Release Of Trace Gases Research Articles

Effectiveness of three nitrification inhibitors on mitigating trace gas emissions from different soil textures under surface and subsurface drip irrigation

Nitrification inhibitors (NIs) and drip irrigation are recommended to mitigate trace gas emissions from agricultural soils. However, studies comparing the effect of different NIs on the release of trace gases from soils with contrasting textures under subsurface (SBD) and surface (SD) drip irrigation are lacking. Therefore, this study aimed to assess the effectiveness of three NIs in mitigating nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4) emissions from two soils with different textures under SBD, with pipe buried in 10 cm depth, and SD. Two greenhouse experiments were carried out with silt loam and loamy sand soil textures cultivated with wheat under SBD and SD to assess the effectiveness of the NIs Dicyandiamide (DCD), 3,4-Dimethylpyrazole phosphate (DMPP), and 3-Methylpyrazol combined with Triazol (MP + TZ). Ammonium sulfate was applied at a rate of 0.18 g N kg soil−1. The measured variables were daily and cumulative N2O–N, CO2–C, and CH4–C emissions, as well as soil NH4+-N and NO3−-N concentrations. The NIs and SBD had additive effects on reducing N2O–N emissions in the silt loam, but not in the loamy sand soil texture. Under SBD, total N2O–N emissions were 44% and 52% lower than under SD in the silt loam and loamy sand soil textures, respectively. Moreover, DMPP kept the highest NH4+-N concentrations and promoted the lowest N2O–N release. CO2–C and CH4–C total emissions were not affected by the treatments. Our findings supported the hypothesis that SBD decreases N2O–N emissions relative to SD. Among the investigated NIs, DMPP has the highest effectiveness in retarding nitrification and mitigating N2O–N release under the studied treatments. Finally, in coarse-textured soils, the use of NIs could be sufficient to significantly abate N2O–N emissions.

TROPOMI-Retrieved Underwater Light Attenuation in Three Spectral Regions in the Ultraviolet and Blue

Ultraviolet (UV) radiation plays an important role in the ocean for the biogeochemical cycling through photooxidation of colored dissolved organic matter and metals. It also influences the release of trace gases into the atmosphere. Understanding interaction and climate feedback mechanisms of these processes requires global long-term monitoring of UV radiation in the ocean. Ocean color sensors provide the diffuse attenuation coefficient for downwelling solar irradiance (Kd) as an estimate for the mean light penetration depth in the surface ocean. However, Kd products for the UV spectral range are currently only indirectly inferred from measurements in the visible spectral region. We exploit the UV and visible wavelengths of the TROPOMI sensor onboard Sentinel-5P to directly infer diffuse attenuation coefficients in the UV and blue spectral range. Our approach is based on Differential Optical Absorption Spectroscopy in combination with radiative transfer modeling which has been used in earlier studies to successfully derive Kd in the blue from the vibrational Raman scattering (VRS) signal. VRS was detected in the backscattered radiances of TROPOMI-like sensors at a spectral resolution around 0.5 nm. We adapt this method for the TROPOMI sensor and more spectral regions to obtain two novel Kd products in the UV range (312.5–338.5 nm and 356.5–390 nm), additionally to the blue Kd (390–423 nm). One month of TROPOMI data show high sensitivity to retrieve VRS (fit errors <15%). The resulting Kd values in the UV and blue agree reasonably (RMSD <0.31 m−1) with in-situ data measured during a transatlantic cruise. Kd-blue compared to wavelength-converted Kd(490) products from OLCI and OC-CCI shows that the differences between the three data sets are within uncertainties given for the OC-CCI product. TROPOMI Kd-blue results have higher quality and much higher spatial coverage and resolution than previous ones from SCIAMACHY, GOME-2, and OMI. Perspectively, global TROPOMI Kd products, once thoroughly validated, can be used as input for modeling, e.g., photochemical reaction rates of climatically important compounds and inhibition of primary productivity.

Water at Interfaces.

