bluuebunny/crossref_metadata_2025 huggingface dataset:
Removal of NOx and NOy in biomass burning plumes in the boundary layer over northern Australia
Authors: Takegawa N. , Kondo Y. , Koike M. , Ko M. , Kita K. , Blake D. R. , Nishi N. , Hu W. , Liley J. B. , Kawakami S. , Shirai T. , Miyazaki Y. , Ikeda H. , Russel‐Smith J. , Ogawa T.
Published: May 2003

The Biomass Burning and Lightning Experiment Phase B (BIBLE‐B) aircraft measurement campaign was conducted over the western Pacific and Australia in August and September 1999. In situ aircraft measurements of carbon monoxide (CO), nitric oxide (NO), total reactive nitrogen (NOy), ozone (O3), nonmethane hydrocarbons (NMHCs), and other species were made during BIBLE‐B. Meteorological analysis shows that the trace gases emitted from biomass burning in northern Australia were mostly confined within the planetary boundary layer (below ∼3 km) by strong subsidence in the free troposphere. Removal processes of NOx (equal to measured NO + calculated NO2) and NOy in biomass burning plumes in the boundary layer are examined on the basis of correlation analysis. The photochemical lifetime of NOx in biomass burning plumes during the daytime is estimated to be 0.1 to 0.3 days using the correlations of NOx with short‐lived NMHCs and hydroxyl radical (OH) concentration calculated from a constrained photochemical model. Correlation of NOy with CO shows that ∼60% of the NOy molecules originating from biomass burning were removed in the boundary layer within 2–3 days. This result is consistent with dry deposition of nitric acid (HNO3) in the plumes. It is likely that only a small fraction of NOy emitted from biomass burning was exported from the boundary layer to the free troposphere during the BIBLE‐B period.
Relation between interannual variations in satellite measures of northern forest greenness and climate between 1982 and 1999
Authors: Zhou L. , Kaufmann R. K. , Tian Y. , Myneni R. B. , Tucker C. J.
Published: May 2003

This paper analyzes the relation between satellite‐based measures of vegetation greenness and climate by land cover type at a regional scale (2° × 2° grid boxes) between 1982 and 1999. We use the normalized difference vegetation index (NDVI) from the Global Inventory Monitoring and Modeling Studies (GIMMS) data set to quantify climate‐induced changes in terrestrial vegetation. Climatic conditions are represented with monthly data for land surface air temperature and precipitation. The relation between NDVI and the climate variables is represented using a quadratic specification, which is consistent with the notion of a physiological optimum. The effects of spatial heterogeneity and unobserved variables are estimated with specifications and statistical techniques that allow coefficients to vary among grid boxes. Using this methodology, we are able to estimate statistically meaningful relations between NDVI and climate during spring, summer, and autumn for forests between 40°N and 70°N in North America and Eurasia. Of the variables examined, changes in temperature account for the largest fraction of the change in NDVI between the early 1980s and the late 1990s. Changes in stratospheric aerosol optical depth and precipitation have a smaller effect, while artifacts associated with variations in solar zenith angle are negligible. These results indicate that temperature changes between the early 1980s and the late 1990s are responsible for much of the observed increase in satellite measures of northern forest greenness.
Leaf‐to‐aircraft measurements of net CO2 exchange in a sagebrush steppe ecosystem
Authors: Smith William K. , Kelly Robert D. , Welker Jeffery M. , Fahnestock Jace T. , Reiners William A. , Raymond Hunt E.
Published: May 2003

Measurements of net CO2 flux density (μmol m−2 s−1) from a high elevation, sagebrush steppe community in southeastern Wyoming (USA) were compared quantitatively among four different instrument systems (leaf cuvette and 1 m2 community chamber for gas exchange measurements; tower and aircraft eddy covariance systems) by minimizing spatial and temporal variability. Ground‐based flux measurements were recorded at an intensive site located near the midpoint and directly beneath an approximate 20‐km flight transect. A high degree of homogeneity in plant species composition, density, cover, and the amount of leaf area per unit ground area, as well as little topographic variability, was measured at the intensive site and along the flight transect. Flux measurements were compared on days with relatively high and low soil moisture availability (predawn plant water potentials >−0.8 MPa and <−2.0 MPa, respectively). Same‐day, mean flux values between the four measurement systems (4.0–4.6 μmol m−2 s−1) over identical time intervals (0900–1100 hours) varied by a maximum of ±9% (maximum range 23%). Ground‐level measurements taken within ±1 day of flight measurements, varied by a minimum of ±7% (25% maximum range) of aircraft values. This difference increased curvilinearly to a maximum of ±31% (38% maximum range) for a 2‐week separation between flight and ground‐based measurements. Thus, under near‐ideal conditions of topographic and vegetative homogeneity, temporal heterogeneity in the measurement regime of only a few days resulted in greater disparity in measured CO2 flux density than occurred among the four instrument types.
Lidar studies of interannual, seasonal, and diurnal variations of polar mesospheric clouds at the South Pole
Authors: Chu Xinzhao , Gardner Chester S. , Roble Raymond G.
Published: May 2003

