bluuebunny/crossref_metadata_2025 huggingface dataset:
Improved air mass factor concepts for scattered radiation differential optical absorption spectroscopy of atmospheric species
Authors: Marquard L. C. , Wagner T. , Platt U.
Published: September 2002

Conventionally, air mass factors are used to relate so‐called “slant” trace‐gas column densities measured by differential optical absorption spectroscopy (DOAS) using extraterrestrial light sources to desired vertical trace‐gas column densities. We show that two fundamental problems arise when the traditional air mass factor A concept, which was originally developed for direct radiation measurements, is applied to spectroscopy of scattered radiation: (1) the inaccuracy of the traditional formula to determine vertical column densities V from measured slant column densities S, i.e., the equation V = S/A and (2) the inaccuracy of the traditional air mass factor computation method itself. We show that the reason for these inaccuracies is the substitution of absolute by differential slant column densities during a DOAS evaluation. It may be carried out and thus the traditional air mass factor concept is valid if and only if the optical density of a certain absorber is proportional to the product of its absorption cross section and its vertical column density. Since for spectroscopy of scattered radiation we have a nonlinear relationship between these quantities, the traditional concept cannot be, in general, extended to this application. The nonlinearity of this relationship is mathematically shown by a path integral formulation of the radiative transfer, which is developed. Its practical relevance is shown by Monte Carlo model calculations of the radiative transfer. The systematic error in both the air mass factor and the vertical column density associated with applying the traditional concept to spectroscopy of scattered radiation is also determined by Monte Carlo model calculations. Improved air mass factor concepts are developed in this work, which are also valid for absorption spectroscopy of scattered radiation (and not only for spectroscopy of direct radiation as the traditional concept) and include the traditional concept as a linear limit case. By applying one of these improved concepts the two fundamental problems mentioned above as well as the fundamental inconsistency of the method, i.e., the a priori knowledge of the vertical column density, which is required when applying the traditional concept, can be resolved.
Analysis of radical propagation efficiency to assess ozone sensitivity to hydrocarbons and NOx: 1. Local indicators of instantaneous odd oxygen production sensitivity
Authors: Tonnesen Gail S. , Dennis Robin L.
Published: September 2002

We used a simple trajectory model and a three‐dimensional grid model to evaluate several indicators that can be used to predict the sensitivity of odd oxygen production (P(Ox)) to changes in emissions of anthropogenic nonmethane hydrocarbons (VOC) and nitrogen oxides (NOx = NO + NO2). To perform the evaluation, we augmented the model to include new diagnostic outputs such as rates of P(Ox), rates of conversion of NOx to unreactive nitrogen, and radical chain length. We used the new diagnostic outputs to explain the model‐predicted sensitivity of P(Ox) to changes in VOC and NOx emissions. We found that the ozone ridgeline, which distinguishes between NOx‐limited and radical‐limited conditions, is determined by a ridgeline of maximum OH chain length. We examined the radical propagation reactions which affect OH chain length, and we developed four indicators related to radical propagation efficiency: I(HC,NO2) which approximates the fraction of OH that reacts with hydrocarbons; I(NO,RO2) which approximates the fraction of HO2 that reacts with NO; the ratio of [O3]/[NOx] which affects the balance between radical initiation and propagation; and [HO2] which is the dominant term in P(H2O2)/P(HNO3). Each of these indicators distinguishes conditions in which instantaneous P(Ox) is primarily sensitive to either NOx emissions reductions or VOC emissions reductions.
Update of Umkehr ozone profile data trend analysis through 1997
Authors: Reinsel G. C. , Tiao G. C. , Miller A. J. , Nagatani R. M. , Wuebbles D. J. , Weatherhead E. C. , Cheang W.‐K. , Zhang L. , Flynn L. E. , Kerr J. B.
Published: September 2002

An extended update of both nonseasonal and seasonal trend analysis for northern hemisphere Umkehr ozone profile data from 1977 through 1997 is presented. The nonseasonal or year‐round trend analysis reaffirms the previous results of significant negative ozone trend in the upper stratospheric Umkehr layers 7, 8, and 8–10 (33–48 km) of about −0.5% per year, with little trend in the middle stratospheric layer 5 (24–28 km). In addition, the Umkehr data show a substantial negative trend in the lower stratospheric Umkehr layer 2–3 (10–19 km) of about −0.4% per year. Two procedures for the correction of the Umkehr measurements for errors due to volcanic aerosols are considered, and trend results are quite similar using both aerosol‐correction procedures. With respect to the seasonal trend aspects, the analysis indicates relatively small trend differences over seasons in the upper layers, where trends in the winter and spring seasons are only slightly more negative than those in the summer and fall seasons, by about −0.1% per year. In the lower stratospheric layer 2–3, however, a strong seasonal pattern in trends is observed in the midlatitude Umkehr data, with the most extreme negative trends, of about −0.6% per year, occurring in the spring and winter, the least negative trends, of about −0.2% per year, occurring in the fall, and the summer trends of about −0.4% per year are intermediate. Also, the trends in layer 2–3 are substantially more negative for the higher latitude range of 40–52°N than for the lower latitude range of 26–39°N, by about −0.3% per year during all seasons, with even larger differences in the spring.
Isoprene emission estimates and uncertainties for the central African EXPRESSO study domain
Authors: Guenther Alex , Baugh Bill , Brasseur Guy , Greenberg Jim , Harley Peter , Klinger Lee , Serça Dominique , Vierling Lee
Published: September 2002

