bluuebunny/crossref_metadata_2025 huggingface dataset:
A global three‐dimensional chemical transport model for the troposphere: 1. Model description and CO and ozone results
Authors: Berntsen Terje K. , Isaksen Ivar S. A.
Published: February 2004

A global three‐dimensional photochemical tracer/transport model (CTM) of the troposphere has been developed. The model is based on the NASA/Goddard Institute for Space Studies (GISS) CTM with the incorporation of an extensive photochemical scheme. The model resolution is 8° latitude and 10° longitude with nine vertical layers below 10 hPa. One year of meteorological data from a free running GCM (NASA/GISS), including advective winds and convection frequency with 8‐hour time resolution, is used as input. Transport of species by advection, convection, and diffusion is included in the model. The chemical scheme consists of 49 components, 85 thermal reactions, and 16 photolytic reactions. The chemical scheme is solved by the quasi steady state approximation (QSSA) method with iterations and chemical families, with a time step of 30 min. The model simulates well the lower tropospheric distribution of key species like carbon monoxide, nonmethane hydrocarbons (NMHCs), and ozone. The model is also able to simulate the important pattern of background NOx distribution [see Jaffe et al. this issue]. In the upper troposphere, coarse model resolution gives some discrepancies between modeled ozone concentrations and observations, especially at high latitudes. A global tropospheric ozone budget is presented. Net ozone production is found in the boundary layer and in the upper troposphere. In the middle free troposphere there is a close balance between chemical loss and production of ozone, giving a small net ozone loss. Hydroxyl (OH) concentrations are found to be sensitive to parameterization of cloud effects on photolysis rates. A global mean tropospheric OH concentration of 1.1×106 molecules/cm3 is calculated, which is about 15% higher than recent estimates from analysis of CH3CCI3 observations indicate.
Photochemical ozone production in the convective mixed layer, studied with a tethered balloon sounding system
Authors: De Muer D. , Heylen R. , Van Loey M. , De Sadelaer G.
Published: February 2004

In 1987, 1988, and 1990, four measuring campaigns with a tethered balloon sounding system were performed in a semirural area in Belgium to study the ozone distribution in the boundary layer in relation to meteorological parameters, during episodes of photochemical ozone production in the convective mixed layer. The detailed profiles of the ozone concentration between sunrise and sunset were used for a calculation of the change rate of the ozone column density in the boundary layer. These values allowed us to determine for the first time the integrated ozone production rates in the convective mixed layer, making use of a simplified form of the continuity equation of ozone mass. From all the measuring campaigns a range of integrated ozone production rates from 0 to 9 μg m−2 s−1 was found. Peak values of the ozone production rate were observed when the direct solar radiation intensity rises for the first time in the course of a day to about 400 W m−2 before noon. The upper envelope of a scatterplot of all ozone production rates as a function of the direct solar radiation shows a linear course up to about 650 W m−2. Beyond this value the intensity of direct solar radiation is no longer a rate‐limiting factor in the photochemical ozone production process in the boundary layer; under these conditions the ozone production is limited by the supply rate of hydrocarbons. In general, the measured ozone production rates also show an increase with increasing ambient temperatures. At temperatures larger than 24°C the integrated net ozone production rate during the campaigns was always at least 3 μg m−2 s−1 when the direct solar radiation values were larger than 350 W m−2 . The surface ozone deposition velocity at the site of the campaigns was determined from the ozone data through an indirect method that does not require any parameterization of the vertical ozone flux; at daylight conditions an upper limit of 1.4 cm s−1 was found.
Estimation of photosynthetically active radiation absorbed at the surface
Authors: Li Zhanqing , Moreau Louis , Cihlar Josef
Published: February 2004

This paper presents a validation and application of an algorithm byLi and Moreau[1996] for retrieving photosynthetically active radiation (PAR) absorbed at the surface (APARSFC). APARSFCis a key input to estimating PAR absorbed by the green canopy during photosynthesis. Extensive ground‐based and space‐borne observations collected during the BOREAS experiment in 1994 were processed, colocated, and analyzed. They include downwelling and upwelling PAR observed at three flux towers, aerosol optical depth from ground‐based photometers, and satellite reflectance measurements at the top of the atmosphere. The effects of three‐dimensional clouds, aerosols, and bidirectional dependence on the retrieval of APARSFCwere examined. While the algorithm is simple and has only three input parameters, the comparison between observed and estimated APARSFCshows a small bias error (<10 W m−2) and moderate random error (36 W m‐2for clear, 61 W m−2for cloudy). Temporal and/or spatial mismatch between satellite and surface observations is a major cause of the random error, especially when broken clouds are present. The algorithm was subsequently employed to map the distribution of monthly mean APARSFCover the 1000×1000 km2BOREAS region. Considerable spatial variation is found due to variable cloudiness, forest fires, and nonuniform surface albedo.
Heterogeneous interaction of formaldehyde with cold sulfuric acid: Implications for the upper troposphere and lower stratosphere
Authors: Iraci Laura T. , Tolbert Margaret A.
Published: February 2004

