bluuebunny/crossref_metadata_2025 huggingface dataset:
Eikonal simulations for the formation and the maintenance of atmospheric gravity wave spectra
Authors: Hertzog Albert , Souprayen Claude , Hauchecorne Alain
Published: October 2002

The atmospheric gravity wave energy spectra often show power law dependencies with wavenumbers and frequencies. A simple mechanism involving off‐resonant scale‐separated interactions is proposed for their formation, namely the refraction of the wave packets in pseudorandom shears encountered during their vertical propagation. In the Boussinesq and rotating frame approximation the evolution of the spectral distribution of wave action is calculated within the eikonal formalism, i.e., via the simulation of the ray paths for an ensemble of elementary wave packets. The energy spectra are then easily built from the wave action spectra. Experiments are conducted where wave packets propagate away from Dirac delta function, or spectrally uniform sources at low altitudes, in realistic atmospheric background flows. The energy spectra show dependencies with the vertical wavenumber m and horizontal wavenumber k that are consistent with the most widely recognized empirical spectral models. A specific focus is given on the vertical evolution of the vertical wavenumber spectrum. The spectrum shows an invariant scaling as N2/m3 at large wavenumbers. It possesses a central wavenumber whose value depends on the total wave energy and is controlled by the statistics of the background mean flow. Similarly, the wave packet azimuths show an increasingly strong anisotropy resulting from the wave mean flow interaction at critical levels.
A comparison of cloud properties at a coastal and inland site at the North Slope of Alaska
Authors: Doran J. C. , Zhong S. , Liljegren J. C. , Jakob C.
Published: October 2002

We have examined differences in cloud liquid water paths (LWPs) at a coastal (Barrow) and an inland (Atqasuk) location on the North Slope of Alaska using microwave radiometer (MWR) data collected by the U.S. Department of Energy's Atmospheric Radiation Measurement Program for the period June–September 1999. Revised retrieval procedures and a filtering algorithm to eliminate data contaminated by wet windows on the MWRs were employed to extract high‐quality data suitable for this study. For clouds with low base heights (<350 m), the LWPs at the coastal site were significantly higher than those at the inland site, but for clouds with higher base heights the differences were small. Air‐surface interactions may account for some of the differences. Comparisons were also made between observed LWPs and those simulated with the European Centre for Medium‐Range Weather Forecasts model. The model usually successfully captured the occurrence of cloudy periods, but it underpredicted the LWPs by approximately a factor of 2. It was also unsuccessful in reproducing the observed differences in LWPs between Barrow and Atqasuk. Some suggestions on possible improvements in the model are presented.
The 6.5‐day wave in the tropical stratosphere and mesosphere
Authors: Talaat E. R. , Yee J.‐H. , Zhu Xun
Published: October 2002

The 6.5‐day wave is a westward propagating zonal wave number 1 (s = 1) planetary wave that is prominent near equinox. The spatial‐temporal structure of the 6.5‐day wave seen in the United Kingdom Meteorological Office (UKMO) data in the stratosphere is found to be consistent with that obtained from Upper Atmosphere Research Satellite/High Resolution Doppler Imager (HRDI) measurements of horizontal winds and temperatures in the mesosphere/lower thermosphere (MLT). Two wave events are commonly observed before and after equinox at low latitudes in both UKMO and HRDI data. This paper focuses on a pair of such events in late 1994. The amplitude growth and phase decrease with altitude of the zonal wind 6.5‐day wave appear to be continuous between the two data sets for the period analyzed. Both wave events appear to grow from the stratosphere to the upper atmosphere at a rate consistent with the suggested vertical phase velocity. Such consistency between the two data sets establishes that the wave previously observed in the MLT is propagating from the stratosphere. Further analyses of the UKMO data shows the presence of the 10‐day s = 1 and 4‐day s = 2 waves that appear to be either nonlinearly coupled with the 6.5‐day wave or related through a common sourcing mechanism.
The impact of increasing carbon dioxide on ozone recovery
Authors: Rosenfield Joan E. , Douglass Anne R. , Considine David B.
Published: October 2002

We have used the Goddard Space Flight Center coupled two‐dimensional model to study the impact of increasing carbon dioxide from 1980 to 2050 on the recovery of ozone to its pre‐1980 amounts. We find that the changes in temperature and circulation arising from increasing CO2 affect ozone recovery in a manner which varies greatly with latitude, altitude, and time of year. Middle and upper stratospheric ozone recovers faster at all latitudes due to a slowing of the ozone catalytic loss cycles. In the lower stratosphere the recovery of tropical ozone is delayed due to a decrease in production and a speed up in the overturning circulation. The recovery of high northern latitude lower stratospheric ozone is delayed in spring and summer due to an increase in springtime heterogeneous chemical loss, and is speeded up in fall and winter due to increased downwelling. The net effect on the higher northern latitude column ozone is to slow down the recovery from late March to late July, while making it faster at other times. In the high southern latitudes the impact of CO2 cooling is negligible. Annual mean column ozone is predicted to recover faster at all latitudes, and globally averaged ozone is predicted to recover approximately 10 years faster as a result of increasing CO2.
Impact of aircraft NOx emissions on tropospheric ozone calculated with a chemistry‐general circulation model: Sensitivity to higher hydrocarbon chemistry
Authors: Kentarchos A. S. , Roelofs G. J.
Published: October 2002

