CH4 Source Strength Research Articles

Stem CH4 and N2O fluxes of Fraxinus excelsior and Populus alba trees along a flooding gradient

Methane (CH4) and nitrous oxide (N2O) fluxes from tree stems are still poorly quantified in temperate floodplain forests. Methane and N2O fluxes were repeatedly measured at 0.3, 1.6 and 3.6 m stem height at three sites along a landscape gradient ranging from non-flooded to frequently flooded forest sites. The non-flooded forest was dominated by Fraxinus excelsior and the infrequently and frequently flooded sites by Populus alba. Stem surfaces were net CH4 and N2O sources at all sites. The CH4 source strength increased towards the wetter sites (non-flooded 2.51±12.71, infrequently-flooded 5.2±17.26, and frequently-flooded 11.15±24.04 μg-C-m−2 h−1), but flooding had no immanent effects on CH4 and N2O fluxes. Methane fluxes from poplar stems were highest at the stem base (0.3 m) and decreased with increasing measurement height. Methane fluxes from ash stems were lowest at the stem base and gradually increased until 3.6 m height. Nitrous oxide fluxes were low and did not show clear spatial patterns. The presence of mosses had no significant effects on CH4 and N2O fluxes. Stem fluxes were small when compared to the corresponding soil fluxes at the non-flooded and infrequently flooded site, but significantly reduced the soil CH4 sink capacity at the frequently-flooded site. Methane flux strongly varied between 0.3 and 3.6 m stem height and showed distinctive tree species specific patterns. Our results therefore suggest that measuring at more than a single location near the stem base is inevitable to obtain any reliable CH4 or N2O flux estimate of a whole tree stem.

Soil Is a Net Source of Methane in Tropical African Forests

Research Highlights: Monitoring of soil CH4 fluxes in African tropical forest conducted run for almost two years, contributing to the scant information on greenhouse gas (GHG) fluxes from forests available from this region. Data showed that the forest soil acted as a net yearly source of CH4. Hotspots of CH4 emissions were measured both in upland and lowland areas of the forest, and on an annual basis they overcame the soil CH4 sink during drier periods or in well-drained areas. Background and Objectives: Atmospheric studies indicate that tropics are a strong CH4 source. Regional budgets attribute the majority of this source to wetland ecosystems and flooded lowland forests, whereas un-flooded forests are considered net CH4 sinks, although few studies in tropical forests, in particular in Africa, are available. The present work aims to contribute to this knowledge gap. Materials and Methods: Monitoring campaigns were conducted along the year in the tropical forest of the Ankasa National Park, Ghana, in two contrasting environments, uphill and downhill, using close static chambers coupled with gas chromatography. Results: The uphill area was a net weak CH4 sink with mean daily fluxes ranging from −1.29 to 0.44 mg CH4 m−2 d−1. The downhill area was a significant CH4 source with mean daily fluxes ranging from −0.67 to 188.09 mg CH4 m−2 d−1 and with peaks up to 1312 mg CH4 m−2 d−1 in the wet season. Conclusions: The net annual soil CH4 budget for the Ankasa Park, normalizing the proportion of downhill areas over the whole park surface, was a source of about 3.3 kg CH4 ha−1 yr−1. Overlooking such areas might lead to underestimates of the total CH4 source strength of forested areas.

Multiple factors dominate the distribution of methane and its sea-to-air flux in the Bohai Sea in summer and autumn of 2014

The Bohai Sea is well-known as a source of atmospheric methane (CH4). However, the main regulate factors of the spatiotemporal distribution of CH4 and its sea-to-air flux remain largely unknown. In this study, the observed CH4 concentration ranged from 4.8 to 32.7 nmol/L and 3.1 to 15.2 nmol/L in August and November of 2014, respectively. The main factors that influence the distribution of CH4 and its sea-to-air flux were stratification, solubility, and current structure for the mid-west depression basins, the permanent well-mixed seawater column and CH4 source strength for the centre shallow ridge zone, and the upwelling for the east depression basin, respectively. Meanwhile, wind also plays an important role in sea-to-air CH4 flux in the study area except the centre shallow ridge zone. Upwelling made the east depression basin the most intensive source of CH4, with a flux of 2 to 4 times higher than the other sub-regions.