A global three‐dimensional (3‐D) chemistry and transport model was used to demonstrate that a factor of 2 decrease in isoprene and monoterpene emissions results in significant (10–30%) changes in predicted concentration distributions of compounds such as OH, MPAN, NOx, H2O2, O3, and CO. Isoprene and monoterpenes were predicted to have a particularly strong impact on tropical regions, including central Africa. The 1996 Experiment for Regional Sources and Sinks of Oxidants (EXPRESSO) study included a number of experiments that improved our ability to predict isoprene and monoterpene emissions from central Africa. The results of these experiments have been incorporated into an isoprene emission model that predicts hourly emissions on a spatial scale of about 1 km2. The model uses procedures that are suitable for estimating global emissions but uses regional measurements to accurately parameterize the model. Comparisons with above‐canopy aircraft and tower flux measurements demonstrate that the model can estimate emissions within a factor of 2 for regions where ground measurements of model inputs are available. The annual central African isoprene emission predicted by our revised model (35 Tg C) is only 14% less than that predicted by our earlier model, but there are considerable differences in estimates of individual model variables. The models differ by more than a factor of 5 for specific times and locations, which indicates that there are large uncertainties in emission estimates for at least some locations and seasons. The good agreement obtained for the EXPRESSO study field sites, however, suggests that the model can predict reasonable estimates if representative field measurements are used to parameterize the model.
Regional aerosol distribution and its long‐range transport over the Indian Ocean
Authors: Rajeev K. , Ramanathan V. , Meywerk J.
Published: September 2002

Aerosol chemical, microphysical, and optical data collected from an island station and a ship during the first field phase of the Indian Ocean Experiment provided a unique opportunity to develop models for retrieving aerosol optical depth from the advanced very high resolution radiometer (AVHRR) onboard NOAA14 during January–March 1998. Columnar aerosol optical depth (AOD) over Arabian Sea, Bay of Bengal, and Indian Ocean was derived for the 630 nm wavelength from the radiance in channel 1 (580–680 nm) of AVHRR. The satellite retrieval model for AOD accounts for several aerosol species (sulfates, nitrates, sea salt, soot, dust, and organics), the in situ measured value of single scattering albedo, and the wind speed dependence of sea surface albedo. Satellite‐retrieved AOD is in good agreement with surface measurements of AOD taken from the Indian Ocean island of Kaashidhoo (4.96°N, 73.46°E) in the Maldives and from the R/V Sagar Kanya cruising between 20°N and 20°S. The success of our model is most likely due to the use of observed single scattering albedo, the use of phase function derived from in situ data, and the limitation of the analysis to the antisolar side of the satellite scan. However, the model relies on atmospheric column data and surface measurements, which need future verification with in situ aircraft data. Regional maps reveal that the entire northern Indian Ocean has large 0.15 AOD with monthly mean values exceeding 0.2 for latitudes north of ∼5°N, for all 3 months. AOD increases northward, reaching values as high as 0.35 toward the coast in the Bay of Bengal and the Arabian Sea. The non‐sea‐salt component of AOD is inferred to be more than 3 times that of the estimated wind‐dependent sea salt component. In the western Indian Ocean and Arabian Sea the high concentration of non‐sea‐salt aerosols are due to transport from the Indian subcontinent and Arabia. The eastern Indian Ocean is influenced by the transport from the Indian subcontinent and southeast Asia, particularly from Indonesia. The 1998 El Niño‐related forest fires from Indonesia resulted in high AOD values (0.25–0.35) in the eastern equatorial Indian Ocean. Minimum AOD is observed between the equator and 10°S, which is the location of the Intertropical Convergence Zone (ITCZ) during the observation period. AOD is generally found to increase to the south of ITCZ and reach a maximum around 20°S.
A study of enhanced fair‐weather electric fields occurring soon after sunrise
Authors: Marshall Thomas C. , Rust W. David , Stolzenburg Maribeth , Roeder William P. , Krehbiel Paul R.
Published: September 2002

In this paper we describe several series of electric field soundings made in the lowest few hundred meters above the ground on 6 days at Kennedy Space Center in Florida. These soundings are used to determine the charge density and thickness of the charged electrode layer just above the surface of the Earth both before and after sunrise during fair weather. On most of the days considered, there was an anomalous enhancement in the ground‐level electric field that was probably associated with the “sunrise effect” previously described by others. At our tether site we found that the electrode‐layer charge density began increasing at about the same time as the local enhancement in the electric field magnitude at the ground. Shortly before the peak in the local E enhancement, the electrode‐layer charge density decreased while the charge thickness increased; these changes were coincident with a decrease in relative humidity, a shift in the average wind direction, and increases in the fluctuations in relative humidity, wind speed, and wind direction. The typical decrease in charge density was from 0.2 to 0.05 nC m−3, while the charge layer thickness increased from less than 20 m to almost 200 m. Our measurements suggest that enhanced positive electrode layers accumulate before sunrise very close to the surface because there is relatively little radioactivity in the soil or air. Local, upward mixing of the denser, low‐lying, electrode‐layer charge may account for the observed sunrise enhancement in electric field. The larger enhancements observed at some sites may indicate that upward convection is supplemented by advection of denser charge from above water surfaces a few tens of meters (or less) from the measurement sites.
A model for studies of tropospheric photochemistry: Description, global distributions, and evaluation
Authors: Lawrence Mark G. , Crutzen Paul J. , Rasch Philip J. , Eaton Brian E. , Mahowald Natalie M.
Published: September 2002

