bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling of gaseous, aerosol, and cloudiness effects on surface solar irradiance measured in Brazil's Amazonia 1992–1995
Authors: Tarasova Tatiana A. , Nobre Carlos A. , Eck Thomas F. , Holben Brent N.
Published: February 2004

The effects of water vapor, aerosol, and cloudiness on the incident surface solar irradiance continuously measured in Brazil's Amazonia at six observational sites from 1992 to 1995 are examined by means of a clear‐sky broadband radiative transfer model. The aerosol optical depth and precipitable water, both retrieved from Sun photometer measurements, serve as inputs to the model. Computed monthly mean values of clear‐sky surface irradiance are analyzed in conjunction with the monthly mean values of all‐sky surface irradiance measured on the ground. To assess the effect of cloudiness, we present the cloud radiative forcing and cloud radiative forcing ratio at the surface, both widely employed in cloud radiation budget studies. By its definition the monthly mean cloud radiative forcing (cloud radiative forcing ratio) is the difference (ratio) between surface solar irradiances under all‐sky and clear‐sky conditions. The analysis of the irradiances, as computed and as measured, shows that during the wet season the gaseous and cloudiness effects on the solar radiation attenuation in the atmosphere are comparable, while the aerosol influence is much smaller. The aerosol effect increases and cloudiness effect decreases in the second half of the dry season. Thus during the biomass burning period in southern Amazonia, the water vapor and aerosol effects become comparable, while the cloudiness impact is 2–3 times smaller. Both cloudiness and aerosol effects have strong seasonal variations, while the gaseous effect changes slightly throughout the year.
Ten‐year east Asian summer monsoon simulation using a regional climate model (RegCM2)
Authors: Lee Dong‐Kyou , Suh Myoung‐Seok
Published: February 2004

This paper presents a 10‐year (1987–1996) summer climate simulation using a regional climate model. The National Center for Atmospheric Research Regional Climate Model (NCAR RegCM2) was designated over east Asia with a horizontal grid spacing of approximately 50 km. The model was initialized at June 1 and integrated up to the end of August for the 10 years. Initial and boundary conditions were the National Centers for Environmental Prediction/NCAR reanalysis data. The 10‐year summer mean biases of the simulated ground temperature and daily precipitation rate over the east Asian land area are about −0.1°C and 1.1 mm/d, respectively. Including the ocean area, the bias of the precipitaion rate is significantly reduced to about 0.05 mm/d. More precipitation is simulated in June over the central part of the model domain. The systematic cold biases of the simulated ground temperature take place over the northern part of the domain. The RegCM2 reproduces fairly well the large‐scale features associated with the east Asian summer monsoon system, including the subtropical high over the northwestern Pacific Ocean, the planetary‐scale trough over Manchuria, the upper level jet and lower‐level southerly or southwesterly flow and the precipitation band extended from south China to Japan. The evolution processes of the onset, break phases and abrupt jumps between two adjacent phases of the east Asian summer monsoon are also well simulated. On the other hand, the intensity and magnitude of the large‐scale features are not well reproduced. For example, the subtropical high over the ocean is too intensified, in association with the enhancement of the warm air advection in the lower level to the precipitation area. The maximum baroclinic zone and the planetary‐scale trough and thus the monsoon rainbelt are shifted northward by 2°–3° compared to reanalysis data. However, some of the systematic errors are hidden in the anomaly analysis over the 10‐year RegCM2 climate period.
Polarization of light in UV‐visible limb radiance measurements
Authors: Oikarinen Liisa
Published: February 2004

Solar light propagating in the atmosphere becomes polarized when it is scattered by air molecules and aerosol particles. Remote‐sensing instruments that use grating spectrometers are sensitive to the polarization state of light. Therefore polarization has to be taken into account in the analysis of measurement data. In this paper we characterize the effects of polarization in UV‐visible radiance measurements by a limb‐viewing satellite‐based spectrometer. Special emphasis is on the Optical Spectrograph and Infra‐Red Imager System instrument designed for measuring ozone profiles in the stratosphere. The variation of polarization as a function of the aerosol conditions of the stratosphere, solar illumination geometry, tangent altitude of the measurement, wavelength, and surface albedo is investigated using a backward Monte Carlo radiative transfer model. Two possible correction schemes for instrument polarization sensitivity are discussed in view of these simulations: scaling of data by a reference spectrum measured at a high altitude and modeling of polarization by a single‐scattering model. Multiple scattering intensities calculated by a radiative transfer model that treats light as a scalar quantity are compared to values obtained by a model that takes polarization into account. In limb‐viewing geometry the scalar approximation is shown to cause an error of maximum ∼6% in the intensity.
Modeling CO2 and water vapor turbulent flux distributions within a forest canopy
Authors: Lai Chun‐Ta , Katul Gabriel , Oren Ram , Ellsworth David , Schäfer Karina
Published: February 2004

