soil VOC emissions to plant invasion is not well understood. We analysed soil VOC emissions and soil respiration of a Mediterranean cork oak (Quercus suber) ecosystem, comparing soil VOC emissions from a non-invaded Q. suber woodland to one invaded by the shrub Cistus ladanifer. Soil VOC emissions were determined under controlled conditions using online proton-transfer time-of-flight mass spectrometry. Net soil VOC emissions were measured by exposing soils with or without litter to different temperature and soil moisture conditions. Soil VOC emissions were sensitive to C. ladanifer invasion. Highest net emission rates were determined for oxygenated VOC (acetaldehyde, acetone, methanol, acetic acid), and high temperatures enhanced total VOC emissions. Invasion affected the relative contribution of various VOC. Methanol and acetaldehyde were emitted exclusively from litter and were associated with the non-invaded sites. In contrast, acetone emissions increased in response to shrub presence. Interestingly, low soil moisture enhanced the effect of shrub invasion on VOC emissions. Our results indicate that shrub invasion substantially influences important belowground processes in cork oak ecosystems, in particular soil VOC emissions. High soil moisture is suggested to diminish the invasion effect through a moisture-induced increase in microbial decomposition rates of soil VOC.

Comprehensive analysis of a novel integration of a biomass-driven combined heat and power plant with a compressed air energy storage (CAES)

Unlike wind and solar energy, biomass is capable of providing stable and controllable energy output in an energy system at zero or even negative net emission rate. The combination of a gasification unit with a Brayton cycle is one of the traditional methods of utilizing biomass resources. This work presents a thorough (Energy, Exergy, Economic, Exergoeconomic, and Environmental) analysis of a biomass-driven combined heat and power (CHP), via a gasifier, with a compressed air energy storage (CAES) unit. The hybrid system is capable of multi-generating heat and power as well as contributing to load shifting and peak shaving for the electricity grid. With about 71% and 47% of total and electrical round-trip efficiencies, the proposed system demonstrates 67% and 12% efficiency improvement in comparison to the stand-alone biomass power plant. 348.4 MWh is the net produced power each day and 50% of this amount is generated during the on-peak period. Also, 6651.4 m3 domestic hot water is produced during the 24-hour operation of the system. The Levelized Cost of Electricity (LCOE) of the system is around 0.05 $/kWh with an average electricity price of 0.1 $/kWh elaborating the economic potentials of the system. The payback period and total profit of around 2 years and 181 M$ further prove the economic feasibility of the proposed system. Moreover, due to obtained results from the environmental analysis, it is proved that the introduced system is capable of capturing 25,764 tonnes/year CO2 emission from the atmosphere.

Soil moisture determines nitrous oxide emission and uptake

Soils are major sources and sinks of nitrous oxide (N2O). The main pathway of N2O emission is performed through soil denitrification; however, the uptake phenomenon in denitrification is overlooked, leading to an underestimation of N2O production. Soil moisture strongly influences denitrification rates, but exact quantifications coupled with nosZ, nirK, and nirS gene analysis remain inadequately unaccounted for. In this study, a 15N-N2O pool dilution (15N2OPD) method was used to measure N2O production rates under different soil moisture levels. Therefore, 20%, 40%, 60%, 80% and 100% soil water holding capacity (WHC) were used. The results revealed that N2O uptake rates increased proportionally with soil moisture content and peaked at 80% WHC with 4.17 ± 2.74 μg N kg−1 soil h−1. The N2O production and net emission rates similarly peaked at 80% WHC, reading at 32.50 ± 4.86 and 27.63 ± 3.09 μg N kg−1 soil h−1 during the incubation period (18 days). Soil moisture content increased the gene copy number of the nosZ, NH4+ content, and denitrification potential in soil. N2O uptake at WHC 80–100% was significantly greater than that at WHC 20–60%. It was attributed to a decrease in O2 and the high NO3− concentration inhibition (> 50 mg N kg−1 of soil NO3−-N content). Principal components analysis (PCA) indicated that the number of nosZ genes was the major driver of N2O uptake, especially nosZ clade II. Thus, the results of this study deepen our understanding of the mechanisms underpinning N2O sources and sinks in soils and provide a useful gene-based indicator to estimate N2O uptake.