A model of atmospheric photochemistry and transport has been developed and applied toward investigating global tropospheric chemistry. The Model of Atmospheric Transport and Chemistry ‐ Max‐Planck‐Institute for Chemistry version (MATCH‐MPIC) is described and key characteristics of its global simulation are presented and compared to available observations. MATCH‐MPIC is an “offline” model which reads in gridded time‐dependent values for the most basic meteorological parameters (e.g., temperature, surface pressure, horizontal winds), then uses these to compute further meteorological parameters required for atmospheric chemistry simulations (convective transport, cloud microphysics, etc.). The meteorology component of MATCH‐MPIC simulates transport by advection, convection, and dry turbulent mixing, as well as the full tropospheric hydrological cycle (water vapor transport, condensation, evaporation, and precipitation). The photochemistry component of MATCH‐MPIC represents the major known sources (e.g., industry, biomass burning), transformations (chemical reactions and photolysis), and sinks (e.g., wet and dry deposition) which affect the O3hyphen;HOx‐NOy‐CH4‐CO photochemical framework of the “background” troposphere. The results of two versions of the model are considered, focusing on the more recent version. O3 is in relatively good agreement with observed soundings, although it is generally underestimated at low levels and overestimated at high levels, particularly for the more recent version of the model. We conclude that the simulated stratosphere‐troposphere flux of O3 is too large, despite the fact that the total flux is 1100 Tg(O3)/yr, whereas the upper limit estimated in recent literature is over 1400 Tg(O3)/yr. The OH distribution yields a tropospheric CH4 lifetime of 10.1 years, in contrast to the lifetime of 7.8 years in the earlier model version, which nearly spans the range of current estimates in the literature (7.5–10.2 years). Surface CO mixing ratios are in good agreement with observations. NO is generally underestimated, a problem similar to what has also been found in several other recent model studies. HNO3 is also considerably underestimated. H2O2 and CH3OOH, on the other hand, are in relatively good agreement with available observations, though both tend to be underestimated at high concentrations and overestimated at low concentrations. Possible reasons for these differences are considered.
Chemical composition of atmospheric aerosol in the European subarctic: Contribution of the Kola Peninsula smelter areas, central Europe, and the Arctic Ocean
Authors: Virkkula Aki , Aurela Minna , Hillamo Risto , Mäkelä Timo , Pakkanen Tuomo , Kerminen Veli‐Matti , Maenhaut Willy , François Filip , Cafmeyer Jan
Published: September 2002

An 18‐month set of concentration data of various elements in fine (diameter D <2.5 μm) and coarse (2.5 μm<D<15 μm) particles is presented. Measurements were done at Sevettijärvi, ∼60 km WNW of Nikel, a large pollution source on the Kola Peninsula, Russia. The concentrations in aerosol arriving from the Norwegian Sea and the Arctic Ocean are very close to the values observed at more remote Arctic sites. In air from the Kola Peninsula, approximately one third of the samples, concentrations of some trace elements were ∼2 orders of magnitude above the background concentrations. The elements most clearly transported in the pollution plumes from Kola Peninsula were Cd, Ni, Cu, V, Pb, As, Fe, and Co. Penner et al. [1993] presented a method for estimating black carbon (BC) emissions by comparing BC/SO2 (S) close to the sources and used a ratio 0.6 for the former USSR. We found that this ratio was <0.1 in the clearest pollution plumes from Kola peninsula. The ratio of the chemical mass to the gravimetric mass of the aerosol samples was ∼80% both for the fine and coarse particle filters, regardless of the source area. Comparison of the aerosol concentrations with the concentrations of elements in snow showed that the deposition was proportional to the aerosol exposure. The contribution of Kola Peninsula to the deposition is high, ∼80% for Ni, Cu, and Co and somewhat lower for other anthropogenic elements.
Radiative forcing from tropospheric ozone calculated with a unified chemistry‐climate model
Authors: Mickley L. J. , Murti P. P. , Jacob D. J. , Logan J. A. , Koch D. M. , Rind D.
Published: September 2002