The interfaces of neat water and aqueous solutions play a prominent role in many technological processes and in the environment. Examples of aqueous interfaces are ultrathin water films that cover most hydrophilic surfaces under ambient relative humidities, the liquid/solid interface which drives many electrochemical reactions, and the liquid/vapor interface, which governs the uptake and release of trace gases by the oceans and cloud droplets. In this article we review some of the recent experimental and theoretical advances in our knowledge of the properties of aqueous interfaces and discuss open questions and gaps in our understanding.

Development and calibration of a novel sensor to quantify the water content of surface soils and biological soil crusts

Summary The surface layer of soil as transition zone between pedosphere and atmosphere plays a crucial role in exchange processes of nutrients, atmospheric gases and water. Knowledge of its water content is essential, as it governs both physiological and transport mechanisms. In arid and semi‐arid regions, this uppermost soil layer is commonly colonized by biological soil crusts (biocrusts), which play major roles in the global terrestrial carbon and nitrogen cycles. The water status of biocrusts is essential as it controls the activity, productivity and surface exchange of these poikilohydric communities. On‐site analyses of the water content of both bare and crusted soils are thus urgently needed to correctly model the exchange processes of water, nutrients and trace gases at the soil surface. In this study, we present the biocrust wetness probe (BWP), which is the first to reliably measure the water content within biocrusts or the uppermost 5 mm of a substrate. Using a weak alternating current, the electrical conductivity is assessed over time. With an automatic calibration routine, conductivity values are temperature‐corrected and converted into water contents and precipitation equivalents. During 1 year of continuous field measurements at 5‐min intervals, 60 BWPs worked reliably without any failure. The probes responded immediately and individually upon rain events, showing substrate‐specific water response curves, which are well represented by linear and exponential calibration curves. The BWP facilitates the spatio‐temporal assessment and interpolation of surface soil wetness and thus biocrust activity, which governs nutrient fluxes, trace gas release and biogeochemical cycles. Its implementation in distributed sensor networks is under development.

Barometric pumping of a fractured porous medium

AbstractBarometric pumping plays a crucial role in the release of trace gases from fractured porous media to the atmosphere, and it requires a rigorous and complete modeling in order to go beyond the approximate schemes available in the literature. Therefore, a coupled set of convection and convection‐diffusion equations for a slightly compressible fluid in unsteady conditions should be solved. The numerical methodology is presented, and it is applied to conditions close to the ones of the Roselend Natural Laboratory (France). The precision of the code is assessed and the mechanism of barometric pumping is explained. The usual schematization by simple vertical fractures is shown to be only qualitative. Finally, barometric pumping is shown to be efficient in a narrow range of parameter values; its efficiency is a decreasing function of the matrix porosity and of the fracture density.

Quantifying methane emission from fugitive sources by combining tracer release and downwind measurements – A sensitivity analysis based on multiple field surveys

Using a dual species methane/acetylene instrument based on cavity ring down spectroscopy (CRDS), the dynamic plume tracer dispersion method for quantifying the emission rate of methane was successfully tested in four measurement campaigns: (1) controlled methane and trace gas release with different trace gas configurations, (2) landfill with unknown emission source locations, (3) landfill with closely located emission sources, and (4) comparing with an Fourier transform infrared spectroscopy (FTIR) instrument using multiple trace gasses for source separation. The new real-time, high precision instrument can measure methane plumes more than 1.2km away from small sources (about 5kgh−1) in urban areas with a measurement frequency allowing plume crossing at normal driving speed. The method can be used for quantification of total methane emissions from diffuse area sources down to 1kg per hour and can be used to quantify individual sources with the right choice of wind direction and road distance. The placement of the trace gas is important for obtaining correct quantification and uncertainty of up to 36% can be incurred when the trace gas is not co-located with the methane source. Measurements made at greater distances are less sensitive to errors in trace gas placement and model calculations showed an uncertainty of less than 5% in both urban and open-country for placing the trace gas 100m from the source, when measurements were done more than 3km away. Using the ratio of the integrated plume concentrations of tracer gas and methane gives the most reliable results for measurements at various distances to the source, compared to the ratio of the highest concentration in the plume, the direct concentration ratio and using a Gaussian plume model. Under suitable weather and road conditions, the CRDS system can quantify the emission from different sources located close to each other using only one kind of trace gas due to the high time resolution, while the FTIR system can measure multiple trace gasses but with a lower time resolution.