One‐dimensional multilayer biosphere‐atmosphere models (e.g., CANVEG) describe ecosystem carbon dioxide (CO2) and water vapor (H2O) fluxes well when cold temperatures or the hydrologic state of the ecosystem do not induce stomatal closure. To investigate the CANVEG model framework under such conditions, CO2, H2O, and sensible heat fluxes were measured with eddy‐covariance methods together with xylem sap flux and leaf‐level gas exchange in a 16‐year‐old (in 1999) southeastern loblolly pine forest. Leaf‐level gas exchange measurements, collected over a 3‐year period, provided all the necessary biochemical and physiological parameters for the CANVEG model. Using temperature‐induced reductions of the biochemical kinetic rate constants, the CANVEG approach closely captures the diurnal patterns of the CO2 and H2O fluxes for two different formulations of the maximum Rubisco catalytic capacity (Vc max) ‐ temperature function, suggesting that the CANVEG approach is not sensitive to Vc max variations for low temperatures. A soil moisture correction (wr) to the Ball‐Berry leaf‐conductance approach was also proposed and tested. The wr magnitude is consistent with values predicted by a root‐xylem hydraulic approach and with leaf‐level measurements. The wr correction significantly improves the model's ability to capture diurnal patterns of H2O fluxes for drought conditions. The modeled bulk canopy conductance (Gm) for pine foliage estimated from the CANVEG‐modeled multilevel resistance values agreed well with canopy conductance (Gc) independently estimated from pine sap flux measurements. Detailed sensitivity analysis suggests that the leaf‐level physiological parameters used in CANVEG are not static. The dynamic property of the conductance parameter, inferred from such sensitivity analysis, was further supported using 3 years of porometry measurements. The CANVEG model also reproduced basic biochemical processes as demonstrated by the agreement between modeled and leaf‐level measured Ci/Ca, where Ci and Ca are the intercellular and atmospheric CO2 concentration, respectively. The model estimated that vapor pressure deficit does not vary significantly within the canopy but that Ci/Ca varied by more than 15%. The broader implication of this variation is that “big‐leaf” approaches that compress physiological and biochemical parameters into bulk canopy stomatal properties may be suitable for estimating water vapor flux but biased for CO2 ecosystem fluxes.
Modeling methane fluxes in wetlands with gas‐transporting plants: 1. Single‐root scale
Authors: Segers Reinoud , Leffelaar Peter A.
Published: February 2004

Methane fluxes were related to the first principles of transport and kinetics (microbial and chemical reactions). The difference between the kinetic scale and the smallest flux scale, the plot scale, is large. A stepwise scaling up procedure was therefore used in a series of three papers. This paper treats kinetics and diffusion around a single gas‐transporting root. Kinetic processes include methane production, methane oxidation, electron acceptor reduction, electron acceptor reoxidation, and aerobic respiration. This is the minimum number of processes needed to relate net methane production to the main driving variables: carbon availability and oxygen inflow. Kinetics were integrated with diffusion, leading to a set of partial differential equations. This set was solved directly and also after simplification to a set of spatially averaged ordinary differential equations. Results of the simplified model closely resembled results of the unsimplified (full) model, which implies that the simplified model covers the main interactions of the full model and is suitable for further scaling up. Model results showed that reduction of methane emission after 100% specific inhibition of methane oxidation may not result in a reliable estimate of methane oxidation, firstly because of changes in the oxygen dynamics which directly or indirectly affects methane production and secondly because of transient effects.
Accuracy of rainfall estimates by two radars in the same Alpine environment using gage adjustment
Authors: Gabella Marco , Joss Jürg , Perona Giovanni , Galli Gianmario
Published: February 2004

A technique is analyzed to mitigate the hostile effects of a complex orography on precipitation estimated by radar. Radar observations are adjusted using a network of gages. The corrections of radar estimates are derived through a Weighted Multiple Regression as a function of (1) the distance from the radar, (2) the minimum height a meteorological target must reach to be visible from the radar site, (3) the height of the ground at each pixel. Two C‐band radars 140 km apart in an Alpine region are analyzed. About 60 radar‐gage data pairs are available for each radar in a 25,000 km2 area during an extreme Mediterranean event (precipitation measured by the gages during the 40‐hour observation period ranges from 36 to 444 mm). In the radar‐gage comparison the data pairs are divided into two groups: one is used for training, the other for testing, and vice versa. All the radar‐gage data pairs are used for training, when comparing the two radars. Then radar‐derived precipitation estimates are compared in a “mutual coverage” area (≈10,000 km2). In all cases the correction significantly reduces the bias and the standard deviation of precipitation difference. In spite of the different ages, technologies, and distances from the area of interest, both radars have shown similar behavior, and the proposed procedure can be successfully applied to both.
Two regimes of the quasi‐biennial oscillation in the equatorial stratospheric wind
Authors: Gruzdev Aleksandr N. , Bezverkhny Vyacheslav A.
Published: February 2004

