bluuebunny/crossref_metadata_2025 huggingface dataset:
Intercomparison of automated methodologies for determination of ambient isoprene during the PROPHET 1998 summer campaign
Authors: Barket Dennis J. , Hurst Julia M. , Couch Tara L. , Colorado Armando , Shepson Paul B. , Riemer Daniel D. , Hills Alan J. , Apel Eric C. , Hafer Rae , Lamb Brian K. , Westberg Hal H. , Farmer Charles T. , Stabenau Erik R. , Zika Rod G.
Published: February 2004

The Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) 1998 summer campaign, conducted at the University of Michigan Biological Station, provided a unique opportunity to compare isoprene measurement techniques that were automated, sampled and analyzed on‐line, and provided relatively fast time resolution. Assessment of the data quality for fast isoprene measurements is important because isoprene dominates the surface chemistry at many rural sites and even some urban environments. An informal intercomparison was conducted by evaluating ambient isoprene mixing ratio data generated by five different instruments: quadrupole ion trap (QIT) MS, the chemiluminescent‐based fast isoprene sensor (FIS), and three gas chromatograph/mass spectrometry (GC/MS) techniques. The GC/MS methods were deployed and maintained by Purdue University (GC/MS‐P), the National Center for Atmospheric Research (GC/MS‐NCAR), and the Rosenstiel School of Marine and Atmospheric Science (GC/MS‐RSMAS). The FIS was deployed and maintained by NCAR, Hills‐Scientific.com and Washington State University, while the QIT was implemented by Purdue University. The GC/MS‐P was chosen as the reference method to evaluate the agreement of the data set. The data were evaluated for time‐matched samples through regression analysis, ratio analysis, and percent difference analysis relative to GC/MS‐P. For measurement data in the central 90th percentile relative to the median, the mean percent difference was 21% for GC/MS‐NCAR, 41% for QIT, 42% for GC/MS‐RSMAS, and 88% for the FIS. Potential sources of disagreement, especially for low‐concentration data, such as variations in sampling time, interferences, method precision and accuracy, and limited cross‐calibration, are discussed.
Occurrence of solid particles in the winter polar stratosphere above the nitric acid trihydrate coexistence temperature inferred from ground‐based polarization lidar observations at Ny‐Ålesund, Spitsbergen
Authors: Beyerle Georg , Deckelmann Holger , Neuber Roland , Rosen James M. , Reimer Eberhard , Schoeberl Mark R.
Published: February 2004

Depolarization data of stratospheric aerosols obtained by ground‐based polarization lidar are analyzed. The measurements were performed at Koldewey station, Ny‐Ålesund, Spitsbergen (78.9°N, 11.9°E), during the five winters of 1994–1995 to 1998–1999. The study is focused on stratospheric sulfuric acid aerosols; interference by nitric acid trihydrate containing particles is excluded by imposing a temperature threshold condition of 2 K above the nitric acid trihydrate coexistence temperature. Under the assumption that large sulfuric acid particles freeze first, T‐matrix scattering calculations show that volume depolarizations start to increase significantly if ∼1% of the aerosols are frozen. Observations in January 1999 and February 1998 indicate the presence of nonspherical particles. We interpret these observations as signatures for the occurrence of sulfuric acid tetrahydrate particles. Temperature histories obtained from back trajectory model calculations and the corresponding polarization lidar measurements are consistent with current knowledge of sulfuric acid tetrahydrate formation.
Climate signals from station arrays with missing data, and an application to winds
Authors: Sherwood Steven C.
Published: February 2004

Typical approaches to climate signal estimation from data are susceptible to biases if the instrument records are incomplete, cover differing periods, if instruments change over time, or if coverage is poor. Here a method (Iterative Universal Kriging, or IUK) is presented for obtaining unbiased, maximum‐likelihood (ML) estimates of the climatology, trends, and/or other desired climatic quantities given the available data from an array of fixed observing stations that report sporadically. The conceptually straightforward method follows a mixed‐model approach, making use of well‐known data analysis concepts, and avoids gridding the data. It is resistant to missing data problems, including “selection bias,” and should also be helpful in dealing with common data heterogeneity issues and gross errors. Perhaps most importantly, the method facilitates quantitative error analysis of the signal being sought, assessing variability directly from the data without the need for any auxiliary model. The method is applied to rawinsonde data to examine weak meridional winds in the equatorial lower stratosphere, providing some improvements on existing climatologies.
Human and natural impacts on the water resources of the Lake Chad basin
Authors: Coe Michael T. , Foley Jonathan A.
Published: February 2004

An integrated biosphere model (IBIS) and a hydrological routing algorithm (HYDRA) are used in conjunction with long time‐series climate data to investigate the response of the Lake Chad drainage basin of northern Africa to climate variability and water use practices over the last 43 years. The simulated discharge, lake level, and lake area of the drainage basin for the period 1953–1979 are in good agreement with the observations. For example, the correlation coefficient (r2) between the simulated and the observed level of Lake Chad for the 288 months of available observations is 0.93. Although irrigation is only a modest portion of the hydrology in the period 1953–1979; representing only 5 of the 30% decrease in simulated lake area for the decade 1966–1975, the simulated lake level and area are in better agreement with the observations when irrigation is included. For the period 1983–1994 the observed water use for irrigation increased fourfold compared to 1953–1979. A comparison of the simulated surface water area, with and without irrigation, suggests that climate variability still controls the interannual fluctuations of the water inflow but that human water use accounts for roughly 50% of the observed decrease in lake area since the 1960s and 1970s.
Sulfur chemistry over the central Arctic Ocean during the summer: Gas‐to‐particle transformation
Authors: Kerminen Veli‐Matti , Leck Caroline
Published: February 2004

