bluuebunny/crossref_metadata_2025 huggingface dataset:
Location of lightning discharges from a single station
Authors: Rafalsky V. A. , Nickolaenko A. P. , Shvets A. V. , Hayakawa M.
Published: February 2004

A new computer‐based ELF/VLF system for locating lightning discharges has been developed. Both the arrival azimuths of atmospherics and the distances to their sources are estimated. The direction‐finding technique uses the Poynting vector calculated directly in the time domain over the full band pass of the receiver. Both the distance of the lightning discharge and the ionospheric height can be estimated from the phase spectrum of the first‐order mode of the Earth‐ionosphere waveguide. The latter is approximated with a model function having the distance and the height as the main parameters. Two ways were applied to obtain the spectrum of the first mode: the radial component of the horizontal magnetic field was used, which contains only a minor component of the zeroth mode, or the mode decomposition problem was solved. The system has been used to locate lightning sources in Africa and Asia from a scientific vessel during its voyage in the Atlantic and Indian Oceans in 1991. The overall uncertainties are estimated to be a few degrees for the source bearing, 5% for distance, and 1% for the effective height of the ionosphere; yet these estimates need an additional confirmation by comparison with independent and more exact techniques.
Mapping the land surface for global atmosphere‐biosphere models: Toward continuous distributions of vegetation's functional properties
Authors: DeFries Ruth S. , Field Christopher B. , Fung Inez , Justice Christopher O. , Los Sietse , Matson Pamela A. , Matthews Elaine , Mooney Harold A. , Potter Christopher S. , Prentice Katharine , Sellers Piers J. , Townshend John R. G. , Tucker Compton J. , Ustin Susan L. , Vitousek Peter M.
Published: February 2004

Global land surface characteristics are important boundary conditions for global models that describe exchanges of water, energy, and carbon dioxide between the atmosphere and biosphere. Existing data sets of global land cover are based on classification schemes that characterize each grid cell as a discrete vegetation type. Consequently, parameter fields derived from these data sets are dependent on the particular scheme and the number of vegetation types it includes. The functional controls on exchanges of water, energy, and carbon dioxide between the atmosphere and biosphere are now well enough understood that it is increasingly feasible to model these exchanges using a small number of vegetation characteristics that either are related to or closely related to the functional controls. Ideally, these characteristics would be mapped as continuous distributions to capture mixtures and gradients in vegetation within the cell size of the model. While such an approach makes it more difficult to build models from detailed observations at a small number of sites, it increases the potential for capturing functionally important variation within, as well as between, vegetation types. Globally, the vegetation characteristics that appear to be most important in controlling fluxes of water, energy, and carbon dioxide include (1) growth form (tree, shrub, herb), (2) seasonality of woody vegetation (deciduous, evergreen), (3) leaf type (broadleaf, coniferous), (4) photosynthetic pathway of nonwoody vegetation (C3, C4), (5) longevity (annual, perennial), and (6) type and intensity of disturbance (e.g., cultivation, fire history). Many of these characteristics can be obtained through remote sensing, though some require ground‐based information. The minimum number and the identity of the required land surface characteristics almost certainly vary with the intended objective, but the philosophy of driving models with continuous distributions of a small number of land surface characteristics is likely to be applicable to a broad range of problems.
Measurement of multioxygen isotopic (δ18O and δ17O) fractionation factors in the stratospheric sink reactions of nitrous oxide
Authors: Johnston Jeffrey C. , Cliff Steven S. , Thiemens Mark H.
Published: February 2004

Nitrous oxide is an important component in the Earth's atmosphere with a global budget that shows significant imbalance between known sources and sinks. A recent isotopic analysis of atmospheric nitrous oxide has suggested that back fluxing from the stratosphere could be an important source of N2O to the troposphere. To quantitatively determine the potential magnitude of this process, the relevant isotopic fractionations must be known. We have measured the isotopic fractionation factors associated with the two known stratospheric sink reactions of N2O: photolysis and photooxidation. It is observed that nitrous oxide photolysis produces no fractionation in δ18O and δ17O, while photooxidation produces a small mass‐dependent enrichment in 18O and 17O in the residual N2O.
Validation of hydrogen chloride measurements made by the Halogen Occultation Experiment from the UARS platform
Authors: Russell James M. , Deaver Lance E. , Luo Mingzhao , Park Jae H. , Gordley Larry L. , Tuck Adrian F. , Toon Geoffrey C. , Gunson Michael R. , Traub Wesley A. , Johnson David G. , Jucks Kenneth W. , Murcray David G. , Zander Rudolphe , Nolt Ira G. , Webster Christopher R.
Published: February 2004