Polar mesospheric clouds (PMC) were observed by an Fe Boltzmann temperature lidar at the South Pole in the 1999–2000 and 2000–2001 austral summer seasons. We report the study of interannual, seasonal, and diurnal variations of PMC using more than 430 h of PMC data. The most significant differences between the two seasons are that in the 2000–2001 season, the PMC mean total backscatter coefficient is 82% larger and the mean centroid altitude is 0.83 km lower than PMC in the 1999–2000 season. Clear seasonal trends in PMC altitudes were observed at the South Pole where maximum altitudes occurred around 10–20 days after summer solstice. Seasonal variations of PMC backscatter coefficient and occurrence probability show maxima around 25–40 days after summer solstice. Strong diurnal and semidiurnal variations in PMC backscatter coefficient and centroid altitude were observed at the South Pole with both in‐phase and out‐of‐phase correlations during different years. A significant hemispheric difference in PMC altitudes was found, that the mean PMC altitude of 85.03 km at the South Pole is about 2–3 km higher than PMC in the northern hemisphere. Through comparisons with the NCAR Thermosphere‐Ionosphere‐Mesosphere‐Electrodynamics General Circulation Model (TIME‐GCM), the hemispheric difference in PMC altitude is attributed to the hemispheric differences in the altitudes of supersaturation region and in the upwelling vertical wind, which are mainly caused by different solar forcing in two hemispheres that the solar flux in January is 6% greater than the solar flux in July due to the Earth's orbital eccentricity. Gravity wave forcing also contributes to these differences.
N2O and NOy observations in the 1999/2000 Arctic polar vortex: Implications for transport processes in a CTM
Authors: Considine D. B. , Kawa S. R. , Schoeberl M. R. , Douglass A. R.
Published: May 2003

We compare the evolution of simulated NOy and N2O with observations made during the 1999/2000 NASA Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment/Third European Stratospheric Experiment on Ozone‐2000 to understand the different effects of transport on these two species and evaluate model transport processes in the Arctic lower stratosphere during mid to late winter. The simulations were made with the Goddard Space Flight Center chemistry and transport model, driven by meteorological analyses from the Goddard Data Assimilation Office. In January, model NOy and N2O distributions agree well with observations, provided a substantial loss of NOy due to polar stratospheric cloud sedimentation is included in the model simulation. In late winter, model NOy is ∼2–3 times larger than observed, despite the continued reasonable agreement of N2O. Anomalously large vertical transport of NOy due to overly vigorous descent in the CTM is primarily responsible for the large late winter NOy mixing ratios. However, the rapid descent is not apparent in the evolution of modeled N2O and presumably other similar species due to compensating horizontal transport of N2O across the vortex edge, which results in the reasonable agreement between modeled and measured N2O at ER‐2 altitudes throughout the winter. The need to properly represent the seasonal evolution of NOy when studying lower stratospheric polar wintertime photochemistry places strong constraints on the meteorological data used to drive global chemistry and transport models.
A Generalized Pareto intensity‐duration model of storm rainfall exploiting 2‐Copulas
Authors: De Michele C. , Salvadori G.
Published: May 2003

Stochastic models of rainfall, usually based on Poisson arrivals of rectangular pulses, generally assume exponential marginal distributions for both storm duration and average rainfall intensity, and the statistical independence between these variables. However, the advent of stochastic multifractals made it clear that rainfall statistical properties are better characterized by heavy tailed Pareto‐like distributions, and also the independence between duration and intensity turned out to be a nonrealistic assumption. In this paper an improved intensity‐duration model is considered, which describes the dependence between these variables by means of a suitable 2‐Copula, and introduces Generalized Pareto marginals for both the storm duration and the average storm intensity. Several theoretical results are derived: in particular, we show how the use of 2‐Copulas allows reproducing not only the marginal variability of both storm average intensity and storm duration, but also their joint variability by describing their statistical dependence; in addition, we point out how the use of heavy tailed Generalized Pareto laws gives the possibility of modeling both the presence of extreme values and the scaling features of the rainfall process, and has interesting connections with the statistical structure of the process of rainfall maxima, which is naturally endowed with a Generalized Extreme Value law. Finally, a case study considering rainfall data is shown, which illustrates how the theoretical results derived in the paper are supported by the practical analysis.
Evolution of chemical peak shapes in the Dome C, Antarctica, ice core
Authors: Barnes P. R. F. , Wolff E. W. , Mader H. M. , Udisti R. , Castellano E. , Röthlisberger R.
Published: May 2003

Interpretation of the chemical layers measured in ice cores requires knowledge of processes occurring after their deposition on the ice sheet. We present evidence for the diffusion of soluble ions in the top 350 m of the Dome C ice core, Antarctica, that helps in explaining the unexpectedly broad volcanic peaks observed at depth. A windowed‐differencing operation applied to chemical time series indicates a damping of the signals over the past 11,000 years, independent of minor climatic variation, for sulfate and chloride, but not sodium. This implies a diffusive process is transporting both sulfate and chloride ions while the sodium ions remain fixed. We estimate the effective diffusivity in the core to be 4.7 × 10−8 m2 yr−1 for sulfate and 2.0 × 10−7 m2 yr−1 for chloride. These values are not high enough to significantly disrupt chemical interpretation in this section of core, but could be significant for older ice. The temperature of this section of ice (−53°C) implies that the predominantly acidic sulfate (and possibly chloride ions) will exist in the liquid phase while the sodium may be solid. We propose and develop two new mechanisms that could explain the observed solute movement. One involves the diffusion of solute through a connected vein network driven by liquid concentration imbalances instigated by the process of grain growth. The other considers a system of discontinuous veins where grain growth increases connectivity between isolated vein clusters allowing the spread of solute. In both mechanisms, the effective diffusivity is governed indirectly by grain growth rate; this may be a significant factor controlling effective diffusion in other cores.
A method for estimating momentum fluxes of deep precipitating convection using profiling Doppler radar
Authors: Mecikalski John R.
Published: May 2003

