Variations In Emission Rates Research Articles

Large-scale circulation of atomic oxygen in the upper mesosphere and lower thermosphere

The variability of the atomic oxygen green line airglow at 557.7 nm, originating from the O( 1S) level, has a long history of observation. Only recently, global observations of the oxygen airglow, interpreted with the help of global circulation models have provided some understanding. Satellite observations of winds and temperatures clearly demonstrate the dynamical influence of tides on the daily variations. Both annual and semi-annual components of the variation have been identified as occurring over the course of the year. From the large-scale circulation of the atmosphere one expects downwelling in the winter at high latitudes, causing enhanced atomic oxygen and enhanced airglow while the opposite is expected in summer. In the present investigation a search is made for this large-scale signature using data from the WIND Imaging Interferometer on the Upper Atmosphere Research Satellite, model results from the Thermosphere–Ionosphere–Mesosphere-Electrodynamics-Global Circulation Model, and earlier published results. A well-defined semi-annual variation of emission rate is found in the tropics, apparently the result of the semi-annual variation of the diurnal tide. Annual and semi-annual patterns are found at mid-latitudes, in satellite and model data, while ground-based observations detect only an annual variation, with a maximum in the autumn. At still higher latitudes the fall peak persists, but with a deep depletion of atomic oxygen in the springtime; this appears to be the signature of the large-scale circulation.

Seasonal variations of monoterpene emissions from coniferous trees of different ages in Korea

Seasonal variations of emission rates and compositions from coniferous species were measured under controlled conditions using a vegetation enclosure method. Total emission rates and compositions of monoterpene compounds from young and adult trees in different seasons were compared. It was found that the total emission rates and the components of monoterpene varied significantly with tree species, age, and season. Total emissions from C. japonica and P. koraiensis were higher for older trees than for younger trees; however, significantly higher emissions were found from younger trees for C. obtusa. Higher monoterpene emission rates from each plant were found in spring and summer compared with autumn and winter emissions.

Integrated odour modelling for sewage treatment works

Odours from sewage treatment works are a significant source of environmental annoyance. There is a need for tools to assess the degree of annoyance caused, and to assess strategies for mitigation of the problem. This is the role of odour modelling. Four main stages are important in the development of an odour problem. Firstly, the odorous molecules must be formed in the liquid phase. They must then transfer from the liquid to the gaseous phase. They are then transported through the atmosphere to the population surrounding the odour source, and are then perceived and assessed by that population. Odour modelling as currently practised tends to concentrate on the transportation of odorants through the atmosphere, with the other areas receiving less attention. Instead, odour modelling should consider each stage in an integrated manner. This paper describes the development of integrated odour models for annoyance prediction. The models describe the liquid-phase transformations and emission of hydrogen sulphide from sewage treatment processes. Model output is in a form suitable for integration with dispersion models, the predictions of which can in turn be used to indicate the probability of annoyance. The models have been applied to both hypothetical and real sewage treatment works cases. Simulation results have highlighted the potential variability of emission rates from sewage treatment works, resulting from flow, quality and meteorological variations. Emission rate variations can have significant effects on annoyance predictions, which is an important finding, as they are usually considered to be fixed and only meteorological variations are considered in predicting the odour footprint. Areas for further development of integrated odour modelling are discussed, in particular the search for improved links between analytical and sensory measurements, and a better understanding of dose/response relationships for odour annoyance.

Time and Space Uniformity of Indoor Bacteria Concentrations in Chicago Area Residences

The objective of this article is to assess spatial and temporal variation of indoor gram-positive bacteria and Staphylococcus sp. in 20 urban residences. At each residence, air was sampled at one outdoor site and four indoor sites (rooms) to assess spatial variation and once each season for five consecutive seasons to assess temporal variation. All temporal and spatial comparisons were performed using data obtained by the Andersen sampling technique. A secondary objective of this study is to evaluate relationships among several sampling methods used to measure bacteria levels; since differences in the measured concentrations are expected, the focus is to discern if corresponding measurements relate to each other. Not surprisingly, comparisons among the four sampling systems revealed statistically significant differences, although levels correlated relatively well. Using data only from the Andersen samplers, we conclude that seasonal variation is residence dependent with typically higher summer levels, but a clear pattern of variation could not be established. Room-to-room difference is not statistically significant, but the basement levels render the basement a distinct microenvironment. Indoor concentrations exceeded corresponding outdoor concentrations 75% of the time. Staphylococcus accounts for approximately 27% of the average indoor bacteria levels. The highest levels of gram-positive bacteria are found in the kitchen. The presence of multiple indoor sources with variable emission rates in multiple indoor locations results in bacteria levels that vary with time and space.