We have developed a global model for the study of chemistry‐climate interactions by incorporating a detailed simulation of tropospheric ozone‐NOx‐hydrocarbon chemistry within a general circulation model (GCM). We present a first application of the model to the calculation of radiative forcing from tropospheric ozone since preindustrial times. Longwave and shortwave radiation fluxes are computed every 5 hours in the GCM using the locally simulated ozone fields. In this manner, the model resolves synoptic‐scale correlations between ozone and meteorological variables. A simulation for present‐day conditions is compared to a preindustrial atmosphere (∼1800 A.D.) with no fossil fuel combustion, 10% of present‐day biomass burning, and 0.7 ppm methane. The two simulations use the same meteorological fields; the radiative forcing does not feed back into the GCM. The model reproduces well the observed distributions of ozone and its precursors in the present‐day atmosphere. Increases in ozone since preindustrial times are 20–200% depending on region and season. The global mean, instantaneous radiative forcing from anthropogenic ozone is 0.44 W m2 (0.35 longwave, 0.09 shortwave). The model reveals large shortwave forcings (0.3–0.7 W m2) over polar regions in summer. The total forcing is greater than 1.0 W m2 over large areas, including the Arctic, during Northern Hemisphere summer. The normalized radiative forcing per unit of added ozone column varies globally from −0.01 to 0.05 W m2. This variance can be explained in large part by the temperature difference between the surface and the tropopause; clouds are an additional factor, particularly at low latitudes. An off‐line radiative calculation using the same ozone fields but averaged monthly shows nearly identical forcings, with differences less than ±2% over most of the Earth. The similarity between the off‐line and on‐line simulations suggests that the common use of off‐line ozone fields is acceptable in radiative forcing calculations. Addition of the direct forcings from anthropogenic sulfate aerosol and tropospheric ozone computed with the same GCM shows compensating effects, with sulfate dominating at northern midlatitudes and ozone usually dominating elsewhere.
Evaluation of origins of CH4 carbon emitted from rice paddies
Authors: Watanabe Akira , Takeda Takuya , Kimura Makoto
Published: September 2002

Possible carbon sources for CH4 emitted from rice paddies are organic matter applied to the fields, such as rice straw (RS), soil organic matter (SOM), and carbon supplied from rice plants (RP), such as exudates and sloughed tissues. To estimate the contribution of each carbon source to CH4 emission, a pot experiment was conducted using 13C‐enriched soil sample and 13C‐enriched RS as tracers. The percentage contribution of RP carbon was estimated by subtraction. When RS was applied at a rate corresponding to 6 t ha−1, the percentage contributions of RS, SOM, and RP carbon to CH4 emission throughout the period of rice growth were 42%, 18–21%, and 37–40%, respectively. The values for SOM and RP carbon for the treatment in which RS was not applied were 15–20% and 80–85%, respectively. Seasonal variations in the percentage contribution of soil organic carbon to CH4 emission were small in the range between 13% and 30% for the pots with RS and between 15% and 24% for the pots without RS. In the RS‐applied treatment, RS and SOM accounted for almost 100% of the CH4 carbon early in the period of rice growth, while 65–70% of the CH4 emission in the milky stage was derived from RP carbon.
Anomalous microwave spectra of snow cover observed from Special Sensor Microwave/Imager measurements
Authors: Rosenfeld Simon , Grody Norman
Published: September 2002

Brightness temperature spectra measured by the Special Sensor Microwave/Imager (SSM/I) flown onboard F8 and F14 satellites of the U.S. Defense Meteorological Satellite Program (DMSP) during the 1987–1988 and 1997–1998 winter periods are analyzed concurrently with the data from snow monitoring stations over the former Soviet Union. Extensive analysis reveals the existence of two anomalies in the microwave thermal radiation spectra of snow cover. It is shown that in the beginning of winter the SSM/I measurements at 19, 37, and 85 GHz generally follow a classical pattern; that is, the brightness temperatures decrease for both increasing snow depth and increasing frequency. Dramatic departures from this behavior is observed around the middle of winter: The brightness temperatures reach a minimum and then begin to increase despite the fact that the snow depth remains constant or even continues to grow. Statistical analysis of the snow pack characteristics and SSM/I measurements is presented around the time when the brightness temperatures reach a minimum. The anomalous spectral characteristics are analyzed using a two‐stream radiative transfer model and dense media theory. It is shown how metamorphic changes in the snow crystalline structure are responsible for the brightness temperature minimum. The second departure from the normal snow signature is the inversion of brightness temperature spectra; that is, the higher‐frequency brightness temperature is greater than the low‐frequency measurements. It is shown that this phenomenon, observed previously over Greenland and Antarctica, is much more extensive. Radiative transfer simulations were used to show that a dense layer of surface crust on top of old coarse‐grained snow can produce the invented brightness temperature spectrum.
Influence of a sea ice ridge on low‐level airflow
Authors: Garbrecht Thomas , Lüpkes Christof , Augstein Ernst , Wamser Christian
Published: September 2002

The influence of a single pressure ridge of 4.5 m height on the structure of the atmospheric surface layer is studied. The field of the mean wind velocity is governed by typical features of a Bernoulli effect with a speedup over the crest and a shadowing effect downwind of the ridge. It is found that the turbulence generated by the ridge compensates for the deformation of the flow field by mixing momentum downward. Both mean and turbulent fields are restored to their upwind values at a distance of ∼300 m downwind of the ridge, which is equivalent to an aspect ratio of ∼0.015. The level of maximum turbulence generated by the ridge is characterized by a linear relationship. A formulation for the determination of the form drag of a single ridge is proposed and generalized toward an ensemble of ridges. We estimate that the form drag contributes <50% to the total drag exerted by a typically ice covered sea surface on the atmospheric flow.
Changes in ultraviolet radiation in the 1990s: Spectral measurements from Reading, England
Authors: Bartlett Lee M. , Webb Ann R.
Published: September 2002