Atmospheric gas‐to‐particle transformation of the oxidation products of dimethyl sulfide (DMS) was investigated over the central Arctic Ocean by using observational data and a coupled gas phase chemistry‐aerosol dynamics model. Chemical compounds investigated were sulfuric acid (H2SO4(g)), methasulfonic acid (MSA(g)), and sulfur dioxide (SO2(g)) in the gas phase, and non‐sea‐salt sulfate (nss‐SO42−) and MSA in the particulate phase. During the advection from the open ocean to over the pack ice region, the particulate phase was found to be influenced significantly by three processes: (1) rapid removal of particulate matter during the first 1–2 days of advection, (2) continual formation of particulate nss‐SO42− and MSA, and (3) local production of particles consisting mostly of matter other than nss‐SO42− and MSA over the pack ice. In the absence of clouds and fogs, the principal sink for the vapors H2SO4(g) and MSA(g) were their condensational transport to submicron particles. SO2(g) was influenced significantly by both dry deposition and gas phase oxidation. Although there is a local source of coarse sea‐salt particles within the pack ice region, the role of these particles in the budgets of nss‐SO42− and SO2(g), and probably also in the budget of MSA, is only minor, especially when compared with marine regions at lower latitudes. The performance of the DMS gas‐phase chemistry scheme was tested against field data using a measurement case influenced minimally by clouds/fogs and the free troposphere. The predicted concentration of nss‐SO42− was within the uncertainties of the analysis, whereas that of MSA was high by a factor of 3–6. This demonstrates that not only heterogeneous reactions involved in atmospheric DMS chemistry but also certain gas‐phase reactions require further investigation.
Comparison of stratospheric air parcel trajectories based on different meteorological analyses
Authors: Knudsen B. M. , Pommereau J.‐P. , Garnier A. , Nunez‐Pinharanda M. , Denis L. , Letrenne G. , Durand M. , Rosen J. M.
Published: February 2004

Calculated trajectories have been compared with the 1995 Polar Vortex Balloon Experiment (POVORBEX) and 1997 and 1999 Montgolfier Infra‐Red (MIR) long‐duration balloon flights in the Arctic stratospheric vortex. The POVORBEX flight lasted 6 days, whereas the MIR flights lasted 13, 22, 7, and 17 days. The trajectories have been run at the pressure of the balloon (10–120 hPa), whereby the dominant contribution of the horizontal trajectory errors can be assessed. To obtain statistically significant results, new trajectories were started every 2 hours along the balloon flight track. Trajectories run at a temporal resolution of the meteorological fields of 24 hours sometimes give substantially larger errors than trajectories run at 12 hours or higher resolution owing to rapid nonlinear temporal developments. In 1995 the European Centre for Medium‐Range Weather Forecasts (ECMWF) trajectories generally have substantially larger errors than the U.K. Meteorological Office (UKMO) and National Centers for Environmental Prediction (NCEP) trajectories, when 24 hourly analyses are used. In 1997 and 1999, when ECMWF had introduced a variational data assimilation, their trajectory errors inside the vortex have apparently reduced considerably and are generally substantially smaller than UKMO and NCEP errors.
Role of equatorially asymmetric sea surface temperature anomalies in the Indian Ocean in the Asian summer monsoon and El Niño‐Southern Oscillation coupling
Authors: Kawamura Ryuichi , Matsuura Tomonori , Iizuka Satoshi
Published: February 2004

Using an ocean general circulation model forced by daily mean wind stresses and heat fluxes derived from the bulk formulation with the National Centers for Environmental Protection/National Center for Atmospheric Research reanalysis, we examined possible roles of the tropical Indian Ocean in the Asian summer monsoon and El Niño‐Southern Oscillation (ENSO) coupling. A dominant precursory signal of anomalous monsoon circulation relevant to ENSO is the presence of latitudinally asymmetric anomalies of tropical convection and sea surface temperature (SST) near the equator in the preceding spring. The model results show that the equatorial asymmetry of surface latent heat flux plays a vital role in generating the equatorially asymmetric SST in the preceding spring, which implies that a positive wind‐evaporation‐SST (WES) feedback proposed by Xie and Philander [1994] is crucially responsible for generation and maintenance of those equatorial asymmetries. The westward extension of anomalous convection from the vicinity of the Philippines into the northern Indian Ocean from the preceding winter to spring, which has a signal of ENSO, can be viewed as a developing process of WES. Persistence of the WES feedback regime affects land surface hydrologic processes in the Asian continent through a Rossby wave response to anomalous convective heating. It is anticipated that the WES mode in the tropical Indian Ocean serves as a bridge between the ENSO prevailing in the preceding winter and the anomalous summer monsoon.
Modeling of annual variations of oak (Quercus robur L.) isoprene synthase activity to predict isoprene emission rates
Authors: Lehning A. , Zimmer W. , Zimmer I. , Schnitzler J.‐P.
Published: February 2004