Soil emissions of nitrous oxide in fire‐prone African savannas

Only few estimates of soil emissions of nitrous oxide (N2O) are available from African savanna ecosystems, which account for two thirds of the savannas worldwide. Field measurements of soil emissions of N2O were conducted in seasonally dry savannas in Ethiopia and Ghana in order to study temporal and spatial variation in emission rates and to examine the effects of fire on the N2O flux. Soil emissions of N2O were measured by an enclosure technique. Soil emission rates were lowest in the dry season averaging 0.5 μg N2O‐N m−2 h−1, and the mean flux increased from 1.7 in the beginning to 3.5 in the middle and 5.5 μg N2O‐N m−2 h−1 by the end of the rainy season. Minor spatial variation in emissions according to differences in drainage capacity, vegetation type, and fire severity was associated with high spatial variation at field scale. The variation in water‐filled pore space in soil explained only 44% of the variation in the N2O flux. In an experiment separating the effect of fire and ash, the N2O flux was not different across treatments 1 week after the low‐intensity burning. In another experiment, the flux at early and late burn sites did not differ and was not different from an unburned site. The study clearly demonstrates that more measurements are needed to fully understand the seasonal and spatial variability of soil emissions of N2O in savanna ecosystems.

Historical and future land use effects on N2O and NO emissions using an ensemble modeling approach: Costa Rica's Caribbean lowlands as an example

[1] The humid tropical zone is a major source area for N2O and NO emissions to the atmosphere. Local emission rates vary widely with local conditions, particularly land use practices which swiftly change with expanding settlement and changing market conditions. The combination of wide variation in emission rates and rapidly changing land use make regional estimation and future prediction of biogenic trace gas emission particularly difficult. This study estimates contemporary, historical, and future N2O and NO emissions from 0.5 million ha of northeastern Costa Rica, a well-documented region in the wet tropics undergoing rapid agricultural development. Estimates were derived by linking spatially distributed environmental data with an ecosystem simulation model in an ensemble estimation approach that incorporates the variance and covariance of spatially distributed driving variables. Results include measures of variance for regional emissions. The formation and aging of pastures from forest provided most of the past temporal change in N2O and NO flux in this region; future changes will be controlled by the degree of nitrogen fertilizer application and extent of intensively managed croplands.

Methane Emission from the Coastal Area in the Southern Baltic Sea

In the shallow coastal area of the brackish water ecosystem of the Baltic Sea, between the Islands of Rügen and Hiddensee (Southern Baltic, Germany), methane emission was continuously determined with floating measuring chambers and automatic sampling equipment. Data were collected during six measuring periods in two years (1995, 1996).The evaluation of more than 5700 measuring series demonstrated very strong seasonal and interannual variations of methane emission, with extremely high rates in the range between 24 and 104mg CH4m−2h−1in June/July 1995, and the lowest values in the range between 0·03 and 0·11mgCH4m−2h−1in October 1996. The highest single emission rate was 243mgCH4m−2h−1. Therefore this shore region represents, at least periodically, a hot spot for methane emission from a brackish water ecosystem into the atmosphere.The crucial controlling factor for the interannual and seasonal variations of methane emission and also for the small spatial differences was found to be the amount of organic matter in the sediment. This exerted a direct influence on methanogenesis and additionally indirect effects on oxygen dynamics and the relationships between methanogenesis and sulphate reduction.The significant correlation between methane emission and temperature in the water and sediment demonstrated that diurnal and long-term variations of emission rates during each season were mainly regulated by temperature. Only at low emission rates did other, indirect ecological factors become more important in controlling methane emission.