A three‐dimensional chemistry‐general circulation model has been employed to estimate the impact of current aircraft NOx emissions on tropospheric ozone. The model contains a representation of higher hydrocarbon chemistry, implemented by means of the Carbon Bond Mechanism 4 (CBM4), in order to investigate the potential effect of higher hydrocarbons on aircraft‐induced ozone changes. Aircraft NOx emissions increase background NOX (= NO + NO2 + NO3 + 2N2O5 + HNO4) concentrations by 50–70 pptv in the upper troposphere over the Northern Hemisphere, and contribute up to 3 ppbv to upper tropospheric background ozone levels. When higher hydrocarbon chemistry is considered in the simulation, the aircraft‐induced ozone perturbations are higher by ∼12% during summer and the aircraft‐induced ozone production efficiency per NOx molecule increases by ∼20%, when compared to a simulation without higher hydrocarbon chemistry.
Cloud condensation nuclei spectra and polluted and clean clouds over the Indian Ocean
Authors: Hudson James G. , Yum Seong Soo
Published: October 2002

Cloud condensation nuclei (CCN) and cloud microphysics measurements with various levels of pollution over the Indian Ocean showed roughly linear relationships. Estimates of adiabatic cloud droplet concentrations make more useful comparisons than average droplet concentrations, which are reduced by entrainment and averaging artifacts. Adiabatic cloud droplet estimates indicate higher cloud supersaturations. As predicted, the supersaturations were suppressed by higher CCN and cloud droplet concentrations. However, the actual suppression of cloud supersaturations was not as great as these comparisons indicated because many small cloud droplets were below the detection limit of the Forward Scattering Spectrometer Probe (FSSP), especially in the polluted air. Predictions of droplet concentrations based on CCN spectra and updraft velocities matched estimates of adiabatic cloud droplet concentrations in the clean air but overpredicted adiabatic estimates of polluted cloud droplet concentrations largely because of the undercounting of smaller droplets in polluted air masses. Thus, when all things were considered, a reasonable level of closure was found between predictions of droplet concentrations and adiabatic estimates of cloud droplet concentrations.
Spring to summer changes in the West African monsoon through NCEP/NCAR reanalyses (1968–1998)
Authors: Fontaine Bernard , Philippon Nathalie , Trzaska Sylwia , Roucou Pascal
Published: October 2002

This article focuses on the spring to summer evolutions of the West African monsoon from an energetic point of view; it uses NCEP/NCAR reanalyses along with observed and simulated rainfall data over the period 1968–1998. The results show that the West African monsoon can be viewed as a coupled ocean‐atmosphere‐land system phased with the zenithal evolution of solar radiation which maintains its thermally direct circulation by pumping, from the surface, enthalpy mainly in spring and latent energy in summer. In this context the horizontal energy gradients in the boundary layer play a key role during the spring to summer transition. At the beginning of spring, the moist static energy (MSE) meridional gradients are the strongest over the continent and are maintained by sensible heating and cooling. Then they relax and the monsoon air mass is transported northward into the continent. However, before the wettest (driest) July to September rainy seasons the MSE meridional gradients tend to be stronger (weaker) and tend to relax later (earlier). It is assumed that these gradients could be controlled by the seasonal evolution of precipitation and soil moisture fields: indeed, the months preceding the beginning of an abnormally wet rainy season are characterized by negative rainfall and soil moisture anomalies over Sudan‐Sahel. Such anomalies could maintain stronger than normal MSE and humidity gradients between the Sahelian and the Guinean areas in late spring before they relax more intensely in summer.
Global distributions of effective cloud fraction and cloud top pressure derived from oxygen A band spectra measured by the Global Ozone Monitoring Experiment: Comparison to ISCCP data
Authors: Koelemeijer R. B. A. , Stammes P. , Hovenier J. W. , de Haan J. F.
Published: October 2002

Satellite measurements of the Earth's reflected sunlight in the oxygen A band around 761 nm, as measured by the Global Ozone Monitoring Experiment (GOME) on board ERS‐2, have been analyzed to obtain global distributions of effective cloud fraction and cloud top pressure for July 1995 and January 1997. These quantities have been compared to effective cloud fractions and cloud top pressures derived by the International Satellite Cloud Climatology Project (ISCCP D2), averaged over the July and January months from 1989 to 1993. We have found a reasonable agreement between the GOME O2 A band cloud results and the climatological ISCCP D2 data. The global monthly average difference in effective cloud fraction is 0.01 for July and vanishes for January; the standard deviation of the difference is 0.10 for both months (absolute values). These effective cloud fractions hold under the assumption of optically thick clouds and are typically 2 times smaller than the real cloud fractions. The global monthly average difference in cloud top pressure is 27 hPa (July) and 36 hPa (January); the standard deviation of the difference is 110 hPa (July) and 104 hPa (January). Largest differences are found over land surfaces.
Mechanistic studies of dimethylsulfide oxidation products using an observationally constrained model
Authors: Lucas Donald D. , Prinn Ronald G.
Published: October 2002