Isoprene plays an important role in regulating the atmospheric trace gas composition, in particular the tropospheric ozone concentrations. Therefore realistic estimates of the seasonal variation of isoprene emission source strengths of strong isoprene‐emitting deciduous trees such as pedunculate oak (Quercus robur L.) are required in temperate regions of Europe. In 1995 to 1997 a study was conducted to survey the annual fluctuations of oak isoprene synthase activity and photosynthetic pigment contents, the latter as a parameter for the development of the photosynthetic apparatus of oak leaves. Depending on annual temperature and light profiles (photosynthetic photon flux densities (PPFD)), different seasonal patterns of isoprene synthase activity were observed with maximum activities of 18.4±10.6 nmol m−2 s−1 14.1±5.8 nmol m−2 s−1 and 19.9±7.9 nmol m−2 s−1 in 1995, 1996, and 1997, respectively. On the basis of isoprene synthase activity, chlorophyll a measurements, and phenological data collected from pedunculate oaks of 89 ecological regions covering all of Germany a model was developed for the calculation of the seasonal variation of oak isoprene synthase activity in relation to annual fluctuation of temperature and PPFD. By coupling this model to a numeric process‐based isoprene emission model it was possible to predict isoprene emission rates of individual pedunculate oak trees with a deviation of 55%.
Ice core studies of anthropogenic sulfate and nitrate trends in the Arctic
Authors: Goto‐Azuma Kumiko , Koerner Roy M.
Published: February 2004

Ice core studies have shown that sulfate and nitrate concentrations in Arctic snow have increased significantly since the end of the 19th century due to the influx of anthropogenic pollutants transported from industrialized regions. Trends of increasing sulfate and nitrate concentrations in snow are evident in all the ice core data from Greenland, the Canadian Arctic, and Svalbard. Temporal patterns, however, show spatial variation. In the area around Dye 3, south Greenland, significant increases in sulfate are found beginning in the 1890s. Increases in nitrate began ∼50 years later. A similar pattern is seen at Penny Ice Cap, Baffin Island, in the Canadian low Arctic. In contrast, both sulfate and nitrate concentrations started to increase significantly in the 1940s on Agassiz Ice Cap, Ellesmere Island, in the Canadian high Arctic; and Snøfjellafonna, Svalbard. At Summit, central Greenland, and sites in north Greenland, sharp sulfate increases occurred at about the turn of the 20th century and again about 1940 or 1950, where the latter increase is the greater of the two. At these central and north Greenland sites, significant increases in nitrate began about 1940 or 1950. The difference between the magnitude and timing of increasing trends of the sulfate ions at these sites can be attributed to their having different source regions and pathways for these pollutant ions. The pollutant sources appear to be North America for south Greenland and Baffin Island, Eurasia, for Ellesmere Island and Svalbard, and both North America and Eurasia for central and north Greenland.
Characterization of oscillation and a period‐doubling transition to chaos reflecting dynamic instability in a simplified model of tropospheric chemistry
Authors: Field Richard J. , Hess Peter G. , Kalachev Leonid V. , Madronich Sasha
Published: February 2004

A simplified box model extracted from tropospheric photochemistry is investigated as the influx of NO (FNO) is increased. A subcritical Hopf bifurcation is encountered asFNOis increased, beyond which the steady state is unstable, and the system evolves to an oscillatory state resulting from alternate dominance of two radical chain processes. The first is an O3‐consuming process of net stoichiometry, CO + O3→ {CO2} + {O2}, occurring at high [CO] and [O3] and very low [NOx], but leading to increased [NOx] viaFNOas CO and O3are depleted. As [NOx] thus grows, passes through a maximum, and then declines, the second, an O3‐producing process of net stoichiometry, CO + 2{O2} + hν → {CO2} + O3, is dominant. It remains so as O3and CO accumulate (CO via its influx,FCO) until [NOx] again reaches very low values at high [O3] and [CO] as a result of the removal of NO2by HO photochemically generated from the increasing [O3]. This alternation occurs because each process affects [NOx] and [HOx] so as to lead to dominance of the other. A period‐doubling transition to chaotic oscillation occurs asFNOis increased further. The embedding dimension of the chaos is estimated to be four, and the original six‐variable model can be reduced to a four‐variable (CO, O3, [NO + NO2] and [HO + HO2]) system that behaves nearly identically to the full six‐variable model. While the oscillatory and chaotic periods seem too long (at least weeks) to be observed in real atmospheres, the model displays the nonlinear nature and dynamic instability of tropospheric photochemistry and offers insight into the behavior of and transitions between higher and lower [NOx] states, which may be observable. The importance of the ratios [CO]/[NO2] and [O3]/[NO] to net O3change is illustrated. The appearance of this instability suggests that predictions based upon the temporal evolution of this or even more complex models of tropospheric chemistry sometimes may be very sensitive to the exact initial conditions of [CO]/[NO2] and [O3]/[NO] prevailing.
Contribution of tropical cyclones to meridional heat transport by the oceans
Authors: Emanuel Kerry
Published: February 2004