Methane emission via sediment and water interface in the Bohai Sea, China

Sediment is recognized as the largest reservoir and source of methane (CH4) in the ocean, especially in the shallow coastal areas. To date, few data of CH4 concentration in sediment have been reported in the China shelf seas. In this study, we measured CH4 concentration in sediment and overlying seawater columns, and conducted an incubation experiment in the Bohai Sea in May 2017. CH4 concentration was found to be ranged from 3.075 to 1.795 μmol/L in sediment, which was 2 to 3 orders of magnitude higher than that in overlying seawater columns. The surface sediment was an important source of CH4, while bottom seawater acted as its sink. Furthermore, the net emission rate via sediment water interface (SWI) was calculated as 2.45 μmol/(m2∙day) based on the incubation experiment at station 73, and the earthquake may enhance CH4 release from sediment to seawater column in the eastern Bohai Sea.

Permafrost thaw driven changes in hydrology and vegetation cover increase trace gas emissions and climate forcing in Stordalen Mire from 1970 to 2014.

Permafrost thaw increases active layer thickness, changes landscape hydrology and influences vegetation species composition. These changes alter belowground microbial and geochemical processes, affecting production, consumption and net emission rates of climate forcing trace gases. Net carbon dioxide (CO2) and methane (CH4) fluxes determine the radiative forcing contribution from these climate-sensitive ecosystems. Permafrost peatlands may be a mosaic of dry frozen hummocks, semi-thawed or perched sphagnum dominated areas, wet permafrost-free sedge dominated sites and open water ponds. We revisited estimates of climate forcing made for 1970 and 2000 for Stordalen Mire in northern Sweden and found the trend of increasing forcing continued into 2014. The Mire continued to transition from dry permafrost to sedge and open water areas, increasing by 100% and 35%, respectively, over the 45-year period, causing the net radiative forcing of Stordalen Mire to shift from negative to positive. This trend is driven by transitioning vegetation community composition, improved estimates of annual CO2 and CH4 exchange and a 22% increase in the IPCC's 100-year global warming potential (GWP_100) value for CH4. These results indicate that discontinuous permafrost ecosystems, while still remaining a net overall sink of C, can become a positive feedback to climate change on decadal timescales.This article is part of a discussion meeting issue ‘Rising methane: is warming feeding warming? (part 2)’.

Intake Fractions for Volatile Organic Compounds in Two Occupied California Residences

Experimental estimates of residential intake fractions for indoor volatile organic compound (VOC) releases are scarce. We evaluated individual intake fractions (iFᵢ, mass inhaled by an individual per unit mass emitted) using approximately five months of time-resolved VOC measurements acquired at two residences. First, we directly estimated iFᵢ using inert tracer gases that were released at fixed rates. Tracer gas iFᵢ values were generally consistent between occupants and comparable across seasons. Furthermore, iFᵢ for sources released on different floors of a residence were statistically indistinguishable, suggesting that source location within the living space was not strongly influential. Emissions from living space sources (iFᵢ ∼ 0.3% = 3000 ppm) contributed to occupant exposures at rates 2–4 times higher than crawl space sources (iFᵢ ∼ 1000 ppm) and greater than 40 times higher than attic sources (iFᵢ < ∼70 ppm). Second, we indirectly estimated iFᵢ for 251 VOCs using net emission rates estimated by indoor–outdoor material balance. Although emission patterns varied between compounds, all VOC-specific iFᵢ estimates were clustered near the values of the living space tracer gases. These experimental observations substantiate the theoretical expectation that iFᵢ values are largely independent of analyte characteristics, a useful simplification for exposure assessments.

Carbon dioxide and methane fluxes at the air–sea interface of Red Sea mangroves

Abstract. Mangrove forests are highly productive tropical and subtropical coastal systems that provide a variety of ecosystem services, including the sequestration of carbon. While mangroves are reported to be the most intense carbon sinks among all forests, they can also support large emissions of greenhouse gases (GHGs), such as carbon dioxide (CO2) and methane (CH4), to the atmosphere. However, data derived from arid mangrove systems like the Red Sea are lacking. Here, we report net emission rates of CO2 and CH4 from mangroves along the eastern coast of the Red Sea and assess the relative role of these two gases in supporting total GHG emissions to the atmosphere. Diel CO2 and CH4 emission rates ranged from −3452 to 7500 µmol CO2 m−2 d−1 and from 0.9 to 13.3 µmol CH4 m−2 d−1 respectively. The rates reported here fall within previously reported ranges for both CO2 and CH4, but maximum CO2 and CH4 flux rates in the Red Sea are 10- to 100-fold below those previously reported for mangroves elsewhere. Based on the isotopic composition of the CO2 and CH4 produced, we identified potential origins of the organic matter that support GHG emissions. In all but one mangrove stand, GHG emissions appear to be supported by organic matter from mixed sources, potentially reducing CO2 fluxes and instead enhancing CH4 production, a finding that highlights the importance of determining the origin of organic matter in GHG emissions. Methane was the main source of CO2 equivalents despite the comparatively low emission rates in most of the sampled mangroves and therefore deserves careful monitoring in this region. By further resolving GHG fluxes in arid mangroves, we will better ascertain the role of these forests in global carbon budgets.