The Halogen Occultation Experiment (HALOE) on UARS uses the method of solar occultation limb sounding to measure the composition and structure of the stratosphere and mesosphere. One of the HALOE channels is spectrally centered at 3.4 μm to measure the vertical profile and global distribution of hydrogen chloride. The mean difference between HALOE and 14 balloon correlative underflight measurements ranges from 8% to 19% throughout most of the stratosphere. This difference is within the limits of error bar overlap for the two data sets. The mean differences between HALOE and HCl data from ATMOS flights on the space shuttle is of the order of 15 to 20% for the 1992 flight and 10% for the 1993 flight. Generally, HALOE results tend to be low in these comparisons. Also, comparisons with two‐dimensional model calculations and HALOE data are in good qualitative agreement regarding vertical profile shapes and features in a pressure versus latitude cross section. HCl values increase from ∼0.3 parts per billion by volume (ppbv) to 1 ppbv in the lower stratosphere to 2.6 ppbv to 3.3 ppbv just above the stratopause which is the upper limit of HALOE single‐profile measurements. There is a dependence of HCl results on the angle between the orbit plane and the Earth‐Sun vector with HCl varying by ±9% in the upper stratosphere. This variation appears to be altitude dependent and it is not discernible in the data below about 10 mbar.
Peroxy radicals measured during Mauna Loa Observatory Photochemistry Experiment 2: The data and first analysis
Authors: Cantrell Christopher A. , Shetter Richard E. , Gilpin Timothy M. , Calvert Jack G.
Published: February 2004

Peroxy radicals were measured using chemical amplification during the four intensive measurement periods of the Mauna Loa Observatory Photochemistry Experiment (MLOPEX 2) campaign from September 1991 to August 1992. One‐minute average radical concentrations were obtained with overall uncertainties of 8 to 16 parts per trillion by volume (pptv), depending on the variability of ambient ozone. Thirty‐minute averaged concentrations have uncertainties of ±30%. The peroxy radicals exhibited diurnal behavior expected of an atmospheric species with photochemical origins and correlated with a simple expression that describes radical formation and loss. The application of a steady state analysis to the radical levels suggested that for some conditions, loss processes other than gas phase chemistry may be required to explain the observed concentrations.
Improvements in Nimbus 7 limb infrared monitor of the stratosphere ozone profiles as obtained with updated spectral line parameters and radiance algorithms
Authors: Remsberg Ellis , Burton John , Gordley Larry , Marshall Benjamin T. , Bhatt Praful , Miles Thomas
Published: February 2004

Ozone distributions from the Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) experiment of 1978–1979 are generally in good agreement with other concurrent ozone measurements in the middle and upper stratosphere, but not at lower altitudes. LIMS ozone is too large below about the 15‐hPa (or millibar) level, particularly at lower latitudes. A new LIMS ozone distribution is presented for 1 day of profiles, May 5, 1979, obtained with an improved forward radiance algorithm and using the spectral line parameters for the 9‐ to 10‐μm region in the HITRAN 92 compilation. However, we also divided our single day of retrieved ozone mixing ratio profiles by a suggested factor of 1.051, which makes them compatible with the ozone distributions from several of the UARS experiments. Our revised distribution still agrees with the archived LIMS ozone to better than ±5% above about the 15‐hPa level, but there is a significant decrease for the revised ozone in the lower stratosphere. That decrease approaches 25% at 50 hPa in the tropics. In general, the revised LIMS ozone is now in accord with the comparison data sets for the stratosphere, except for the lowest levels (Umkehr layer 3 or 63–127 hPa), where the effects of interfering species and small biases in LIMS temperatures are most pronounced. It is concluded that the current 9‐ to 10‐μm ozone line list is adequate for obtaining good quality ozone mixing ratio profiles from satellite broadband limb‐infrared measurements.
Ozone in the marine atmosphere observed during the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange
Authors: Noone Kevin J. , Schillawski Richard D. , Kok Gregory L. , Bretherton Christopher S. , Huebert Barry J.
Published: February 2004

Measurements of vertical profiles and level averages of ozone over the north central Atlantic near the Azores are presented. Lagrangian experiments were carried out in clean air and polluted air, and differences between the cases are discussed. Significant vertical structure was found in the profiles from both cases. Trajectory analysis indicates that the source regions for ozone in the marine boundary layer and in the air above the inversion capping the boundary layer were different. No significant production or destruction of ozone in the boundary layer was observed in the clean case. No substantial differences in ozone concentration between cloudy and clear air were observed, although cloud processes in the polluted case may have been partially responsible for an observed overall reduction in boundary layer ozone concentration.
Rocket and balloon observations of electric field in two thunderstorms
Authors: Marshall Thomas C. , Rison William , Rust W. David , Stolzenburg Maribeth , Willett John C. , Winn William P.
Published: February 2004

Instruments that measure the intense electric field strengths in thunderclouds (∼100 kV m−1) are designed to minimize the production of ions by small electrical discharges (coronas) emanating from the instruments themselves. The nearby charge of these ions would unpredictably disturb the natural field of the cloud. In an attempt to assess this disturbance, two different instruments (one carried by a rocket and one carried by a balloon) were launched on two occasions into thunderstorms. In spite of differing trajectories, the soundings were similar, which gives us some confidence in both instruments. In addition, the measurements revealed some interesting features of the two storms. Each storm appeared to have six significant and distinct regions of charge. The balloon soundings also revealed that lightning flashes temporarily increased the electric field strength above the thunderclouds (at altitudes from 9.7 to 14.3 km) by amounts up to 10 kV m−1, after which the fields decayed away in 50 to 125 s. One pair of ascent and descent rocket soundings, separated in time by a maximum of 60 s and horizontally by 1 to 3 km, showed little change in the thunderstorm electric field between ground and 7.5 km altitude.
Airborne hydrogen peroxide measurements in supercooled clouds
Authors: Snider Jefferson R. , Murphy Todd
Published: February 2004