A multiple source approach to acute human health risk assessments

The US Army Center for Health Promotion and Preventive Medicine (USACHPPM) has developed a three-tiered approach to perform an acute noncarcinogenic health risk assessment to comply with requirements contained in USEPA’s ‘Addendum to the Methodology for Assessing Health Risks Associated with Indirect Exposure to Combustor Emissions' (USEPA, Draft strategy for combustion of hazardous waste, May 1993). The addendum document drafted in November 1993 suggests evaluation of short-term emissions and exposures, but it does not provide necessary guidance or methodology. This evaluation becomes extremely complex when several sources contribute to the overall concentration of contaminants in the air. Because each source has a different emission rate, location, and dispersion profile, the compound-specific maximum concentrations are rarely located at the same receptor location, as determined by using the Industrial Source Complex Short Term (ISCST3) dispersion model. Furthermore, evaluation of hazard quotients at various receptor locations from a large number of contaminants from multiple sources is an extremely complex and tedious process. This paper will detail a three-tiered approach which was developed to perform the acute risk assessment, quantify possible advantages and disadvantages associated with each tier, and demonstrate the effects of contributing factors, such as distance, emission rate variability, population/exposure scenarios, and compound speciation. In order to accomplish this task, a sample facility with four different combustion sources was modeled and processed according to the three-tiered approach.

Quantifying the effects of water table and soil temperature on the emission of methane from peat wetland at the field scale

Methane emissions from peat wetlands have been shown to be very sensitive to soil temperature and the position of the water table in controlled environment studies and in the field. However, these studies have been made using small chambers which average fluxes over areas ranging from 0.05 to 0.3 m 2 and spatial variability in methane emission from these ecosystems commonly spans several orders of magnitude over linear scales of a few metres. In this study, fluxes of methane from peat wetlands in Caithness, Scotland were measured over a 14 d period in May and June 1994 using eddy covariance methods, which provided 30 min mean methane fluxes averaged over areas of typically 10 4 to 10 5 m 2, overcoming the small-scale spatial variability in emission. The mean methane emissions varied with wind direction over the areas of bog with different water tables. Within each sector of the fetch, the mean methane emission increased with temperature in the range 7–11°C by approximately a factor of two (5 μmol m -2 h -1 °C -1), close to the response obtained in controlled conditions. The rates of methane emission from the bog averaged 39 μmol m -2 h -1 with peak values approaching 170 μmol m -2 h -1. The responses of the methane emission to temperature and water table at this site have been used to calculate the seasonal variability in emission rates and annual emissions from continuous measurements of water table and meteorological variables over the Caithness peatlands. Annual emission from these peatlands was estimated to be 0.43 mol m -2 yr -1, and from U.K. peatlands a range of 40–141 kT yr -1 was predicted. The responses were used to simulate the effects of increasing mean temperature by up to 4°C and varying the water table by up to ±2 cm.

Deep Defect Relaxation in Hydrogenated Amorphous Silicon: New Experimental Evidence and Implications

ABSTRACTWe review modulated photocurrent experiments which indicate that thermal emission rate for Do defects in intrinsic samples varies in response to changes in the Fermi-level or quasi-Fermi position. This apparent shift in energy threshold is confirmed using time resolved sub-band-gap spectroscopy. We also demonstrate that such a variation of emission rate with changes in the Fermi-level position, if present within the depletion region near a barrier junction, is consistent with the details of the temperature dependence of the junction capacitance in intrinsic samples.

Tidal mechanisms of dynamical influence on oxygen recombination airglow in the mesosphere and lower thermosphere

The specific mechanism causing local time variations in the atomic oxygen recombination airglow emissions is investigated by examining the Lagrangian histories of air parcels in the wind fields of the diurnal tide generated by the Global Scale Wind Model of Hagan et al. (1995). The emission rate variations are found to be due to the vertical displacements of air parcels associated with the diurnal tide. The density and temperature induced airglow perturbations associated with the tide are out of phase with that associated with the vertical displacement and hence act to supress the intensity variations. The modelled emission rate variations with local time correspond closely to those deduced from recent observations in the mesosphere and lower thermosphere by the Wind Imaging Interferometer and the High Resolution Doppler Imager on the Upper Atmosphere Research Satellite. The observed latitudinal variations in the local time behaviour of these airglow emissions are due to latitudinal variations in the vertical velocity associated with the tide.

Sources of Fine Organic Aerosol. 8. Boilers Burning No. 2 Distillate Fuel Oil

Fine organic particulate matter emitted from an industrial-scale boiler burning no. 2 distillate fuel oil has been characterized on a molecular basis using GC/MS techniques. Most of the identified compound mass consists of n-alkanoic acids (42.0−51.5%), aromatic acids (5.8−22.6%), and n-alkanes (6.7−25.0%). Polycyclic aromatic hydrocarbons (PAH) and oxygenated PAH (oxy-PAH) together comprise 3.1−8.6% of the identifiable mass and together with chlorinated compounds (5.8−16.4%) show the largest variations in emission rates between the two experiments reported here. An increase in chlorinated compound emissions between tests is accompanied by a similar increase in elemental carbon (i.e., soot) and PAH emissions, which may follow the results of laboratory experiments that suggest that the presence of chlorinated compounds can enhance both soot and PAH formation. Differences between the hopanes distribution in the boiler exhaust versus that found in both vehicle exhaust and in the southern California atmosphere suggest that the oil-fired boiler exhaust is at most a minor contributor to the atmospheric aerosol, which is consistent with inferences drawn from local emission inventories.