Indoor and outdoor particulate matter in primary school classrooms with fan-assisted natural ventilation in Singapore.

We conducted multiday continuous monitoring of indoor and outdoor particulate matter (PM) in classrooms with fan-assisted natural ventilation (NV) at five primary schools in Singapore. We monitored size-resolved number concentration of PM with diameter 0.3-10μm at all schools and alveolar deposited surface area concentrations of PM with diameter 0.01-1.0μm (SA0.01-1.0) at two schools. Results show that, during the monitoring period, schools closer to expressways and in the downtown area had 2-3 times higher outdoor PM0.3-1.0 number concentrations than schools located in suburban areas. Average indoor SA0.01-1.0 was 115-118μm(2)cm(-3) during periods of occupancy and 72-87μm(2)cm(-3) during unoccupied periods. There were close indoor and outdoor correlations for fine PM during both occupied and unoccupied periods (Pearson's r = 0.84-1.0) while the correlations for coarse PM were weak during the occupied periods (r = 0.13-0.74). Across all the schools, the size-resolved indoor/outdoor PM ratios (I/O ratios) were 0.81 to 1.58 and 0.61 to 0.95 during occupied and unoccupied periods, respectively, and average infiltration factors were 0.64 to 0.94. Average PM net emission rates, calculated during periods of occupancy in the classrooms, were lower than or in the lower range of emission rates reported in the literature. This study also reveals that indoor fine and submicron PM predominantly come from outdoor sources, while indoor sources associated with occupancy may be important for coarse PM even when the classrooms have high air exchange rates.

Carbon Dioxide and Methane Emissions from a Wet-Dry Tropical Floodplain in Northern Australia

In order to better understand how global climate change will affect greenhouse gas dynamics in wetland systems, an accurate quantification of global greenhouse gas emissions from these areas is essential. Despite a large proportion of wetlands occurring in tropical areas, data on greenhouse gas fluxes from these areas is limited. This study aimed to quantify carbon dioxide (CO2) and methane (CH4) fluxes from an undisturbed tropical wetland environment in northern Australia, and to evaluate the influence of different habitat types on net emission rates. Fluxes were measured at seven sites, representing different habitat types during the inundation season of 2012. Highest CO2 fluxes, with a maximum at 199.4 mg CO2-C m−2 h−1, were measured in open water areas. This likely corresponded to increased mobilisation of sediment organic matter and high water turbulence, as inferred from turbidity measurement. CH4 fluxes, however, were greatest in densely vegetated areas and peaked at 153.2 mg CH4-C m−2 h−1, a possible result of increased transport through plant stems. Carbon dioxide and methane fluxes over the whole wetland averaged 86.0 mg CO2-C m−2 h−1 and 25.3 mg CH4-C m−2 h−1, respectively.

Halogen biogeochemistry of invasive perennial pepperweed (Lepidium latifolium) in a peatland pasture

[1] Perennial pepperweed (Lepidium latifolium) is a widespread invasive plant in North America. This yearlong field study at a pasture peatland infested with L. latifolium demonstrates that these plants are large emitters, on a per area basis, of methyl chloride (CH3Cl) and methyl bromide (CH3Br), compounds that contribute to the destruction of stratospheric ozone. Annually averaged net emission rates were 9.0 ± 11.8 µmol m−2 d−1 for CH3Cl and 460 ± 430 nmol m−2 d−1 for CH3Br, comparable to observed coastal salt marsh emission rates. A stable isotope tracer technique was used to distinguish between simultaneous production and consumption processes for CH3Cl and CH3Br. Over the course of the year, gross production rates for methyl halides varied widely over different life cycle stages, with the highest fluxes surprisingly occurring at senescence. Maximum emissions of CH3Cl and CH3Br occurred at midday, the time of highest solar radiation. During the growing season, methyl halide gross production rates were positively correlated with temperature and live biomass, suggesting that the production of methyl halides from L. latifolium at this time was mostly biotic. During plant senescence, the peak emissions of CH3Cl and CH3Br are unexplained but may be released from L. latifolium associated with transitions in biochemistry. Overall, these flux measurements show expected diel and growing season trends based on plant production for most of the year, overlaid by a high emission peak during senescence, suggesting that both plant biochemical shifts and varying environmental factors need to be considered as important internal and external controls on emissions.