Laboratory studies of formaldehyde uptake on thin sulfuric acid films indicate that HCHO can be taken up for long periods of time without saturation. Initial Henry's law solubility coefficients are large (H* = 106–107 M atm−1) at stratospheric temperatures. This uptake is due to rapid formation of HCHO(aq), CH2(OH)2, and possibly HOCH2OSO2(OH) in solution. Over a longer time period, steady‐state uptake continues with γ≈0.002 due to formaldehyde polymerization. Although uptake by stratospheric sulfate aerosols will not provide a significant sink for atmospheric formaldehyde, it may lead to high condensed‐phase concentrations of HCHO and polymerization in solution. Reaction of condensed HCHO with nitric acid in sulfuric acid has also been observed, converting HNO3 to odd nitrogen (NOx) and formaldehyde to formic acid. This reaction may have considerable impact on the HNO3/NOx ratio, as its rate is likely comparable to the HNO3 photolysis rate.
Modeling the airborne particle complex as a source‐oriented external mixture
Authors: Kleeman Michael J. , Cass Glen R. , Eldering Annmarie
Published: February 2004

A Lagrangian air quality model is developed which represents the airborne particle complex as a source‐oriented external mixture. In a source‐oriented external mixture, particles of the same size can evolve to display different chemical compositions that depend on the chemical and hygroscopic properties of the primary seed particles initially emitted from different sources. In contrast, previous models initialize the airborne particles as an internal mixture in which all particles of the same size are assumed to have the same chemical composition. Test cases show that representation of the aerosol as an internal mixture can distort the predicted particle composition and concentration in the HNO3/NH3/HCl/H2SO4/aerosol Cl−/SO4=/NO3−/NH4+/Na+ system when Na+ and SO4= exist in separate particles, as may occur when sea spray coexists with long‐distance transport of anthropogenic sulfates. Tests also indicate that the external mixture model can predict the evolution of a nearly monodisperse aerosol into a bimodally distributed aerosol as relative humidity increases, qualitatively matching observations. The source‐oriented external mixture model is applied to predict the size and composition distribution of airborne particles observed at Claremont, California, on August 28, 1987. Calculations produce an aerosol mass distribution that is distinctly bimodal in the size range from 0.1 μm to 1.0 μm particle diameter, matching field observations. External mixture calculations also predict specific differences in composition between particles of the same diameter. The external mixture model is expected to have applications including exploration of the cause of the particle‐to‐particle differences seen by time‐of‐flight mass spectrometers that measure single particle size and composition in the atmosphere.
Chemical and physical characterizations of atmospheric aerosols over southern California
Authors: Li Shao‐Meng , Macdonald A. M. , Strapp J. W. , Lee Y.‐N. , Zhou X.‐L.
Published: February 2004

In September 1994 the chemical and physical properties of aerosol particles over southern California were characterized. Concentrations of particle chemical species were higher near the surface than at higher altitudes. In these particles, measured organic and inorganic water soluble masses were 7±1% and 75±12%, respectively, of the dried total particle mass (TPM). Non‐sea‐salt (nss) SO4=, NO3−, NH4+, and H+ were the major contributors to the mass and ionic equivalence of the particles. The ratio of the soluble organics (SumOrg) to nss SO4= was found to be a function of the TPM, which was similar to the simple relationship found over the northwestern Atlantic Ocean for altitudes <3 km. When the H+ ion was included, there was a good ionic balance between the cations and anions in the particles. The pH of the particles ranged from −2.4 to 0.20 (averaging −0.85). The particle volume size distribution dV/d log D shows distinct features according to the altitude and location of sample collection. In urban plumes, dV/d log D was significantly higher than at high altitudes and had a consistent accumulation mode peaking at 0.24 μm. For all samples, nss SO4= and NH4+ comprised 59% of the accumulation mode particle mass on average. Samples showed two groups with high and low NH4+/nss SO4= ratios of 0.85 and 0.01, respectively. For the first group the low ambient relative humidities indicate that NH4+ and nss SO4= were in crystallized ammonium sulfate and letovicite in equilibrium with solution, while the second group was probably close to H2SO4 droplets. On the basis of these compositions the refractive index η was found to be about 1.5 for the first group and <1.4 for the second group. The water contents of the NH4+ − nss SO4= particles were estimated to be 35±24%, leading to an average ratio of H2O/nss SO4==0.9±1.2.
Results of the Gas‐Phase Sulfur Intercomparison Experiment (GASIE): Overview of experimental setup, results and general conclusions
Authors: Stecher Hilmar A. , Luther George W. , MacTaggart Douglas L. , Farwell Sherry O. , Crosley David R. , Dorko William D. , Goldan Paul D. , Beltz Norbert , Krischke Udo , Luke Winston T. , Thornton Donald C. , Talbot Robert W. , Lefer Barry L. , Scheuer Eric M. , Benner Richard L. , Wu Jainguo , Saltzman Eric S. , Gallagher Michael S. , Ferek Ronald J.
Published: February 2004

Seven techniques for the field measurement of trace atmospheric SO2 were compared simultaneously over 1 month in 1994 using samples produced in situ by dynamic dilution. Samples included SO2 in dry air, in humid air, and in air with potentially interfering gases added. In addition, 2 days of comparison using diluted ambient air were conducted. Six of the seven techniques compared well, with good linear response and no serious interferences but with a range of calibration differences of about 50%.
Influence of continental outflow events on the aerosol composition at Cheju Island, South Korea
Authors: Chen Li‐Ling , Carmichael Gregory R. , Hong Min‐Sun , Ueda Hiromasa , Shim Shang , Song Chul H. , Kim Y. P. , Arimoto Richard , Prospero Joseph , Savoie Dennis , Murano Kentaro , Park John K. , Lee Ho‐geun , Kang C.
Published: February 2004