Spectral, ultraviolet irradiance measurements have been made since 1990 at Reading (England). Ancillary measurements using a photodiode show that UV measurements are frequently affected by clouds. Consequently, it is not practical to simply use irradiance measurements to study possible ozone‐related changes in UV‐B over the timescale of the measurements. However, taking ratios of irradiances in bands covering UV‐B and UV‐A wavelengths can be used to reduce the cloud influence on measurements. Using these ratios and ozone vertical column measurements, the anticorrelation between ozone and UV‐B irradiances measured at the ground is demonstrated. These data also show that between 1993 and 1997, UV‐B at Reading has increased by 4.3%, compared to a decrease in ozone of 5.9%. The UV‐B increase is not a statistically significant trend, however, and serves only as an indication of the changes observed.
An examination of chemistry and transport processes in the tropical lower stratosphere using observations of long‐lived and short‐lived compounds obtained during STRAT and POLARIS
Authors: Flocke F. , Herman R. L. , Salawitch R. J. , Atlas E. , Webster C. R. , Schauffler S. M. , Lueb R. A. , May R. D. , Moyer E. J. , Rosenlof K. H. , Scott D. C. , Blake D. R. , Bui T. P.
Published: September 2002

A suite of compounds with a wide range of photochemical lifetimes (3 months to several decades) was measured in the tropical and midlatitude upper troposphere and lower stratosphere during the Stratospheric Tracers of Atmospheric Transport (STRAT) experiment (fall 1995 and winter, summer, and fall 1996) and the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) deployment in late summer 1997. These species include various chlorofluorocarbons, hydrocarbons, halocarbons, and halons measured in whole air samples and CO measured in situ by tunable diode laser spectroscopy. Mixing ratio profiles of long‐lived species in the tropical lower stratosphere are examined using a one‐dimensional (1‐D) photochemical model that includes entrainment from the extratropical stratosphere and is constrained by measured concentrations of OH. Profiles of tracers found using the 1‐D model agree well with all the observed tropical profiles for an entrainment time scale of 8.5−4+6 months, independent of altitude between potential temperatures of 370 and 500 K. The tropical profile of CO is used to show that the annually averaged ascent rate profile, on the basis of a set of radiative heating calculations, is accurate to approximately ±44%, a smaller uncertainty than found by considering the uncertainties in the radiative model and its inputs. Tropical profiles of ethane and C2Cl4 reveal that the concentration of Cl is higher than expected on the basis of photochemical model simulations using standard gas phase kinetics and established relationships between total inorganic chlorine and CFC‐11. Our observations suggest that short‐lived organic chlorinated compounds and HCl carried across the tropical tropopause may provide an important source of inorganic chlorine to the tropical lower stratosphere that has been largely unappreciated in previous studies. The entrainment timescale found here is considerably less than the value found by a similar study that focused on observations obtained in the lower stratosphere during 1994. Several possible explanations for this difference are discussed.
An evaluation of historical methane emissions from the Soviet gas industry
Authors: Reshetnikov A. I. , Paramonova N. N. , Shashkov A. A.
Published: September 2002

An inventory of natural gas losses from the former Soviet Union's gas industry has been constructed from published Russian‐language sources. The results imply that in the late 1980s/early 1990s annual losses from Russia were in the range 35–59×109 cubic meters (24–40 Tg of CH4): estimates based on what are thought to be the more reliable sources place annual losses in the range 37–52×109 cubic meters (25–35 Tg of CH4). Of this amount, one half to two thirds of the emissions may have been from the extremely long and ageing gas pipeline system. Extrapolation of the estimates for Russian losses to the whole territory of the former Soviet Union suggests a probable total annual emission level from the whole ex‐Soviet gas industry in the range 47–67×109 cubic meters of natural gas or 31–45 Tg of CH4 in these years. The envelope of minimum and maximum estimates for emissions from the former Soviet Union ranges from 29 to 50 Tg of methane. The limited availability of systematic and accurate published information on the emissions introduces significant uncertainty into the estimate. In an attempt to constrain emissions better, estimates of losses from specific causes were made using two or more independent approaches, where possible. A reasonable agreement between estimates was achieved in those cases. Our results imply that substantial reductions in emissions could be achieved by investment to reduce losses. Because of the high global warming potential and short lifetime of methane compared to carbon dioxide, reducing the large losses from the FSU may be among the most cost‐effective short‐term approaches available to reduce global anthropogenic greenhouse warming.
A novel approach to atmospheric profiling with a mountain‐based or airborne GPS receiver
Authors: Zuffada Cinzia , Hajj George A. , Kursinski E. Robert
Published: September 2002