Diffusion of volatile organics through porous snow: impact of surface adsorption and grain boundaries

Abstract. Release of trace gases from surface snow on earth drives atmospheric chemistry, especially in the polar regions. The gas-phase diffusion of methanol and of acetone through the interstitial air of snow was investigated in a well-controlled laboratory study in the temperature range of 223 to 263 K. The aim of this study was to evaluate how the structure of the snowpack, the interaction of the trace gases with the snow surface, and the grain boundaries influence the diffusion on timescales up to 1 h. The diffusive loss of these two volatile organics into packed snow samples was measured using a chemical ionization mass spectrometer. The structure of the snow was analysed by means of X-ray-computed micro-tomography. The observed diffusion profiles could be well described based on gas-phase diffusion and the known structure of the snow sample at temperatures ≥ 253 K. At colder temperatures, surface interactions start to dominate the diffusive transport. Parameterizing these interactions in terms of adsorption to the solid ice surface, i.e. using temperature-dependent air–ice partitioning coefficients, better described the observed diffusion profiles than the use of air–liquid partitioning coefficients. No changes in the diffusive fluxes were observed by increasing the number of grain boundaries in the snow sample by a factor of 7, indicating that for these volatile organic trace gases, uptake into grain boundaries does not play a role on the timescale of diffusion through porous surface snow. For this, a snow sample with an artificially high amount of ice grains was produced and the grain boundary surface measured using thin sections. In conclusion, we have shown that the diffusivity can be predicted when the structure of the snowpack and the partitioning of the trace gas to solid ice is known.

Predicting long‐term carbon mineralization and trace gas production from thawing permafrost of Northeast Siberia

The currently observed Arctic warming will increase permafrost degradation followed by mineralization of formerly frozen organic matter to carbon dioxide (CO2 ) and methane (CH4 ). Despite increasing awareness of permafrost carbon vulnerability, the potential long-term formation of trace gases from thawing permafrost remains unclear. The objective of the current study is to quantify the potential long-term release of trace gases from permafrost organic matter. Therefore, Holocene and Pleistocene permafrost deposits were sampled in the Lena River Delta, Northeast Siberia. The sampled permafrost contained between 0.6% and 12.4% organic carbon. CO2 and CH4 production was measured for 1200 days in aerobic and anaerobic incubations at 4 °C. The derived fluxes were used to estimate parameters of a two pool carbon degradation model. Total CO2 production was similar in Holocene permafrost (1.3 ± 0.8 mg CO2 -C gdw(-1) aerobically, 0.25 ± 0.13 mg CO2 -C gdw(-1) anaerobically) as in 34 000-42 000-year-old Pleistocene permafrost (1.6 ± 1.2 mg CO2 -C gdw(-1) aerobically, 0.26 ± 0.10 mg CO2 -C gdw(-1) anaerobically). The main predictor for carbon mineralization was the content of organic matter. Anaerobic conditions strongly reduced carbon mineralization since only 25% of aerobically mineralized carbon was released as CO2 and CH4 in the absence of oxygen. CH4 production was low or absent in most of the Pleistocene permafrost and always started after a significant delay. After 1200 days on average 3.1% of initial carbon was mineralized to CO2 under aerobic conditions while without oxygen 0.55% were released as CO2 and 0.28% as CH4 . The calibrated carbon degradation model predicted cumulative CO2 production over a period of 100 years accounting for 15.1% (aerobic) and 1.8% (anaerobic) of initial organic carbon, which is significantly less than recent estimates. The multiyear time series from the incubation experiments helps to more reliably constrain projections of future trace gas fluxes from thawing permafrost landscapes.