Two day wave induced variations in the oxygen green line volume emission rate: WINDII observations

Variations in the oxygen green line emission rate (at 5577 Å) which correspond to a two day wave signature were observed from January 20 to January 30, 1993 by the Wind Imaging Interferometer (WINDII) on the Upper Atmosphere Research Satellite (UARS). The latitude and height extent of these emission variations are summarized. From the volume emission rate data, fields of oxygen mixing ratio using Mass Spectrometer and Incoherent Scatter (MSIS) model data as the background atmosphere were calculated. Analyses of the mixing ratio fields show the variations of emission rate to be primarily the result (to first order) of vertical motions associated with the quasi two day wave (QTDW), not mixing events associated with wave breaking. The mean peak‐to‐peak amplitude of the vertical displacement is of the order of 1 km although at times this amplitude is as much as 4 km. The phase of the mixing ratio variations approaches the two day wave zonal wind phase above 100 km as is expected from simple considerations of the phase relationships associated with planetary waves and the lifetime of atomic oxygen.

SEASONAL AND SPATIAL VARIATIONS IN NATURAL VOLATILE ORGANIC COMPOUND EMISSIONS

Atmospheric concentrations of ozone and other air pollutants, in some regions, are sensitive to surface fluxes of volatile organic compounds (VOCs). Plant foliage is the source of at least half of all VOC emissions in the United States and more than two-thirds of global VOC emissions. Observed spatial and seasonal variations in foliar VOC emissions range over several orders of magnitude. Land characteristics data are an important component of the modeling techniques used to estimate VOC emission rate variations due to seasonal and spatial changes in species composition, foliar density, and other factors. Model techniques and land characteristics databases are compared and evaluated in this paper. Significant differences in VOC fluxes are predicted depending on spatial resolution, procedures used to develop land characteristics databases, and foliar density models. Satellite and ground observations can be combined to generate the accurate estimates of the species composition and foliar density required for natural VOC emission models.

The BEMA-project—A North American perspective

Until recently, European regulatory agencies and photochemical modelers had to use biogenic hydrocarbon emission models that were based almost entirely on the results of investigations in North America. Field studies of biogenic hydrocarbon emissions have been conducted in North America and Europe for more than two decades but it is only recently that there has been an attempt at multidisciplinary (plant physiologists, micrometeorologists, atmospheric scientists, landscape ecologists) field studies. Whereas past studies involved a few scientists with similar backgrounds, recent studies have been much more multi-disciplinary efforts involving more scientists. In North America, investigations of biogenic hydrocarbon emissions have been important, but relatively minor, components of regional studies such as the Southern Oxidant Study in the United States and the BOREAS experiment in Canada. None of these studies approach the level of participation in biogenic emission studies that occurred in the BEMA program. The BEMA project also addressed another issue that is gaining interest in North America, seasonality. Variation in emission rates through the year need to be incorporated into models and longer field campaigns need to be incorporated into future experimental programs to better determine seasonality of hydrocarbon emissions. The BEMA study has provided valuable insight into hydrocarbon emissions in the Mediterranean ecosystem. Both the similarities and differences between this ecosystem and typical North America ecosystems help us to understand biogenic hydrocarbon emissions.

Effect of habitat and age on variations in volatile organic compound (VOC) emissions from Quercus ilex and Pinus pinea

A dynamic branch enclosure was used to measure emission rates of volatile organic compounds (VOCs) under field conditions from two common native Mediterranean species, Quercus ilex and Pinus pinea. In addition to α-pinene, β-pinene, sabinene, limonene and cineole, a suite of lesser known compounds were tentatively identified including cis- and trans-ocimene, cis- and trans-linalool oxide and sabinaketone. Emissions of isoprene from Quercus ilex were insignificant in comparison to those of the monoterpenes and were not detected from Pinus pinea. Variability in emission rates between two habitats, the forest and the dunes, were assessed for Quercus ilex. Temperature sensitivities of emissions and total summed emission rates from Quercus ilex were clearly related to environmental conditions. Emission rates from Pinus pinea showed great variability, but differences between normalised mean emission rates from mature forest and young plantation trees may be significant. Existing emission rate models were found to inadequately describe the observed data.