Tropical cyclones mix warm surface waters with cooler water within the thermocline, leaving pronounced, cold wakes that over a period of weeks are restored to normal conditions by mixing and surface fluxes. This restoration is associated with net, vertically integrated heating of ocean columns, which in statistical equilibrium must be balanced by oceanic heat transport out of the regions affected by the storms. Observed tropical cyclone tracks together with coupled ocean‐atmosphere hurricane models are used to estimate the net ocean heating induced by global tropical cyclone activity during one calendar year (1996). This estimate, amounting to (1.4 ± 0.7) × l015 W, represents a substantial fraction of the observed peak poleward heat flux by the oceans, suggesting that tropical cyclones may play an important role in driving the thermohaline circulation and thereby in regulating climate. In particular, the strong sensitivity of tropical cyclone intensity to tropical ocean temperatures in turn implies that the net poleward heat flux by the ocean is sensitive to tropical temperature, reducing tropical climate sensitivity and increasing climate sensitivity at higher latitudes.
Polarized light scattering by inhomogeneous hexagonal monocrystals: Validation with ADEOS‐POLDER measurements
Authors: C.‐Labonnote Laurent , Brogniez Gérard , Buriez Jean‐Claude , Doutriaux‐Boucher Marie , Gayet Jean‐François , Macke Andreas
Published: February 2004

Various in situ measurements of the light‐scattering diagram in ice clouds were performed with a new nephelometer during several airborne campaigns. These measurements were favorably compared with a theoretical scattering model called Inhomogeneous Hexagonal Monocrystal (IHM) model. This model consists in computing the scattering of light by an ensemble of randomly oriented hexagonal ice crystals containing spherical impurities of soot and air bubbles. It is achieved by using a combination of ray tracing, Mie theory, and Monte Carlo techniques and enables to retrieve the six independent elements of the scattering matrix. This good agreement between nephelometer measurements and IHM model provides an opportunity to use this model in order to analyze ADEOS‐POLDER total and polarized reflectance measurements over ice clouds. POLDER uses an original concept to observe ice cloud properties, enabling to measure reflectances and polarized reflectances, for a given scene, under several (up to 14) viewing directions. A first analysis of ice cloud spherical albedoes over the terrestrial globe for November 10, 1996, and April 23, 1997, shows a rather good agreement between measurements and modeling. Moreover, polarized reflectances are also calculated and show a satisfactory agreement with measurements.
A conceptual model of the dehydration of air due to freeze‐drying by optically thin, laminar cirrus rising slowly across the tropical tropopause
Authors: Jensen Eric J. , Pfister Leonhard , Ackerman Andrew S. , Tabazadeh Azadeh , Toon Owen B.
Published: February 2004

In this study, we use a cloud model to simulate dehydration which occurs due to formation of optically thin, laminar cirrus as air rises slowly across the tropopause. The slow ascent and adiabatic cooling, which balances the radiative heating near the tropopause, drives nucleation of a very small number of ice crystals (<1 L−1). These crystals grow rapidly and sediment out within a few hours. The clouds never become optically thick enough to be visible from the ground. The ice crystal nucleation and growth prevents the relative humidity with respect to ice (RHI) from rising more than a few percent above the threshold for ice nucleation (RHInuc ≃ 110–160%, depending upon the aerosol composition); hence, laminar cirrus can limit the mixing ratio of water vapor entering the stratosphere. However, the ice number densities are too low and their sedimentation is too rapid to allow dehydration of the air from RHInuc down to saturation (RHI = 100%). The net result is that air crosses the tropopause with water vapor mixing ratios about 1.1 to 1.6 times the ice saturation mixing ratio corresponding to the tropopause temperature, depending on the threshold of ice nucleation on aerosols in the tropopause region. If the cross‐tropopause ascent rate is larger than that calculated to balance radiative heating (0.2 cm s−1), then larger ice crystal number densities are generated, and more effective dehydration is possible (assuming a fixed temperature). The water vapor mixing ratio entering the stratosphere decreases with increasing ascent rate (approaching the tropopause ice saturation mixing ratio) until the vertical wind speed exceeds the ice crystal terminal velocity (about 10 cm s−1). More effective dehydration can also be provided by temperature oscillations associated with wave motions. The water vapor mixing ratio entering the stratosphere is essentially controlled by the tropopause temperature at the coldest point in the wave. Hence, the efficiency of dehydration at the tropopause depends upon both the effectiveness of upper tropospheric aerosols as ice nuclei and the occurrence of wave motions in the tropopause region. In situ humidity observations from tropical aircraft campaigns and balloon launches over the past several years have provided a few examples of ice‐supersaturated air near the tropopause. However, given the scarcity of data and the uncertainties in water vapor measurements, we lack definitive evidence that air entering the stratosphere is supersaturated with respect to ice.
Arctic ozone loss in threshold conditions: Match observations in 1997/1998 and 1998/1999
Authors: Schulz A. , Rex M. , Harris N. R. P. , Braathen G. O. , Reimer E. , Alfier R. , Kilbane‐Dawe I. , Eckermann S. , Allaart M. , Alpers M. , Bojkov B. , Cisneros J. , Claude H. , Cuevas E. , Davies J. , De Backer H. , Dier H. , Dorokhov V. , Fast H. , Godin S. , Johnson B. , Kois B. , Kondo Y. , Kosmidis E. , Kyrö E. , Litynska Z. , Mikkelsen I. S. , Molyneux M. J. , Murphy G. , Nagai T. , Nakane H. , O'Connor F. , Parrondo C. , Schmidlin F. J. , Skrivankova P. , Varotsos C. , Vialle C. , Viatte P. , Yushkov V. , Zerefos C. , von der Gathen P.
Published: February 2004