Assessing diel variation of CH4 flux from rice paddies through temperature patterns

The diel variation in methane (CH4) flux from irrigated rice was characterized during the dry and wet cropping seasons in 2013 and 2014 using the eddy covariance (EC) technique. The EC technique has the advantage of obtaining measurements of fluxes at an extremely high temporal resolution (10Hz), meaning it records 36,000 measurements per hour. The EC measurements can very well capture the temporal variations of the diel (both diurnal and nocturnal) fluxes of CH4 and the environmental factors (temperature, surface energy flux, and gross ecosystem photosynthesis) at 30-min intervals. The information generated by this technique is important to enhance our mechanistic understanding of the different factors affecting the landscape scale diel CH4 flux. Distinct diel patterns of CH4 flux were observed when the data were partitioned into different cropping periods (pre-planting, growth, and fallow). The temporal variations of the diel CH4 flux during the dry seasons were more pronounced than during the wet seasons because the latter had so much climatic disturbance from heavy monsoon rains and occasional typhoons. Pearson correlation analysis and Granger causality test were used to confirm if the environmental factors evaluated were not only correlated with but also Granger-causing the diel CH4 flux. Soil temperature at 2.5 cm depth (Ts 2.5 cm) can be used as simple proxy for predicting diel variations of CH4 fluxes in rice paddies using simple linear regression during both the dry and wet seasons. This simple site-specific temperature response function can be used for gap-filling CH4 flux data for improving the estimates of CH4 source strength from irrigated rice production.

Winter precipitation and snow accumulation drive the methane sink or source strength of Arctic tussock tundra.

Arctic winter precipitation is projected to increase with global warming, but some areas will experience decreases in snow accumulation. Although Arctic CH4 emissions may represent a significant climate forcing feedback, long-term impacts of changes in snow accumulation on CH4 fluxes remain uncertain. We measured ecosystem CH4 fluxes and soil CH4 and CO2 concentrations and (13) C composition to investigate the metabolic pathways and transport mechanisms driving moist acidic tundra CH4 flux over the growing season (Jun-Aug) after 18years of experimental snow depth increases and decreases. Deeper snow increased soil wetness and warming, reducing soil %O2 levels and increasing thaw depth. Soil moisture, through changes in soil %O2 saturation, determined predominance of methanotrophy or methanogenesis, with soil temperature regulating the ecosystem CH4 sink or source strength. Reduced snow (RS) increased the fraction of oxidized CH4 (Fox) by 75-120% compared to Ambient, switching the system from a small source to a net CH4 sink (21±2 and -31±1mgCH4 m(-2) season(-1) at Ambient and RS). Deeper snow reduced Fox by 35-40% and 90-100% in medium- (MS) and high- (HS) snow additions relative to Ambient, contributing to increasing the CH4 source strength of moist acidic tundra (464±15 and 3561±97mgCH4 m(-2) season(-1) at MS and HS). Decreases in Fox with deeper snow were partly due to increases in plant-mediated CH4 transport associated with the expansion of tall graminoids. Deeper snow enhanced CH4 production within newly thawed soils, responding mainly to soil warming rather than to increases in acetate fermentation expected from thaw-induced increases in SOC availability. Our results suggest that increased winter precipitation will increase the CH4 source strength of Arctic tundra, but the resulting positive feedback on climate change will depend on the balance between areas with more or less snow accumulation than they are currently facing.

Reduction of the temperature sensitivity of minerotrophic fen methane emissions by simulated glacial atmospheric carbon dioxide starvation