This paper outlines a novel approach for obtaining estimates of momentum fluxes for deep precipitating convection. This method relies on data collected by vertically incident profiling Doppler radars but may be applied to aircraft‐ or space‐based radar systems. This work represents a significant step forward toward the evaluation of convective momentum transports across regional scales using data from operational satellite systems (e.g., the Tropical Rainfall Measuring Mission (TRMM) instrument). The new method is demonstrated as convective momentum flux terms and are estimated for deep convection occurring during the Third Convection and Atmospheric Moisture Experiment (CAMEX‐3), which was conducted during August and September 1998. These terms contain the most information on the momentum transported in the updrafts and downdrafts of cumulus convection. Key to the method is that the tilt and orientation of the convective drafts, relative to Earth's surface, are quantified from radar information into perturbation horizontal wind components once an accurate storm motion estimate is provided. For perturbation vertical velocities, the evaluation of drop size diameters from radar reflectivities and the subsequent evaluation of terminal velocities of hydrometeors and draft motions are necessary information toward applying this technique. Doppler velocity measurements from the upward looking Rd‐69 Disdromet disdrometer (Joss‐Waldvogel disdrometer) calibrated 915 MHz profiler are used to test the procedures for estimating w′. The method is tested against numerically simulated convection using the University of Wisconsin‐Nonhydrostatic Modeling System (UW‐NMS) mesoscale and cloud‐resolving model. The radar‐estimated momentum fluxes are found to match in sign and relative magnitude through the depth of the convective clouds studied compared to those obtained from model simulation.
Global modeling of the isotopic analogues of N2O: Stratospheric distributions, budgets, and the 17O–18O mass‐independent anomaly
Authors: McLinden Chris A. , Prather Michael J. , Johnson Matthew S.
Published: May 2003

A three‐dimensional chemical transport model (CTM) is used to study the stratospheric distributions and global budgets of the five most abundant isotopic analogues of N2O: 14N14N16O, 14N15N16O, 15N14N16O, 14N14N18O, and 14N14N17O. Two different chemistry models are used to derive photolysis cross sections for the analogues of N2O: (1) the zero‐point energy shift model, scaled by a factor of 2 to give better agreement with recent laboratory measurements and (2) the time‐dependent Hermite propagator model. Overall, the CTM predicts stratospheric enrichments that are in good agreement with most measurements, with the latter model performing slightly better. Combining the CTM‐calculated stratospheric losses for each N2O species with current estimates of tropospheric N2O sources defines a budget of flux‐weighted enrichment factors for each. These N2O budgets are not in balance, and trends of −0.04 to −0.06 ‰/yr for the mean of 14N15N16O and 15N14N16O and −0.01 to −0.02 ‰/yr for 14N14N18O are predicted, although each has large uncertainties associated with the sources. The CTM also predicts that 14N14N17O and 14N14N18O will be fractionated by photolysis in a manner that produces a nonzero mass‐independent anomaly. This effect can account for up to half of the observed anomaly in the stratosphere without invoking chemical sources. In addition, a simple one‐dimensional model is used to investigate a number of chemical scenarios for the mass‐independent composition of stratospheric N2O.
Micropulse lidar observations of tropospheric aerosols over northeastern South Africa during the ARREX and SAFARI 2000 dry season experiments
Authors: Campbell James R. , Welton Ellsworth J. , Spinhirne James D. , Ji Qiang , Tsay Si‐Chee , Piketh Stuart J. , Barenbrug Marguerite , Holben Brent N.
Published: May 2003

During the Aerosol Recirculation and Rainfall Experiment (ARREX 1999) and Southern African Regional Science Initiative (SAFARI 2000) dry season experiments, a micropulse lidar (523 nm) instrument was operated at the Skukuza Airport in northeastern South Africa. The lidar was colocated with a diverse array of passive radiometric equipment. For SAFARI 2000, a daytime time series of layer mean aerosol optical properties, including layer mean extinction‐to‐backscatter ratios and vertical extinction cross‐section profiles are derived from the synthesis of the lidar data and aerosol optical depths from available AERONET Sun photometer data. Combined with derived spectral Angstrom exponents, normalized broadband flux measurements, and calculated air mass back‐trajectories, the temporal evolution of the surface aerosol layer optical properties is analyzed for climatological trends. For dense biomass smoke events the extinction‐to‐backscatter ratio is between 50 and 90 sr, and corresponding spectral Angstrom exponent values are between 1.50 and 2.00. Observations of an advecting smoke event during SAFARI 2000 are shown. The smoke was embedded within two distinct stratified thermodynamic layers causing the particulate mass to advect over the instrument array in an incoherent manner on the afternoon of 1 September 2000. Significant surface broadband flux forcing of over −50 W/m2 was measured in this event. The evolution of the vertical aerosol extinction profile is profiled using the lidar data. Finally, observations of persistent elevated aerosol layers during ARREX 1999 are presented and discussed. Back‐trajectory analyses combined with lidar and Sun photometer measurements indicate the likelihood for these aerosols being the result of long‐range particulate transport from the southern and central South America.
Precipitation chemistry at a rural Mediterranean site: Between anthropogenic pollution and natural emissions
Authors: Avila Anna , Alarcón Marta
Published: May 2003