An analysis of changes in the period of the quasi‐biennial oscillation (QBO) in the zonal velocity of the equatorial stratospheric wind on isobaric surfaces 70, 50, 40, 30, 20, 15, and 10 hPa for 1953–1997 is made. In particular, wavelet analysis, high‐resolution spectral analysis, and circle map analysis have been made. Different methods reveal that the QBO in the equatorial wind velocity at the noted stratospheric levels manifests itself mainly as two prevailing regimes with periods of about 2 and 2.5 years alternating with each other. At middle‐stratosphere levels the QBO amplitude changes as well with the changing period (increases with the period increase). The two periods are in a rational ratio to the annual period (with the ratios of corresponding frequencies to the annual frequency equal to one half and two fifths), thus pointing out the phase locking of the QBO to the annual cycle. Spectra of wind velocity exhibit, in addition to the main QBO periods, the annual and semiannual oscillations as well as 20‐, 8‐, and 8.6‐month oscillations corresponding to oscillations with combination frequencies equal to the difference and sum of the annual frequency and the main QBO frequencies. Hypothetical conceptual mechanisms of the two regimes of the QBO are discussed.
Methane and soil and plant community respiration from wetlands, Kejimkujik National Park, Nova Scotia: Measurements, predictions, and climatic change
Authors: Dalva Moshe , Moore Timothy R. , Arp Paul , Clair Thomas A.
Published: February 2004

A static, dark chamber technique was used weekly from mid‐July to mid‐November in 1995 and biweekly from mid‐May to late November in 1996, to measure methane (CH4) flux and soil and plant community respiration of CO2 from 36 sites in two wetlands in Kejimkujik National Park in south‐central Nova Scotia, Canada. Overall mean fluxes of CH4 were 43 mg m−2 d−1 in 1995 and 20 mg m−2 d−1 in 1996. Respiration rates were 5.1 g CO2 m−2 d−1 in 1995 and 3.2 g CO2 m−2 d−1 in 1996. Fluxes of CH4 and CO2 were related to microtopography and ecological grouping, depth to water table, and air and peat temperatures. Edge and hummock sites showed the lowest CH4 flux and the highest respiration rate, while pools showed the highest CH4 and lowest respiration rate. Gas emissions displayed a strong seasonal pattern with highest values occurring during the summer (June to August) and with a marked reduction in late fall. Depth to water table and air temperature explained 34 to 43% of the variance in CH4 flux and respiration from the sites over the 2 years (n = 666 to 824). We developed algorithms relating the daily mean flux of CH4 and respiration from the wetlands to an aspatial soil moisture, water table, and temperature model (ForHyM2) applied to the wetland basins. We then applied this model to calculated May to October fluxes of CH4 and CO2 from 1966 to 1998. We estimated that CH4 fluxes ranged from 2.8 to 7.4 g m−2, with a mean of 3.7 g m−2 and a standard deviation of 1.2 g m−2 over the 1966–1998 period. Respiration estimates ranged from 0.60 to 1.16 kg CO2 m−2, with a mean of 0.74 kg CO2 m−2 and a standard deviation of 0.11 kg CO2 m−2. Application of a 2xCO2 General Circulation Model scenario to temperature and precipitation for this part of eastern Canada resulted in increases of growing season CH4 emissions from 4.7 to 11.4 g m−2 and respiration from 0.77 to 1.32 kg CO2 m−2.
Evaluation of aerosol direct radiative forcing in MIRAGE
Authors: Ghan Steven , Laulainen Nels , Easter Richard , Wagener Richard , Nemesure Seth , Chapman Elaine , Zhang Yang , Leung Ruby
Published: February 2004

A variety of measurements have been used to evaluate the treatment of aerosol radiative properties and radiative impacts of aerosols simulated by the Model for Integrated Research on Atmospheric Global Exchange (MIRAGE). The treatment of water uptake in MIRAGE agrees with laboratory measurements, and the growth of aerosol extinction with relative humidity in MIRAGE simulations agrees with field measurements. The simulated frequency of relative humidity near 100% is about twice that of analyzed relative humidity. When the analyzed relative humidity is used to calculate aerosol water uptake in MIRAGE, the simulated aerosol optical depth agrees with most surface measurements after cloudy conditions are filtered out and differences between model and station elevations are accounted for, but simulated optical depths are too low over Brazil and central Canada. Simulated optical depths are mostly within a factor of 2 of satellite estimates, but are too high off the east coasts of the United States and China and too low off the coast of West Africa and in the Arabian Sea. The simulated single‐scatter albedo is consistent with surface measurements. MIRAGE correctly simulates a larger Ångström exponent near regions with emissions of submicron particles and aerosol precursor gases, and a smaller exponent near regions with emissions of coarse particles. The simulated sensitivity of radiative forcing to aerosol optical depth is consistent with estimates from measurements. The simulated direct forcing is within the uncertainty of estimates from measurements in the North Atlantic.
Methyl bromide, other brominated methanes, and methyl iodide in polar firn air
Authors: Sturges W. T. , McIntyre H. P. , Penkett S. A. , Chappellaz J. , Barnola J.‐M. , Mulvaney R. , Atlas E. , Stroud V.
Published: February 2004

We report measurements of brominated, bromochlorinated, and iodinated methanes in air extracted from deep firn at three polar locations (two Antarctic and one Arctic). Using a firn diffusion model, we are able to reconstruct a consistent temporal trend for methyl bromide from the two Antarctic sites. This indicates a steady increase by about 2 ppt from the midtwentieth century to 8 ppt today. The Arctic firn, however, contained extremely high levels of methyl bromide as well as numerous other organic gases, which are evidently produced in situ. The other brominated species (dibromomethane, bromochloromethane, bromodichloromethane, dibromochloromethane, and bromoform) showed little or no long‐term trend in Antarctic firn and therefore are evidently of entirely natural origin in the Southern Hemisphere. A clear seasonal trend was observed in the upper firn for the shortest‐lived halocarbons (notably bromoform and methyl iodide). The same species were present at lower abundance at the higher altitude and more inland Antarctic site, possibly due to their origin from more distant oceanic sources.
General circulation model assessment of direct radiative forcing by the sulfate‐nitrate‐ammonium‐water inorganic aerosol system
Authors: Adams Peter J. , Seinfeld John H. , Koch Dorothy , Mickley Loretta , Jacob Daniel
Published: February 2004