AbstractVariations to the global wetland CH4 source strength in response to changes in orbital insolation patterns and atmospheric CO2 concentration ([CO2]a) are hypothesized to play an important role in determining glacial‐interglacial variations in atmospheric CH4 concentration ([CH4]a). Here the interactive effects of temperature, a major controlling variable determining wetland CH4 flux, and the low [CO2]a of glacial intervals are investigated for the first time. We measured the temperature dependence of CH4 emissions from replicated mesocosms (n = 8 per CO2 treatment) collected from a minerotrophic fen and an ombrotrophic bog incubated in either ambient (c. 400 ppm) or glacial (c. 200 ppm) [CO2]a located in the United Kingdom. CH4 fluxes were measured at 5°C, 10°C, 15°C, 20°C, and 25°C and then in reverse order over a 20 day period under each [CO2]a treatment. Results showed that the Q10 temperature response of CH4 emissions from the Carex/Juncus‐dominated fen declined significantly by approximately 39% under glacial [CO2]a (ambient [CO2]a = 2.60, glacial [CO2]a = 1.60; P < 0.01). By contrast, the response of CH4 emissions from the Sphagnum‐dominated bog remained unaltered (ambient [CO2]a = 3.67, glacial [CO2]a = 3.67; P > 0.05). This contrasting response may be linked to differences in plant species assemblage and the varying impact of CO2 starvation on plant productivity and carbon availability in the rhizosphere. Furthermore, our results provide empirical evidence to support recent model‐based indications that glacial‐interglacial variations in [CH4]a may be explained by changes in wetland CH4 source strength in response to orbitally forced changes in climate and [CO2]a.

Large methane emission upon spring thaw from natural wetlands in the northern permafrost region

The permafrost carbon–climate feedback is one of the major mechanisms in controlling the climate–ecosystem interactions in northern high latitudes. Of this feedback, methane (CH4) emission from natural wetlands is critically important due to its high warming potential. The freeze–thaw transition has been confirmed to play an important role in annual CH4 budget, yet the magnitude of this effect is uncertain. An intensive field campaign was carried out in the Sanjiang Plain, Northeast China to estimate the CH4 emission in the spring freeze–thaw transition period. The observation concluded that a large CH4 source was caused by spring thaw; the maximum hourly emission rate was 48.6 g C m−2 h−1, more than three orders of the regularly observed CH4 emission rate in the growing season. In some sporadically observed ‘hot spots’, the spring thawing effect contributed to a large CH4 source of 31.3± 10.1 g C m −2, which is approximately 80% of the previously calculated annual CH4 emission in the same study area. If our results are typical for natural wetlands in the Northern Hemisphere permafrost region, we estimate a global CH4 source strength of 0.5–1.0 Tg C (1 Tg =1012 g) caused by spring thaw in the Northern Hemisphere permafrost region in the year 2011. Combining with available satellite and flask data, a regional extrapolation reaches a temporal pattern of CH4 emission during 2003–2009 which is consistent with recently observed changes in atmospheric CH4 concentration in the high latitudes. This suggests that the CH4 emission upon spring thaw in the high latitudes might be enhanced by the projected climate warming. These findings indicate that the spring thawing effect is an important mechanism in the permafrost carbon–climate feedback and needs to be incorporated in Earth system models.

Methane source strength and energy flow shape methanotrophic communities in oxygen–methane counter‐gradients

The role of microbial diversity for ecosystem functioning has become an important subject in microbial ecology. Recent work indicates that microbial communities and microbial processes can be very sensitive to anthropogenic disturbances. However, to what extent microbial communities may change upon, resist to, or overcome disturbances might differ depending on substrate availability. We used soil from an Italian rice field in gradient microcosms, and analysed the response of methanotrophic communities to an NH4 (+) pulse as a potential disturbance under two different CH4 source strengths. We found a significant influence of source strength, i.e. the energy flow through the methanotrophic community, while NH4 (+) had no effect. Our data suggest that historical contingencies, i.e. nitrogen fertilization, led to an ammonium-tolerant MOB community. Methanotrophs were able to oxidize virtually all CH4 diffusing into the oxic-anoxic boundary layer regardless of NH4 (+) addition. Total and active methanotrophic communities were assessed by a pmoA-specific microarray. From the reservoir of dormant methanotrophs, different species became active with Methylobacter and an environmental cluster affiliated with paddy soils being indicative for high CH4 source strength. Thus, a microbial seed bank is an important prerequisite to maintain functioning in a fluctuating environment.