Pollution monitoring by dosimetry and passive diffusion sampling for evaluation of environmental conditions for paintings in microclimate frames

Pollutants and their potential degradation of paintings have been measured for the first time in microclimate frames (mc-frames), which are used to protect paintings. The pollutants that were measured include both inorganic pollutants, which originate mainly from external sources, and organic pollutants from mainly internal sources. Those originating from the outdoors enter rooms and subsequently mc-frames at a rate depending on the ventilation rates (air exchange rates) of the mc-frames. The concentration of gaseous pollutants emitted within the mc-frames will depend on net emission rates of the materials used to make the mc-frames, their design, and their ventilation rates. In the EU PROPAINT project measurements of gaseous air pollutants and climatic conditions were performed at various locations both inside and outside different state-of-the-art mc-frames. Diffusive passive pollution gas samplers were used together with different types of dosimeters. Results show that the dosimeters respond to either the photo-oxidizing conditions or the level of volatile organic acids in the environments both in the museums and within the mc-frames. Two dosimeters, the Early Warning Organic (EWO) made from a synthetic polymer and the Resin Mastic coated Piezo electric Quartz Crystals (RM-PQC) respond to photo-oxidation and showed higher values outside than inside the mc-frames. Two other dosimeters, the Glass Slide Dosimeter (GSD) and the Lead coated Piezo electric Quartz Crystals (L-PQC) respond to volatile organic acids and yielded higher values inside than outside the mc-frames. This study emphasizes the need for further work to determine environmental damage functions for paintings, in particular for the effects of organic acids. Such information is essential for the evaluation of the protective effects of mc-frames for paintings. The use of mc-frames is increasing and it is very important to know that this protective measure does not introduce new risks.

Seasonal soil VOC exchange rates in a Mediterranean holm oak forest and their responses to drought conditions

Available information on soil volatile organic compound (VOC) exchange, emissions and uptake, is very scarce. We here describe the amounts and seasonality of soil VOC exchange during a year in a natural Mediterranean holm oak forest growing in Southern Catalonia. We investigated changes in soil VOC dynamics in drought conditions by decreasing the soil moisture to 30% of ambient conditions by artificially excluding rainfall and water runoff, and predicted the response of VOC exchange to the drought forecasted in the Mediterranean region for the next decades by GCM and ecophysiological models. The annual average of the total (detected) soil VOC and total monoterpene exchange rates were 3.2±3.2 and −0.4±0.3 μg m −2 h −1, respectively, in control plots. These values represent 0.003% of the total C emitted by soil at the study site as CO 2 whereas the annual mean of soil monoterpene exchange represents 0.0004% of total C. Total soil VOC exchange rates in control plots showed seasonal variations following changes in soil moisture and phenology. Maximum values were found in spring (17±8 μg m −2 h −1). Although there was no significant global effect of drought treatment on the total soil VOC exchange rates, annual average of total VOC exchange rates in drought plots resulted in an uptake rate (−0.5±1.8 μg m −2 h −1) instead of positive net emission rates. Larger soil VOC and monoterpene exchanges were measured in drought plots than in control plots in summer, which might be mostly attributable to autotrophic (roots) metabolism. The results show that the diversity and magnitude of monoterpene and VOC soil emissions are low compared with plant emissions, that they are driven by soil moisture, that they represent a very small part of the soil-released carbon and that they may be strongly reduced or even reversed into net uptakes by the predicted decreases of soil water availability in the next decades. In all cases, it seems that VOC fluxes in soil might have greater impact on soil ecology than on atmospheric chemistry.