The delay induced by the Earth's atmosphere on the Global Positioning System (GPS) signal has been exploited in the last decade for atmospheric remote sensing. Ground‐based GPS measurements are traditionally used to derive columnar water vapor content, while space‐based GPS measurements, obtained by a receiver in a low‐Earth orbit tracking GPS satellites occulting behind the Earth's atmosphere, yield accurate, high‐resolution profiles of refractivity, temperature, and water vapor. A GPS receiver on a mountain top or an airplane with a “downward looking” field of view toward the Earth's limb is a novel concept presented here. We describe a generalized ray‐tracing inversion scheme where spherical symmetry is assumed for the atmosphere, and the refractivity is modeled as piecewise exponential, with scale height changing from one atmospheric layer to the next. Additional refractivity data, derived from a model, might be introduced above the receiver as an a priori constraint, and are treated as properly weighted additional measurements. The exponential scale heights and a normalizing value of refractivity are retrieved by minimizing, in a least squares sense, the residuals between measured bending angles and refractivity and those calculated on the basis of the exponential model and ray‐tracing. As a first validation step, we illustrate results comparing refractivity and temperature profiles obtained by this generalized ray‐tracing scheme against those derived via the Abel inversion for the GPS/MET experiment. Additionally, we present results for a hypothetical situation where the receiver is located within the atmosphere at a height of 5 km. For the last case we investigate the accuracy of the retrieval both below and above the receiver at a set of locations in the atmosphere ranging from middle to tropical latitudes. The main objective is that of establishing whether the bending measurements have sufficient strength to allow for retrieval of refractivity below and possibly above the receiver location. Our findings suggest that accurate profiles of refractivity at heights ranging from the Earth's surface to slighly above the receiver location can be derived by GPS data collected from within the atmosphere.
Modeling OH, HO2, and RO2 radicals in the marine boundary layer: 1. Model construction and comparison with field measurements
Authors: Carslaw N. , Creasey D. J. , Heard D. E. , Lewis A. C. , McQuaid J. B. , Pilling M. J. , Monks P. S. , Bandy B. J. , Penkett S. A.
Published: September 2002

An observationally constrained box model has been constructed in order to investigate the chemistry of the marine boundary layer at Mace Head, a remote location on the west coast of Ireland. The primary aim of the model is to reproduce concentrations of the hydroxyl (OH) and hydroperoxy (HO2) radicals measured by an in situ fluorescence assay by gas expansion (PAGE) instrument, and the sum of peroxy radicals ∑([HO2]+[RO2]) determined by a peroxy radical chemical amplification (PERCA) instrument. The model has been constructed based on observed concentrations of a suite of non‐methane hydrocarbons, measured in situ by gas chromatography. The chemical mechanism for the model is a subset of a comprehensive master chemical mechanism (MCM). This paper describes in detail the construction of the model, as well as the underlying approach. Comparisons of modeled and measured concentrations of radical species, from a recent field campaign held at the Mace Head Atmospheric Observatory during July and August 1996 (EASE 96), are also presented. For the limited OH data available from this campaign, the model tends to overestimate the observations by about 40%, although this discrepancy is within the uncertainties of the model (±31%, 2σ) and the PAGE measurements (±75% on average, 2σ). For HO2 the model reproduces the concentrations well on one day but less well on another. Low HOx concentrations compared to model results have been observed previously, with greater than expected heterogeneous losses invoked to explain the differences. Comparisons between measurements of peroxy radicals made by chemical amplification and model predictions show good agreement over a wide range of conditions.
Molecular hydrogen in the troposphere: Global distribution and budget
Authors: Novelli Paul C. , Lang Patricia M. , Masarie Kenneth A. , Hurst Dale F. , Myers Richard , Elkins James W.
Published: September 2002

Molecular hydrogen (H2) has been measured since 1989 in air samples collected using a globally distributed sampling network. Time series from 50 locations are used to better define the distribution and recent changes of H2in the remote lower troposphere. These data show that the globally averaged H2mixing ratio between 1991 and 1996 was about 531±6 parts per billion (ppb). Hydrogen exhibited well‐defined seasonal cycles in each hemisphere, with similar seasonal maxima (530–550 ppb). However, in the Northern Hemisphere the seasonal minimum was 70 ppb deeper than in the Southern Hemisphere (∼450 and 520 ppb, respectively), resulting in ∼3% more H2in the south than in the north. With these data we have reevaluated the global H2budget. Methane oxidation is the largest source of H2to the troposphere, and soil uptake accounts for much of its sink. The global annual turnover is estimated as ∼75 Tg H2yr−1. The annual turnover, combined with a calculated tropospheric burden of 155 Tg, indicates a lifetime of ∼2 years. While our understanding of the global distribution of the sources and sinks of H2is still incomplete, the lower annual minimum in the north may be reasonably attributed to hemispheric asymmetry in uptake by soils. The seasonal cycles in the two hemispheres show unusual similarities: the northern and the southern seasonal maxima and minima were offset by only a few months. We suggest that the seasonal cycle in the Southern Hemisphere is dominated by H2emissions from biomass burning.
Studying ozone climatology with a regional climate model: 1. Model description and evaluation
Authors: Bouchet V. S. , Laprise R. , Torlaschi E. , McConnell J. C.
Published: September 2002