The chemistry of weekly rainwater collected at a rural site (La Castanya, Montseny Mountains, northeast Spain), exposed both to anthropogenic pollutants and natural African dust, was analyzed for the period 1983–2000. On the basis of back trajectories and synoptic charts we distinguished four source regions: (1) African, (2) European, (3) Oceanic, and (4) Local. The chemical characteristics of each provenance were interpreted in relation to the emission processes at each source region. The temporal trends between 1983 and 2000 in the volume weighted mean (VWM) concentrations showed an upward trend for alkalinity in European, Local, and Oceanic provenances, while they showed a slight (nonsignificant) downward trend for African rains. All provenances exhibited a significant decline in excSO4−2, which was most pronounced for European rains (b = −5.7 μeqL−1 yr−1). Because NO3− remained about constant (except for European rains), NO3− acquired a more important role in the acid anion contribution. A reduction of the acid deposition for European, Local, and Oceanic rains was observed, as well as an increase in the global deposition of alkalinity. The role of African rains appeared as a determinant in the accumulated alkalinity during the period. A quantification of the transport of pollutant species (SO4−2, NO3−, and NH4+) from eight contributing European subregions was obtained on the basis of the residence time of the trajectories in each subregion. It suggested that not only subregions closer to the receptor point, such as the Iberian Peninsula, France, and the United Kingdom, were important contributors to the deposition of SO4−2, NO3−, and NH4+ but also distant sites such as Scandinavia contributed considerably.
Sources of atmospheric acidity in an agricultural‐industrial region of São Paulo State, Brazil
Authors: da Rocha G. O. , Franco A. , Allen A. G. , Cardoso A. A.
Published: May 2003

Surface‐based measurements of atmospheric formic acid (HCOOH), acetic acid (CH3COOH), sulfur dioxide (SO2), hydrogen chloride (HCl), and nitric acid (HNO3) were made in central São Paulo State, Brazil, between April 1999 and March 2000. Mean concentrations were 9.0 ppb (HCOOH), 1.3 ppb (CH3COOH), 4.9 ppb (SO2), 0.3 ppb (HCl), and 0.5 ppb (HNO3). Concentrations in sugar cane burning plumes were 1160–4230 ppb (HCOOH), 360–1750 ppb (CH3COOH), 10–630 ppb (SO2), 4–210 ppb (HCl), and 14–90 ppb (HNO3). Higher ambient concentrations of SO2, HCl and HNO3 were measured during the burning season (May–November). Concentrations of SO2 and HCl increased during the evening, and of HCOOH and CH3COOH were lowest in the morning, with peak levels in the afternoon. Ratios obtained between different species showed either nighttime maxima (SO2/HCOOH, SO2/CH3COOH, SO2/HNO3, CH3COOH/HNO3, SO2/HCl and HCOOH/HNO3), daytime maxima (HCOOH/HCl, CH3COOH/HCl and HNO3/HCl), or no clear trends (HCOOH/CH3COOH). Correlation analysis showed that SO2 and HCl were primary emissions from biomass burning and road transport; HCOOH, HNO3 and CH3COOH were products of photochemistry; HCOOH and CH3COOH were emitted directly during combustion as well as from biogenic sources. Biomass burning affected atmospheric acidity on a regional scale, while vehicular emissions had greater impact in urban and adjacent areas. Atmospheric ammonia levels were insufficient to neutralize atmospheric acidity, which was mainly removed by deposition to the surface.
Mineral aerosol contamination of TIROS Operational Vertical Sounder (TOVS) temperature and moisture retrievals
Authors: Weaver C. J. , Joiner J. , Ginoux P.
Published: May 2003

Since mineral aerosols absorb significant amounts of infrared radiation, they may contribute to errors in the retrievals of atmospheric and surface parameters from the TIROS Operational Vertical Sounder (TOVS) High‐Resolution Infrared Radiation Sounder (HIRS) if the atmosphere is assumed clear. TOVS is an operational sounder on NOAA polar satellites. To see if observed brightness temperatures are reduced by mineral aerosol, we analyzed results from the Data Assimilation Office (DAO) Finite Volume Data Assimilation System (fvDAS). Every 6 hours the assimilated temperature and moisture profiles are used as a first guess in the DAO interactive cloud‐clearing TOVS retrieval system. The observed minus the forecast (O–F) brightness temperature, which is a measure of the accuracy of the first guess and radiative transfer parameters, becomes more negative with increasing dust concentrations. Dust concentrations are from the Goddard Ozone Chemistry Aerosol Radiation Transport (GOCART) model. Since there was no account of dust during this fvDAS run, the dependence of O–F on the estimated atmospheric dust concentrations from GOCART indicates that the dust is affecting the TOVS brightness temperatures. HIRS channels that are sensitive to the surface temperature, lower tropospheric temperature, and moisture are subject to a 0.5 K or more reduction in the brightness temperature during heavy dust loading conditions. The radiative transfer module used in the TOVS retrieval system was modified to account for dust assuming a composition of illite, and the fvDAS run was repeated. Accounting for dust absorption in the retrieval system yields warmer surface temperatures (0.4 K) and warmer lower tropospheric temperatures in regions of moderate dust loading over the tropical Atlantic.
Trapped waves in the edge region of stratospheric polar vortices
Authors: Tomikawa Yoshihiro , Sato Kaoru
Published: May 2003