Hybrid simulation study of ion escape at Titan for different orbital positions

Titan, the largest moon of Saturn, forms a unique plasma interaction with the Kronian corotating plasma. Titan’s dense and nitrogen rich atmosphere is the primary source of the nitrogen torus around Titan’s orbit through the subsonic and super-Alfvénic interaction between the corotating plasma of the Kronian system and the exosphere and ionosphere of Titan. We have studied this magnetic interaction of Titan with a global hybrid simulation model and especially the effect of the orbital position of Titan with four different Saturn local time. The hybrid simulation includes the various drifts important in Titan’s plasma environment. Under the assumption of uniform magnetospheric properties along Titan’s orbit, differences were found in the tail structure while the net emission rates varied only little. For all four studied orbital positions the magnetic field maximum was located on the anti-Saturn side of Titan. The iono- and magnetotails were co-aligned and tilted towards Saturn up to 45°.

Methane fluxes between terrestrial ecosystems and the atmosphere at northern high latitudes during the past century: A retrospective analysis with a process‐based biogeochemistry model

We develop and use a new version of the Terrestrial Ecosystem Model (TEM) to study how rates of methane (CH4) emissions and consumption in high‐latitude soils of the Northern Hemisphere have changed over the past century in response to observed changes in the region's climate. We estimate that the net emissions of CH4 (emissions minus consumption) from these soils have increased by an average 0.08 Tg CH4 yr−1 during the twentieth century. Our estimate of the annual net emission rate at the end of the century for the region is 51 Tg CH4 yr−1. Russia, Canada, and Alaska are the major CH4 regional sources to the atmosphere, responsible for 64%, 11%, and 7% of these net emissions, respectively. Our simulations indicate that large interannual variability in net CH4 emissions occurred over the last century. Our analyses of the responses of net CH4 emissions to the past climate change suggest that future global warming will increase net CH4 emissions from the Pan‐Arctic region. The higher net CH4 emissions may increase atmospheric CH4 concentrations to provide a major positive feedback to the climate system.

Effects of changes in environmental conditions on atmospheric mercury exchange: Comparative analysis from a rice paddy field during the two spring periods of 2001 and 2002

The environmental mobilization characteristics of Hg were investigated in a rice paddy field in the Hari district of Kang Hwa Island, Korea during two field campaigns held in late March 2001 and late April 2002. In the course of these two field campaigns, we measured the concentration, concentration gradients (between two different heights of 1 and 5 m), and fluxes of Hg with the aid of micrometeorological approaches. The results of our experiments consistently indicate that the study area behaved as both source and sink of Hg, yielding a net emission rate of 183 (1st) to 28 ng m−2 h−1 (2nd campaign). It was however found that the soil‐air exchange patterns of Hg during the two study periods, although investigated in similar seasonal cycles, contrast to a large extent. Upward emission prevailed in both the frequency and magnitude during the 1st study period, but dry deposition was seen more dominantly in terms of frequency during the 2nd study period. In light of the fact that the occurrence patterns of Asian Dust events differed greatly between the two study periods, the possible role of long‐range transport of airborne pollutants (LRTAP) as sources of Hg into the study area has also been examined using information on air mass movement patterns. Because observed differences in Hg flux patterns generally reflect changes in environmental conditions across study periods, the bi‐directional exchange patterns of Hg during the two field campaigns were explained in relation to highly diverse environmental conditions that persisted in each study period.

Towards a spatial CO 2 budget of a metropolitan region based on textural image classification and flux measurements

A project on monitoring urban CO 2 budgets has been conducted since the year 2000 focusing on the Metropolitan area of Copenhagen, Denmark. Methodologically, the project combines remote sensing with CO 2 fluxes measured by eddy covariance technique from the top of a 40-m mast located in the center of Copenhagen. These data are supplemented by flux measurements from a mobile system on a 10-m mast which is moved between different urban types, including a major entrance road, and residential and industrial areas. By comparing the time series of vertical CO 2 exchange and the number of cars on the major entrance roads, it is demonstrated that the traffic intensity has a major impact on the urban carbon budget. The spatial distribution of the CO 2 emission rates is examined through texture-based classification of Landsat-TM satellite images. Local traffic intensity and local heating is seen as a function of specific local urban land use and activities, and the corresponding satellite image texture is used as a proxy for the CO 2 emission from these components. The urban scene is divided into urban land use classes that constitute homogenous areas in terms of main types of activity and these are linked to specific levels of CO 2 emission. For this purpose, a multi-scale approach based on co-occurrence matrices has been developed and applied. The paper outlines how the CO 2 exchange from the urban sources and sinks can be estimated from continuous flux measurements in central Copenhagen. It is shown that traffic is the largest single CO 2 source in the city. The mobile measurements demonstrate that the emission rates ranges from less than 0.8 g CO 2 m −2 h −1 in the residential areas up to a maximum of 16 g CO 2 m −2 h −1 along the major entrance roads in the city center. An average annual CO 2 exchange rate of 35 g CO 2 m −2 day −1 is calculated by assigning fluxes to each land use type and excluding the effect of remote sources (power plants, air and sea traffic). This value can be compared to a carbon budget recently calculated from national statistics showing that the local urban sources (road traffic, industry, service and household) have a comparable net emission rate of 38 g CO 2 m −2 day −1. The perspective of having more precise knowledge of the distribution of sources and sinks is finally discussed in relation to changing land use patterns.