The chemical composition of aerosols measured at Cheju Island, Korea, over the 3‐year period March 1992 to February 1995 are presented and discussed, with a particular emphasis on the Pacific Exploratory Mission in the Western Pacific (PEM‐West B) time period. Cheju Island is under the influence of continental outflow conditions nearly 70% of the year, and as a result the aerosol loading of sea salt as well as continental aerosol components is high. The 3‐year mean values derived from the daily tape filters are non‐sea‐salt (nss) SO22− = 6.8 μg/m3, NO3− = 1.2 μg/m3, Cl− = 1.9 μg/m3, Na+ = 1.7 μg/m3, Ca2+ = 0.5 μg/m3, NH4+ = 1.3 μg/m3, Mg2+ = 0.3 μg/m3, and K+ = 0.4 μg/m3. Sea‐salt components show peak values in winter, while calcium, nitrate, potassium, and to a lesser extent, sulfate and ammonium, show higher values in the spring, and all species exhibit a pronounced minimum in summer. Trajectory and principal component analysis show that elevated levels of primary aerosols (both sea salt and soil‐derived) occur with strong wind conditions associated with winter and spring, and high concentrations of non‐sea‐salt components are most strongly associated with springtime continental outflow events. During the PEM‐West B period, nitrate and calcium are found to be ∼60% higher than the annual mean, sea‐salt components ∼30% higher, and sulfate ∼10% higher. The aerosol data are combined with gaseous SO2 concentrations, precipitation chemistry data and companion aerosol measurements taken at the same site, to provide further insights into the aerosol composition at Cheju. At Cheju, sulfate and ammonium are found mostly in the fine fraction (∼80 to 90%), while calcium and nitrate reside in the coarse size fraction. Sulfate and nitrate are not associated with the primary aerosols, but rather become associated with the aerosol during the long‐range transport process.
A 6.5‐day westward propagating planetary wave: Origin and characteristics
Authors: Meyer Christian K. , Forbes Jeffrey M.
Published: February 2004

A large (10–30 m s−1) 6–7 day period westward propagating planetary wave was seen in both ground radar and satellite data within the 80–100 km height regime during August and September 1993. Previous analyses of these data argue that it was most likely a manifestation of the (1–2) Rossby planetary wave commonly referred to as the 5‐day wave, perhaps shifted to longer periods by interaction with the mean zonal winds. In this study the global scale wave model (GSWM [Hagan et al., 1993]) is used to analyze the characteristics of this westward propagating wave. We propose that the observed wave is not a Doppler‐shifted 5‐day wave but rather an unstable mode, drawing energy from unstable regions in the upper mesosphere, whose realization is global in scale. The concept of atmospheric instability is used to determine a likely source region, and the model is used determine the characteristic period, growth rate, and perturbation fields of the instability‐driven wave.
Observations of gas‐phase hydrogen peroxide at an elevated rural site in New York
Authors: Balasubramanian R. , Husain Liaquat
Published: February 2004

Concurrent measurements of gas‐phase hydrogen peroxide and ozone, along with meteorological parameters, were made from July 1 to July 28, 1995, at Whiteface Mountain, a rural, high‐elevation site in the northeastern United States. H2O2 and O3 exhibited variability from day to day, with concentrations ranging from 0.15 to 5.15 parts per billion by volume (ppbv) (mean, 1.61±0.90 ppbv) and from 29 to 108 ppbv (mean, 50.2±14.6 ppbv), respectively. We present three case studies to elucidate the influence of atmospheric chemical and dynamical processes on the ambient concentrations of H2O2 and O3. Elevated levels of these two photochemical oxidants were observed during July 12–14, owing to an intense photochemical activity in the presence of a synoptic high‐pressure system centered over the northeast. Nocturnal maxima in H2O2 and O3 concentrations were observed and are thought to be due to mixing processes injecting air from aloft into the nocturnal boundary layer. While H2O2 showed a weak diurnal variation with a maximum concentration during daytime, O3 showed a weak reverse diurnal variation with nighttime concentrations higher than daytime. A strong positive correlation was observed between H2O2 and O3, suggesting that O3 serves as a source of hydroperoxy radicals (HO2), the precursor of H2O2. The concentration of HO2 in the atmosphere was estimated to be 23 parts per trillion by volume from the observed rate of production of hydrogen peroxide, which is in good agreement with the values reported based on direct measurements and modeling studies.
Large‐scale ice clouds in the GFDL SKYHI general circulation model
Authors: Donner Leo J. , Seman Charles J. , Soden Brian J. , Hemler Richard S. , Warren James C. , Ström Johan , Liou Kuo‐Nan
Published: February 2004