A global distribution and seasonal variability of short‐period (<2 days) disturbances in the lower and middle stratosphere are investigated using 6‐hourly European Centre for Medium‐Range Weather Forecasts reanalysis data over 15 years (1979–1993). Two‐dimensional spectral analysis and a recently developed Lagrangian‐mean diagnostic are used for the analysis. It is shown that the short‐period disturbances are most active around the polar‐night jet in each winter hemisphere. They have a wavelike structure in the longitudinal direction with typical zonal wavelengths of 1,700–2,000 km (i.e., zonal wave number 10–12) and a latitudinally evanescent structure with a half width of about 1,500 km, suggesting that they are waves trapped in the edge region of the polar vortex, where the latitudinal gradient of isentropic potential vorticity is maximized. The trapped waves have a nearly barotropic structure over a depth greater than 10 km. Because the polar vortex in the Northern Hemisphere is much deformed and not symmetric around the pole, dynamical characteristics of the trapped waves in the Southern Hemisphere alone are investigated by applying a lag‐correlation analysis to the reanalysis data. The characteristics of trapped waves are fairly dependent on the state of the polar vortex, such as zonal wind speed and its vertical shear. They have a significant westward intrinsic phase velocity from June through October, when the polar vortex is most stable. This indicates that the trapped waves during this period have their own dynamics, contrary to the filaments being passively advected by the large‐scale flow.
Micrometeorological and leaf‐level measurements of isoprene emissions from a southern African savanna
Authors: Harley Peter , Otter Luanne , Guenther Alex , Greenberg James
Published: May 2003

In February 2001, as part of the Southern African Regional Science Initiative (SAFARI 2000), isoprene fluxes were measured for 8 days using the relaxed eddy accumulation technique from a 21‐m tower in a Combretum‐Acacia savanna in Kruger National Park, 13 km from Skukuza, RSA. Despite warm and sunny conditions, midday isoprene concentrations were low, averaging 0.39 nL/L. Fluxes of isoprene increased through the morning hours, with midday fluxes averaging 0.34 mg m−2 h−1 and a maximum measured flux of approximately 1.0 mg m−2 h−1. Consistent with these low fluxes, leaf enclosure measurements of woody species within the tower footprint determined that only one isoprene‐emitting species, Acacia nigrescens, was present in significant numbers, comprising less than 10% of the woody biomass. Combining enclosure data with species composition and leaf area index data from the site, we estimated that the isoprene emission capacity of the vegetation within the vicinity of the tower was very low, approximately 0.47 mg m−2 h−1, and patchy. Under these circumstances, low and variable fluxes are expected. Additional leaf enclosure measurements, for a total of 121 species, were made at other locations, and approximately 35% of the species was found to emit significant amounts of isoprene. Important isoprene emitting plant families included Caesalpinaceae, Mimosaceae, Papilionaceae, Euphorbiaceae, Moraceae, and Myrtaceae. Twelve members of the important savanna genus Acacia were measured, of which five species, all belonging in Subgenus Aculeiferum, Section Aculeiferum, were found to emit significant amounts of isoprene. In contrast, the plant family, Combretaceae, dominant in many savanna ecosystems, was found to contain no species which emit isoprene.
Observations of stratosphere‐to‐troposphere transport events over the eastern Mediterranean using a ground‐based lidar system
Authors: Galani E. , Balis D. , Zanis P. , Zerefos C. , Papayannis A. , Wernli H. , Gerasopoulos E.
Published: May 2003

In order to cover a substantial amount of stratospheric intrusions during the project Influence of Stratosphere‐Troposphere Exchange in a Changing Climate on Atmospheric Transport and Oxidation Capacity (STACCATO), coordinated measurements were carried out at several locations in central and southern Europe, based on quasi‐operational stratosphere‐troposphere exchange forecasts. In this context, lidar measurements of tropospheric ozone were performed at Thessaloniki (23°E, 40.5°N), Greece, for the investigation of stratosphere‐to‐troposphere transport (STT) events over the southeastern Mediterranean region, during 2000–2002. The study of STT in this area is of particular interest not only because of its geographic location, which is more southern than the typical position of the polar front jet, but also because of the sparseness of detailed studies in this area. A summary of the main characteristics of the STT events that were detected by lidar during the investigation period reveals a tropospheric ozone increase of the order of 10% between 4.5 and 6.5 km above sea level, a coincident decrease in relative humidity, and elevated values of potential vorticity, thus providing a direct indication of the occurrence of stratospheric air in the middle troposphere. No direct effect on surface ozone was recorded. Furthermore, the majority of the STT cases detected reveal a common pathway of the stratospheric air masses that reached Thessaloniki originating from the North Sea. In addition, the event of 9 January 2001, during which the clearest and strongest descent of stratospheric air occurred, is further analyzed. An ozone‐rich layer of the order of 60–90 ppbv between 5 and 6.5 km above sea level is clearly depicted, which was the result of the intermixture of originally stratospheric and boundary layer air originating from North America.
Stratosphere‐to‐troposphere transport: A model and method evaluation
Authors: Cristofanelli P. , Bonasoni P. , Collins W. , Feichter J. , Forster C. , James P. , Kentarchos A. , Kubik P. W. , Land C. , Meloen J. , Roelofs G. J. , Siegmund P. , Sprenger M. , Schnabel C. , Stohl A. , Tobler L. , Tositti L. , Trickl T. , Zanis P.
Published: May 2003