Airborne measurements of hydrogen peroxide and its partitioning between the aqueous and gas phases were conducted during studies of wintertime orographic clouds at temperatures ranging between −9° and −24°C. A subset of the derived values of the H2O2 Henry's law coefficient, based on measurement of gas‐phase H2O2 and the physical properties of the cloud, were found to agree with laboratory measurements extrapolated to the cloud temperature. Disparities between the derived and temperature‐extrapolated values were also observed, but this was attributed to overestimates of the in‐cloud H2O2 mixing ratio that result from solute H2O2 volatilization on the reverse‐flow air sample inlet during cloud droplet impaction and freezing. This hypothesis is supported by the fact that the derived Henry's law coefficients which exhibit agreement with the temperature‐extrapolated values were conducted using a narrower inlet which intercepts less cloud water. Also discussed are vertical profiles of gaseous H2O2 within and above a cloud‐capped boundary layer and in‐cloud measurements which reveal that H2O2 scavenging by ice hydrometeors is minimal in comparison to uptake by an equivalent mass of liquid cloud droplets.
Lower stratospheric stationary waves and the longitude dependence of ozone trends in winter
Authors: Hood L. L. , Zaff D. A.
Published: February 2004

At northern midlatitudes, total ozone trends are largest in winter when the longitudinal distribution of ozone is highly asymmetric owing to dynamical forcing by stationary, planetary‐scale waves. A distinct longitude dependence of these midlatitude trends for the 1979 to 1991 time period has previously been derived statistically from Nimbus 7 total ozone mapping spectrometer data. Here we apply a mechanistic ozone transport model to investigate the extent to which the geographic dependence of total ozone trends in January can be explained as being due to decadal changes in the amplitudes and phases of stationary planetary waves. Results indicate that most or all of the statistically derived geographic dependence can be explained in this manner. In support of the mechanistic model calculations, lower stratospheric geopotential height differences or “anomalies” between groups of Januaries near the end of the 1980s and near the beginning of the 1980s are inversely correlated with the geographic distribution of ozone trends. The observed decadal changes in upper tropospheric and lower stratospheric stationary wave properties are most probably a consequence of decadal climate variability in the troposphere.
Fluxes of nitric oxide from soils following the clearing and burning of a secondary tropical rain forest
Authors: Neff J. C. , Keller M. , Holland E. A. , Weitz A. W. , Veldkamp E.
Published: February 2004

At sites in the Atlantic Lowlands of Costa Rica, clearing and burning of a secondary tropical rain forest caused a significant increase in soil nitric oxide (NO) emissions. Soil‐atmosphere NO fluxes averaged 0.5 ng N cm−2 hr−1 prior to clearing and increased to 4.1 ng N cm−2 hr−1 following clearing and to greater than 12.0 ng N cm−2 hr−1 following burning. Soil NO emissions were elevated for a period of 3–4 months following clearing, and fluxes peaked for 1–3 days following burning. We conducted a series of experiments with intact soil cores to determine the probable mechanism responsible for elevated NO emissions from soils. In one set of experiments we added substrates for microbial nitrification (ammonium), denitrification (nitrate), and chemical denitrification (nitrite) to autoclaved (killed) and nonautoclaved (live) soil cores. Water‐only additions were used as controls. Compared to water or nitrate additions, ammonium caused a significant increase in NO emissions from live cores. Water, ammonium, and nitrate additions had no effect on emissions from autoclaved cores. Nitrite solution additions resulted in highly significant increases in NO emissions from both autoclaved and nonautoclaved soil cores. In a second set of experiments we treated intact soil cores with acetylene (1 kPa C2H2) to selectively inhibit nitrification and oxygen to inhibit denitrification. The oxygen treatment had no effect on NO production while acetylene significantly reduced NO emissions. The results from the substrate addition and inhibition experiments demonstrate that microbial denitrification is not a major pathway for NO production in these soils. In contrast, microbial nitrification appears to be a critical process responsible for NO emissions throughout the clearing and burning period. Field experiments with acetylene as an inhibitor show that immediately following burning, chemical denitriflcation of nitrite deposited in ash supports a large peak in NO fluxes.
Process modeling of controls on nitrogen trace gas emissions from soils worldwide
Authors: Potter Christopher S. , Matson Pamela A. , Vitousek Peter M. , Davidson Eric A.
Published: February 2004