An on‐line simulation of aerosol sulfate, nitrate, ammonium, and water in the Goddard Institute for Space Studies general circulation model (GCM II‐prime) has been used to estimate direct aerosol radiative forcing for the years 1800, 2000, and 2100. This is the first direct forcing estimate based on the equilibrium water content of a changing SO42−‐NO3−‐NH4+ mixture and the first estimate of nitrate forcing based on a global model of nitrate aerosol. Present‐day global and annual average anthropogenic direct forcing is estimated to be −0.95 and −0.19 W/m2 for sulfate and nitrate, respectively. Simulations with a future emissions scenario indicate that nitrate forcing could increase to −1.28 W/m2 by 2100, while sulfate forcing declines to −0.85 W/m2. This result shows that future estimates of aerosol forcing based solely on predicted sulfate concentrations may be misleading and that the potential for significant concentrations of ammonium nitrate needs to be considered in estimates of future climate change. Calculated direct aerosol forcing is highly sensitive to the model treatment of water uptake. By calculating the equilibrium water content of a SO42−‐NH4+ aerosol mixture and the optical properties of the wet aerosol, we estimate a forcing that is almost 35% greater than that derived from correcting a low relative humidity scattering coefficient with an empirical f(RH) factor. The discrepancy stems from the failure of the empirical parameterization to adequately account for water uptake above about 90% relative humidity. These results suggest that water uptake above 90% RH may make a substantial contribution to average direct forcing, although subgrid‐scale variability makes it difficult to represent humid areas in a GCM.
Simulation of large‐scale dynamics in the mesosphere and lower thermosphere with the Doppler‐spread parameterization of gravity waves: 2. Eddy mixing and the diurnal tide
Authors: Akmaev R. A.
Published: February 2004

Turbulence is thought to be produced by gravity waves that become unstable in the upper atmosphere. Owing to a relatively short vertical wavelength of the solar diurnal tide, turbulent viscous‐type mixing is expected to be particularly important for tidal dissipation in the mesosphere and lower thermosphere (MLT). Within the Doppler‐spread parameterization (DSP) of gravity waves, the vertical eddy exchange coefficients may be calculated from the GW energy deposition rate. The DSP, as implemented in the Spectral Mesosphere/Lower Thermosphere Model, is shown to provide realistic simulations of both the diurnal tide and turbulent mixing in the MLT. In particular, the vertical structure and seasonal variations of eddy mixing agree well with in situ observations and independent estimates from global thermal balance. The seasonal variations of turbulence are found to be primarily responsible for a strong semiannual variation of the diurnal tide in agreement with recent satellite observations.
Estimating scalar sources, sinks, and fluxes in a forest canopy using Lagrangian, Eulerian, and hybrid inverse models
Authors: Siqueira Mario , Lai Chun‐Ta , Katul Gabriel
Published: February 2004

A new method was developed to estimate canopy sources and sinks from measured mean concentration profiles within the canopy (referred to as the “inverse” problem). The proposed method combined many of the practical advantages of the Lagrangian localized near‐field (LNF) theory and higher‐order Eulerian (EUL) closure principles. Particularly, this “hybrid” method successfully combined the essential conservation equations of closure modeling and the robustness of the regression source inversion developed for LNF theory. The proposed method along with LNF and EUL were tested using measurements from two field experiments collected in a pine forest and published measurements from a wind tunnel experiment. The field experiments were conducted to investigate the vertical distribution of the scalar fluxes within the canopy and the temporal patterns of the scalar fluxes above the canopy. This comparison constitutes the first “inverse method” comparison performed using the same data sets on all three models. For the wind tunnel data, all three models well reproduced the measured flux distribution. For the field experiments, all three models recovered the measured spatial and temporal flux distribution in an ensemble sense. The agreement between these three models is desirable to the inverse problem because it adds the necessary confidence in the computed flux distributions. However, the agreement among all three models with the field measurements, on a 30‐min time step, was less than satisfactory. Additionally, the divergence between models and measurements increased with departure from a near‐neutral atmospheric state. Despite fundamental differences in these model approximations, this similarity in model performance suggests that the source information recovered from a measured one‐dimensional mean concentration profile will not be further enhanced by a one‐dimensional steady state, planar homogeneous model of neutral flows.
An extreme wind event at Casey Station, Antarctica
Authors: Turner John , Lachlan‐Cope Tom A. , Marshall Gareth J. , Pendlebury Stephen , Adams Neil
Published: February 2004