During the EU‐project Influence of Stratosphere‐Troposphere exchange in a Changing Climate on Atmospheric Transport and Oxidation Capacity (STACCATO), a combined approach of a measurement network and numerical simulations was used to estimate the strength and frequency of stratosphere‐to‐troposphere transport (STT) events and their influence on tropospheric chemistry. Measurements of surface ozone, beryllium‐7, and beryllium‐10 concentrations and meteorological parameters at four European high mountain stations, as well as atmospheric profiles obtained by ozone soundings and a high‐resolution lidar, were carried out. In order to simulate STT events, seven different models have been applied by the STACCATO partners. These are two trajectory models (LAGRANTO and FLEXTRA), a Lagrangian transport model (FLEXPART), a Lagrangian chemistry‐transport model (STOCHEM), a Eulerian transport model (TM3), and two general circulation models (ECHAM4 and MA‐ECHAM4). In order to investigate the strengths and weaknesses of each of these models and to identify the reasons for their discrepancies, a detailed comparison with measured data is presented in this paper. These models provided fluxes and concentrations of a stratospheric tracer, as well as the vertical profiles of ozone and radionuclides for a stratospheric intrusion case study that occurred over Europe in the year 1996. The comparison of the model results with the measurement data and the satellite observations revealed that all the models captured the general behavior of the event. However, great differences were found in the intensity and spatial development of the simulated intrusion event.
What controls photochemical NO and NO2 production from Antarctic snow? Laboratory investigation assessing the wavelength and temperature dependence
Authors: Cotter E. S. N. , Jones A. E. , Wolff E. W. , Bauguitte S. J.‐B.
Published: May 2003

Laboratory experiments were conducted to determine the wavelength and temperature dependence of NO and NO2 release from Antarctic snow. This photochemically driven process has been observed during recent field measurements in polar regions. In this work the photochemical production of NO and NO2 was reproduced under laboratory conditions with NO2 dominating the production. The wavelength of incident light was varied over the range 295–385 nm. We observed a λ dependence where NO and NO2 release ceases when the snow was illuminated with λ > 345 nm. Comparing these data with the aqueous absorption cross section of the nitrate ion (NO3−) indicates that NO3− is the precursor N‐oxide species, which is photolyzed in snow. The temperature of the experimental system was varied over the range 253–243 K with no effect on NO and NO2 production. The occurrence of these photochemical processes followed by release to the atmosphere will impact the chemistry of the boundary layer over any snow‐covered region. In addition, understanding these processes is essential for accurate ice core interpretation.
Study of finely divided aqueous systems as an aid to understanding the formation mechanism of polar stratospheric clouds: Case of HNO3/H2O and H2SO4/H2O systems
Authors: Bogdan A. , Molina M. J. , Kulmala M. , MacKenzie A. R. , Laaksonen A.
Published: May 2003

The study of nanometer‐scale aqueous systems (finely divided aqueous systems (FDAS)) can be achieved using the absorption of vapors on fumed silica (SiO2) powder. Being a product of flame synthesis technology, fumed silica particles (6–11 nm) can be considered to be analogous to the silica smoke particles of anthropogenic and extraterrestrial origin that are supposed to be widely present in the stratosphere and mesosphere. Here, we describe the freezing and melting behavior of nanometer‐scale pure H2O and binary HNO3/H2O and H2SO4/H2O systems of varying acid content, using differential scanning calorimetry (DSC). Besides reductions of melting temperature, Tm, large reductions in freezing and melting enthalpies, with ΔHf < ΔHm, in comparison with bulk solutions have also been detected. Experiments showed that fumed silica can serve as a freezing nucleus for heterogeneous ice nucleation from dilute HNO3/H2O droplets. The onset of freezing of a silica/HNO3/H2O sample with HNO3/H2O stoichiometry close to that of NAT (53 ± 5 wt % HNO3) at temperatures ≈7 K warmer than the ice frost point suggests that silica particles can promote heterogeneous freezing of nitric acid hydrates in the stratosphere. Freezing of bulk droplets (53.2 wt % HNO3) supported on Al substrate at temperatures warmer than −73°C (200 K) suggests that in principle, Al2O3 surface may induce freezing of HNO3 hydrates as well. DSC measurements performed on the silica/H2SO4/H2O nanosystem showed that at stratospheric temperatures, silica particles cannot induce heterogeneous formation of sulfuric acid hydrates.
Potential impact of subsonic and supersonic aircraft exhaust on water vapor in the lower stratosphere assessed via a trajectory model
Authors: Morris Gary A. , Rosenfield Joan E. , Schoeberl Mark R. , Jackman Charles H.
Published: May 2003