We report on an ecosystem modeling approach that integrates global satellite, climate, vegetation, and soil data sets to (1) examine conceptual controls on nitrogen trace gas (NO, N2O, and N2) emissions from soils and (2) identify weaknesses in our bases of knowledge and data for these fluxes. Nitrous and nitric oxide emissions from well‐drained soils were estimated by using an expanded version of the Carnegie‐Ames‐Stanford (CASA) Biosphere model, a coupled ecosystem production and soil carbon‐nitrogen model on a 1° global grid. We estimate monthly production of NO, N2O, and N2 based on predicted rates of gross N mineralization, together with an index of transient water‐filled pore space in soils. Analyses of model performance along selected climate gradients support the hypothesis that low temperature restricts predicted N mineralization and trace gas emission rates in moist northern temperate and boreal forest ecosystems, whereas in tropical zones, seasonal patterns in N mineralization result in emission peaks for N2O that coincide with wetting and high soil moisture content. The model predicts the annual N2O:NO flux ratio at a mean value of 1.2 in wet tropical forests, decreasing to around 0.6 in the seasonally dry savannas. Global emission estimates at the soil surface are 6.1 Tg N and 9.7 Tg N yr−1 for N2O and NO, respectively. Tropical dry forests and savannas are identified by using this formulation as important source areas for nitrogen trace gas emissions. Because humans continue to alter these ecosystems extensively for agricultural uses, our results suggest that more study is needed in seasonally dry ecosystems of the tropics in order to understand the global impacts of land use change on soil sources for N2O and NO.
Evolution of alkyl nitrates with air mass age
Authors: Bertman Steven B. , Roberts James M. , Parrish David D. , Buhr Martin P. , Goldan Paul D. , Kuster William C. , Fehsenfeld Fred C. , Montzka Stephen A. , Westberg Hal
Published: February 2004

Alkyl nitrates (RONO2) and alkanes (RH) were measured at Scotia, Pennsylvania, in 1988 and at the Kinterbish Wildlife Area, Alabama, in 1992. A simple kinetic analysis was developed to relate the two species' concentrations based on the analytical solution of two sequential first‐order reactions using OH chemistry, RONO2 photolysis, and assuming peroxy radicals (ROO) + NO as the source of RONO2. The analysis predicts an increase in RONO2/RH ratio with time. The C4 and C5 secondary alkyl nitrate/alkane ratios vary in a manner consistent with ROO + NO chemistry as their only source and photolysis and OH reaction as their only sinks. Comparison of ambient measurements of these compounds with predictions indicate that measured air masses experienced photochemical processing times consistent with alkyl nitrate evolution times between 0.1 and 5 days. The relationships of ethyl nitrate, n‐propyl nitrate and 2‐propyl nitrate ratios to their parent alkanes all indicate additional sources of RONO2, probably of a photochemical nature. Decomposition of larger alkoxy radicals are discussed as one possible source of smaller ROO radicals.
A residual circulation derived from an energy balance climate model
Authors: Chen D.
Published: February 2004

A modelling approach treating the problem of changing climate and trace gases is presented. The approach uses the atmospheric temperature and diabatic heating rate computed by the seasonal, multi‐layer GLAS energy balance climate model to derive a seasonal residual circulation. The derived circulation corresponding to the present climate is found to be comparable to similar previous computations. This circulation is used to estimate air mass exchange among principal atmospheric domains. It is estimated that the mean turnover time for the air exchange between the troposphere and stratosphere is about 3.5 years and that between the two hemispheres, about 1.2 years. Sensitivities of the circulation to incoming solar radiation and CO2 concentration are investigated. As the forcings increase, the residual circulation is intensified, which implies an enhancement of the air mass exchange between the troposphere and stratosphere and between the two hemispheres. This result may have implications for atmospheric chemistry.
Mathematical modeling of atmospheric fine particle‐associated primary organic compound concentrations
Authors: Rogge Wolfgang F. , Hildemann Lynn M. , Mazurek Monica A. , Cass Glen R. , Simoneit Bernd R. T.
Published: February 2004

An atmospheric transport model has been used to explore the relationship between source emissions and ambient air quality for individual particle phase organic compounds present in primary aerosol source emissions. An inventory of fine particulate organic compound emissions was assembled for the Los Angeles area in the year 1982. Sources characterized included noncatalyst‐ and catalyst‐equipped autos, diesel trucks, paved road dust, tire wear, brake lining dust, meat cooking operations, industrial oil‐fired boilers, roofing tar pots, natural gas combustion in residential homes, cigarette smoke, fireplaces burning oak and pine wood, and plant leaf abrasion products. These primary fine particle source emissions were supplied to a computer‐based model that simulates atmospheric transport, dispersion, and dry deposition based on the time series of hourly wind observations and mixing depths. Monthly average fine particle organic compound concentrations that would prevail if the primary organic aerosol were transported without chemical reaction were computed for more than 100 organic compounds within an 80 km × 80 km modeling area centered over Los Angeles. The monthly average compound concentrations predicted by the transport model were compared to atmospheric measurements made at monitoring sites within the study area during 1982. The predicted seasonal variation and absolute values of the concentrations of the more stable compounds are found to be in reasonable agreement with the ambient observations. While model predictions for the higher molecular weight polycyclic aromatic hydrocarbons (PAH) are in agreement with ambient observations, lower molecular weight PAH show much higher predicted than measured atmospheric concentrations in the particle phase, indicating atmospheric decay by chemical reactions or evaporation from the particle phase. The atmospheric concentrations of dicarboxylic acids and aromatic polycarboxylic acids greatly exceed the contributions that are due to direct emissions from primary sources, confirming that these compounds are principally formed by atmospheric chemical reactions.
The land surface‐atmosphere interaction: A review based on observational and global modeling perspectives
Authors: Betts Alan K. , Ball John H. , Beljaars Anton C. M. , Miller Martin J. , Viterbo Pedro A.
Published: February 2004