An accounting of C‐based trace gas release during abiotic plant litter degradation

AbstractRecent studies showed that photochemical breakdown (photodegradation) of plant material accounts for a substantial portion of litter decomposition and subsequent trace gas release in ecosystems under high radiative load and low precipitation. In the absence of solar radiation, thermal degradation may also cause trace gas release at temperatures below the ignition point. These observations suggest that the abiotic processes of photodegradation and thermal degradation of plant litter may be important in understanding global trace gas budgets. In a laboratory incubation study, we performed a simultaneous carbon (C) accounting of CO2, CO, and CH4 produced as a byproduct of photodegradation and thermal degradation of six different plant litter types that varied in chemical composition. The patterns of trace gas release during photodegradation and thermal degradation differed considerably across the six plant materials, suggesting that chemical composition of litter may influence the rates of abiotic degradation. There was a strong positive correlation between the rates of trace gas release during photodegradation and temperature. A significant portion of trace gases were produced during low temperature (< 100 °C) thermal degradation of litter in the absence of solar radiation, which was also positively correlated to temperature. In addition, both thermal degradation and photodegradation occurred in the absence of O2. This indicates that the mechanism formerly accepted as photo‐oxidation may only be one of several photodegradation processes. We speculate that the direct breakdown of chemical groups such as carboxyl, carbonyl, and methoxyl groups may result in CO2, CO, and CH4 release. We suggest that the combined processes of thermal and photodegradation of litter may be a previously under accounted source of C‐based trace gases from terrestrial systems.

Field determination of biomass burning emission ratios and factors via open-path FTIR spectroscopy and fire radiative power assessment: headfire, backfire and residual smouldering combustion in African savannahs

Abstract. Biomass burning emissions factors are vital to quantifying trace gas release from vegetation fires. Here we evaluate emissions factors for a series of savannah fires in Kruger National Park (KNP), South Africa using ground-based open path Fourier transform infrared (FTIR) spectroscopy and an IR source separated by 150–250 m distance. Molecular abundances along the extended open path are retrieved using a spectral forward model coupled to a non-linear least squares fitting approach. We demonstrate derivation of trace gas column amounts for horizontal paths transecting the width of the advected plume, and find for example that CO mixing ratio changes of ~0.01 μmol mol−1 [10 ppbv] can be detected across the relatively long optical paths used here. Though FTIR spectroscopy can detect dozens of different chemical species present in vegetation fire smoke, we focus our analysis on five key combustion products released preferentially during the pyrolysis (CH2O), flaming (CO2) and smoldering (CO, CH4, NH3) processes. We demonstrate that well constrained emissions ratios for these gases to both CO2 and CO can be derived for the backfire, headfire and residual smouldering combustion (RSC) stages of these savannah fires, from which stage-specific emission factors can then be calculated. Headfires and backfires often show similar emission ratios and emission factors, but those of the RSC stage can differ substantially. The timing of each fire stage was identified via airborne optical and thermal IR imagery and ground-observer reports, with the airborne IR imagery also used to derive estimates of fire radiative energy (FRE), allowing the relative amount of fuel burned in each stage to be calculated and "fire averaged" emission ratios and emission factors to be determined. These "fire averaged" metrics are dominated by the headfire contribution, since the FRE data indicate that the vast majority of the fuel is burned in this stage. Our fire averaged emission ratios and factors for CO2 and CH4 agree well with those from prior studies conducted in the same area using e.g. airborne plume sampling. We also concur with past suggestions that emission factors for formaldehyde in this environment appear substantially underestimated in widely used databases, but see no evidence to support suggestions by Sinha et al. (2003) of a major overestimation in the emission factor of ammonia in works such as Andreae and Merlet (2001) and Akagi et al. (2011). We also measure somewhat higher CO and NH3 emission ratios and factors than are usually reported for this environment, which is interpreted to result from the OP-FTIR ground-based technique sampling a greater proportion of smoke from smouldering processes than is generally the case with methods such as airborne sampling. Finally, our results suggest that the contribution of burning animal (elephant) dung can be a significant factor in the emissions characteristics of certain KNP fires, and that the ability of remotely sensed fire temperatures to provide information useful in tailoring modified combustion efficiency (MCE) and emissions factor estimates maybe rather limited, at least until the generally available precision of such temperature estimates can be substantially improved. One limitation of the OP-FTIR method is its ability to sample only near-ground level smoke, which may limit application at more intense fires where the majority of smoke is released into a vertically rising convection column. Nevertheless, even in such cases the method potentially enables a much better assessment of the emissions contribution of the RSC stage than is typically conducted currently.