Model output, satellite data, and in situ observations are used to investigate the conditions that gave rise to an extreme wind event at the Australian Casey Station (66.27°S, 110.53°E) on the coast of East Antarctica. The event took place over the period March 20–22, 1992, and resulted in Casey Station's highest ever wind gust for March (66.9 m s−1, 130 knots) and 10 m mean winds of near 50 m s−1. The event occurred when a deep low was located just north of the coast and there was high surface pressure inland. The rapid deepening of the low took place within a strong baroclinic zone lying north‐south between a cold trough and a ridge bringing very warm air southward. A conceptual model is proposed for the very strong winds experienced at Casey Station. Key elements of the model are (1) a synoptic‐scale high‐low pressure couplet, providing a strengthening pressure gradient; (2) entrainment of radiatively cooled air by the supercritical synoptic gradient, leading to downslope flow; (3) the acceleration of the wind down the lee slope of Law Dome, occurring primarily in response to a topographically induced, long‐period, vertically propagating gravity wave; and (4) sources of negative buoyancy, including prestorm radiatively cooled air and, later in the storm, maritime air cooled by heat flux to the ice surface. The topographically induced gravity wave increases the horizontal temperature difference, thus increasing the negative buoyancy of the surface airflow.
Development of an inexpensive handheld LED‐based Sun photometer for the GLOBE program
Authors: Brooks David R. , Mims Forrest M.
Published: February 2004

Sun photometers that use light‐emitting diode (LED) detectors in place of optical interference filters and photo diodes have significant potential advantages, including low cost, durability, and long‐term optical stability. However, their relatively wide spectral response bandwidth poses some challenges in calibration and interpretation. Analysis of LED‐based Sun photometers developed for the Global Learning and Observations to Benefit the Environment (GLOBE) program has demonstrated that such instruments can, in fact, be calibrated using the standard Langley plot method and that their performance can be described in terms of effective response wavelengths. Several GLOBE Sun photometers have been calibrated at Mauna Loa Observatory on two separate occasions. The derived extraterrestrial constants are essentially the same in spite of the fact that the calibrations were performed under significantly different atmospheric conditions. These reference instruments have been used to transfer calibrations to other optically and electronically identical Sun photometers, thereby making it possible to establish a large network of inexpensive LED‐based Sun photometers. Data collected by students at a GLOBE high school near NASA's Goddard Space Flight Center (GSFC) and compared against aerosol optical thickness measurements from Sun photometers at GSFC demonstrate both that students can reliably make the required measurements and that LED‐based Sun photometers give results that compare favorably with conventional filter‐based instruments, even though their optical properties are significantly different.
Introduction to special section: FIRE Arctic Clouds Experiment
Authors: Curry Judith A.
Published: February 2004

The sensitivity of climate models seems to be particularly dependent on their treatment of radiation feedback processes in the Arctic. However, there are substantial uncertainties in the cloud, sea ice, and radiation properties of the Arctic, which is an environment where almost all measurement systems and analyses are working at their limit of capability. Motivated by these sensitivities and uncertainties, an intense effort was jointly sponsored by the National Science Foundation (NSF), the National Aeronautics and Space Administration (NASA), the Department of Energy (DOE), and the Office of Naval Research (ONR) to design a research program to address these issues. This planning culminated in three closely coordinated projects: the Surface Heat Budget of the Arctic Ocean (SHEBA) (sponsored primarily by NSF and ONR), the FIRE Arctic Clouds Experiment (sponsored primarily by NASA), and the Atmospheric Radiation Measurement (ARM) Program (sponsored by DOE).
SUSPEN intercomparison of ultraviolet spectroradiometers
Authors: Bais A. F. , Gardiner B. G. , Slaper H. , Blumthaler M. , Bernhard G. , McKenzie R. , Webb A. R. , Seckmeyer G. , Kjeldstad B. , Koskela T. , Kirsch P. J. , Gröbner J. , Kerr J. B. , Kazadzis S. , Leszczynski K. , Wardle D. , Josefsson W. , Brogniez C. , Gillotay D. , Reinen H. , Weihs P. , Svenoe T. , Eriksen P. , Kuik F. , Redondas A.
Published: February 2004

Results from an intercomparison campaign of ultraviolet spectroradiometers that was organized at Nea Michaniona, Greece, July 1–13, 1997, are presented. Nineteen instrument systems from 15 different countries took part and provided spectra of global solar UV irradiance for two consecutive days from sunrise to sunset every half hour. No data exchange was allowed between participants in order to achieve absolutely independent results among the instruments. The data analysis procedure included the determination of wavelength shifts and the application of suitable corrections to the measured spectra, their standardization to common spectral resolution of 1 nm full width at half maximum and the application of cosine corrections. Reference spectra were calculated for each observational time, derived for a set of instruments which were objectively selected and used as comparison norms for the assessment of the relative agreement among the various instruments. With regard to the absolute irradiance measurements, the range of the deviations from the reference for all spectra was within ±20%. About half of the instruments agreed to within ±5%, while only three fell outside the ±10% agreement limit. As for the accuracy of the wavelength registration of the recorded spectra, for most of the spectroradiometers (14) the calculated wavelength shifts were smaller than 0.2 nm. The overall outcome of the campaign was very encouraging, as it was proven that the agreement among the majority of the instruments was good and comparable to the commonly accepted uncertainties of spectral UV measurements. In addition, many of the instruments provided consistent results relative to at least the previous two intercomparison campaigns, held in 1995 in Ispra, Italy, and in 1993 in Garmisch‐Partenkirchen, Germany. As a result of this series of intercomparison campaigns, several of the currently operating spectroradiometers operating may be regarded as a core group of instruments, which with the employment of proper operational procedures are capable of providing quality spectral solar UV measurements.
A 1‐year Lagrangian “climatology” of airstreams in the northern hemisphere troposphere and lowermost stratosphere
Authors: Stohl Andreas
Published: February 2004