This review discusses the land‐surface‐atmosphere interaction using observations from two North American field experiments (First International Satellite Land Surface Climatology Project Field Experiment (FIFE) and Boreal Ecosystem Atmosphere Study (BOREAS)) and the application of research data to the improvement of land surface and boundary layer parameterizations in the European Centre for Medium‐Range Weather Forecast (ECMWF) global forecast model. Using field data, we discuss some of the diurnal and seasonal feedback loops controlling the net surface radiation and its partition into the surface sensible and latent heat fluxes and the ground heat flux. We consider the impact on the boundary layer evolution and show the changes in the diurnal cycle with soil moisture in midsummer. We contrast the surface energy budget over the tropical oceans with that over both dry and wet land surfaces in summer. Results from a new ECMWF model with four predicted soil layers illustrate the interaction between the soil moisture reservoir, evaporation and precipitation on different timescales and space scales. An analysis of an ensemble of 30‐day integrations for July 1993 (the month of the Mississippi flood) showed a large sensitivity of the monthly precipitation pattern (and amount) to different initial soil moisture conditions. Short‐range forecasts with old and new land surface and boundary layer schemes showed that the new scheme produced much better precipitation forecasts for the central United States because of a more realistic thermodynamic structure, which in turn resulted from improved evaporation in an area that is about 1‐day upstream. The results suggest that some predictability exists in the extended range as a result of the memory of the soil moisture reservoir. We also discuss briefly the problem of soil moisture initialization in a global forecast model and summarize recent experience with nudging of soil moisture at ECMWF and improvements in the surface energy budget coming from the better prediction of clouds.
Production of a filtered and standardized surface flux data set for FIFE 1987
Authors: Heiser Mark D. , Sellers Piers J.
Published: February 2004

A filtered and standardized surface heat flux data set, which includes net radiation (Rn), latent heat flux (λE), sensible heat flux (H), and ground heat flux (G), was produced from an analysis of field measurements taken by a network of 16 Bowen ratio and 6 eddy correlation instruments deployed during the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment in 1987 (FIFE 1987). The objective was to produce time series of flux data that could be used for studies involving surface‐vegetation‐atmosphere‐transfer (SVAT) models operating in the inverse mode (see Sellers et al. 1992b). To be useful for this work, erroneous flux data had to be identified and systematic instrument‐related differences between the net radiation values reported by flux stations had to be accounted for. The following procedures were carried out on the original data sets as contributed by individual FIFE scientists: (1) A flag (IDATA) was assigned to unreliable flux data as defined by the presence of anomalies (“spikes”) in the sensible and latent heat flux time series, discontinuities generated when the temperature and humidity profiles crossed over at dawn and dusk, and data points isolated by the erasure of neighboring unreliable data. In addition, Bowen ratio data were also considered to be unreliable whenever the surface energy budget was reported as nonzero (Rn + λE + H + G ≠ 0). (2) A simple procedure was used to adjust the net radiation values reported by two of the Bowen ratio stations to appear as if they were using the same type of net radiometer as most of the other Bowen ratio stations deployed in FIFE 1987. Corresponding changes were made to the associated sensible and latent heat fluxes for these two stations; the ground heat fluxes were left unchanged. At one eddy correlation site (site 26), the sensible and latent heat fluxes were suspected to be systematically underestimated; these were proportionately increased to match the site net radiation flux minus a FIFE‐area‐average ground heat flux. (3) An additional flag (IDF) was assigned to data based on modeling results obtained from the simple biosphere (SiB) model (see Sellers et al., 1992b). Nonzero values of IDF denote periods when the flux data time series diverge significantly from a set of calibrated model results, suggesting a bias in the data or indicating that SiB could not reproduce the observed latent and sensible heat flux time series using a reasonable set of surface parameters. An IDF flag was assigned to each flux station for each intensive field campaign (IFC). (4) Using the IDATA and IDF flags to identify and exclude questionable data points, a site‐averaged surface heat flux data set was computed for each IFC from the remaining data. This is compared to a similar “hand‐filtered” data set produced by Betts and Ball as published by Strebel et al. (1994).
Biogenic NO emissions from savanna soils as a function of fire regime, soil type, soil nitrogen, and water status
Authors: Parsons Dirk A. B. , Scholes Mary C. , Scholes Robert J. , Levine Joel S.
Published: February 2004

A study of NOx emissions from soils representative of nutrient‐poor and nutrient‐rich savannas and their response to burning and soil water content was carried out in the southern Kruger National Park, South Africa. The study spanned the end of the dry season and the beginning of the wet season (September–December 1992). Nitrogen mineralization rates were measured using an in situ technique simultaneously with measurements of NOx emissions. NOx emissions were almost entirely as NO. The relationship between NO emission rate and soil moisture was parabolic regardless of soil type and management practice, with the lowest NO emission rates being measured at low (<0.087) and high (>0.542) water‐filled pore space values. The initial increase in NO emission rates with increasing soil moisture are paralleled by increases in the nitrate concentration in the soil. The highest NO emission rates (20 ng N‐NO m−2 s−1—excluding the brief initial peak) were measured on plots from which fire had been excluded for 35 years. The next highest rates (8 ng N‐NO m−2 s−1) were measured on the more fertile soils. Infertile soils, burned every second year, had rates of 3.5 ng N‐NO m−2 s−1. The NO emission rates show a positive correlation with soil total N content and N nitrification rate. The effect of excluding fire from a savanna is to increase the soil nitrogen content through increased litter inputs, which in turn increases nitrification rates and soil NO emissions.
Measurements of stratospheric winds by the high resolution Doppler imager
Authors: Ortland David A. , Skinner Wilbert R. , Hays Paul B. , Burrage Mark D. , Lieberman Ruth S. , Marshall Alan R. , Gell David A.
Published: February 2004