On the basis of the Canadian Regional Climate Model (CRCM) a new regional oxidant model has been developed to study ozone climatology in eastern Canada. In addition to the semi‐Lagrangian advection and vertical diffusion schemes already present for tracers, a chemical module, dry deposition parameterization, and anthropogenic and on‐line biogenic emissions were added to the CRCM. The complete model forms a single system which integrates meteorological and chemical variables simultaneously. Transport of 25 chemical species is evaluated on a 80 × 80 horizontal grid at 42.3 km resolution and for the 24 unequally spaced levels. The chemical scheme includes 47 species and 114 reactions used in the Acid Deposition and Oxidant Model (ADOM) gas‐phase mechanism. Precalculated photolysis rates, corrected for the model cloud cover through the variations of the simulated solar radiation penetration, are used in the model. This limited‐area model is driven at its boundaries by objective reanalyses for the meteorological fields and by climatological concentrations for the chemical counterparts. A time step of 15 min common to all processes is currently used. A two‐step validation procedure, which includes specific cases and climatological simulations, was adopted. The first test consists of a week‐long simulation for the first week of August 1988 when an intense ozone episode affected most of northeastern America during the first Eulerian Model Evaluation and Field Study (EMEFS) campaign. Results show that the CRCM ability to simulate this episode is comparable to other existing models using off‐line approaches. The CRCM performance was substantially improved by the addition of the biogenic emission parameterization. Climatology‐specific results are presented in the companion paper by Bouchet et al. [this issue].
Uptake on fractal particles: 2. Applications
Authors: Bekki Slimane , David Christine , Law Kathy , Smith Dwight M. , Coelho Daniel , Thovert Jean‐Francois , Adler Pierre M.
Published: September 2002

This first part of the paper is devoted to the validation of the theoretical framework developed in the companion paper [Coelho et al., this issue] against laboratory data. Although there are limited data suitable for a full‐scale validation of the model, model‐calculated surface to mass ratios of soot aggregates are found to be consistent with laboratory measurements. Then we use the framework to estimate errors which can be generated in the derivation of fractal aggregate properties from measurements of equivalent sizes. It is shown that the derivations of the aggregate mass and the surface area enhancement factor can be in error by at least an order of magnitude. The calculations are substantially improved when the fractal character of soot is accounted for. Since the gyration radius is a key parameter of the model, useful relationships are provided for converting aggregate equivalent sizes (mass transfer equivalent radius, hydrodynamic radius) into gyration radius. Finally, uptake of chemical species on atmospheric soot is investigated for the lower stratosphere and the boundary layer. The uptake is found to be mostly reaction limited, justifying the assumption of proportionality between the soot surface area and the uptake rate. However, the uptake occurs in the transition regime for the relatively compact aggregates found in urban areas.
Properties of aerosols aloft relevant to direct radiative forcing off the mid‐Atlantic coast of the United States
Authors: Hartley W. Seth , Hobbs Peter V. , Ross John. L. , Russell Philip B. , Livingston John M.
Published: September 2002

Comparisons are presented of ambient aerosol optical depths (AOD) derived from airborne in situ measurements of aerosol properties and from Sun photometer measurements off the mideastern seaboard of the United States in July 1996. At 450 nm the Sun photometer measurements of AOD were greater than the in situ measurements by, on average, (12±5)%. Possible reasons for this difference are discussed. Comparisons of Angstrom coefficients for the ambient aerosol, based on in situ and Sun photometer measurements, are presented, as well as estimates of the ambient aerosol single‐scattering albedo. At a wavelength of 550 nm, the mean value of the ambient single‐scattering albedo was (0.95±0.03). Also compared are various optical properties of the dry aerosol derived from airborne in situ measurements with values calculated from a model of the dry aerosol. At a wavelength of 550 nm, measurements and calculations of the total light scattering and light backscattering coefficients for the dry aerosol agree, on average, to within (23±2)% and (8±2)%. The calculated and measured dry aerosol absorption coefficients at 535 nm are in agreement for an assumed value of the mass absorption efficiency for the black carbon content of the dry aerosol of 14 m2 g−1.
Distributions of NO, NOx, and NOy in the upper troposphere and lower stratosphere between 28° and 61°N during POLINAT 2
Authors: Ziereis H. , Schlager H. , Schulte P. , van Velthoven P. F. J. , Slemr F.
Published: September 2002