A one‐dimensional model of dimethylsulfide (DMS) oxidation chemistry and simultaneous observations from Flight 24 of the First Aerosol Characterization Experiment are used to test the oxidation of DMS by OH and the gas‐phase production of SO2, H2SO4, and methanesulfonic acid (MSA) in the remote marine atmosphere. The model includes a comprehensive chemical mechanism (55 sulfur reactions, 28 sulfur species), vertical mixing, scavenging by background aerosols, and surface losses and emissions. Model parameter uncertainties have been estimated and are used to compute probability distribution functions of observable model outputs using a Monte Carlo method. Seven mechanistic scenarios are considered, which include a baseline case incorporating our best current knowledge and six cases that test novel MSA production reactions involving a newly proposed MSA isomer and the oxidation of methanesulfenic (MSEA) and methanesulfinic (MSIA) acids. The results show that for each of the seven scenarios the modeled DMS and SO2 concentrations agree statistically with the Flight 24 observations. For MSA, however, the observations are a factor of 104 to 105 larger than the baseline mean model predictions and lie 3–4 orders of magnitude outside of the one‐sigma model uncertainty range. Statistical agreement between the boundary layer MSA observations and the model is achieved only for the mechanism scenarios that invoke new MSA production pathways, with the best agreement occurring when MSA is produced from the oxidation of MSIA or through a path involving the DMS‐OH adduct and MSA isomer. For H2SO4 this study finds that even though the majority of the observations lie within the one‐sigma model uncertainty range, the baseline scenario systematically underproduces H2SO4 in the boundary layer. These systematic differences are removed when the production of SO3 is enhanced through a pathway that is independent of SO2. This provides evidence for an efficient H2SO4 production pathway that does not involve SO2 as a precursor. Sensitivity studies are also presented, the results of which suggest observables that are most effective at distinguishing between our seven DMS mechanistic scenarios.
A comparison of HALOE V19 with SAGE II V6.00 ozone observations using trajectory mapping
Authors: Morris Gary A. , Gleason James F. , Russell James M. , Schoeberl Mark R. , McCormick M. Patrick
Published: October 2002

We apply trajectory mapping to an 8‐year intercomparison of ozone observations from the Halogen Occultation Experiment (HALOE) (V19) and Stratospheric Aerosol and Gas Experiment (SAGE) II (V6.00) for the months March, May, June, September, October, and December from the period December 1991 to October 1999. Our results, which represent the most extensive such intercomparison of these two data sets to date, suggest a root‐mean‐square difference between the two data sets of >15% below 22 km in the tropics and of 4–12% throughout most of the rest of the stratosphere. In addition, we find a bias with HALOE ozone low relative to SAGE II by 5–20% below 22 km between 40°S and 40°N. Biases throughout most of the rest of the stratosphere are nearly nonexistent. Finally, our analysis suggests almost no drift in the bias between the data sets is observed over the period of study. In the course of our study, we also determine that the employment of the Wang‐Cunnold criteria is still recommended with the V6.00 SAGE II ozone data. Results with the new versions of the data sets show significant improvement over previous versions, particularly in the elimination of midstratospheric biases and the elimination of the previously observed drifts in the biases between the data sets in the lower stratosphere. Since HALOE V19 and V18 ozone are very similar, these changes can likely be attributed to improvements in the SAGE II retrieval.
On the changing nature of the regional connection between the North Atlantic Oscillation and sea surface temperature
Authors: Walter Katrin , Graf Hans‐F.
Published: October 2002

Evidence is presented that the correlation between the North Atlantic Oscillation (NAO), in terms of the NAO index, and the North Atlantic sea surface temperature (SST) is not stationary. This is inferred from both reanalysis data from the U.S. National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) and the Kaplan sea surface temperature data set. Two phases of winterly North Atlantic atmosphere‐ocean covariability are identified by means of linear regression and correlation analysis. During the recent decades since the late 1960s/early 1970s and during the first 3 decades of the twentieth century, the North Atlantic SST is strongly correlated to the regional atmospheric circulation in the North Atlantic sector, i.e., the North Atlantic Oscillation. During these periods the NAO index, defined as the difference of normalized sea level pressures on the Azores and Iceland, is characterized by pronounced decadal variability and by mainly positive values. In contrast, the NAO index is only weakly correlated to the North Atlantic SST from the 1930s to the early 1960s, when the NAO index is characterized by weak decadal variability. Remote influences, in particular from the tropical Pacific region, become important, especially for the SST in the western tropical North Atlantic.
Warm season tree growth and precipitation over Mexico
Authors: Therrell Matthew D. , Stahle David W. , Cleaveland Malcolm K. , Villanueva‐Diaz Jose
Published: October 2002

We have developed a network of 18 new tree ring chronologies to examine the history of warm season tree growth over Mexico from 1780 to 1992. The chronologies include Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) and Montezuma pine (Pinus montezumae Lamb.) latewood width, and Montezuma bald cypress (Taxodium mucronatum Ten.) total ring width. They are located in southwestern Texas, the Sierra Madre Oriental, Sierra Madre Occidental, and southern Mexico as far south as Oaxaca. Seven of these chronologies are among the first precipitation sensitive tree ring records from the American tropics. Principal component analysis of the chronologies indicates that the primary modes of tree growth variability are divided north and south by the Tropic of Cancer. The tree ring data in northern Mexico (PC1) are most sensitive to June–August rainfall, while the data from southern Mexico (PC2) are sensitive to rainfall in April–June. We find that the mode of tree growth variability over southern Mexico is significantly correlated with the onset of the North American Monsoon. Anomalies in monsoon onset, spring precipitation, and tree growth in southern Mexico all tend to be followed by precipitation anomalies of opposite sign later in the summer over most of central Mexico.
Modeling the heterogeneous spatial structure of temperature observations from a meteorological network in Spain
Authors: Capilla Carmen , Capilla Raquel
Published: October 2002