Chemical ozone loss rates inside the Arctic polar vortex were determined in early 1998 and early 1999 by using the Match technique based on coordinated ozonesonde measurements. These two winters provide the only opportunities in recent years to investigate chemical ozone loss in a warm Arctic vortex under threshold conditions, i.e., where the preconditions for chlorine activation, and hence ozone destruction, only occurred occasionally. In 1998, results were obtained in January and February between 410 and 520 K. The overall ozone loss was observed to be largely insignificant, with the exception of late February, when those air parcels exposed to temperatures below 195 K were affected by chemical ozone loss. In 1999, results are confined to the 475 K isentropic level, where no significant ozone loss was observed. Average temperatures were some 8°–10° higher than those in 1995, 1996, and 1997, when substantial chemical ozone loss occurred. The results underline the strong dependence of the chemical ozone loss on the stratospheric temperatures. This study shows that enhanced chlorine alone does not provide a sufficient condition for ozone loss. The evolution of stratospheric temperatures over the next decade will be the determining factor for the amount of wintertime chemical ozone loss in the Arctic stratosphere.
Organic nitrogen in Hawaiian rain and aerosol
Authors: Cornell S. , Mace K. , Coeppicus S. , Duce R. , Huebert B. , Jickells T. , Zhuang L.‐Z.
Published: February 2004

Water‐soluble organic nitrogen (ON) is an important component of fixed nitrogen in clean marine aerosol and rainwater collected at a site on the windward coast of Oahu, Hawaii. Aerosol material associated with the predominant trade winds carries 3.3±2.0 nmol ON m−3, which makes up roughly one third of the total nitrogen in aerosol (11±4 nmol N m−3). The inorganic nitrogen (65% as nitrate) is predominantly found in coarse‐mode aerosol, consistent with displacement reactions of sea‐salt chloride. In contrast, most of the ON is found in fine particle (submicrometer) aerosol, and may be associated with gas‐to‐particle conversions and with long‐range transport in the atmosphere. At times, aerosol ON also appears to have a local, anthropogenic source, and when meteorological conditions are favorable, large pulses of ON from these local sources can dominate the total fixed nitrogen in the sampled aerosol (30–50 nmol ON m−3, contributing about 80% of the total aerosol nitrogen). About one fifth of rainwater dissolved nitrogen at this site is organic nitrogen. The average rainwater concentration of dissolved ON was 2.8 μmol N L−1, and of inorganic nitrogen (nitrate plus ammonium) was 15 μmol N L−1. In both rainwater and aerosol, urea was a major component of the ON, contributing about half of the ON and about 15% of total nitrogen. This quantitative importance of urea as a component of ON has not previously been seen in continental locations.
Turbulent transport of carbon dioxide and water vapor within a vegetation canopy during unstable conditions: Identification of episodes using wavelet analysis
Authors: Scanlon Todd M. , Albertson John D.
Published: February 2004

The net exchange of CO2 between the biosphere and atmosphere is realized as a difference between the fluxes associated with photosynthesis and respiration. This paper contrasts the turbulent transport mechanics of two dominant pathways affecting this exchange. Using high‐frequency measurements from an experiment conducted at the Duke Forest in North Carolina, wavelet analysis is applied to time series of carbon dioxide and water vapor concentrations in order to (1) determine the dominant eddy sizes involved in the net exchange of these constituents, (2) resolve the eddy size and timescales involved in the intermittent release of CO2 from the forest floor to the atmosphere, and (3) relate the boundary layer turbulent characteristics to the transport of air enriched in CO2 from soil respiration. During the daytime hours, when photosynthesis and soil respiration are active in this pine forest and evapotranspiration is taking place, air enriched in both CO2 and water vapor is indicative of transport from the forest floor. Thus the coherent turbulent structures associated with these transport events are identified and conditionally analyzed from the time series by wavelet transforms, which retain information in the time domain as well as the frequency domain. The dominant flux‐carrying eddies between the canopy and atmosphere were approximately 63 m in diameter, about four times the height of the canopy. Eddies that were most effective in transporting air enriched in CO2 from below the canopy to the atmosphere were found to be approximately 8 m in diameter, on the order of one half the canopy height. Conditional sampling shows that the prevalence of air enriched in both CO2 and water vapor is related to the rate of turbulent kinetic energy production measured from 24 approximately half‐hour time series corresponding to unstable atmospheric conditions.
Diurnal variations of the atmospheric electric field and conductivity at Maitri, Antarctica
Authors: Deshpande C. G. , Kamra A. K.
Published: February 2004