We employ a trajectory model to assess the impact on the stratosphere of water vapor present in the exhaust of subsonic and a proposed fleet of supersonic aircraft. Air parcels into which water vapor from aircraft exhaust has been injected are run through a 6‐year simulation in the trajectory model using meteorological data from the UKMO analyses with emissions dictated by the standard 2015 emissions scenario. For the subsonic aircraft, our results suggest maximum enhancements of ∼150 ppbv just above the Northern Hemisphere tropopause and of much less than 50 ppbv in most other regions. Inserting the perturbed water vapor profiles into a radiative transfer model, but not considering the impact of additional cirrus formation resulting from emissions by subsonic aircraft, we find that the impact of subsonic water vapor emissions on the radiative balance is negligible. For the supersonic case, our results show maximum enhancements of ∼1.5 ppmv in the tropical stratosphere near 20 km. Much of the remaining stratosphere between 12 and 25 km sees enhancements of greater than 0.1 ppmv, although enhancements above 35 km are generally less than 50 ppbv, in contrast to previous 2‐D and 3‐D model studies. Radiative calculations based upon these projected water vapor perturbations indicate they may cause a nonnegligible impact on tropical temperature profiles. Since our trajectory model includes no chemistry and our radiative calculations use the most extreme water vapor perturbations, our results should be viewed as upper limits on the potential impacts.
Controls on the emission of plant volatiles through stomata: Differential sensitivity of emission rates to stomatal closure explained
Authors: Niinemets Ülo , Reichstein Markus
Published: May 2003

Volatile (VOC) flux from leaves may be expressed as GSΔP, where GS is stomatal conductance to specific compound and ΔP partial pressure gradient between the atmosphere and substomatal cavities. It has been suggested that decreases in GS are balanced by increases in ΔP such that stomata cannot control VOC emission. Yet, responses of emission rates of various volatiles to experimental manipulations of stomatal aperture are contrasting. To explain these controversies, a dynamic emission model was developed considering VOC distribution between gas and liquid phases using Henry's law constant (H, Pa m3 mol−1). Our analysis demonstrates that highly volatile compounds such as isoprene and monoterpenes with H values on the order of 103 have gas and liquid pool half‐times of a few seconds, and thus cannot be controlled by stomata. More soluble compounds such as alcohols and carboxylic acids with H values of 10−2–101 are controlled by stomata with the degree of stomatal sensitivity varying with H. Inability of compounds with high solubility to support a high partial pressure, and thus to balance ΔP in response to a decrease in GS is the primary explanation for different stomatal sensitivities. For compounds with low H, the analysis predicts bursts of emission after stomatal opening that accord with experimental observations, but that cannot be currently explained. Large within‐leaf VOC pool sizes in compounds with low H also increase the system inertia to environmental fluctuations. In conclusion, dynamic models are necessary to simulate diurnal variability of the emissions of compounds that preferably partition to aqueous phase.
Photochemical age determinations in the Phoenix metropolitan area
Authors: Kleinman L. I. , Daum P. H. , Lee Y.‐N. , Nunnermacker L. J. , Springston S. R. , Weinstein‐Lloyd J. , Hyde P. , Doskey P. , Rudolph J. , Fast J. , Berkowitz C.
Published: May 2003

An extensive VOC data set was gathered as part of a photochemical oxidant field campaign conducted in the Phoenix air basin in the late spring of 1998. Sampling was done at the surface and by aircraft at midboundary layer height; in regions with emission sources and downwind in the urban plume. VOC concentration ratios were used to calculate photochemical age, defined as the time integrated exposure of an air mass to OH radical. Based on the VOC ratios of 15 compounds (with OH reactivity varying between acetylene and p, m‐xylene), we present estimates for photochemical age and dilution factors for several regions within the air basin. Geographic trends are in agreement with the expectation that pollutants are transported in a generally eastward direction so that older and more dilute mixtures occur to the east of the city. Photochemical ages determined from aircraft samples agree with those determined at a downwind surface site. The bias in photochemical age that occurs because fresh pollutants are added to an aged mixture has been quantified by using a particle trajectory model. A combination of trajectory results (actual age of the pollutants in an air mass) and photochemical age yields an estimate of the average OH concentration experienced by the air parcel. OH obtained in this way is somewhat lower, but has the same trends as OH concentrations calculated using a photochemical box model that is constrained with observed concentrations coincident with the VOC samples.
Controls on the emission of plant volatiles through stomata: A sensitivity analysis
Authors: Niinemets Ülo , Reichstein Markus
Published: May 2003

According to experimental studies, plant emissions of volatile organic compounds (VOC) are controlled by stomata to a varying extent, but the differing responses could not be explained so far. A dynamic emission model developed in a previous study indicated that stomata may limit the emission rate in a nonsteady state conditions, whereas the rate of increase of liquid‐phase volatile concentrations controls the degree to which stomata temporarily curtail the emission. Despite its large predictive capability, potentially large number of volatile physico‐chemical and leaf structural variables are needed for parameterization of such dynamic models, limiting the usefulness of the approach. We conducted a sensitivity analysis to determine the effect of varying VOC distribution between gas‐ and liquid‐phases (Henry's law constant, H, Pa m3 mol−1) and varying internal diffusion conductances in the liquid‐ and gas‐phases. The model was parameterized for three contrasting leaf architectures (conifer, sclerophyll, and mesophytic leaves). The sensitivity analysis indicated that the volatile H value is the key variable affecting the stomatal sensitivity of VOC emissions. Differences in leaf architecture, in particular in leaf liquid volume to area ratio, also modified the emission responses to changes in stomatal aperture, but these structural effects were superimposed by compound gas/liquid phase partitioning. The results of this analysis indicate that major effort in parameterization of dynamic VOC emission models should be directed toward obtaining reliable gas/liquid‐phase equilibria for various plant volatiles, and that these models may readily be applied for leaves with contrasting architecture.
Characterization of soil dust aerosol in China and its transport and distribution during 2001 ACE‐Asia: 1. Network observations
Authors: Zhang X. Y. , Gong S. L. , Shen Z. X. , Mei F. M. , Xi X. X. , Liu L. C. , Zhou Z. J. , Wang D. , Wang Y. Q. , Cheng Y.
Published: May 2003