Controls on the production of organohalogens by phytoplankton: Effect of nitrate concentration and grazing

Using a number of laboratory culture experiments we have been able to show that trace gas release by phytoplankton is affected by changes in ambient nitrate conditions. Nutrient concentrations are fundamentally important to phytoplankton community structure and primary production, so it is important to understand if changes in nitrate levels are likely to affect metabolic processes. Recent human activities have added a large quantity of atmospheric nitrogen to the open ocean and this, in turn, will affect both carbon dioxide drawdown and nitrous oxide release. In contrast, ocean prediction models tell us that large parts of the ocean are going to become more oligotrophic. How these changes will affect other climatically active gases, e.g., organohalogens, is unclear. In this study we look at the effect of varying the nitrate to phosphate ratio on the release of three methyl halides. Some attention is also given to the effects of grazing as a means of gas release. Assuming that a 16:1 nitrate:phosphate ratio is the norm, we show that increasing the nitrate level above 16 μmol L−1 can lead to up to an 85% increase in trace gas production per unit cell while lowering the concentration can result in up to a 65% reduction. This is the first time that such a nutrient effect has been observed. Grazing by the microzooplankton Oxyrrhis marina also increases release, but whether this is due to enhanced metabolic processes caused by stress or the breaking down of cells due to grazing is unclear.

Halocarbon and dimethyl sulphide studies around the Mascarene Plateau

Air-sea exchange is thought to be one of the major routes by which halocarbons and dimethyl sulphide reach the troposphere and stratosphere. Once there, in different ways, they participate in chemical reactions that have implications for ozone depletion and climate change. The gases are released by phytoplankton and other algae, but our present understanding of the sources and sinks is insufficient to establish a balanced global budget. Published data suggest that there are regions of coastal and ocean waters that constitute a major source, but, for halocarbons, in other regions the ocean is a net sink. For example, in many open oceanic areas, the rate of degradation of methyl bromide outweighs production. Here we present data from the Central Indian Ocean, a region considered to be low in terms of biological productivity. Little is known about trace-gas release from the Central Indian Ocean and without such data it is impossible to even hazard a guess at the global ocean source to the atmosphere.

A Local-Scale Study of the Trace Gas Emissions from Vegetation Burning around the Village of Dalun, Ghana, with Respect to Seasonal Vegetation Changes and Burning Practices

The annual trace gas emissions from a West African rural region were calculated using direct observations of gas emissions and burning practices, and the findings compared to the guidelines published by the IPCC. This local-scale study was conducted around the village of Dalun in the Northern Region of Ghana, near the regional capital of Tamale. Two types of fires were found in the region – agricultural fires andwildfires. Agricultural fires are intentionally set in order to remove shrub and crop residues; wildfires are mostly ignited by herders to remove inedible grasses and to promote the growth of fresh grass. An agricultural fire is ignited with a fire front moving against the wind (backfire), whereas a wildfire moves with the wind (headfire). Gas emissions (CO2, CO and NO) weremeasured by burning eight experimental plots, simulating both headfires and backfires. A common method of evaluating burning conditions is to calculate modified combustion efficiency (MCE), which expresses the percentage of the trace gases released as CO2. Modified combustion efficiency was95% in the wildfires burned as headfires, but only 90% in the backfires.The burned area in the study region was determined by classifying a SPOT HRV satellite image taken about two months into the dry season. Fires were classified as either old burned areas or new burned areas as determined by the gradient in moisture content in the vegetation from the onset of the dry season. Classified burned areas were subsequently divided into two classes depending on whether the location was in the cultivated area or in the rangeland area, this sub-classification thus indicating whether the fire had been burned as a backfire or headfire. Findings showed that the burned area was 48% of the total region, and that the ratio of lowland wildfiresto agricultural fires was 3:1. The net trace gas release from the classified vegetation burnings were extrapolated to 26–46×108 gCO2, 78–302×106 g CO,17–156×105 g CH4,16–168×105 g NMHC and 11–72×103 NOx. Calculation of the emissionsusing proposed IPCC default values on burned area and average biomass resulted in a net emission 5 to 10 times higher than the measured emission values. It was found that the main reason for this discrepancy was not the emission factorsused by the IPCC, but an exaggerated fuel load estimate.