At some stage, investigations of regional variability in climatological information require the analysis of spatial structure and interpolation between data points. In this paper we study both spatial analysis and interpolation using the example of temperature observations from a fixed surface meteorological network. Initial analyses indicate that the mean field can be modeled with a deterministic term plus a nonstationary stochastic component. The deterministic component models the influence of site location in terms of geographic coordinates, elevation, and local environment. The stochastic component presents anisotropy and nonstationarity through second‐order moments and accounts for high variability between subareas in the studied region. The heterogeneous spatial structure of the region is modeled with a technique that transforms the site geographic map into a new site representation with an isotropic and stationary covariance structure. The transformed representation provides valuable information on the influence of microclimatic conditions in the spatial covariance structure. The model captures the main spatial structure characteristics over the region and allows optimal prediction of the spatial mean field at unobserved locations.
Application of neural networks for studying nonlinear relationships between ozone and its precursors
Authors: Konovalov Igor B.
Published: October 2002

Neural networks are used for extracting information on the sensitivity of surface ozone to changes of ozone precursors directly from the data of routine air quality measurements. Three neural network‐based (perceptron type) empirical models are created following original methodology and using independent data sets of long‐term (1980–1995) air quality monitoring collected at three sites (Azusa, Riverside‐Rubidoux, and Reseda) at South Coast Air Basin, California. The qualitative features of the nonlinear relationships between ozone and its precursors captured by the empirical models are compared with theoretical ones under approximated zero‐wind conditions. Fair agreement with the photochemical theory is found for the results demonstrated by two models, and the results of the third model (for Reseda) are found to be in poor agreement with the theory. It is found also that in accordance with the theoretical expectations all the empirical models demonstrate a seasonal transition from NOx‐sensitive to NOx‐saturated regimes of ozone photochemistry. A comparison between the results obtained with and without the zero‐wind approximation shows that windy conditions usually are a benefit to the NOx‐sensitive regime. With regard to the problem of reducing ozone concentrations, it is assessed that a dominant fraction of days on which 8‐hour‐averaged ozone concentrations exceeded a level of 80 ppb is associated with the NOx‐sensitive regime at Riverside and Reseda sites (89% and 93%, respectively), while at Azusa, both regimes appear on those days with the same frequency.
A numerical study of propagation characteristics of gravity wave packets propagating in a dissipative atmosphere
Authors: Zhang Shao Dong , Yi Fan
Published: October 2002

By using a two‐dimensional, full‐implicit‐continuous‐Eulerian (FICE) scheme, we simulated the nonlinear propagation and evolution of gravity wave packets in a compressible, nonisothermal and dissipative atmosphere. The numerical results show that when an upgoing gravity wave packet is generated in the lower mesosphere, it can propagate along its ray path until it reaches lower thermosphere. However, upon reaching the lower thermosphere, the wave packet and associated energy propagate almost horizontally, which departs obviously from the prediction of linear gravity wave theory under WKB approximation in the nondissipative case. Further discussion indicates that the influences of nonlinearity and background temperature are not strong enough to restrict completely the upward energy propagation of the wave packet and that the influence of constant molecular viscosity on the characteristics (energy propagation path and wave parameters) of gravity waves is insignificant. It is the vertical inhomogeneity of molecular viscosity that causes the restriction of upward energy propagation of the gravity wave packet. Moreover, throughout propagation, the dominant vertical wavelength of the wave packet decreases with time, as it is affected by the joint actions of nonlinearity, background temperature, and dissipation. These results indicate that the molecular viscosity, especially the vertical inhomogeneity of molecular viscosity, plays an important role in the nonlinear propagation of gravity wave packets.
Modeling Cl2 formation from aqueous NaCl particles: Evidence for interfacial reactions and importance of Cl2 decomposition in alkaline solution
Authors: Knipping Eladio M. , Dabdub Donald
Published: October 2002

A series of experiments have demonstrated that a significant yield of chlorine gas is produced when mixtures of ozone and sodium chloride particles above their deliquescence point are irradiated at 254 nm. In order to obtain expressions for future modeling studies, a comprehensive model is used to analyze the system and to determine its sensitivity. This work reexamines and expands previous studies [Knipping et al., 2000]. The enhanced model, described in detail herein, reaffirms that current known physical and chemical processes fail to reproduce the observed Cl2 formation in the experiments. A methodological analysis, proposed as a framework for similar studies, of the physicochemical system supports the accountability of an overall mechanism initiated by the formation of a relatively stable complex of the hydroxyl radical and chloride ions at the gas–liquid interface for the observed chlorine generation. Different potential fates of the OH•••Clsurface•− intermediate are discussed. A rate expression and kinetic parameters are presented for the overall reaction of the interfacial mechanism. In addition, sensitivity studies underscore the importance of accurately modeling chlorine decomposition processes in alkaline solution—in particular, the reactions of chlorine with hydroxide, carbonate, and basic hydrogen peroxide. Recommended aqueous‐phase rate constants for these reactions are drawn from a literature evaluation illustrating the limited availability and lack of agreement of related kinetic data.
Water‐soluble organic nitrogen in atmospheric fine particles (PM2.5) from northern California
Authors: Zhang Qi , Anastasio Cort , Jimenez‐Cruz Mike
Published: October 2002