This paper presents a 1‐year “climatology” of airstreams, based on a large number of three‐dimensional air parcel trajectories started daily from points on a grid covering the Northern Hemisphere troposphere and lowermost stratosphere. Different types of airstreams (warm conveyor belts, dry intrusions, stratosphere‐to‐troposphere flows, and troposphere‐to‐stratosphere flows) are identified, and their geographical distribution and associated mass fluxes are examined. Warm conveyor belt air parcels, which are deposited in the uppermost troposphere and even the lowermost stratosphere, originate most frequently in the boundary layer over the warm water pools at the eastern seaboards of North America and Asia, close to the worldwide highest anthropogenic emissions. Therefore their chemical characteristics likely range from very clean to highly polluted. Because warm conveyor belts rarely start over Europe, European trace gas emissions have fewer chances of reaching the upper troposphere and lowermost stratosphere with warm conveyor belts than emissions from other densely populated regions. On the other hand, the upper troposphere over Europe frequently receives outflow from warm conveyor belts originating over the North American seaboard. Dry intrusion activity is highest in winter and lowest in summer and shows maxima over western North America and eastern Asia. Deep intrusions from the stratosphere into the lower troposphere are almost nonexistent during the summer. Mass fluxes associated with each of the airstream types are reported and are an alternative to Eulerian mass flux estimates. All types of airstreams with strong vertical motions are highly incoherent. Rather than depositing compact air parcels, they stir air masses into a new environment and create filamentary tracer structures.
Heterogeneous reactions of volatile organic compounds on oxide particles of the most abundant crustal elements: Surface reactions of acetaldehyde, acetone, and propionaldehyde on SiO2, Al2O3, Fe2O3, TiO2, and CaO
Authors: Li P. , Perreau K. A. , Covington E. , Song C. H. , Carmichael G. R. , Grassian V. H.
Published: February 2004

The heterogeneous reactivity of volatile organic compounds (VOCs) on oxide particles has been investigated. Oxide particles composed of the most abundant elements present in the Earth's crust were used to identify potentially important reactions of VOCs on mineral dust. In particular, heterogeneous reactions of acetaldehyde, acetone, and propionaldehyde on α‐Al2O3, α‐Fe2O3, TiO2, CaO, and SiO2 particle surfaces have been investigated. Fourier transform infrared and UV/visible spectroscopic measurements show that these carbonyl compounds weakly and reversibly adsorbed on SiO2. However, on the more basic and acidic oxides investigated, the data show that these carbonyl compounds irreversibly adsorb and can, in part, react on the surface to form larger molecular weight compounds. The kinetics of the heterogeneous reaction of acetaldehyde, acetone, and propionaldehyde on α‐Al2O3, α‐Fe2O3, TiO2, CaO, and SiO2 were measured with a Knudsen cell reactor at a gas concentration of 6×1010 molecules/cm3. Using the Brunauer‐Emmett‐Teller surface area of the powdered sample, initial uptake coefficients were determined to be in the 10−4 to 10−6 range for the adsorption of carbonyl compounds on the oxides at 298 K. By using these values for the uptake coefficient, it is found that the heterogeneous loss of these trace volatile organic compounds is comparable to that due to photolysis and reaction with OH in the middle to upper troposphere. Although surface adsorption can occur, the measured second‐order pressure dependence to form higher molecular weight compounds suggests that these reaction products will less likely form under atmospheric conditions.
Regional physical and chemical properties of the marine boundary layer aerosol across the Atlantic during Aerosols99: An overview
Authors: Bates Timothy S. , Quinn Patricia K. , Coffman Derek J. , Johnson James E. , Miller Theresa L. , Covert David S. , Wiedensohler Alfred , Leinert Stephan , Nowak Andreas , Neusüss Christian
Published: February 2004

The Aerosols99 cruise crossed the Atlantic Ocean from Norfolk, Virginia, to Cape Town, South Africa, between January 14 and February 8, 1999. The goals of the cruise were to determine the chemical, physical, and optical properties of the marine boundary layer (MBL) aerosol, the vertical distribution of aerosols and ozone, the column‐integrated aerosol optical depth, and the ozone, CO, and peroxy radical chemistry in the MBL. Sampling strategies were optimized to obtain data sets to evaluate satellite‐derived ocean color (Sea‐viewing Wide Field‐of‐view Sensor), aerosol optical depth (advanced very high resolution radiometer) and total column ozone (Total Ozone Mapping Spectrometer). The cruise track crossed through seven different meteorological/oceanographic regimes ranging from background marine air masses in the Northern and Southern Hemispheres to air masses containing mineral dust and the products of biomass burning. This overview discusses the seven regimes encountered enroute and the chemical and physical properties of the MBL aerosol in each regime.
A study of the reaction between NaHCO3 and H: Apparent closure on the chemistry of mesospheric Na
Authors: Cox Rachel M. , Self Daniel E. , Plane John M. C.
Published: February 2004