Ice clouds associated with large‐scale atmospheric processes are studied using the SKYHI general circulation model (GCM) and parameterizations for their microphysical and radiative properties. The ice source is deposition from vapor, and the ice sinks are gravitational settling and sublimation. Effective particle sizes for ice distributions are related empirically to temperature. Radiative properties are evaluated as functions of ice path and effective size using approximations to detailed radiative‐transfer solutions (Mie theory and geometric ray tracing). The distributions of atmospheric ice and their impact on climate and climate sensitivity are evaluated by integrating the SKYHI GCM (developed at the Geophysical Fluid Dynamics Laboratory) for six model months. Most of the major climatological cirrus regions revealed by satellite observations appear in the GCM. The radiative forcing associated with ice clouds acts to warm the Earth‐atmosphere system. Relative to a SKYHI integration without these clouds, zonally averaged temperatures are warmer in the upper tropical troposphere with ice clouds. The presence of ice produced small net changes in the sensitivity of SKYHI climate to radiative perturbations, but this represents an intricate balance among changes in clear‐, cloud‐, solar‐, and longwave‐sensitivity components. Deficiencies in the representation of ice clouds are identified as results of biases in the large‐scale GCM fields which drive the parameterization and neglect of subgrid variations in these fields, as well as parameterization simplifications of complex microphysical and radiative processes.
NOx emission indices of subsonic long‐range jet aircraft at cruise altitude: In situ measurements and predictions
Authors: Schulte P. , Schlager H. , Ziereis H. , Schumann U. , Baughcum S. L. , Deidewig F.
Published: February 2004

In the course of the Commissions of the European Communities project “Pollution From Aircraft Emissions in the North Atlantic Flight Corridor (POLINAT)”, in situ measurements of NO, NOx, and CO2 volume mixing ratios in the near‐field exhaust plumes of seven subsonic long‐range jet aircraft have been carried out by using the research aircraft Falcon of the Deutsche Forschungsanstalt für Luft‐ und Raumfahrt (DLR). For three additional aircraft, only NO and CO2 were measured. Plume ages of 50 s to 150 s have been covered, with maximum observed exhaust gas enhancements of 319 parts per billion by volume and 51 parts per million by volume for Δ[NOx] and Δ[CO2], respectively, in relation to ambient values. Aircraft cruising altitudes and Mach numbers ranged from 9.1 to 11.3 km and from 0.77 to 0.85, respectively. These measurements are used to derive NOx emission indices for seven of the individual aircraft/engine combinations. The NOx emission indices derived range from 12.3 g/kg to 30.4 g/kg. They are compared with predicted emission index values, calculated for the same aircraft engine and the actual conditions by using two newly developed fuel flow correlation methods. The calculated emission indices were mostly within or close to the error limits of the measured values. On average, the predictions from both methods were 12% lower than the measured values, with an observed maximum deviation of 25%. The ratio γ=[NO2]/[NOx] found during the present measurements ranged from 0.06 to 0.11 for five daytime cases and was around 0.22 for two nighttime cases. By use of a simple box model of the plume chemistry and dilution these data were used to estimate the initial value γ0 present at the engine exit plane. We found γ0 values between 0 and 0.15. These were applied to estimate the corresponding NO2 for the three cases in which only NO was measured.
Effects of interannual variation of temperature on heterogeneous reactions and stratospheric ozone
Authors: Tie XueXi , Granier Claire , Randel William , Brasseur Guy P.
Published: February 2004

A two‐dimensional chemical/dynamical/microphysical model is used to calculate the effect of temperature interannual variability on stratospheric ozone. Two effects associated with temperature variations are considered in this paper: First, the effect on the rate coefficient of heterogeneous reactions occurring at the surface of sulfate aerosols, in particular for the enhanced sulfate aerosols following the eruption of Mount Pinatubo, and second, the effect of temperature interannual variations on the formation of polar stratospheric clouds (PSCs) and the spring Antarctic ozone depletion. The interannual variabilities of ozone concentration affected by both the processes are evaluated. The model results show that during winter 1992–1993, stratospheric temperature at high latitudes in the northern hemisphere is colder than during winter 1991–1992, leading to a larger rate of heterogeneous conversion from inactive chlorine to active chlorine in sulfate aerosols during winter 1992–1993 than during winter 1991–1992 at northern high latitudes. As a result, ozone reduction due to heterogeneous reactions following the eruption of Mount Pinatubo is enhanced in a colder winter (1992–1993) and reduced in a warmer winter (1991–1992). This result indicates that the enhanced heterogeneous conversion associated with colder temperature can explain, in part, the large ozone depletion observed by the total ozone mapping spectrometer (TOMS) experiment during winter 1992–1993. The model results also show that during winters 1986 to 1990, the formation of PSCs over Antarctica exhibits a strong interannual variation, due to temperature variability. In the colder years (1987 and 1989) the surface area of PSCs (a major factor to determine the rate of heterogeneous conversion from inactive chlorine to active chlorine) increases, and ozone concentration decreases. In the warmer years (1986 and 1988) the results are opposite. The calculated ozone interannual variation is consistent with the satellite observation (TOMS), indicating that the interannual variation of the formation of PSCs likely plays an important role for the interannual variability of the spring Antarctic ozone hole.
Arctic boundary layer in the fall season over open and frozen sea
Authors: Paluch I. R. , Lenschow D. H. , Wang Q.
Published: February 2004