During the Pollution From Aircraft Emissions in the North Atlantic Flight Corridor 2 (POLINAT 2) field campaign the distribution of NO, NOx, and NOy in the upper troposphere and lowermost stratosphere over the eastern North Atlantic was measured using the Deutsches Zentrum für Luft‐und Raumfahrt research aircraft Falcon. Based from Shannon Airport in Ireland, 14 flights were carried out between September 19 and October 25, 1997. The measurements were performed in and out of the North Atlantic flight corridor covering latitudes between 28°N and 61°N. A marked latitudinal gradient in NO, NOx, and NOy, the sum of all reactive nitrogen compounds, was observed. Mean NO volume mixing ratios in the upper troposphere increased from about 50 parts per trillion by volume (pptv) at 28°N to about 180 pptv at 59°N. A similar latitude dependence was also found for NOx and NOy. In the northern part of the POLINAT 2 measuring area, NO and NOx volume mixing ratios increased significantly with increasing altitude with maximum values around the tropopause, while in the southern part of the measuring area no strong altitude gradient was observed. NO and NOx did not show a substantial gradient across the tropopause. NO/NOy and NOy/O3 ratios showed maximum values of about 0.30 ppbv/ppbv and 0.012 ppbv/ppbv, respectively, around the tropopause. The POLINAT 2 observations suggest that aircraft emissions are an important source of NOx and NOy in the region studied. Also, the present measurements contribute to the data set obtained in the North Atlantic flight corridor during the last few years and help to establish a NOx climatology around the tropopause for this region.
Aircraft observations of δ13C of atmospheric methane over the Pacific in August 1991 and 1993: Evidence of an enrichment in 13CH4 in the southern hemisphere
Authors: Mak John E. , Manning Martin R. , Lowe David C.
Published: September 2002

Measurements of atmospheric methane and its carbon isotopic composition from aircraft samples collected across a broad latitude range during August 1991 and 1993 show both latitudinal and temporal structure. Latitudinal gradients in August 1993 are generally consistent with three dimensional model simulations using the atmospheric transport model TM2 and previously developed methane budgets. However, there is a significant enrichment in δ13C values in 1991 compared to 1993, and this difference is more pronounced with increasing latitude in the Southern Hemisphere (SH). In the extratropical SH, δ13C values are about 0.25‰ larger in August 1991 than in August 1993, consistent with previously published results. The observed 13C enrichment decreases with northerly latitude and reaches zero in the subtropical Northern Hemisphere. Sensitivity tests using transient sources in the TM2 model suggest that a regional source of isotopically enriched methane located in the SH (e.g., savannah burning from southern Africa) cannot explain the observed spatial and temporal pattern of enrichment. We conclude that the anomalous behavior of methane between 1991 and 1993 is likely due to several concurrent factors involving both isotopically light and heavy sources.
Role of lee waves in the formation of solid polar stratospheric clouds: Case studies from February 1997
Authors: Rivière E. D. , Huret N. , G.‐Taupin F. , Renard J.‐B. , Pirre M. , Eckermann S. D. , Larsen N. , Deshler T. , Lefèvre F. , Payan S. , Camy‐Peyret C.
Published: September 2002

Recent theories of solid polar stratospheric clouds (PSCs) formation have shown that particles could remain liquid down to 3 K or 4 K below the ice frost point. Such temperatures are rarely reached in the Arctic stratosphere at synoptic scale, but nevertheless, solid PSCs are frequently observed. Mesoscale processes such as mountain‐induced gravity waves could be responsible for their formation. In this paper, a microphysical‐chemical Lagrangian model (MiPLaSMO) and a mountain wave model (NRL/MWFM) are used to interpret balloon‐borne measurements made by an optical particle counter (OPC) and by the Absorption par Minoritaires Ozone et NOx (AMON) instrument above Kiruna on February 25 and 26, 1997, respectively. The model results show good agreement with the particle size distributions obtained by the OPC in a layer of large particles, and allow us to interpret this layer as an evaporating mesoscale type Ia PSC (nitric acid trihydrate) mixed with liquid particles. The detection of a layer of solid particles by AMON is also qualitatively reproduced by the model and is interpreted to be frozen sulfate acid aerosols (SAT). In this situation, the impact of mountain waves on chlorine activation is studied. It appears that mesoscale perturbations amplify significantly the amount of computed ClO, as compared to synoptic runs. Moreover, MiPLaSMO chemical results concerning HNO3 and HCl agree with measurements made by the Limb Profile Monitor of the Atmosphere (LPMA) instrument on February 26 at a very close location to AMON, and explain part of the differences between LPMA measurement and Reactive Processes Ruling the Ozone Budget in the Stratosphere (REPROBUS) model outputs.
Schauinsland Ozone Precursor Experiment (SLOPE96): Scientific background and main results
Authors: Volz‐Thomas Andreas , Geiß Heiner , Kalthoff Norbert
Published: September 2002

The Schauinsland Ozone Precursor Experiment (SLOPE) was conducted in June 1996 as part of the former European Experiment on Transport and Transformation of Environmentally Relevant Trace Constituents in the Troposphere Over Europe (EUROTRAC) subproject Tropospheric Ozone Research (TOR) in order to study the photochemical formation of ozone and the degradation of the precursors in the plume of the city of Freiburg on its way to Schauinsland. Chemical and meteorological measurements were made at several surface sites in a valley that channels the airflow to Schauinsland, including vertical profiles at several locations, and aboard a small aircraft. A tracer experiment with SF6 was made in order to establish transport and dispersion in the valley. Particular emphasis was on the determination of the OH concentration from the decay of hydrocarbons during transport. The relatively high OH concentration of 7×106 cm−3 in the presence of high NOx concentrations (3–15 ppb) indicates the potential for unidentified processes in the HOx budget in the high‐NOx regime which may have important consequences for the ozone budget and smog chemistry in the urban/suburban environment.