The reaction between sodium bicarbonate (NaHCO3) and atomic H is the only likely route (apart possibly from daytime photolysis) for recycling this major sodium reservoir back to atomic Na in the upper mesosphere. The rate coefficient was determined for the first time by reacting NaHCO3 in a known excess of H in a fast flow tube, yielding k(227 K) = (1.7 ± 0.6) × 10−13 and k(307 K) = (5.2 ± 2.5) × 10−13 cm3 molecule−1 s−1. These measurements are very well described by transition state theory, using ab initio quantum calculations of the saddle points on the reaction potential energy surface. The resulting temperature dependence of the rate coefficient is then given by k(100–400 K) = 1.84 × 10−13 T0.78exp −(1014+130−96/T) cm3 molecule−1 s−1. When k(T) is extrapolated to 180–220 K, there is excellent agreement with the value predicted from atmospheric models of the nighttime Na layer. Hence this measurement has, to a first order, demonstrated closure of the neutral chemistry governing the underside of the Na layer above 80 km.
Deuterium excess record from central Greenland over the last millennium: Hints of a North Atlantic signal during the Little Ice Age
Authors: Hoffmann Georg , Jouzel Jean , Johnsen Sigfus
Published: February 2004

A stacked water isotope record for Summit, central Greenland, has been established covering approximately the last 900 years. Measurements of δ18O and δD in the GRIP deep ice core and the 230‐m core, S93, allow the reconstruction of a millennial record of the deuterium excess in a near‐annual resolution. A short period of particularly high values of the Deuterium Excess at the beginning of the fourteenth century may be associated with the medieval warm period (MWP). The Little Ice Age (LIA) might be represented by a 100‐year period of very low excess values in the sixteenth and seventeenth centuries. Using a simple isotope model, δ18O and deuterium excess are interpreted in terms of surface temperature variations over central Greenland and over the subtropical North Atlantic, Greenland's principal vapor source region. An estimated cooling of −0.7°C of subtropical sea surface temperatures (SSTs) during the Little Ice Age and a warming of 0.6°C during the short warm period in the medieval is in agreement with previous studies. Over periods of about 100 years, an antiphasing between gradually decreasing δ18O and increasing deuterium excess is observed. Interannual‐to‐decadal‐scale variability associated to the North Atlantic Oscillation may be responsible for this anti‐phase relationship. An alternative explication is a North‐South oscillation in North Atlantic sea surface temperatures associated with short‐term changes in the thermohaline circulation.
Some results relevant to the discussion of a possible link between cosmic rays and the Earth's climate
Authors: Wagner Gerhard , Livingstone David M. , Masarik Jozef , Muscheler Raimund , Beer Jürg
Published: February 2004

Based on a 16‐year observation period (1980–1995), it was claimed recently that Earth's climate was linked to variations in the flux of cosmic rays penetrating into the atmosphere via their postulated effect on global cloud cover. Data from three independent studies yield information relevant to the ongoing discussion of the likelihood of the existence of such a link. (1) Model calculations show that the relative change in the ion production rate from a solar maximum to a solar minimum is of the same order as, or even greater than, the corresponding change in global cloud cover. (2) However, the smoothed combined flux of 10Be and 36Cl at Summit, Greenland, from 20–60 kyr B.P. (proportional to the geomagnetically modulated cosmic ray flux) is unrelated to the corresponding δ18O and CH4 data (interpreted as supraregional climate proxies). (3) Furthermore, although a comparison of the incoming neutron flux with cloud cover in Switzerland over the last 5 decades shows a significant correlation at times during the 1980s and 1990s, this does not occur during the rest of the period.
A composite view of ozone evolution in the 1995–1996 northern winter polar vortex developed from airborne lidar and satellite observations
Authors: Douglass A. R. , Schoeberl M. R. , Kawa S. R. , Browell E. V.
Published: February 2004

A three‐dimensional model using winds and temperatures from the Goddard Earth Observing System Data Assimilation System is used to simulate ozone evolution in the winter high‐latitude northern lower stratosphere. The simulation results are compared with ozone observations from three platforms: the differential absorption lidar (DIAL), the Microwave Limb Sounder (MLS), and the Polar Ozone and Aerosol Measurement (POAM II). Time series for the different data sets are consistent with each other, and diverge from model time series during December and January. The model ozone in December and January is much less sensitive to the model photochemistry than to the model vertical transport. Simulations with different initial conditions for ozone demonstrate sensivitity to the model ozone profile shape. The modeled ozone throughout December and January most closely resembles observed ozone when the vertical profiles between 12 and 20 km within the polar vortex closely match December DIAL observations. We make a quantitative estimate of the uncertainty in the vertical advection using diabatic trajectory calculations. The net transport uncertainty is significant and should be accounted for when comparing observations with model ozone. The observed and modeled ozone time series during December and January are not inconsistent when these transport uncertainties are taken into account.
Ozone changes in the lower stratosphere from the Halogen Occultation Experiment for 1991 through 1999
Authors: Remsberg Ellis E. , Bhatt Praful P. , Deaver Lance E.
Published: February 2004