MAMAP – a new spectrometer system for column-averaged methane and carbon dioxide observations from aircraft: instrument description and performance analysis

Abstract. Carbon dioxide (CO2) and Methane (CH4) are the two most important anthropogenic greenhouse gases. CH4 is furthermore one of the most potent present and future contributors to global warming because of its large global warming potential (GWP). Our knowledge of CH4 and CO2 source strengths is based primarily on bottom-up scaling of sparse in-situ local point measurements of emissions and up-scaling of emission factor estimates or top-down modeling incorporating data from surface networks and more recently also by incorporating data from low spatial resolution satellite observations for CH4. There is a need to measure and retrieve the dry columns of CO2 and CH4 having high spatial resolution and spatial coverage. In order to fill this gap a new passive airborne 2-channel grating spectrometer instrument for remote sensing of small scale and mesoscale column-averaged CH4 and CO2 observations has been developed. This Methane Airborne MAPper (MAMAP) instrument measures reflected and scattered solar radiation in the short wave infrared (SWIR) and near-infrared (NIR) parts of the electro-magnetic spectrum at moderate spectral resolution. The SWIR channel yields measurements of atmospheric absorption bands of CH4 and CO2 in the spectral range between 1.59 and 1.69 μm at a spectral resolution of 0.82 nm. The NIR channel around 0.76 μm measures the atmospheric O2-A-band absorption with a resolution of 0.46 nm. MAMAP has been designed for flexible operation aboard a variety of airborne platforms. The instrument design and the performance of the SWIR channel, together with some results from on-ground and in-flight engineering tests are presented. The SWIR channel performance has been analyzed using a retrieval algorithm applied to the nadir measured spectra. Dry air column-averaged mole fractions are obtained from SWIR data only by dividing the retrieved CH4 columns by the simultaneously retrieved CO2 columns for dry air column CH4 (XCH4) and vice versa for dry air column CO2 (XCO2). The signal-to-noise ratio (SNR) of the SWIR channel is approximately 1000 for integration times (tint) in the range of 0.6–0.8 s for scenes with surface spectral reflectances (SSR)/albedo of around 0.18. At these integration times the ground scene size is about 23 × 33 m2 for an aircraft altitude of 1 km and a ground speed of 200 km/h. For these scenes the actual XCH4 or XCO2 dry air column retrieval precisions are typically about 1% (1 σ). Elevated levels of CH4 have been retrieved above a CH4 emitting landfill. Similarly the plume of CO2 from coal-fired power plants can be well detected and tracked. The measurements by the MAMAP sensor could enable estimates of anthropogenic, biogenic and geological emissions of localized intense CH4 and CO2 sources such as anthropogenic fugitive CH4 emissions from oil and gas industry, coal mining, disposal of organic waste, CO2 emissions from coal-fired power plants, steel production or geologic CH4 and CO2 emissions from seepage and volcanoes. Appropriate analysis of the measurements of MAMAP potentially also yields natural CH4 emissions from less intense but extensive sources such as wetlands.

Methane and carbon dioxide emissions and nitrogen turnover during liquid manure storage

Animal slurry stored in-house and outside is a significant source of atmospheric methane (CH4). The CH4 source strength of stored slurry is greatly affected by temperature. To improve emission calculations on a global scale there is a need for knowledge about the relationship between production of CH4 in slurry and temperature. In this study, the filling of slurry channels was reproduced in the laboratory by gradually filling 1 m-high PVC vessels during 9 days followed by incubation for 100–200 days. A preliminary test showed that little CH4 was produced from animal slurry during 10 days of incubation at 20°C, if no inoculum (slurry incubated anaerobically at the test temperature for 1.5–2 months) was present. However, the addition of 7.6% inoculum supported an immediate production of CH4. Vessels amended with inoculum and gradually filled with cattle or pig slurry were then incubated at 10, 15 and 20°C. Methane production from stored pig and cattle slurry was not significant at temperatures below 15°C, where CO2 was the main product of decomposition processes. In contrast, the anaerobic production of CH4 was high and significant relative to the production of CO2 at 20°C. Peak emissions of CH4 averaging 0.012 and 0.02 g C h−1 kg−1 volatile solids (VS) were reached within about 10 days at 10 and 15°C, respectively. At 20°C, the emission of CH4 from pig slurry was about 0.01 g C h−1 kg−1 for 10 days, and thereafter emissions increased to about 0.10 g C h−1 kg−1 VS. For cattle slurry a peak emission of 0.08 g C h−1 kg−1 VS was measured after 180 days. Degradation of organic nitrogen (N) in cattle slurry was related to the reduction of organic material as reflected in CO2 and CH4 emission. The mineralization of organic N during storage represented 10–80% of organic N in cattle slurry, and 40–80% of the organic N in pig slurry.