EFFECTS ON THE IONOSPHERE DUE TO PHENOMENA OCCURRING BELOW IT

The terrestrial thermosphere and ionosphere form the most variable part of theEarth's atmosphere. Because our society depends on technological systems thatcan be affected by thermospheric and ionospheric phenomena, understanding,monitoring and ultimately forecasting the changes of the thermosphere–ionosphere system are of crucial importance to communications, navigation and the exploration of near-Earth space. The reason for the extreme variability of the thermosphere–ionosphere system isits rapid response to external forcing from various sources, i.e., thesolar ionizing flux, energetic charged particles and electric fields imposed via the interaction between the solar wind, magnetosphere and ionosphere, as well as coupling from below (“meteorological influences”) by the upward propagating, broad spectrum,internal atmospheric waves (planetary waves, tides, gravity waves) generated in thestratosphere and troposphere. Thunderstorms, typhoons, hurricanes, tornadoes andeven seismological events may also have observable consequences in the ionosphere.The release of trace gases due to human activity have the potential to cause changes inthe lower and the upper atmosphere.A brief overview is presented concerning the discoveries and experimentalresults that have confirmed that the ionosphere is subject to meteorologicalcontrol (especially for geomagnetic quiet conditions and for middle latitudes).D-region aeronomy, the winter anomaly of radiowave absorption, wave-liketravelling ionospheric disturbances, the non-zonality and regional peculiaritiesof lower thermospheric winds, sporadic-E occurrence and structure, spread-Fevents, the variability of ionospheric electron density profiles and Total ElectronContent, the variability of foF2, etc., should all be considered in connection withtropospheric and stratospheric processes. “Ionospheric weather”, as a part of spaceweather, (i.e., hour-to-hour and day-to-day variability of the ionospheric parameters)awaits explanation and prediction within the framework of the climatological, seasonal,and solar-cycle variations.

Structure, microphysics, and surface area of the Arctic snowpack near Alert during the ALERT 2000 campaign

The seasonal snowpack at Alert (North coast of Ellesmere Island, 82°29.94′N, 62°20.55′W) was studied in February and April 2000, on land and on sea ice. The stratigraphy was studied, and the density and specific surface area (SSA) of each snow layer were measured. SSA was measured by CH 4 adsorption at 77 K using a volumetric method. On land, the snowpack was 10–50 cm thick and consisted of a depth hoar layer covered by one or more hard wind-packed layers with densities between 0.35 and 0.52. These were sometimes separated by soft layers of more or less faceted crystals. The surface was covered by recent precipitation and surface hoar. The stratigraphy on sea ice was more variable, with numerous hard wind-packed layers alternating with soft layers of depth hoar or faceted crystals. SSA values ranged from 125 cm 2/g for depth hoar to 1500 cm 2/g for diamond dust and dendritic snow. The total surface area of the snowpack was calculated from the thickness, density, and SSA of each layer, and ranged from 1160 to 3710 m 2 of snow surface area per m 2 of ground. These values were used to estimate the potential impact of the snowpack on atmospheric chemistry, by adsorption/desorption of trace gases. Using the example of acetone, whose adsorption behavior on ice is estimated, it is found that the snowpack may sequester most of the acetone of the (snow+boundary layer) system most of the year. The release during metamorphism of trace gases dissolved in snow is also discussed. We propose that the frequency and intensity of wind storms will strongly affect the release of trace gases, as this will determine whether intense metamorphism leading to depositional depth hoar can happen.

Soil gas fluxes of N2O, CH4 and CO2 beneath Lolium perenne under elevated CO2: The Swiss free air carbon dioxide enrichment experiment