The Upper Atmospheric Research Satellite (UARS) Halogen Occultation Experiment (HALOE) Version 19 ozone profile data set was analyzed for both periodic and nonseasonal polynomial changes in lower stratospheric ozone for 1991–1999. The profile data were screened for cloud contamination at the lowest levels and were then integrated within six half‐Umkehr (each ∼2.5 km thick) layers from 32 to 253 hPa. The column values were then binned and averaged into 10°‐wide latitude regions but separated into sunrise and sunset measurements, yielding an effective time series of zonal‐averaged ozone that spans 8 years. The results extend to near tropopause levels, 253 hPa in the extratropics and 127 hPa in the tropics. Multiple linear regression techniques were applied to the data of each latitude zone and half‐Umkehr layer. We developed models that included annual, semiannual, and interannual periodic terms plus polynomial terms at 50°N, 30°N, equator, 30°S, and 50°S. The amplitudes of the three periodic terms vary with latitude and pressure altitude and do not maintain their same order of importance in all cases, emphasizing the changes and hemispheric asymmetry of the transport mechanisms in the lower stratosphere. We find no clear evidence for long‐term change at most latitudes and layers. Steady declines in ozone of 0.5% yr−1 were found in the 32–45 hPa layer at 50°S, most likely due to transport of ozone‐depleted air from higher latitudes. Our lowest tropical layer (90–127 hPa) shows an ozone decline in the early 1990s but an increase in the last half of the decade. Steady increases of ozone of 3–4% yr−1 were found at 50°N from 127 to 253 hPa and at 30°N from 179 to 253 hPa, probably in response to a changing net circulation for the Northern Hemisphere lower stratosphere. However, there is no highly significant trend at 50°N for the column ozone over the deeper layer from 8 to 253 hPa. For Northern Hemisphere middle latitudes it is concluded that the previously reported declines of zonal‐average ozone in the lower stratosphere for the 1980s and early 1990s have not continued over the decade of the 1990s.
Total VOC reactivity in the planetary boundary layer: 2. A new indicator for determining the sensitivity of the ozone production to VOC and NOx
Authors: Kirchner Frank , Jeanneret Francois , Clappier Alain , Krüger Bernd , van den Bergh Hubert , Calpini Bertrand
Published: February 2004

A new indicator is proposed for determining if tropospheric ozone production in a specific area is limited by VOC or NOx. The indicator Θ = τOHVOC/τOHNOx describes the ratio of the lifetimes of OH against the losses by reacting with VOC and NOx. Whereas τOHNOx can be obtained by conventional measurements, the new pump and probe OH approach which is described in part one of this publication makes it now possible to obtain also τOHVOC. Indicator values above a threshold value of 0.2±50% are representative of NOx‐saturated conditions where an increase of NOx emissions causes lower ozone production. For values below 0.01 the ozone production is very insensitive to changes of VOC emissions. The robustness of this indicator against several parameters such as temperature, humidity, photolysis, and initial ozone concentrations is tested in a box model and compared to the robustness of other earlier proposed indicators. In contrast to earlier proposed indicators, this new one is not based on photochemically produced long‐lived species but describes the instantaneous regime of an air parcel. Three‐dimensional simulation shows that this indicator is quite successful in estimating the impact of increased or reduced emissions on the ozone concentrations for each location in the modeling area. This will make it a very helpful tool for developing ozone abatement strategies.
Nonmethane hydrocarbons in the rural southeast United States national parks
Authors: Kang Daiwen , Aneja Viney P. , Zika Rod G. , Farmer Charles , Ray John D.
Published: February 2004

Measurements of volatile organic compounds (VOCs) were made at three rural sites in the southeast U.S. national parks: Mammoth Cave National Park, Kentucky; Cove Mountain, Great Smoky Mountains National Park, Tennessee; and Big Meadows, Shenandoah National Park, Virginia. In 1995 the three locations were sampling sites for the Southern Oxidants Study (SOS) Nashville Intensive, and the measurements of VOCs for Shenandoah were also made under contract with the National Park Service. Starting in 1996, the National Park Service added the other two parks to the monitoring contract. Hydrocarbon measurements made during June through September for the years 1995, 1996, and 1997 were analyzed in this study. Source classification techniques based on correlation coefficient, chemical reactivity, and ratioing were developed and applied to these data. The results show that anthropogenic VOCs from automobile exhaust appeared to be dominant at Mammoth Cave National Park, and at Cove Mountain, Great Smoky Mountains National Park, but other sources were also important at Big Meadows, Shenandoah National Park. Correlation and ratio analysis based on chemical reactivity provides a basis for source‐receptor relationship. The most abundant ambient VOCs varied both in concentration and order depending on park and year, but the following VOCs appeared on the top 10 list for all three sites: isoprene (6.3 to 18.4 ppbv), propane (2.1 to 12.9 ppbv), isopentane (1.3 to 5.7 ppbv), and toluene (1.0 to 7.2 ppbv). Isoprene is naturally emitted by vegetation, and the others are produced mainly by fossil fuel combustion and industrial processes. Propylene‐equivalent concentrations were calculated to account for differences in reaction rates between the hydroxyl radical and individual hydrocarbons, and to thereby estimate their relative contributions to ozone formation.