The high resolution Doppler imager (HRDI) on the Upper Atmosphere Research Satellite (UARS) determines winds in the stratosphere (10–40 km) by measuring the Doppler shift of the rotational lines in the O2 atmospheric B and γ bands. These lines are observed as absorption features in scattered sunlight. The Doppler shifts are found by fitting the observed high‐resolution spectrum to a single‐scattering model. The model includes effects due to scattering from clouds and the ground. Results are compared to radiosonde measurements and the standard deviation between the two measurement systems is found to be between 8 m/s and 12 m/s. This standard deviation includes the errors in both measurement systems as well as geophysical noise due to the different sampling times, sampling locations, and sampling volumes. Monthly averages of HRDI stratospheric winds near the equator are examined from December 1991 to April 1995. These averages reveal the three‐dimensional structure of the quasi‐biennial oscillation (QBO) and its evolution through one full cycle. In particular, we note that the QBO extends over a wide latitude and altitude range and that the easterly and westerly descent rates are similar above 25 km. HRDI measurements show that there is strong meridional shear in the zonal winds above 35 km during the solstices, indicating that inertial instability may play a role in the dynamics of the upper stratosphere. The HRDI wind measurements also show that there is a significant annual cycle in the tropics and that there is substantial interannual variability in the semiannual oscillation in the upper stratosphere.
Thoron and radon fluxes from the island of Hawaii
Authors: Whittlestone S. , Schery S. D. , Li Y.
Published: February 2004

The accumulator method has been used to determine the flux of thoron and radon from 42 varied sites in Hawaii. This is the first part, aimed at measuring source terms, of a two‐part study of the use of the decay product of thoron, Pb‐212, as an atmospheric tracer. It was of particular interest to scientists at the baseline atmospheric sampling laboratory on Mauna Loa to know the origin of Pb‐212 in their air samples. The observed average flux from lowland soils was 173 mBq m−2s−1 (4.7 pCi m−2s−1) on the wet eastern side of the island and 854 mBq m−2s−1 (23 pCi m−2s−1) on the west. Barren lava yielded 20 mBq m−2s−1, which is thought sufficient to contribute a significant amount of Pb‐212 to the air stream after a path of a few miles over lava. A comparison of the radon flux measurements with a previous survey indicated that there are seasonal variations of radon and thoron fluxes.
Wind and temperature profiles in the boundary layer above the Kruger National Park during SAFARI‐92
Authors: Held G.
Published: February 2004

The experimental phase of SAFARI‐92 in the Kruger National Park took place from September 7 to 26, 1992. Eskom's Environmental Sciences was committed to provide meteorological support during the experimental burns in the Pretoriuskop area of the KNP and to characterize the boundary layer during the field campaign. Surface temperature inversions were found during most nights when vertical soundings were available. The inversion strength was generally ≤3.5° with a depth of ≤270 m above ground level (agl). Low‐level elevated inversions with a base height of 350 to 500 m agl and a strength of ≤3.6°C were found on four occasions. The base height of the subsidence inversion, when observed, varied between 1500 and 2500 m agl. Significant superadiabatic temperature gradients, impacting directly on the vertical dispersion of pyrogenic products, have been observed to reach as high as several hundred meters above ground level. Vertical profiles of wind speed and direction varied greatly from day to day in response to the changes in the synoptic pattern. However, weak low‐level wind maxima just above the surface inversion were observed during most nights, generally with speeds of <10 m s−1. Detailed case studies of boundary layer conditions during the major burns have been included. Since the observations were made during spring it is suggested that the results could be more characteristic of summer conditions. It can be assumed that the inversions will be stronger and the low‐level wind maxima more pronounced during winter.
Rapid vertical trace gas transport by an isolated midlatitude thunderstorm
Authors: Hauf Thomas , Schulte Peter , Alheit Reiner , Schlager Hans
Published: February 2004