Recent studies have suggested that organic nitrogen (ON) is a ubiquitous and significant component of atmospheric dry and wet deposition, but very little is known about the concentrations and speciation of organic nitrogen in aerosol particles. In addition, while amino compounds also appear to be ubiquitous in atmospheric condensed phases, their contribution to organic nitrogen has not been previously quantified. To address these issues, we have characterized the water‐soluble organic nitrogen and amino compounds in fine particles (PM2.5) collected in Davis, California, over a period of 1 year. Concentrations of water‐soluble organic nitrogen (WSON) ranged from 3.1–57.8 nmol N m−3 air, peaking during winter and early spring, and typically accounted for ∼20% of total nitrogen in Davis PM2.5. Assuming an average N‐normalized molecular weight of 100 Da per N atom for WSON, particulate organic nitrogen had a median mass concentration of 1.6 μg m−3 air, and typically represented 18% of the total fine particle mass. The average mass of water‐soluble ON in Davis PM2.5 was comparable to that of sulfate during the summer, but was significantly higher in winter. Total amino compounds (free plus combined forms) made up a significant portion of particulate organic nitrogen (median value equal to 23%), primarily due to the presence of combined amino compounds such as proteins and peptides. Total amino compounds had a median mass concentration of 290 ng m−3 air, and typically accounted for 3.3% of the total fine particle mass. These results indicate that organic nitrogen is a significant component of fine particles in northern California, and suggest that this group of compounds might play an important role in the ecological, radiative, and potential health effects of atmospheric fine particles in this region.
Validation of the TOPEX rain algorithm: Comparison with ground‐based radar
Authors: McMillan A. C. , Quartly G. D. , Srokosz M. A. , Tournadre J.
Published: October 2002

Recently developed algorithms have shown the potential recovery of rainfall information from spaceborne dual‐frequency altimeters. Given the long mission achieved with TOPEX and the prospect of several other dual‐frequency altimeters, we need to validate the altimetrically derived values so as to foster their integration with rain information from different sensors. Comparison with some alternative climatologies shows the bimonthly means for TOPEX to be low. Rather than apply a bulk correction we investigate individual rain events to understand the cause of TOPEX's underestimation. In this paper we compare TOPEX with near‐simultaneous ground‐based rain radars based at a number of locations, examining both the detection of rain and the quantitative values inferred. The altimeter‐only algorithm is found to flag false rain events in very low wind states (<3.8 m s−1); the application of an extra test, involving the liquid water path as sensed by the microwave radiometer, removes the spurious detections. Some false detections of rain also occur at high wind speeds (>20 m s−1), where the empirical dual‐frequency relationship is less well defined. In the intermediate range of wind speeds, the TOPEX detections are usually good, with the instrument picking up small‐scale variations that cannot be recovered from infrared or passive microwave techniques. The magnitude of TOPEX's rain retrievals can differ by a factor of 2 from the ground‐based radar, but this may reflect the uncertainties in the validation data. In general, over these individual point comparisons TOPEX values appear to exceed the “ground truth.” Taking account of all the factors affecting the comparisons, we conclude that the TOPEX climatology could be improved by, in the first instance, incorporating the radiometric test and employing a better estimate of the melting layer height. Appropriate corrections for nonuniform beam filling and drizzle fraction are harder to define globally.
Optical properties of boreal forest fire smoke derived from Sun photometry
Authors: O'Neill N. T. , Eck T. F. , Holben B. N. , Smirnov A. , Royer A. , Li Z.
Published: October 2002

Aerosol optical properties derived fromSun photometry were investigated in terms of climatological trends at twoSun photometer sites significantly affected by western Canadian boreal forest fire smoke and in terms of a 2‐week series of smoke events observed at stations near and distant from boreal forest fires. Aerosol optical depth (τa) statistics for Waskesiu, Saskatchewan, and Thompson, Manitoba, were analyzed for summer data acquired between 1994 and 1999. A significant correlation between the geometric mean and the forest fire frequency indices (hot spots) was found; on the average, 80% of summertime optical depth variation in western Canada can be linked to forest fire sources. The average geometric mean and geometric standard deviation at 500 nm was observed to be 0.074 and 1.7 for the clearest, relatively smoke‐free summer and 0.23 and 3.0 for the summer most influenced by smoke. A systematic decrease of fine mode Angstrom exponent (αf) was noted (dαf/dlog τa∼ −0.6). This decrease roughly corresponds to an increase in the fine mode effective radius (reff) from 0.09 to 0.15 μm and an abundance (A) to size rate increase near 2.0 (dlogA/dlogreff). A 1998 series of forest fire events was tracked using TOMS, AVHRR, and GOES imagery, back trajectories, and data from six Sun photometer sites in Canada and eastern United States. The results showed rates of decrease of αfwith increasing τawhich were similar to the climatological data. An analysis in terms of source to station distance showed a decrease in αfand an increase inreffwith increasing distance. This observation was coherent with previous observations on the particle growth effects of aging.
Radio‐holographic analysis of Microlab‐1 radio occultation data in the lower troposphere
Authors: Gorbunov M. E.
Published: October 2002