Fluxes of nitrous oxide, methane and carbon dioxide were measured from soils under ambient (350 µL L-1) and enhanced (600 µL L-1) carbon dioxide partial pressures (pCO2) at the ‘Free Air Carbon Dioxide Enrichment’ (FACE) experiment, Eidgenossische Technische Hochschule (ETH), Eschikon, Switzerland in July 1995, using a GC housed in a mobile laboratory. Measurements were made in plots of Lolium perenne maintained under high N input. During the data collection period N fertiliser was applied at a rate of 14 g m-2 of N. Elevated pCO2 appeared to result in an increased (27%) output of N2O, thought to be the consequence of enhanced root-derived available soil C, acting as an energy source for denitrification. The climate, agricultural practices and soils at the FACE experiment combined to give rise to some of the largest N2O emissions recorded for any terrestrial ecosystem. The amount of CO2–C being lost from the control plot was higher (10%) than for the enhanced CO2 plot, and is the reverse of that predicted. The control plot oxidised consistently more CH4 than the enhanced plot, oxidising 25.5 ± 0.8 µg m-2 hr-1 of CH4 for the control plot, with an average of 8.5 ± 0.4 µg m-2 hr-1 of CH4 for the enhanced CO2 plot. This suggests that elevated pCO2 may lead to a feedback whereby less CH4 is removed from the atmosphere. Despite the limited nature of the current study (in time and space), the observations made here on the interactions of elevated pCO2 and soil trace gas release suggest that significant interactions are occurring. The feedbacks involved could have importance at the global scale.

A model investigation of turbulence‐driven pressure‐pumping effects on the rate of diffusion of CO2, N2O, and CH4 through layered snowpacks

Pressure pumping at the Earth's surface is caused by short‐period atmospheric turbulence, longer‐period barometric changes, and quasi‐static pressure fields induced by wind blowing across irregular topography. These naturally occurring atmospheric pressure variations induce periodic fluctuations in airflow through snowpacks, soils, and any other porous media at the Earth's surface. Consequently, the uptake or release of trace gases from soils and snowpacks is a combination of molecular diffusion and advection forced by pressure pumping. Using model‐estimated fluxes, this study attempts to quantify the influence that turbulent pressure fluctuations with periods between 0.1 and 1000 s can have on the rate of exchange of CO2, N2O, and CH4 through a seasonal snowpack. Data for this study were collected at a forested subalpine meadow site in the Rocky Mountains of southern Wyoming, during February 1995 when the snowpack was distinctly layered and approximately 1.4 m deep. The data include mole fraction of CO2, N2O, and CH4 just above and at the base of the snowpack, several profiles of CO2, N2O, and CH4 mole fraction in the top l m of the snowpack, and a profile of snowpack density and tortuosity. Turbulent atmospheric pressure‐pumping fluctuations, sampled at approximately 11 Hz for several hours, were obtained with a fast response differential pressure sensor. A one‐dimensional steady state diffusion model and one‐ and three‐dimensional time‐dependent pressure‐pumping models are used to estimate the gas fluxes through the snowpack. Boundary conditions are provided by grab samples just above the snowpack and at the soil/snow interface. The pressure‐pumping models are driven by the observed pressure fluctuations, and all models include the observed layering of the snowpack. As with previous studies the present results indicate that the effects of pressure pumping are diminished with increasingly strong gradients. Furthermore, we conclude that unless pressure pumping influences the gas concentrations at the boundaries of the snowpack, it appears unlikely that it can significantly impact the rate of gaseous diffusion through the snowpack. Even two‐ and three‐dimensional effects, which can have a significant short‐term impact on the fluxes and concentration profiles, are nearly eliminated when averaged over a period of hours. It is also suggested that vertical layering is important for three‐dimensional pressure‐pumping studies and that the time‐dependent temperature term, which is traditionally ignored when modeling dynamic pressure variations, may in fact be dominant in some situations and probably should be incorporated in future modeling studies of pressure pumping.

Atmospheric circulation and the tropospheric ozone maximum over the tropical South Atlantic Ocean

Ozone for a ten year period obtained from the Total Ozone Mapping Spectrometer (TOMS) has been compared with velocity potential for the same period. It has been found that the global-scale circulation serves to modulate the ozone concentration over the tropical Atlantic Ocean. Examination of the low frequency modes reveal close correspondence between the two parameters, especially at the annual timescale. Descending motion over southern Africa reduces precipitation, thereby enhancing biomass burning and the subsequent release of trace gases and the possible photochemical production of ozone. Transport to the ozone maximum region over the tropical Atlantic Ocean occurs mainly from tropical Africa. Little evidence has been found on a climatological time-scale to support the theory of stratospheric infusion into the troposphere above the tropical Atlantic Ocean.