Methane Budget of a Black Forest Spruce Ecosystem Considering Soil Pattern

Study included seven soils, an adjacent spring and brook and was conducted to estimate CH4 source and sink strengths of forest soils along a wetness gradient, i.e. their exchange with atmosphere (direct emission), and hydrosphere (indirect emission). Soils are represented by anaerobic Histosol, oxic Cambisols, Histosol with degraded peatlayers and Gleysols having intermediate redox state. They could be separated into three emission groups: CH4 emitting (248–318 kg C ha−1 a−1), CH4 uptake (−0.1 to −5 kg C ha−1 a−1), and soils on the edge of CH4 uptake and release (−0.2–20 kg C ha−1 a−1). Although soils with CH4 uptake were dominant (75%), the soil specific CH4 budget identified the study field (6.53 ha) as CH4 source (40.9 kg C ha−1 a−1). Not only CH4 emissions, but also dissolved CH4 in soil solution varied regularly with soil type. Individual soil solutions contained 0.008–151 μmol CH4 l−1. CH4 vanished to negligible loads, when dissolved CH4 passed an oxidative downslope soil zone, but promoted CH4 uptake was measured at this soil. In turn, CH4 was discharged to the atmosphere, when the soil solution left the pedosphere across an anaerobic soil zone. These measured indirect emissions were low (34 g C a−1), but the values of individual soil solution indicate possible higher discharges (3.9 kg a−1) at a different soil pattern. The results suggest that CH4 uptake rates of temperate forests are overestimated.

Numerical Model for Biological Oxidation and Migration of Methane in Soils

Concern over the potentially negative impacts of climate change has resulted in a search for techniques to reduce anthropogenic emissions of methane, a primary greenhouse gas. Microbial oxidation of methane in soils may serve as an inexpensive technique for reducing CH4 emissions from sources such as landfills and heavy oil wells. To gain a better quantitative understanding of the biological and physical processes limiting CH4 oxidation in soils and biofilters, a numerical reactive-transport model was developed. The model inputs include CH4 source strength, soil bulk density, moisture content, and biological kinetic parameters. The outputs consist of gas concentration profiles, CH4 oxidation rates, and surface flux rates. A series of soil column and batch incubation experiments were performed on a variety of soil types to calibrate and verify the model. The model was also verified by reproducing experimental results found in the literature.

Combining upscaling and downscaling of methane emissions from rice fields: methodologies and preliminary results.

The uncertainty in the methane (CH4) source strength of rice fields is among the highest of all sources in the global CH4 budget. Methods to estimate the source strength of rice fields can be divided into two scaling categories: bottom-up (upscaling) and top-down (downscaling). A brief review of upscaling and downscaling methodologies is presented. The combination of upscaling and downscaling methodologies is proposed as a potential method to reduce the uncertainty in the regional CH4 source strength of rice fields. Some preliminary results based on upscaling and downscaling are presented and the limitations of the approaches are discussed. The first case study focuses on upscaling by using a field-scale model in combination with spatial databases to calculate CH4 emissions for the island of Java. The reliability of upscaling results is limited by the uncertainty in model input parameters such as soil properties and organic carbon management. Because controlling variables such as harvested rice area may change on relatively short time scales, a land use change model (CLUE) was used to quantify the potential land use changes on Java in the period 1994—2010. The predicted changes were evaluated using the CH4 emission model. Temporal scaling by coupling land use change models and emission models is necessary to answer policy-related questions on future greenhouse gas emissions. In a downscaling case study, we investigate if inverse modeling can constrain the emissions from rice fields by testing a standard CH4 from rice scenario and a low CH4 from rice scenario (80 and 30 Tg CH4 yr-1, respectively). The results of this study are not yet conclusive; to obtain fine-resolution CH4 emission estimates over the Southeast Asian continent, the monitoring network atmospheric mixing ratios need to be extended and located closer to the continental sources.