Surface measurements of the atmospheric electric potential gradient and conductivity have been made at the Indian station, Maitri (70°45′52″S, 11°44′3″E, 117 m above mean sea level), Antarctica, from January 10 to February 24, 1997. The diurnal variation curve of potential gradient averaged for 20 fair weather days is single periodic with a maximum at 1300 UT and another secondary but distinct maximum at 1900 UT and a minimum at 0100 UT. The mean value of the potential gradient is 83 V m−1 and the maximum and minimum values are 1.55 and 0.73 times the mean value, respectively. The total conductivity is 2.1×10−14 S m−1, and does not show any significant diurnal variation. The diurnal variation of the potential gradient observed at Maitri is similar to the one observed a few days later in the southern Indian Ocean and to the one reported by Kamra et al. [1994] with a few hours difference in phase in the northern Indian Ocean. The results are discussed in terms of the seasonal variation of the longitudinal distribution of thunderstorm activity. It is proposed that the diurnal variation of flash activity when averaged over several days or weeks may cause a peak at an averaged time in the diurnal variation of ionospheric potential.
Atmospheric dispersion modeling for assessment of exposure to arsenic for epidemiological studies in the Nitra Valley, Slovakia
Authors: Colvile R. N. , Stevens E. C. , Nieuwenhuijsen M. J. , Keegan T. J.
Published: February 2004

The short‐range atmospheric dispersion model UK‐ADMS has been applied to calculate ground‐level concentrations of arsenic within 20 km of the Nováky Power Plant, Nitra Valley, Slovakia, as a function of distance and direction from source and year of operation, for application to an assessment of local population exposure to arsenic, and hence to two epidemiological studies of the effect of arsenic exposure on skin, lung, and bladder cancer in an east European population. A simple approach, where the effect of complex terrain is included by using local observations for meteorological input to the model, is sufficient for initial screening to compare atmospheric dispersion with other exposure pathways. This shows that isopleths of ground‐level annual average arsenic concentration in air (excluding periods when there is no wind) are elongated aligned with the axis of the valley. The maxima of concentration occur about 2 km to the northeast and southwest of the source, and a factor of 30 reduction in concentration is found from there to a distance 10 km away. The value of this information for epidemiological studies is discussed. If atmospheric concentration is found to be an important determinant of arsenic exposure for the Nitra Valley population, it will be necessary to carry out further investigation of the dispersion of the plume in complex terrain when there is no wind in the valley, which is the case in 13% of the observations and is estimated could contribute more than 50% of the annual average arsenic concentration close to the source.
Neural network analysis of the radiative interaction between neighboring pixels in inhomogeneous clouds
Authors: Faure Thierry , Isaka Harumi , Guillemet Bernard
Published: February 2004

In this study, we analyzed the effect of the radiative interaction between neighboring pixels on the high‐resolution radiant flux of bounded cascade inhomogeneous clouds by using a one‐layer mapping neural network as generalized regression analysis. The analysis was done for reflectance, transmittance, and absorptance at different wavelengths under different conditions of illumination. The sign and magnitude of output coefficients indicate how neighboring pixels contribute to the radiant flux of a target pixel. We found that the variation of output coefficients with the distance from the target pixel changes significantly in its shape and horizontal extent not only with the type of radiant flux we consider but also with the wavelength and solar zenith angle. The mapping neural network clearly reveals the asymmetric feature of radiative interaction between neighboring pixels under oblique illumination, which illustrates the shadowing and enhancing effects of local cloud inhomogeneity. The present analysis shows that the mapping neural network is a flexible method of analysis when used as a generalized regression analysis.
Three‐dimensional model simulations of SF6 with mesospheric chemistry
Authors: Reddmann Thomas , Ruhnke Roland , Kouker Wolfgang
Published: February 2004

Multiannual integrations with the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA) have been performed using meteorological analyses of vorticity and divergence up to 10 hPa to analyze the influence of a simplified SF6 mesospheric chemistry on estimation of mean age of air and to compare profiles of SF6 mixing ratios observed in the stratosphere with model simulations. The chemical degradation scheme includes electron attachment of SF6 and subsequent reactions of SF−6, such as photodetachment and charge transfer with ozone. Several combinations of reaction rate constants and electron profiles have been tested. Good agreement with observations is found for inert SF6 transport. However, when mesospheric loss is included in the model, significant deviations are found for polar winter observations above 25 km. Chemical loss by electron attachment without reactions yielding SF6 again is not compatible with observations. The atmospheric lifetime of SF6 spans 400 to 10,000 years, depending on the assumed loss mechanism and the value for the electron density in the stratosphere.
Midstratospheric ozone variability over Bern related to planetary wave activity during the winters 1994–1995 to 1998–1999
Authors: Calisesi Yasmine , Wernli Heini , Kämpfer Niklaus
Published: February 2004