This study is based on aircraft data collected on research flights in the Beaufort Arctic Storms Experiment (BASE). The experiment was conducted during the fall, when the frozen parts of the sea surface and the lower troposphere are subject to strong radiative cooling, while the temperature of the open water stays near −1 or −2°C, leading to the formation of two different types of boundary layers. Data from a boundary layer over open water and a boundary layer over frozen sea surface with open leads are examined in detail. The boundary layer over open water was capped by a stratus layer in which large temperature fluxes were observed. Recent advection of air from the colder coastal regions over warm water was probably responsible for the large temperature differences at the sea‐air interface. The boundary layer over ice was clear and very stably stratified, with a low‐level potential temperature lapse rate in the 20–57 K km−12 range. Turbulent fluxes were very weak or nonexistent, except at the lowest observation level, 40 m above surface, in an area over many open leads. In this boundary layer, wave‐like features were common. Some were identified as gravity waves, but others with shorter wavelengths could have been due to horizontal roll vortices generated by convection from the leads.
Bursts of intermediate ions in atmospheric air
Authors: Hõrrak U. , Salm J. , Tammet H.
Published: February 2004

The mobility spectrum of air ions has been measured at Tahkuse Observatory in Estonia for several years. The average concentration of intermediate ions with mobilities of 0.05–0.5 cm2 V−1 s−1 in atmospheric air is about 50 cm−3. On the level of this low background, high concentration bursts of intermediate air ions occur occasionally. A burst can be followed by subsequent evolution of intermediate ions into larger ones. To explain the bursts of intermediate air ions, two hypotheses can be advanced: (1)A burst of neutral particles occurs due to homogeneous nucleation, and the particles are charged by the attachment of cluster ions. (2) The cluster ions grow by ion‐induced nucleation in proper environmental conditions.
Temperature histories in liquid and solid polar stratospheric cloud formation
Authors: Larsen Niels , Knudsen Bjørn M. , Rosen James M. , Kjome Norman T. , Neuber Roland , Kyrö Esko
Published: February 2004

Polar stratospheric clouds (PSCs) have been observed by balloonborne backscatter sondes from Alert, Thule, Heiss Island, Scoresbysund, Sodankylä, Søndre Strømfjord, and Ny Ålesund during winters 1989, 1990, 1995, and 1996 in 30 flights. The observations can be categorized into two main groups: type 1a and type 1b PSC particles. Type 1b PSCs show the characteristics expected from liquid ternary solution (HNO3/H2SO4/H2O) particles, consistent with model simulations. Type 1a PSCs are observed at all temperatures below the condensation temperature TNAT of nitric acid trihydrate (NAT), consistent with solid NAT composition. Air parcel trajectories have been calculated for all observations to provide synoptic temperature histories of the observed particles. A number of cases have been identified, where the particles have experienced temperatures close to or above the sulfuric acid tetrahydrate melting temperatures within 20 days prior to observation. This assures a knowledge of the physical phase (liquid) of the particles at this time, prior to observation. The subsequent synoptic temperature histories, between melting and the time of observation, show pronounced differences for type 1a and type 1b PSC particles, indicating the qualitative temperature conditions, necessary to generate solid type 1a PSCs. The temperature histories of type 1b particles show smoothly, in most cases monotonie, decreasing temperatures. The temperature can apparently decrease to the frost point without causing the particles to freeze. The type 1b PSC particles are mostly observed shortly after entering a cold region. The observed type 1a particles have spent several days at temperatures close to or below TNAT prior to observation, often associated with several synoptic temperature oscillations around TNAT, and the particles are observed in aged clouds. It appears that the PSC particles may freeze, if they experience synoptic temperatures below TNAT with a duration of at least 1 day, possibly accompanied by several temperature oscillations. However, liquid particles that experience a smooth cooling, even to very low temperatures, or single smooth cooling/heating below TNAT without synoptic temperature fluctuations do not seem to freeze.
Scavenging of aerosol particles by large water drops: 2. The effect of electrical forces
Authors: Pranesha T. S. , Kamra A. K.
Published: February 2004

The effect of electrical forces on the collection efficiency of millimeter‐sized water drops collecting micron‐sized aerosol particles has been investigated in a laboratory experiment. The observations show higher collection efficiencies for drops of 3.6‐ to 4.8‐mm diameters than reported in some of the earlier studies for smaller drops. The limited and sparse data obtained in our experiments show that the collection efficiency of a drop is higher when it is charged or interacts with the aerosol in the presence of an electric field. The collection efficiency shows a maximum when the drop charge of either polarity is in the range of 10−12 to 10−11 C. The data show that the drop surface charge density required for this maximum decreases with the increase in drop size but is independent of the particle size. However, the peak value of collection efficiency is higher for larger particles. Moreover, the total charge on the drop required for this maximum remains almost constant at about 2–3×10−12C. The collection efficiency increases with the increase in the electric field, and the effect of the electric field is stronger for larger drops. In high fields, the drop collection efficiency shows a maximum for particles of diameter between 3.5 and 5 μm. The change in collection efficiency for the same change in particle size is larger for higher electric fields. Distortion of large drops and the consequent charge accumulation on the rim of the drop has been proposed to explain the results. The decrease in collection efficiency for large values of drop charge and electric field support the drop‐to‐particle charge transfer during their interactions.
Luminosity characteristics of dart leaders and return strokes in natural lightning
Authors: Jordan Douglas M. , Rakov Vladimir A. , Beasley William H. , Uman Martin A.
Published: February 2004