Changes in CH4 emission from rice fields From 1960 to 1990s: 2. The declining use of organic inputs in rice farming

Previous studies have shown that organic amendments used in paddy rice cultivation strongly enhance methane emission from rice fields. Because of the increased availability of mineral fertilizer and the labor‐intensive nature of many organic amendments, it is likely that the use of organic amendments in rice farming has decreased in recent years. If so, a significant decrease in CH4 emissions from rice fields could be the result, but quantification is difficult because data on organic amendments are scarce. In this paper data on green manure application in China are used to quantify the impact of changes in an organic amendment on CH4 emission. The trend in CH4 emissions over time, taking green manure application explicitly into account, suggests that CH4 emissions from Chinese rice fields from the 1970s to the early 1980s may have been considerably higher than at present. The results presented here and in a previous study [Denier van der Gon, 1999] indicate that changes in agricultural management and technology need to be considered in assessments of the CH4 source strength of wetland rice fields and their role in the global CH4 budget of the past, present, and future.

Methane emissions from above ground open manure slurry tanks

Estimates of methane (CH4) emissions from slurry tanks or lagoons in the past have been made primarily by using chamber techniques, which are point specific and interfere with conditions at the slurry‐atmosphere interface. This study is based on the use of sulphur hexafluoride (SF6) as a tracer gas to estimate CH4 transport from the slurry surface. The tracer was released from the surface of the swine slurry, and air samples were taken at the downwind rim of the tank over a 165 day period from June 12 to November 20, 1995, at the McGiIl University ‐ Macdonald Campus Farm, Ste‐Anne‐de‐Bellevue, Quebec, and analyzed for CH4 and SF6 concentrations (C). Knowing the SF6 source strength (Q), CH4 source strength was then determined from the measured downwind concentrations using the C/Q ratio. Using this method, annual CH4 emission from the swine slurry tank was estimated at 56.5 kg CH4 m−2 tank surface yr−1 (± 20%). Assuming that diffusion processes at the nearby dairy tank were similar to those at the swine tank, the same C/Q ratio was used to determine CH4 emissions from the dairy slurry based on downwind CH4 concentrations measured over the same period as at the swine tank. Annual methane emission from the dairy tank was estimated at 74 kg CH4m−2 tank surface yr−1 (± 45%). On the basis of these estimates, CH4 emissions from outdoor holding tanks for swine and dairy slurry in Canada were approximated at 0.71 Tg CH4 yr−1 (± 40%) and 0.24 Tg CH4 yr−1 (± 70%), respectively, giving a combined annual emission of approximately 0.95 Tg yr−1(± 50%).

Potential methane emission from north‐temperate lakes following ice melt

Methane, a radiatively active “greenhouse” gas, is emitted from lakes to the atmosphere throughout the open‐water season. However, annual lake CH4 emissions calculated solely from open‐water measurements that exclude the time of spring ice melt may substantially underestimate the lake CH4 source strength. We estimated potential spring CH4 emission at the time of ice melt for 19 lakes in northern Minnesota and Wisconsin. Lakes ranged in area from 2.7 to 57,300 ha and varied in littoral zone sediment type. Regression analyses indicated that lake area explained 38% of the variance in potential CH4 emission for relatively undisturbed lakes; as lake area increases potential CH4 emission per unit area decreases. Inclusion of a second term accounting for the presence or absence of soft organic‐rich littoral‐zone sediments explained 83% of the variance in potential spring CH4 emission. Total estimated spring CH4 emission for 1993 for all Minnesota lakes north of 45° with areas ≥4 ha was 1.5 × 108 mol CH4 assuming a 1 : 1 ratio of soft littoral sediment to hard littoral sediment lakes. Emission estimates ranged from 5.3 × 107 mol assuming no lakes have soft organic‐rich littoral sediments to 4.5 × 108 mol assuming all lakes have soft organic‐rich littoral sediments. This spring CH4 pulse may make up as much as 40% of the CH4 annually emitted to the atmosphere by small lakes.

Airborne boundary layer flux measurements of trace species over Canadian boreal forest and northern wetland regions