Since November 1994, continuous observations of the stratospheric and mesospheric ozone volume mixing ratio (VMR) profiles over Bern (46.95°N, 7.45°E), Switzerland, are performed using the Ground‐based Millimeter‐Wave Ozone Spectrometer (GROMOS), an instrument of the Network for the Detection of Stratospheric Change. We report on large episodic perturbations of the midstratospheric (25–40 km) ozone VMR values observed during the winters 1994–1995 through 1998–1999. Backward trajectory calculations show that the observed episodes are coincident with periods of enhanced meridional transport. Representations of the isentropic potential vorticity field indicate that this transport goes along with significant deformations and southward excursions of the polar vortex in association with strong planetary wave activity. Along the eastern edge of the distorted vortex, northward advection of subtropical air leads to anomalously high ozone VMR values in the midlatitudes middle stratosphere, whereas the passage of polar vortex air over Bern leads to midstratospheric ozone minima. Besides a comprehensive analysis of all extreme episodes detected between November 1994 and June 1999, details are presented for one specific episode. For another episode the influence of photochemical processes is investigated, and it is found that photochemistry acts to damp (rather than to enhance) the effects of planetary‐wave‐driven meridional transport. It is concluded that the extreme ozone episodes observed over Bern during winter are primarily a dynamical feature, their amplitude being determined by the meridional ozone VMR gradient rather than by photochemical processes.
Effective albedo derived from UV measurements in the Swiss Alps
Authors: Schmucki Daniel , Voigt Stefan , Philipona Rolf , Fröhlich Claus , Lenoble Jacqueline , Ohmura Atsumu , Wehrli Christoph
Published: February 2004

The Alps are one of the regions in Europe showing very high erythemal UV irradiance (UVery) which is a consequence of low aerosol levels, high altitude and snow‐covered surfaces. Effective albedo plays a key role in understanding the radiative transfer over an Alpine terrain. The difficulty in determining the effective albedo is predominantly due to inhomogeneous surfaces in Alpine regions and a number of interrelated parameters. Most of these parameters can be corrected or normalized by simple methods. The effect of total ozone on different components of the erythemal UV irradiance and the influence of aerosols have been largely investigated and are determined by model calculations. After these atmospheric corrections or normalizations the albedo of the surrounding surfaces within a radius of about 25 km is the only remaining influence to be taken into account on the ratio between direct and diffuse UVery radiation. Satellite data from selected days allowed us to determine the fraction of snow coverage within this radius and to weight the effect of the surrounding surfaces on UVery radiation. With this information from space and data from local albedo measurements in the erythemal UV, it is possible to calculate effective albedo values for these selected days. A correlation between effective albedo and the ratio direct to diffuse was found to be linear for constant solar elevations. Hence this correlation allows us to determine effective albedo at any Alpine location with accurate measurements of direct and diffuse UVery radiation. It is shown from this investigation that the albedo effect can enhance diffuse UVery radiation by up to 57% and global UVery radiation by 30%, especially in spring.
An open chamber system for measuring soil surface CO2 efflux: Analysis of error sources related to the chamber system
Authors: Pumpanen J. , Ilvesniemi H. , Keronen P. , Nissinen A. , Pohja T. , Vesala T. , Hari P.
Published: February 2004

An open dynamic chamber is a widely used method for soil CO2 efflux measurements. However, any uncontrolled air flow between the inside and outside of the chamber that is created by differences between the inflow and outflow rates can cause errors in soil CO2 efflux measurements. Estimates of these errors are reported here. A system was constructed in which compensation air of known CO2 concentration was introduced into the chamber and the CO2 concentration was measured in the outflow. The flows of air into and out of the chamber were controlled by two separate mass flow controllers, and the pressure difference between the chamber and the ambient air was measured. When the inflow was larger than the outflow, the measured efflux decreased, whereas when the inflow was smaller than the outflow, the efflux increased. With low fluxes this error was statistically significant when the difference between the flow rates was >30%. The difference between flow rates had a larger effect on efflux rates during high efflux than during low efflux. The effect of air mixing inside the chamber on the reliability of the efflux measurement was tested. Without sufficient mixing it was not possible to get stable efflux readings with this type of chamber technique. The open chamber measurements described here were also compared to closed static measurements based on syringe samples taken manually from the chamber. On average the CO2 efflux values measured with closed static technique were 11% lower than those measured with open dynamic method.
Simulation of LIDAR returns from pristine and deformed hexagonal ice prisms in cold cirrus by means of “face tracing”
Authors: Del Guasta Massimo
Published: February 2004

The simulation of the optical properties of ice clouds plays a crucial role in the interpretation of remote sensing data of cirrus clouds. In this work, a novel simulation code (“face tracing”) derived from ray tracing was described and used to compute LIDAR depolarization and the extinction/backscatter ratio at 532 nm, as expected from randomly oriented pristine and slightly deformed hexagonal prisms of various sizes and aspect ratios. By increasing the aspect ratio the depolarization of pristine crystals was found to increase sharply from zero (thin plates) to a maximum value (columns) at an aspect ratio of around 1, where an absolute minimum of extinction/backscatter ratio (corresponding to a maximum LIDAR backscatter efficiency) was also found. When including the far‐field diffraction in backscattering simulations, pristine particles smaller than 100 μm showed depolarization and extinction/backscatter ratios comparable with the experimental LIDAR data of cold (T < 30°C) polar cirrus. Recent in situ observations showed the widespread presence of nonpristine hexagonal crystals in cirrus, stimulating the calculation of the backscatter properties of deformed particles by “face tracing.” Simulations for deformed hexagonal prisms showed in most cases a smaller depolarization and a higher extinction/backscatter ratio compared with those obtained for pristine crystals. A mixture containing variable proportions of pristine and deformed hexagonal prisms (with an aspect ratio of 1–2) resulted in a depolarization‐extinction/backscatter scatterplot similar to the experimental one for cold polar cirrus.
Investigation of the nighttime decay of isoprene
Authors: Hurst Julia M. , Barket Dennis J. , Herrera‐Gomez Orlando , Couch Tara L. , Shepson Paul B. , Faloona I. , Tan D. , Brune W. , Westberg H. , Lamb B. , Biesenthal T. , Young V. , Goldstein Allen , Munger J. W. , Thornberry T. , Carroll M. A.
Published: February 2004