Streak‐camera photographs were obtained in daylight for 23 subsequent strokes in five Florida negative cloud‐to‐ground flashes. Out of the 23 return‐stroke streaked images, only 11 were accompanied by leader streaked images, while all 23 leaders were identified in corresponding electric field records. Thus, 12 subsequent leaders (one of which created a new channel to ground) failed to produce luminosity above the daylight background level. The brightest three dart‐leader/return‐stroke sequences from two flashes have been examined for relative light intensity as a function of time and height. Dartleader light waveforms appear as sharp pulses with 20‐to‐80% risetimes of about 0.5–1 μs and widths of 2–6 μs followed by a more or less constant light level (plateau). The plateau continues until it is overridden by the return‐stroke light waveform, suggesting that a steady leader current flows through any channel section behind the downward moving leader tip before the return‐stroke front has passed that channel section. Return‐stroke light pulses near ground have 20‐to‐80% risetimes of about 1–2 μs and amplitudes a factor of 2 to 3 greater than those of the dart‐leader light pulses. As opposed to the return‐stroke light pulses that suffer appreciable degradation during the upward propagation of the return‐stroke front, the dart‐leader light pulses preserve their shape, and the pulse amplitude is either more or less constant or increases as the leader approaches ground. The average electric field intensity across the dart‐leader front, whose length is inferred from measured light‐pulse risetimes and propagation speed to be of the order of 10 m, should be at least an order of magnitude greater than the average electric field intensity across the return‐stroke front, whose length is inferred to be of the order of 100 m.
Variations in western Arctic temperatures in response to cloud radiative and synoptic‐scale influences
Authors: Stone Robert S.
Published: February 2004

The analysis focuses on Barrow, Alaska, a site that is sensitive to changing conditions because it is located near cryospheric boundaries and is influenced by both extratropical and Arctic synoptic activity. Surface and upper air meteorological data for a 31‐year period (1965–1995) are used to evaluate temperature variations as they relate to dynamical and radiative processes. Both annual and monthly analyses indicate a tendency toward warming overall. However, the annual warming is not monotonic over time and varies seasonally. Comparisons of temperature time series from four sites along the Siberian‐Alaskan coastline show that Barrow is a representative site to evaluate climate change in the western Arctic coastal zone. Regionally, the warming is dominated by significant temperature increases during winter and spring, but cooling is indicated for autumn. These results are not entirely consistent with model predictions of a more uniform high‐latitude warming during the cold season in response to increasing concentrations of greenhouse gases in the atmosphere. Rather, the observed changes are attributed to well‐known natural processes that affect regional cloud distributions in response to changing circulation patterns. Coincident daily and hourly meteorological and radiation data are also used to demonstrate empirically how clouds modulate Arctic temperatures.
A calibrated peroxy radical chemical amplifier for ground‐based tropospheric measurements
Authors: Clemitshaw Kevin C. , Carpenter Lucy J. , Penkett Stuart A. , Jenkin Michael E.
Published: February 2004

A calibrated peroxy radical chemical amplifier (PERCA) instrument has been developed at the University of East Anglia (UEA) and deployed in several recent field measurement campaigns. The UEA PERCA utilizes modulated chemical amplification of NO2 by HO2 and RO2 radicals in the presence of 3.0 ppmv NO and 7.0% vol/vol CO, with the amplification factor or chain length approximately equal to ΔNO2/([H2] + [RO2]). NO2 measurements and calibrations are made in the linear response regime of an improved Scintrex LMA‐3 NO2‐luminol chemiluminescence detector with an estimated systematic uncertainty of ±10% and a precision of 5%. A novel calibration source of CH3O2 radicals has been developed. It is used routinely during field campaigns to measure chain lengths of 175±15 for a new inlet system. The source is based upon 253.7 nm photolysis of CH3I in air and is capable of producing between 25 and 100 pptv CH3O2 radicals. The detection limit of the PERCA varies from 0.2 to 2 pptv for a 30 min averaging time and is largely dependent upon the stability of the sum of the concentrations of ambient NO2 and O3 (Ox) which contribute to a background signal. The estimated systematic uncertainty in the measurements of HO2 and RO2 is ±30%. Observations of peroxy radicals made during the recent WAOWE'94 and WAOSE'95 field measurement campaign on the north Norfolk coast of England are presented to demonstrate the performance of the PERCA and are discussed briefly in terms of nighttime radical chemistry and net photochemical production of O3 and peroxyacetyl nitrate (PAN).
Dust deposition downwind of Owens (dry) Lake, 1991–1994: Preliminary findings
Authors: Reheis Marith C.
Published: February 2004

Salt‐rich dust derived from the Owens Lake playa is deposited in significant quantities to distances of at least 40 km north and south of the playa. Semiannual measurements from 1991 to 1994 of dust deposition rates (dust flux) and composition 2 m above the ground at seven sites in Owens Valley show that (1) dust in Owens Valley is derived mainly from the playa, although areas closer to the sites can also be sources; (2) south of the playa, dust flux is higher in the winter than in the summer, but north of the playa, dust flux is about the same or slightly lower in the winter; (3) on the playa, interannual variation in dust flux is large (factors of 5–10 during the 3 years), but at downwind sites, the variation is much smaller; (4) the dust typically has total soluble salt content as high as 30%, generally much higher than that of dust elsewhere in southern Nevada and California; and (5) to a distance of at least 40 km south of the playa, soluble salt flux is significantly higher than regional rates. The dust flux measurements indicate that significant quantities of salt‐rich dust are probably being added to the soils in the region around Owens Lake playa; these dust additions may affect soil pH and vegetation.
Impact of lightning and convection on reactive nitrogen in the tropical free troposphere
Authors: Kawakami S. , Kondo Y. , Koike M. , Nakajima H. , Gregory G. L. , Sachse G. W. , Newell R. E. , Browell E. V. , Blake D. R. , Rodriguez J. M. , Merrill J. T.
Published: February 2004