The interpretation of radio occultation data in the lower troposphere is a complicated problem due to strong multipath effects. This problem can be solved on the basis of the wave optics. We analyze Microlab‐1 radio occultation using two radio‐holographic approaches: the radio‐optics method, which employs the analysis of the local spatial spectra of the registered wave field, and the canonical transform method, which is based on the principles of the theory of Fourier integral operators. The radio‐optics method is a means of data visualizaion, while the canonical transform method must be used for achieving accurate numerical results in processing measurements in multipath areas. The comparison of the results of the application of these two methods is a useful method of data quality control. We show examples of radio occultations with multipath propagation and reflection from the Earth's surface. The application of the radio‐holographic methods also allows for the detection of corrupted data caused by phase lock loop failure.
Eastern Atlantic Spring Experiment 1997 (EASE97) 1. Measurements of OH and HO2 concentrations at Mace Head, Ireland
Authors: Creasey D. J. , Heard D. E. , Lee J. D.
Published: October 2002

We report measurements of the hydroxyl (OH) and hydroperoxy (HO2) radicals, taken over 20 days, in the remote marine boundary layer at Mace Head, Ireland, during April and May 1997. OH was monitored directly by laser‐induced fluorescence (LIF) spectroscopy at 308 nm, and HO2 was measured by chemical conversion to OH upon the addition of NO, with subsequent detection by LIF. The detection limit of the instrument at midday for OH was 6.0 × 105 molecule cm−3 (0.0024 parts per trillion by volume (pptv)) and for HO2 was 3.0 × 106 molecule cm−3 (0.12 pptv), as defined for a signal integration period of 2.5 min and a signal‐to‐noise ratio of 1. Midday OH and HO2 concentrations were between 2.0–6.0 × 106 molecule cm−3 (0.08–0.24 pptv) and 0.5–3.5 × 108 molecule cm−3 (2.5–14 pptv), respectively. OH concentrations correlated well with the rate of OH production from ozone photolysis for clean air from the Arctic containing low concentrations of both NOx and nonmethane hydrocarbons. A lower correlation was observed in more polluted air that originated from the United Kingdom and continental Europe. Measurements of OH and HO2 were made throughout two nights, and although no evidence was seen for OH above the detection limit, up to 2 pptv of HO2 was observed. The measured HO2/OH ratio was in good agreement with the predictions of a steady state expression for NO in the range 75–400 pptv.
Comparisons of box model calculations and measurements of formaldehyde from the 1997 North Atlantic Regional Experiment
Authors: Frost G. J. , Fried A. , Lee Y.‐N. , Wert B. , Henry B. , Drummond J. R. , Evans M. J. , Fehsenfeld F. C. , Goldan P. D. , Holloway J. S. , Hübler G. , Jakoubek R. , Jobson B. T. , Knapp K. , Kuster W. C. , Roberts J. , Rudolph J. , Ryerson T. B. , Stohl A. , Stroud C. , Sueper D. T. , Trainer M. , Williams J.
Published: October 2002

Formaldehyde (CH2O) measurements from two independent instruments are compared with photochemical box model calculations. The measurements were made on the National Oceanic and Atmospheric Administration P‐3 aircraft as part of the 1997 North Atlantic Regional Experiment (NARE 97). The data set considered here consists of air masses sampled between 0 and 8 km over the North Atlantic Ocean which do not show recent influence from emissions or transport. These air masses therefore should be in photochemical steady state with respect to CH2O when constrained by the other P‐3 measurements, and methane oxidation was expected to be the predominant source of CH2O in these air masses. For this data set both instruments measured identical CH2O concentrations to within 40 parts per trillion by volume (pptv) on average over the 0–800 pptv range, although differences larger than the combined 2σ total uncertainty estimates were observed between the two instruments in 11% of the data. Both instruments produced higher CH2O concentrations than the model in more than 90% of this data set, with a median measured‐modeled [CH2O] difference of 0.13 or 0.18 ppbv (depending on the instrument), or about a factor of 2. Such large differences cannot be accounted for by varying model input parameters within their respective uncertainty ranges. After examining the possible reasons for the model‐measurement discrepancy, we conclude that there are probably one or more additional unknown sources of CH2O in the North Atlantic troposphere.
Trace gas composition of midlatitude cyclones over the western North Atlantic Ocean: A conceptual model
Authors: Cooper O. R. , Moody J. L. , Parrish D. D. , Trainer M. , Ryerson T. B. , Holloway J. S. , Hübler G. , Fehsenfeld F. C. , Evans M. J.
Published: October 2002