Mass loading, 20 elemental concentrations, and time series of aerosol particles were investigated over the China Dust Storm Research (ChinaDSR) observational network stations from March to May 2001 during the intensive field campaign period of ACE‐Asia. Four extensive and several minor dust storm (DS) events were observed. Mass balance calculations showed that 45–82% of the observed aerosol mass was attributable to Asian soil dust particles among the sites, in which Ca and Fe contents are enriched to 12% and 6%, respectively, in the Western High‐Dust source regions compared with dust aerosols ejected from the Northern High‐Dust source regions. For the latter areas, elemental contents exhibited high Si (30%) and low Fe (4%). For all major source areas and depositional regions, aluminium (Al) comprises 7% of Asian dust. Air mass back‐trajectory analysis showed that five major transport pathways of Asian dust storms dominated dust transport in China during spring 2001, all of which passed over Beijing. Measurements also suggest that the sand land in northeastern China is a potential source for Asian dust. The size distribution for estimating vertical dust flux was derived from the observed surface dust size distributions in the desert regions. For particle diameters between 0.25 and 16 μm, a lognormal distribution was obtained from averaging observations at various deserts with a mass mean diameter of 4.5 μm and a standard deviation of 1.5. This range of soil dust constitutes about 69% of the total dust loading. The fractions for particles in the size ranges of <2.5 μm and >16 μm are around 1.7% and 30%, respectively.
A 15‐year climatology of stratosphere–troposphere exchange with a Lagrangian particle dispersion model 2. Mean climate and seasonal variability
Authors: James P. , Stohl A. , Forster C. , Eckhardt S. , Seibert P. , Frank A.
Published: May 2003

A comprehensive climatology of stratosphere–troposphere exchange (STE), based on 15 years of ECMWF global atmospheric reanalysis data for the period 1979–1993, has been carried out with a specially tuned version of the Lagrangian particle dispersion model, FLEXPART. The typical amplitudes, timescales, seasonality, and spatial variability of STE revealed in the climatology are described. Mean distributions of STE air masses of various ages relate to aspects of the global circulation, while anomaly patterns, such as the North Atlantic Oscillation (NAO) and El Niño, influence STE variability. Characteristic differences are seen between shallow STE flows remaining within the tropopause region (TPR) and deep STE transports, which include cases of stratospheric air reaching the lower troposphere. Unlike for shallow STE events, such deep intrusions show a distinct winter maximum. This subsequently impacts on the seasonal cycle of stratospheric ozone in the troposphere, which thus depends on altitude but also on the decay timescale of ozone. A much broader view of STE than has previously been possible is enabled. The inherent scope of STE is far wider than a discussion of cross‐tropopause fluxes alone. Indeed, more than 90% of all cross‐tropopause flows return within 6 hours. It is thus crucial to distinguish between short‐term STE, during which air parcels rapidly recross the tropopause, and deep and/or long‐term STE, in which air parcels have subsequently long residence times away from their source.
A new METEOSAT “water vapor” archive for climate studies
Authors: Picon L. , Roca R. , Serrar S. , Monge J. L. , Desbois M.
Published: May 2003

Water vapor plays an important role in the climate system through a number of mechanisms spanning a wide range of space and timescale. Since 1977, the METEOSAT satellites are equipped with a radiometer dedicated to the measurements of upper tropospheric humidity (UTH) which covers a relevant range of scales for a better understanding of the water vapor role in the climate. Due to the changes of the satellites and the calibration techniques over the last 20 years, this water vapor METEOSAT archive is not homogeneous and cannot be directly used for climatic studies. Hence the authors present in this paper a newly homogenized METEOSAT water vapor channel archive. Two main types of anomalies entail the original METEOSAT archive. The first one corresponds to the successive improvements of the calibration procedure. In this case, a statistical correction technique based on comparisons between ECMWF‐simulated brightness temperature (BT) and water vapor METEOSAT‐observed BT is developed. The second type of anomaly concerns the METEOSAT radiometer changes over the time. While still measuring the UTH, the details of the filter function, indeed, evolved over the last 20 years. In this second case, the correction is based on a physical method implying simulations of the same scene by different radiometer filter functions. Two major cases are documented in detail for September 1987 and for February 1994. Sensitivity analysis of the techniques is conducted and the methods are shown to be robust with respect to the details of their implementations. The efficiency of the two methods is then evaluated. The resulting archive reveals water vapor seasonal cycle features in better agreement with climatological estimates. The new homogenized METEOSAT archive consists of 3‐hourly total sky radiance at the 0.625° × 0.625° resolution over the July 1983 to February 1994 period, offering the opportunity to investigate the variability of the regional UTH from synoptic scales to interannual and interdecadal scales.