An Enclosure Technique for Measuring Nonmethane Organic Compound Emissions from Grasslands

AbstractA static chamber technique for measuring emissions of nonmethane organic compounds (NMOCs) from soil surfaces and herbaceous vegetation in a natural setting has been developed. The technique involves the insertion of a polymethyl methacrylate frame into the ground. A polycarbonate chamber is placed in a water‐containing trough on the frame to seal the chamber to the surface. The soil surface and vegetation are covered for various lengths of time, and samples of the chamber air are passively collected in preevacuated stainless steel canisters. The samples are analyzed in the laboratory by a whole‐air cryogenic‐preconcentration/high‐resolution gas chromatographic technique with flame ionization detection. Changes in the concentrations of the NMOCs with time in the chamber are used to derive emission rates. The technique is sensitive and precise, but the occasional observation of nonlinear changes in NMOC concentrations in the chamber with the length of the covering period limits the accuracy of emission rates derived by this technique. Despite this limitation, the method is effective for identifying NMOC emissions and investigating seasonal variations of emission rates.

Assessment of denitrification in organogenic forest soil by regulating factors

One of the dominating sources of the increase in atmospheric N2O are elevated emissions from terrestrial ecosystems. The N2O production in soil is probably dominated by two microbial processes, i.e. nitrification and denitrification, and their N2O production is regulated by an array of soil and environmental factors. This contributes to the large both spatial and temporal variability of emission rates measured in the field. To obtain reasonable estimates of large-scale emission rates this variability has to be dealt with, and one possible way could be to relate measured emission rates to relevant soil and/or environmental factors. In this study the relationship between soil factors (temperature, soil organic matter, soil moisture, pH, and NH4 +, NO3 - and NO2 - contents) and the denitrification activity in organogenic forest soil was investigated. About 40% or less of the variation of the denitrification rates could be accounted for by soil factors, either by multiple regression or by partial least squares analysis of latent variables (PLS analysis). Interpretation of the relationships obtained was, however, difficult since many of them were unexpected, and some of the soil factors were internally related. In conclusion, the possibility to obtain significant and useful relationships between momentary denitrification rates and soil factors for spatial and temporal extrapolations was limited in this system.

Feedlot Odor Emissions Following Heavy Rainfall

There is a need to develop predictive models for odor emissions from cattle feedlots. Surface emissions of odor from a feedlot pen were sampled under standardized aerodynamic conditions using a portable wind tunnel and tested with a forced-choice, dynamic-dilution olfactometer over a five-day period following 64 mm of rain. Pad moisture contents were monitored. No relationship between odor emission and pad moisture content alone was found even though the emission from the pad was up to 60 times greater under wet than dry conditions. A diurnal variation in emission rates was observed on some days. A hypothesis explaining the pattern of odor emission over time is proposed using an understanding of the kinetics of anaerobic fermentation. If shown correct by further research, the hypothesis will provide the basis for a model for the prediction of odor generation.

Emissions of NO from soil at a rural site in central Tennessee

Field measurements of soil emissions of NO from a Mountview silt loam soil with three land uses (forest, fertilized pasture, and fertilized corn) were made on a commercial farm during a summer and autumn sampling period. A new automated closed‐chamber sampling system was developed to allow simultaneous measurements on five chambers per 100 m2 plot. Individual chambers with hinged tops, covering 0.3 m2 of soil area were pneumatically operated via data logger control to sample soil NO flux every third hour. Spatial variability in emission rates was high. For each land use type the range from the lowest to the highest emitting chamber was approximately threefold. Land use type significantly affected soil NO emissions. The fertilized pasture had the highest mean emission rate (44.1 ng N m−2 s−1), followed by the fertilized corn (27.0 ng N m−2 s−1), and the forest (8.4 ng N m−2 s−1). NO emission rates and soil nitrate levels at the forest plot were considerably higher than at other forest sites in the region, possibly due to runoff from an adjacent fertilized hayfield. The results of this study, when extrapolated to a regional estimate, suggest that emissions of NO from soils could play a significant role in summertime tropospheric ozone photochemistry in the southeastern United States.