Latitudinal distributions of NO, NOy, O3, CO, CH3I, and H2O mixing ratios at 8.9–12 km were obtained between 30°N and 10°S by DC‐8 aircraft measurements made in February 1994 during Pacific Exploratory Mission‐West B (PEM‐West B). Very low NOy mixing ratios with a median value of 51 parts per trillion by volume (pptv) were observed at 9.5–12 km at 1°N–14°N during two flights made within 3 days. A very low median O3 mixing ratio of 19 parts per billion by volume (ppbv) and high mixing ratios of H2O and CH3I were simultaneously observed, suggesting that the low NOy values were probably due to the convective transport of air from the tropical marine boundary layer to this altitude. The median NOy/O3 ratio being a factor of 2 smaller than in the air masses in the tropical marine boundary layer might suggest the possibility that the heterogeneous removal of HNO3 during convective transport further reduced NOy levels. In addition to the measurements between 9.5 and 12 km, low values of NOy and O3 were observed between 4 and 12 km at 1°N. Divergent wind fields at 200 and 1000 hPa and infrared (IR) cloud images show that there was large scale convection (>1000 km × 1000 km) in the northeast of New Guinea Island centered around 0°S and 150°E as part of systematic convective activity of the Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ). This type of large scale convection could have transported air with low levels of NOy and O3 to the middle and upper troposphere over a wide area in the tropics. On the other hand, NO mixing ratios of 50–200 pptv and high NOx/NOy ratios of 0.4–0.6 were observed at 9.5 km between 4°S and 10°S. High H2O mixing ratios of 600–1200 parts per million by volume (ppmv) and low CO mixing ratios of 65 ppbv observed in the air mass indicated that the high NO values were probably due to NO production by lightning. Satellite observations showed relatively frequent lightning flashes over the New Guinea Island for 3 days prior to the aircraft measurements. These results are considered to be consistent with the idea that, in general, marine convection is not accompanied by lightning activity, whereas convection over land is. Because of the large areal extent of the influences from these processes, the convective transport of low NOy air and NO production by lightning should play critical roles in controlling the abundance of reactive nitrogen in the equatorial region.
Rainwater chemistry at the eastern flanks of the Sierra Madre Oriental, Veracruz, Mexico
Authors: Báez A. P. , Padilla H. , Cervantes J. , Pereyra D. , Belmont R.
Published: February 2004

A rainwater chemistry study was carried out at a single site in Xalapa located at the eastern flanks of the Sierra Madre Oriental, Veracruz, Mexico, during the rainy and dry seasons under the influence of different meteorological conditions. Rain samples were analyzed for the most important ions. The statistical results indicated that there were significant differences at the 95% confidence level between the ionic concentration measured in samples collected in both seasons. The lower concentration values found during the rainy season could be attributed to dilution processes since in this season 79% of the annual precipitation occurs, compared to only 21% in the dry season. High and positive ion‐pair correlations were found among the most important ions in both seasons. Although Xalapa is situated at less than 100 km from the coast, marine contribution of SO42−and Ca2+is negligible compared with the nonmarine contribution. In the rainy season, enrichment of SO42−, NH4+, and NO3−is due to upwind medium‐ and long‐distance emission sources of Veracruz, as back trajectories of the predominant winds from the southeast indicate. On the other hand, in the dry season this enrichment is due to emission sources situated in Tampico, Madero, and Poza Rica and south‐southeastern Texas, as indicated by the predominant winds from the northeast. Finally, wet deposition rates were, as expected, higher during the rainy season in spite of the dilution processes.
Simple models of tidal transience: The steady state signal
Authors: Walterscheid R. L.
Published: February 2004

The nature of the tidal response to latitudinally localized transient changes in tidal forcing is examined using simple theoretical models. The major conclusions are (1) the initial tidal response is an outward propagating signal that oscillates with tidal frequency (steady state signal); (2) the diurnal steady state signal propagating from low latitudes is cut off at latitudes poleward of turning latitudes and exhibits exponential decay; (3) a few thousand kilometers from a localized source the steady state signal should exhibit long vertical wavelengths over a few scale heights, with shorter wavelength variations above and below; (4) the unsteadiness of tides observed locally can depend upon the spectrum of free waves excited; and (5) the setup times suggest that the tide can be unsteady to a significant extent. Theory suggests that tidal transience might be an important generator of normal modes, contribute significantly to short‐term tidal variability, and help explain the complex tidal behavior observed at high latitudes.
Detection and parameterization of variations in solar mid‐ and near‐ultraviolet radiation (200–400 nm)
Authors: Lean Judith L. , Rottman Gary J. , Kyle H. Lee , Woods Thomas N. , Hickey John R. , Puga Lawrence C.
Published: February 2004