A rapid nighttime decay of isoprene (2‐methyl‐1,3‐butadiene) has been observed at several forest sites. Data from the Program for Research on Oxidants: PHotochemistry, Emissions, and Transport (PROPHET) have been carefully examined with respect to this phenomenon. Essentially every evening (at PROPHET), isoprene concentrations fall from several ppb to levels below 100 ppt, with an average lifetime of 2.7 hours. Since this decay rate exceeds that expected from established nighttime chemistry, other possible mechanisms are suggested and discussed. Reaction with ozone will not occur at a rate consistent with the observed decay. Calculations of nitrate radical concentrations reveal that this oxidant only becomes an important sink for isoprene after the majority of the isoprene decay has taken place. The isoprene flux data were not consistent with dry deposition playing a significant role in nighttime forest loss. On the basis of ambient measurements of OH radical concentrations at the PROPHET site, calculated isoprene decay rates were compared with observations. For some nights the observed decay can be fit strictly by OH consumption; however, the reported OH data overpredict the isoprene loss rate on most nights. We estimate that vertical mixing with isoprene‐depleted air probably contributes to the fast isoprene decay observed; however, the measurements needed to support this suggestion have yet to be made.
Ground‐based determination of the spectral ultraviolet extraterrestrial solar irradiance: Providing a link between space‐based and ground‐based solar UV measurements
Authors: Gröbner J. , Kerr J. B.
Published: February 2004

The extraterrestrial solar spectrum between 295 and 355 nm has been determined from direct irradiance measurements made with a Brewer double spectrophotometer, using the Langley method. The measurements in this study consist of 449 half days of data collected during 1998 at Mauna Loa Observatory, Hawaii. The ±2.3% accuracy of this extraterrestrial solar spectrum is obtained by a careful investigation of the instrument calibration and the systematic errors that can arise because of atmospheric and instrument instabilities as well as finite slit width effects and is limited by the uncertainty of the absolute irradiance scale transfer between the standard laboratory and the instrument. A comparison between this extraterrestrial solar spectrum measured from the ground with the mean UARS and ATLAS‐1 spectrum show an agreement better than 3%. The mean ratios are 1.002 for the mean UARS spectrum, 1.003 for the mean ATLAS‐1 spectrum, 1.013 for the SOLSPEC spectrum, and 1.017 for the ATLAS‐3 spectrum.
“Cloud slicing”: A new technique to derive upper tropospheric ozone from satellite measurements
Authors: Ziemke J. R. , Chandra S. , Bhartia P. K.
Published: February 2004

A new technique called cloud slicing has been developed for measuring upper tropospheric O3. Cloud slicing takes advantage of the opaque property of water vapor clouds to ultraviolet wavelength radiation. Measurements of above‐cloud column O3 from the Nimbus 7 Total Ozone Mapping Spectrometer (TOMS) instrument are combined together with Nimbus 7 temperature‐humidity and infrared radiometer (THIR) cloud‐top pressure data to derive O3 column amounts in the upper troposphere. In this study, tropical TOMS and THIR data for the period 1979–1984 are analyzed. By combining total tropospheric column ozone (referred to as TCO) measurements from the convective cloud differential (CCD) method with 100‐ to 400‐hPa upper tropospheric column O3 amounts from cloud slicing, it is possible to estimate 400‐ to 1000‐hPa lower tropospheric column O3 and evaluate its spatial and temporal variability. Results for both the upper and lower tropical troposphere show a year‐round zonal wave number 1 pattern in column O3 with the largest amounts in the Atlantic region (up to ∼15 DU in the 100‐ to 400‐hPa pressure band and ∼25–30 DU in the 400‐ to 1000‐hPa pressure band). Upper tropospheric O3 derived from cloud slicing shows maximum column amounts in the Atlantic region in the June‐August and September‐November seasons which are similar to the seasonal variability of CCD‐derived TCO in the region. For the lower troposphere, the largest column amounts occur in the September‐November season over Brazil in South America and also southern Africa. Localized increases in the tropics in the lower tropospheric O3 are found over the northern region of South America around August and off the west coast of equatorial Africa in the March‐May season. Time series analysis for several regions in South America and Africa show an anomalous increase in O3 in the lower troposphere around the month of March which is not observed in the upper troposphere. The eastern Pacific indicates weak seasonal variability of upper, lower, and total tropospheric O3 compared with the western Pacific, which shows the largest TCO amounts in both hemispheres around spring months. O3 variability in the western Pacific is expected to have greater variability caused by strong convection, pollution and biomass burning, land‐sea contrast, and monsoon developments.