Midlatitude cyclones provide the energy necessary for most of the trace gas transport from North America to the western North Atlantic Ocean (WNAO). These cyclones are composed of four primary airstreams: warm conveyor belt (WCB), cold conveyor belt (CCB), dry airstream (DA), and post cold front (PCF) airstream. This study is the first to present a conceptual model of the chemical composition of a midlatitude cyclone tracking from North America to the WNAO. The model, a composite of chemical measurements from several cyclones, establishes the fundamental relationships between large‐scale chemical transport and midlatitude cyclone structure. It also separates the meteorological influences on airstream trace gas signatures from the influence of surface emissions heterogeneity and presents characteristic mixing ratios of ozone, CO, NOx, and NOy within the four types of airstream during late summer/early autumn. While cyclone track and surface emissions heterogeneity impact the median trace gas values within airstreams, the O3/CO and O3/NOy slopes remain fairly constant. Several characteristics of the conceptual model impact trace gas signatures, regardless of cyclone track: (1) the DA always advects stratospheric ozone into the middle and upper troposphere; (2) the WCB is a more favorable location for photochemical ozone production than the CCB or PCF; (3) the PCF originates to the northwest, is unaffected by wet deposition, and the sunny conditions may allow for some photochemical ozone production; (4) the CCB is generally cloudy and does not show signs of significant photochemical ozone production; (5) both the CCB and the WCB experience wet deposition resulting in little NOy export from the lower troposphere.
Trace gas composition of midlatitude cyclones over the western North Atlantic Ocean: A seasonal comparison of O3 and CO
Authors: Cooper O. R. , Moody J. L. , Parrish D. D. , Trainer M. , Holloway J. S. , Hübler G. , Fehsenfeld F. C. , Stohl Andreas
Published: October 2002

The regional‐ to synoptic‐scale transport of trace gases from North America to the western North Atlantic Ocean (WNAO) is largely controlled by midlatitude cyclones. The four primary airstreams that compose these cyclones, the warm conveyor belt (WCB), cold conveyor belt (CCB), dry airstream (DA), and post cold front (PCF) airstream, exhibit characteristic trace gas mixing ratios that vary seasonally. The present study compares ozone and CO mixing ratios measured in these four airstreams during spring 1996 and late summer/early autumn 1997. The three main influences on this seasonal variation of ozone and CO are surface emissions heterogeneity, photochemistry, and stratosphere/troposphere exchange efficiency. The more southerly springtime cyclone tracks account for nearly 50% of the increase of lower troposphere CO from late summer/early autumn to spring. The remainder of the variation is due to the seasonal cycle of background CO. Stratosphere/troposphere exchange occurs in every DA; however, the seasonal cycle of ozone in the lowermost stratosphere allows greater quantities of ozone to enter the troposphere during spring. Net photochemical ozone production occurs in the late summer/early autumn WCB at all levels and in the lower troposphere PCF. In contrast, springtime net ozone production appears absent from all airstreams, with the CCB influenced by ozone destruction. Ozone and CO are greater in spring, but the relative mixing ratios between airstreams are roughly the same in both seasons. NOx/CO emissions ratios vary across the midlatitudes according to socioeconomic factors. It is expected that the emissions variation influences the ozone production efficiency of the cyclone airstreams that draw from these regions.
Determination of top‐of‐atmosphere longwave radiative fluxes: A comparison between two approaches using ScaRaB data
Authors: Chen Ting , Rossow William B.
Published: October 2002

Two conceptually different approaches (broadband‐based ERBE (Earth Radiation Budget Experiment) and narrowband‐based ISCCP (International Satellite Cloud Climatology Project) approaches), used to derive the TOA (top of atmosphere) longwave radiative fluxes, are compared using the ScaRaB simultaneous narrowband and broadband measurements. Except for very thin cirrus clouds, differences between the ERBE and the ISCCP approaches are in general <10 W m−2 for the TOA LW radiative fluxes. For clear pixels the model‐calculated (ISCCP approach) TOA LW radiances are systematically smaller than the observations. Compared with the radiative transfer model used in this study, the ERBE LW angular dependence models are too weakly limb darkened for optically thin clouds but too strongly limb darkened for optically thick clouds, suggesting that more accurate instantaneous TOA LW flux estimations from the ERBE approach would require additional cloud classes based on cloud height and optical thickness.
Neutral air turbulence and temperatures in the vicinity of polar mesosphere summer echoes
Authors: Lübken Franz‐Josef , Rapp Markus , Hoffmann Peter
Published: October 2002

A total of 8 sounding rocket flights with measurements of neutral air turbulence in the upper mesosphere have been performed in the past 10 years with simultaneous and nearly co‐located radar measurements of polar mesosphere summer echoes (PMSE). These measurements took place close to the rocket ranges in northern Norway (Andøya Rocket Range, 69°N) and in northern Sweden (Esrange, 68°N). A detailed comparison demonstrates that there is no apparent correlation between PMSE and neutral air turbulence and that in fact turbulence is absent in the majority of all PMSE events (no turbulence in 7 out of 10 PMSE layers). This suggests that neutral turbulence and other mechanisms affecting the neutral atmosphere at very small spatial scales play a minor role in creating PMSE, contrary to the speculations published in the literature. The main mechanism for creating PMSE remains unidentified. A comparison of PMSE with simultaneous temperature profiles derived from falling sphere and ionization gauge measurements shows that PMSE are practically always present at altitudes where the temperature is low enough for water ice particles to exist. This supports the general understanding that PMSE are closely related to charged water ice particles. On the other hand, the measurements also demonstrate that low enough temperatures are not sufficient for PMSE to exist. Temperature lapse rates were deduced from the high‐altitude‐resolution ionization gauge measurements. Within the PMSE layers the temperature lapse rate is typically +1–2 K/km with a rather large variability of ±5–10 K/km. Adiabatic lapse rates have never been found within a PMSE layer, which suggests that turbulence cannot have been active for a substantial period. This again supports the idea that neutral air turbulence plays a minor role in creating PMSE. Probably the only common physical reason for PMSE and turbulence is the background temperature profile, which supports the creation of ice particles (since temperatures are very low) and which provokes the breaking of gravity waves and creation of turbulence since the temperature gradient changes at the mesopause.
Horizontal wind disturbances induced by inertial instability in the equatorial middle atmosphere as seen in rocketsonde observations
Authors: Hayashi Hiroo , Shiotani Masato , Gille John C.
Published: October 2002