Nimbus 7 and Solar Stellar Irradiance Comparison Experiment (SOLSTICE) spacecraft measurements of solar irradiance both exhibit variability at mid (200–300 nm) and near (300–400 nm) ultraviolet (UV) wavelengths that are attributable to the Sun's 27‐day solar rotation, even though instrument sensitivity drifts obscure longer‐term, 11‐year cycle variations. Competing influences of dark sunspots and bright faculae are the dominant causes of this rotational modulation. Parameterizations of these influences using a newly developed UV sunspot darkening index and the Mg index facular proxy replicate the rotational modulation detected in both the broadband Nimbus 7 filter data (275–360 nm and 300–410 nm) and in SOLSTICE 1‐nm spectra from 200 to 400 nm. Assuming that these rotational modulation influences scale linearly over the solar cycle, long–term databases of sunspot and global facular proxies permit estimation of 11‐year cycle amplitudes of the mid– and near–UV solar spectrum, unmeasured at wavelengths longward of 300 nm because of insufficient long‐term repeatability (relative accuracy) of state‐of‐the‐art solar radiometers at these wavelengths. Reconstructions of UV irradiances throughout the 11‐year solar cycle indicate variabilities of 0.173 W/m2 (1.1%) in the integrated radiation from 200 to 300 nm and 0.24 W/m2 (0.25%) in radiation from 300 to 400 nm. These two UV bands thus contribute about 13% and 18%, respectively, to the 1.34 W/m2 (0.1%) total (spectrally integrated) radiative output solar cycle. The parameterizations allow customization of UV irradiance time series for specific wavelength bands required as inputs to general circulation model simulations of solar cycle forcing of global climate change, and have practical implications regarding the long‐term repeatability required for future solar monitoring.
Microphysical and radiative properties of boundary layer stratiform clouds deduced from ground‐based measurements
Authors: Dong Xiquan , Ackerman Thomas P. , Clothiaux Eugene E. , Pilewskie Peter , Han Yong
Published: February 2004

Two methods for retrieving the microphysical and radiative properties of marine and continental boundary layer stratiform clouds from ground‐based measurements are implemented. The first method uses measurements of the cloud liquid water path and the cloud nadir radiance at 1 μm to infer the cloud optical depth, cloud droplet effective radius, and cloud droplet concentration. In the second method a <52‐stream radiative transfer model is used to retrieve the microphysical and radiative properties of stratiform clouds from measurements of the cloud liquid water path, the cloud geometric thickness, the downward shortwave irradiance at the surface, and atmospheric profiles of temperature, pressure, and relative humidity. In all of the retrievals the cloud droplets are assumed to have a lognormal size distribution with a logarithmic width of 0.35; the cloud droplet modal radius and cloud droplet concentration are free parameters. Data obtained in the Azores and Oklahoma from marine and continental boundary layer stratiform clouds, respectively, are used in the study. Results of the analysis demonstrated that the retrieved cloud droplet effective radii from the marine clouds (13 μm) were generally larger than the values from continental clouds (7 μm), while cloud droplet number concentrations were much less in marine clouds (150 cm−3) than in continental clouds (500 cm−3). The retrieved cloud droplet effective radius had a strong negative correlation with the cloud droplet concentration. The effects of uncertainties in the measurements, the cloud droplet distribution width, and the surface albedo on the retrieved properties were evaluated by using the 52‐stream model. The errors in the retrieved cloud radiative properties were generally less than 5%, while the errors in the retrieved cloud microphysical properties were considerably larger.
Evaluation of source gas lifetimes from stratospheric observations
Authors: Volk C. M. , Elkins J. W. , Fahey D. W. , Dutton G. S. , Gilligan J. M. , Loewenstein M. , Podolske J. R. , Chan K. R. , Gunson M. R.
Published: February 2004

Simultaneous in situ measurements of the long‐lived trace species N2O, CH4, 12, CFC‐113, CFC‐11, CCl4, CH3CCl3, H‐1211, and SF6 were made in the lower stratosphere and upper troposphere on board the NASA ER‐2 high‐altitude aircraft during the 1994 campaign Airborne Southern Hemisphere Ozone Experiment/ Measurements for Assessing the Effects of Stratospheric Aircraft. The observed extratropical tracer abundances exhibit compact mutual correlations that show little interhemispheric difference or seasonal variability except at higher altitudes in southern hemisphere spring. The environmental impact of the measured source gases depends, among other factors, on the rate at which they release ozone‐depleting chemicals in the stratosphere, that is, on their stratospheric lifetimes. We calculate the mean age of the air from the SF6 measurements and show how stratospheric lifetimes of the other species may be derived semiempirically from their observed gradients with respect to mean age at the extratropical tropopause. We also derive independent stratospheric lifetimes using the CFC‐11 lifetime and the slopes of the tracer's correlations with CFC‐11. In both cases, we correct for the influence of tropospheric growth on stratospheric tracer gradients using the observed mean age of the air, time series of observed tropospheric abundances, and model‐derived estimates of the width of the stratospheric age spectrum. Lifetime results from the two methods are consistent with each other. Our best estimates for stratospheric lifetimes are 122±24 years for N2O, 93±18 years for CH4, 87±17 years for CFC‐12, 100±32 years for CFC‐113, 32±6 years for CCl4, 34±7 years for CH3CCl3, and 24±6 years for H‐1211. Most of these estimates are significantly smaller than currently recommended lifetimes, which are based largely on photochemical model calculations. Because the derived stratospheric lifetimes are identical to atmospheric lifetimes for most of the species considered, the shorter lifetimes would imply a faster recovery of the ozone layer following the phaseout of industrial halocarbons than currently predicted.