During the cloud dynamics and chemistry field experiment CLEOPATRA in the summer of 1992 in southern Germany, the Deutsche Forschungsanstalt für Luft‐ und Raumfahrt (DLR) (German Aerospace Research Establishment) research aircraft Falcon traversed four times the anvil of a severe, isolated thunderstorm. The first two traverses were at 8 km altitude and close to the anvil cloud base, while the second two traverses were at 10 km. During the 8‐km traverse, measured ozone mixing ratios dropped by 13 parts per billion by volume (ppbv) from the ambient cloud free environment to the anvil cloud, while water vapor increased by 0.3 g kg−1. At the 10‐km traverses, ozone dropped by 25 ppbv, while water vapor increased by 0.18 g kg−1. Three‐dimensional numerical thunderstorm simulations were performed to understand the cause of these changes. The simulations included the transport of two chemical inert tracers. Ozone was assumed to be one of them. The initial ozone profile was composed from an ozone routine sounding and the in situ Falcon measurements prior to the thunderstorm development. The second tracer is typical for a surface released pollutant with a nonzero, constant value in the boundary layer but zero above it. The redistribution of both tracers by the storm is calculated and compared with the observations. For the anvil penetration at 10 km, the calculated difference in ozone mixing ratios is 21 ppbv, while for water vapor an increase of 0.25 g kg−1 was found, in good agreement with the observations. To validate the model results, the radar reflectivity was calculated from simulated fields of cloud water, rain, graupel, hail, and snow and ice crystals and compared with observed values. With respect to maximum reflectivity values and spatial scales, again, excellent agreement was achieved. It is concluded that the rapid transport from the boundary layer directly into the anvil level is the most likely cause of the observed ozone decrease and water vapor increase. Entrainment of ozone‐rich environmental air into the anvil cloud occurred but left a protected core with undiluted boundary layer air in the anvil cloud even at a distance of 120 km from the main updraft. Processes such as production of O3 by electrical discharges, chemical reactions of ozone with boundary layer‐released or lightning‐produced nitrogen compounds, scavenging by hydrometeors, and heterogeneous reactions at the surface of ice crystals may occur, but on the timescale of 0.5–1 hour seem to have a negligible influence on the observed ozone drop.
Experimental and theoretical light scattering profiles from spherical and nonspherical particles
Authors: Hirst Edwin , Kaye Paul H.
Published: February 2004

A laser light scattering instrument has been designed to allow an investigation of the wide angle spatial intensity distribution, or scattering profile, of light scattered by individual airborne particles constrained within a laminar flow sample airstream, with a view to providing a means of classifying the particles in terms of their shape and size. Such an instrument may find application in areas of environmental monitoring such as studies of cloud interstitial aerosol or the detection of small concentrations of potentially hazardous particles such as respirable fibers. The instrument has been challenged with both spherical and nonspherical particle types and appropriate theoretical comparisons have been made with the experimental profiles using either Mie theory or a simple treatment based on the Rayleigh‐Gans formalism. Both experimental and theoretical scattering data are presented, showing good agreement for all particle types examined.
Millimeter wave spectroscopic measurements over the South Pole: 1. A study of stratospheric dynamics using N2O observations
Authors: Crewell Susanne , Cheng Dongjie , de Zafra Robert L. , Trimble Curt
Published: February 2004

Millimeter wave measurements of N2O and O3 [Cheng et al., 1995], along with several other trace gases, have been made nearly continuously from February 1993 through early January 1994 at the Amundsen‐Scott Station, South Pole. In order to separate chemical and dynamical effects, this paper uses the observations of the long‐lived tracer N2O to study stratospheric dynamics. The main emphasis is on the synoptic evolution of the polar vortex over an entire winter period, and quantitative results are given for various times and altitudes. Diabatic descent rates derived for different altitude levels showed the strongest descent in austral fall at high altitudes, agreeing fairly well with model predictions by Rosenfield et al. (1994). Subsidence was observed to continue until late October, well after polar sunrise. The breakdown of the vortex occurred first in the upper stratosphere, marked in the intrusion of N2O rich air at these altitudes, consistent with trajectory calculations. Our calculated descent rates are not consistent with the idea that the polar vortex is a “flowing processor”, but instead should be viewed as an isolated system.
Spectral optical properties of the polluted atmosphere of Mexico City (spring‐summer 1992)
Authors: Vasilyev O. B. , Contreras A. Leyva , Velazquez A. Muhlia , Fabi R. Peralta y , Ivlev L. S. , Kovalenko A. P. , Vasilyev A. V. , Jukov V. M. , Welch Ronald M.
Published: February 2004

A joint Mexican, Russian, and American research effort has been initiated to develop new methods to remotely sense atmospheric parameters using ground‐based, aircraft, and satellite observations. As a first step in this program, ground‐based spectrophotometric measurements of the direct solar radiation have been obtained for the extremely polluted Mexico City atmosphere for the period of April–June 1992. These observations were made at mote than 1300 channels in the spectral range of 0.35–0.95 μm. In the ultraviolet (UV) portions of the spectrum (e.g., 0.35 μm), aerosol optical thicknesses were found to range between 0.6 and 1.2; in the visible portion of the spectrum (e.g., 0.5 μm) they ranged from 0.5 to 0.8; and in the near‐infrared (NIR) spectra (e.g., 0.85 μm), values of 0.3–0.5 were found. Applying a spectral optical depth (SOD) model of τ(λ) = C + A λ−∝, values of 1.55 < ∝ < 1.85 were obtained for polluted, cloudless days, with values of 1.25 < ∝ < 1.60 on days with haze. The aerosol particles in the polluted Mexico City atmosphere were found to be strongly absorbing, with a single‐scattering albedo of 0.7–0.9 in the UV, 0.6–0.8 in the visible portion of the spectrum, and 0.4–0.7 in the NIR. These values are possibly consistent with a high soot concentration, contributed both by vehicular traffic and heavy industry. Analysis of the measured aerosol SOD using the optical parameters of an urban aerosol model permits the concentration of aerosol particles to be estimated in the vertical column; a maximum value of 3×109 cm−2 was found. This concentration of aerosol particles exceeds that found in most other regions of the globe by at least an order of magnitude. Near the ground the aerosol size distributions measured using an optical particle counter were found to be strongly multimodal.
General circulation model assessment of the sensitivity of direct climate forcing by anthropogenic sulfate aerosols to aerosol size and chemistry
Authors: Boucher Olivier , Anderson Theodore L.
Published: February 2004