Rocketsonde observations at Kwajalein (8.7°N, 167.7°E) were used for investigation of horizontal wind disturbances induced by inertial instability. Two characteristic cases were found in the 1978–1979 northern winter during which the limb infrared monitor of the stratosphere (LIMS) made observations. In both the cases the eastward component and the northward component of horizontal winds are negatively correlated in the vertical direction near the stratopause level, which is thought to be due to inertial instability. The wind disturbances have a wavelength of about 10 km, and their maximum amplitudes are estimated as over 10 ms−1. Examinations of the LIMS temperatures support a possibility that the wind disturbances are induced by inertial instability. It was revealed that temperature disturbances characteristic of inertial instability, called “pancake structures,” appear in the same height and time as the wind disturbances do. The relationship between the rocketsonde wind disturbances and the LIMS pancake structures is 90° out of phase, as expected by the theory of inertial instability. In addition, it was confirmed that the two inertial instability events follow enhancements of the midlatitude planetary wave. This is consistent with a mechanism of the inertially unstable disturbances in the equatorial region inferred in some observational and numerical studies. Analyses of rocketsonde data were extended to the entire observation period at Kwajalein, which suggests that the occurrence of inertially unstable disturbances is biased toward the winter season. However, at some other stations in the tropics, wind disturbances which may be induced by inertial instability do not appear so frequently as they do at Kwajalein. This seasonality and regionality of inertially unstable disturbances would be due to the seasonality and regionality of planetary wave breakings at midlatitude.
Ozone production rates as a function of NOx abundances and HOx production rates in the Nashville urban plume
Authors: Thornton J. A. , Wooldridge P. J. , Cohen R. C. , Martinez M. , Harder H. , Brune W. H. , Williams E. J. , Roberts J. M. , Fehsenfeld F. C. , Hall S. R. , Shetter R. E. , Wert B. P. , Fried A.
Published: October 2002

Tropospheric O3 concentrations are functions of the chain lengths of NOx (NOx ≡ NO + NO2) and HOx (HOx ≡ OH + HO2 + RO2) radical catalytic cycles. For a fixed HOx source at low NOx concentrations, kinetic models indicate the rate of O3 production increases linearly with increases in NOx concentrations (NOx limited). At higher NOx concentrations, kinetic models predict ozone production rates decrease with increasing NOx (NOx saturated). We present observations of NO, NO2, O3, OH, HO2, H2CO, actinic flux, and temperature obtained during the 1999 Southern Oxidant Study from June 15 to July 15, 1999, at Cornelia Fort Airpark, Nashville, Tennessee. The observations are used to evaluate the instantaneous ozone production rate (PO3) as a function of NO abundances and the primary HOx production rate (PHOx). These observations provide quantitative evidence for the response of PO3 to NOx. For high PHOx (0.5 < PHOx < 0.7 ppt/s), O3 production at this site increases linearly with NO to ∼500 ppt. PO3 levels out in the range 500–1000 ppt NO and decreases for NO above 1000 ppt. An analysis along chemical coordinates indicates that models of chemistry controlling peroxy radical abundances, and consequently PO3, have a large error in the rate or product yield of the RO2 + HO2 reaction for the classes of RO2 that predominate in Nashville. Photochemical models and our measurements can be forced into agreement if the product of the branching ratio and rate constant for organic peroxide formation, via RO2 + HO2 → ROOH + O2, is reduced by a factor of 3–12. Alternatively, these peroxides could be rapidly photolyzed under atmospheric conditions making them at best a temporary HOx reservoir. This result implies that O3 production in or near urban areas with similar hydrocarbon reactivity and HOx production rates may be NOx saturated more often than current models suggest.
Modeling halogen chemistry in the marine boundary layer 1. Cloud‐free MBL
Authors: von Glasow Roland , Sander Rolf , Bott Andreas , Crutzen Paul J.
Published: October 2002

A numerical one‐dimensional model of the marine boundary layer (MBL) is presented. It includes chemical reactions in the gas phase and aerosol particles, focusing on the reaction cycles of halogen compounds. Results of earlier box model studies were confirmed. They showed the acid‐catalyzed activation of bromine from sea salt aerosol, and the role of halogen radicals in the destruction of O3. A distinct diurnal variation in BrO mixing ratios with maxima at sunrise and sunset was found which might be the cause of the recently published “sunrise ozone destruction.” Maxima of BrO and sea salt acidity are predicted at the top of the MBL and not close to the sea surface where sea salt spray is produced. The presence of sulfate aerosol was found to be important for the recycling of less reactive to photolyzable bromine species. Day/night and seasonal differences in halogen chemistry are shown.