Airborne heat, moisture, O3, CO, and CH4 flux measurements were obtained over the Hudson Bay lowlands (HBL) and northern boreal forest regions of Canada during July–August 1990. The airborne flux measurements were an integral part of the NASA/Arctic Boundary Layer Expedition (ABLE) 3B field experiment executed in collaboration with the Canadian Northern Wetlands Study (NOWES). Airborne CH4 flux measurements were taken over a large portion of the HBL. The surface level flux of CH4 was obtained from downward extrapolations of multiple‐level CH4 flux measurements. Methane source strengths ranged from −1 to 31 mg m2− d−1, with the higher values occurring in relatively small, isolated areas. Similar measurements of the CH4 source strength in the boreal forest region of Schefferville, Quebec, ranged from 6 to 27 mg m−2 d−1 and exhibited a diurnal dependence. The CH4 source strengths found during the ABLE 3B expedition were much lower than the seasonally averaged source strength of 51 mg m−2 d−1 found for the Yukon‐Kuskokwim delta region of Alaska during the previous ABLE 3A study. Large positive CO fluxes (0.31 to 0.53 parts per billion by volume (ppbv) m s−1) were observed over the inland, forested regions of the HBL study area, although the mechanism for the generation of these fluxes was not identified. Repetitive measurements along the same ground track at various times of day near the Schefferville site also suggested a diurnal dependence for CO emissions. Measurements of surface resistance to the uptake of O3 (1.91 to 0.80 s cm−1) for the HBL areas investigated were comparable to those observed near the Schefferville site (3.40 to 1.10s cm−1). Surface resistance values for the ABLE 3B study area were somewhat less than those observed over the Yukon‐Kuskokwim delta during the previous ABLE 3A study. The budgets for heat, moisture, O3, CO, and CH4 were evaluated. The residuals from these budget studies indicated, for the cases selected, a moderate net photochemical production of O3 present in the boundary layer over the HBL that coincided with an in situ destruction of CO, although the mechanism responsible for the destruction of CO was not identified. Results from the O3 budget analysis indicate the importance of in situ photochemical production and its possible dominance over surface deposition to the local O3 budget at the Schefferville site. Measurements of the in situ production of O3 indicated a direct relationship between the presence of biomass burning or large‐scale pollution effects. Residuals from budget calculations for conserved quantities (heat, moisture, and CH4) were compared with their respective surface fluxes to provide a measure of the internal selfconsistency of the flux measurements.

Airborne flux measurements of trace species in an Arctic boundary layer

In situ airborne measurements of the turbulent flux and mean values for O3, CO, and CH4 were obtained in the boundary layer over selected wetland systems in Alaska. These measurements were obtained in July–August 1988 as part of the NASA Global Tropospheric Experiment Program's Arctic Boundary Layer Expedition (ABLE 3A). The flux measurements obtained from this study provide information on the source/sink distribution of O3 and CH4 over the Yukon‐Kuskokwim Delta (YKD) and Alaskan North Slope (ANS) regions of Alaska. The source/sink distribution over the YKD is qualitatively correlated with surface vegetation type, identified from multispectral scanner imagery. Direct measurements of the spatial variation in the CH4 source strength were obtained over the YKD. The CH4 source strength over the YKD ranged from 25 to 85 mg m−2 d−1 during a flux survey flight which spanned a considerable portion of the YKD. A spatially averaged, seasonally adjusted source strength of 51 mg m−2 d−1 was established for the YKD. Indirect CH4 flux estimates obtained over the ANS indicate a much lower (∼10 mg m−2 d−1) source strength. The global CH4 emissions from tundra were estimated to be 44 Tg/a based on (1) the spatially averaged source strength obtained over the YKD, (2) current estimates of the global coverage of tundra, and (3) assuming a similarity between other tundra areas and that of the YKD. This estimate is taken to be an upper limit due to possible sampling inadequacies and because the spatial distribution of the CH4 source function over the YKD may not extend to all other northern wetland regions. This estimate is, however, in reasonable agreement with previous estimates. Airborne CO flux measurements over the YKD indicated low negative flux values over the coastal areas, while some positive fluxes were observed in the inland, sparsely forested regions. An inspection of the cospectrum of CO with vertical velocity for sample runs in coastal areas indicated a minimum at wavelengths which were noticeably shorter (70–400 m) than where any prominent feature could be found for similar cospectrums of heat, moisture, O3, or CH4 with vertical velocity. Similar analyses for transects over inland areas indicated occasional peaks in this same, short wavelength band. These features indicate the possibility of in situ photochemical destruction/production of CO, although the identification of a possible chemical mechanism was not attempted at this time. Tundra surfaces are estimated to be responsible for ∼32% of the total deposition loss of O3 poleward of 60°N. Data from the airborne O3 flux measurements made during this study indicate similar values of Rc for the ANS and YKD regions.