Climate response to atmospheric changes brought about by human activity may depend strongly on the geographical and temporal pattern of radiative forcing [Taylor and Penner, 1994]. In the case of aerosols stemming from anthropogenic sulfur emissions, geographical and temporal variations are certainly caused by variations in local mass concentration [Charlson et el., 1991; Kiehl and Briegleb, 1993], but could also arise from variations in the optical properties of sulfate aerosols. Since optical properties (including their relative humidity (RH) variation) depend fundamentally on aerosol size and chemical form and since size and chemical form are features of the aerosol which are not likely to be modeled on the global scale in the near future, geographical and temporal variations in optical properties could represent a stumbling block to accurate climate change forecasts. While extensive measurements of aerosol optical properties are needed to fully assess this problem, a preliminary assessment can be gained by considering the sensitivity of climate forcing to realistic variations in sulfate aerosol size and chemical form. Within a plausible set of assumptions (sulfate aerosol resides in the accumulation mode size range and only interacts with water vapor and ammonia vapor), we show that this sensitivity is fairly small (±20%). This low sensitivity derives from a number of compensating factors linking the three optical parameters identified by Charlson et al. [1991]. By implication, these optical parameters, low RH scattering efficiency, the ratio of hemispheric backscatter to total scatter, and the RH dependence of scattering efficiency, should not be treated independently in either theoretical or experimental investigations of direct climate forcing. A suggested logical focus for such investigations is the backscatter efficiency at high RH. If borne out by future research, low sensitivity to sulfate aerosol size and chemistry would mean that direct sulfate climate forcing can be incorporated in global climate models with only a knowledge of sulfate mass concentration. We emphasize, therefore, the need to study the extent to which our assumptions break down, in particular, the fraction of anthropogenic sulfate that forms on coarse mode particles (i.e., those with diameters >1 μm) and the extent and effects of sulfate interactions with other accumulation mode components. Finally, we find that a significant fraction of direct aerosol forcing occurs in cloud‐covered regions, according to a simple bulk parameterization.
Effect of fuel composition on combustion efficiency and emission factors for African savanna ecosystems
Authors: Ward D. E. , Hao W. M. , Susott R. A. , Babbitt R. E. , Shea R. W. , Kauffman J. B. , Justice C. O.
Published: February 2004

Savanna burning in Africa occurs over a wide range of environmental, vegetation, and land use conditions. The emission factors for trace emissions from these fires can vary by a factor of 6 to 8, depending on whether the fires burn in miombo woodlands or in ecosystems where grass vegetation dominates. Ground‐based measurements of smoke emissions and aboveground biomass were made for fires in grassland and woodland savanna ecosystems in South Africa and Zambia. A high combustion efficiency () was measured for the pure grassland; i.e., a high proportion of carbon was released as CO2. Thewas lower for woodland savanna ecosystems with variable amounts of grass and with a more compact layer of leaf material and litter lying near the ground. Thewas found to be dependent on the ratio of grass to the sum of grass and litter. Models developed for estimating emissions were integrated in a nomogram for estimating total emissions of CO2, CO, CH4, nonmethane hydrocarbons, and particles of less than 2.5 μm diameter per unit area.
Three‐dimensional model study of organized mesoscale circulations induced by vegetation
Authors: Seth A. , Giorgi F.
Published: February 2004

Organized mesoscale circulations induced by vegetation discontinuities and their effects on the vertical transport of heat and moisture are investigated under realistic atmospheric conditions. A regional climate model (RegCM2) is used to simulate local circulations which result from contrasts in surface thermal forcing over land. The limited area domain, with a grid resolution of 10 km, is forced at the boundaries by analyses of observations for the 30‐day period of June 1990. Surface vegetation and soil moisture are idealized to provide strong, geometric contrasts in surface heating. Control (homogeneous surface) and perturbation (surface thermal contrast) simulations are required to isolate the circulation signal. Differences in the wind, temperature, and humidity fields show the diurnal evolution of coherent circulations during the simulated month. On three days, circulations having strength similar to a land‐sea breeze occur. Vertical fluxes of heat and moisture, which result from the circulations, show a different vertical structure and diurnal development than turbulent fluxes. The vertical transport of moisture is found to be comparable to or larger than transport by turbulent processes. Vertical transport of heat by the circulations is small. An increase is seen in the monthly total large‐scale precipitation downwind of the ascending region of the circulations. Of the large‐scale forcings studied, the circulation is most closely correlated to the mean horizontal temperature gradient.