CO2 emission from mineral soils following land‐cover change in Brazil

AbstractThis paper presents a first approximation of net CO2 fluxes from mineral soil due to land use, land‐use changes and forestry (LULUCF) activities in Brazil for the periods 1970–90 and 1975–95. The methodology employed is an adaptation of the approach proposed by the IPCC in ‘Revised 1996 guidelines for national greenhouse gas inventories’, which is based on the variations in soil C stocks as a function of changes in land‐use activities. The calculation was done separately for each Brazilian state and subsequently summarized for all of Brazil. The annual fluxes for Brazil indicate a net emission of CO2 to the atmosphere, which decreased from 93.3 Tg CO2 for the period 1970–90 to 46.4 Tg CO2 for the period 1975–95. This corresponded to yearly net emission rates of 11.0 g CO2 m−2 year−1 for the 1970–90 period and 5.5 g CO2 m−2 year−1 for the 1975–95 period. Within each administrative region, considerable differences in the yearly emission rates between the states could be observed. Several sources of uncertainties could be identified. The most important uncertainties were linked to the impact factor values, which represent changes in native C stock associated with conversion of the native vegetation to agricultural use.

Net methane emissions from grazing sheep

SummaryMethane emissions from ruminant livestock are responsible for 45 % of New Zealand’s combined CO2‐equivalent greenhouse gas inventory, and arise principally from sheep. Using a flock of 6‐month old sheep (20 ha–1) grazing abundant pasture, we compare micrometeorological measurements of net methane emission rates with measurements from individual sheep based on a sulphur‐hexafluoride tracer technique. Individual sheep emission rates were highly variable and averaged 19.5 ± 4.8 (SD) g CH4 sheep–1 d–1, or 39 ± 9.6 mg CH4 m–2 d–1 on an areal basis. Emission rates were poorly correlated with animal live weight or dry matter intake but represented an average dietary energy loss of 3.6%. Methane fluxes from the surface were determined as half hourly averages by a flux‐gradient technique using temperature and methane gradients. Soil methane consumption was measured using chambers and found to be negligible (&lt; 0.09 mg CH4 m–2 d–1) in comparison with the animal contribution. Daily net emission rates averaged 46 mg m–2 d–1 and exhibited a broad peak in the early afternoon which corresponded with animal activity patterns. On average, net emisssion rates were 40% higher during the day than at night. Stable nocturnal conditions led to a separation of the micrometeorological measurements from the methane source and hence highly variable results. Based on two corroborating techniques, the average net methane emission rate was c. 43 mg CH4 m–2 d–1 or 155 kg CH4 ha–1 y–1.

Simulation of sputter deposition at high pressures

Sputtering, although often optimized by trial and error, increasingly needs sound modeling techniques because of the high capital investment. Usually the simulation problem is broken down into three stages: emission at the target, transport through the host gas, and deposition at the substrate. It turns out that these stages are necessarily linked and must be properly decoupled. Here we are mainly interested in the transport through the vapor phase, which may be simulated by our Monte Carlo model, SIMSPUD. However, at high operating pressures, significant deposition takes place onto the target. To modify our Monte Carlo routine, rather than emitting particles in proportion to the erosion profile of the target and waiting for some of these to completely escape, all particles which land on the target are re-emitted until they ultimately escape to the substrate or chamber walls. This modification makes the net emission rate equal to the target erosion rate. At sufficiently high pressures, the Monte Carlo routine becomes very slow. In fact, the problem then becomes a diffusion problem best handled by numerical solution of the Laplace equation. At any rate, it is shown that when SIMSPUD has been modified in this way, good agreement with experiment is obtained over a wide range of working pressures.