bluuebunny/crossref_metadata_2025 huggingface dataset:
Multiangle observations of Arctic clouds from FIRE ACE: June 3, 1998, case study
Authors: Marchand Roger T. , Ackerman Thomas P. , King Michael D. , Moroney Catherine , Davies Roger , Muller Jan‐Peter A. L. , Gerber Hermann
Published: February 2004

In May and June 1998 the Airborne Multiangle Imaging Spectroradiometer (AirMISR) participated in the FIRE Arctic Cloud Experiment (ACE). AirMISR is an airborne instrument for obtaining multiangle imagery similar to that of the satellite‐borne MISR instrument. This paper presents a detailed analysis of the data collected on June 3, 1998. In particular, AirMISR radiance measurements are compared with measurements made by two other instruments, the Cloud Absorption Radiometer (CAR) and the MODIS airborne simulator (MAS), as well as to plane‐parallel radiative transfer simulations. It is found that the AirMISR radiance measurements and albedo estimates compare favorably both with the other instruments and with the radiative transfer simulations. In addition to radiance and albedo, the multiangle AirMISR data can be used to obtain estimates of cloud top height using stereoimaging techniques. Comparison of AirMISR retrieved cloud top height (using the complete MISR‐based stereoimaging approach) shows excellent agreement with the measurements from the airborne Cloud Lidar System (CLS) and ground‐based millimeter‐wave cloud radar.
Large‐eddy simulations of entrainment of cloud condensation nuclei into the Arctic boundary layer: May 18, 1998, FIRE/SHEBA case study
Authors: Jiang Hongli , Feingold Graham , Cotton William R. , Duynkerke Peter G.
Published: February 2004

Two sets of three‐dimensional simulations of a springtime Arctic boundary layer cloud observed during the FIRE/SHEBA 1998 spring IOP were conducted to study the influence of entrainment of cloud condensation nuclei (CCN) at cloud top on cloud microphysical and dynamical structure, radiative properties, and cloud evolution. The model is a large‐eddy version of the Regional Atmospheric Modeling System (RAMS) with explicit representation of the CCN spectrum and cloud droplet spectrum. The initial CCN concentration is a constant value of 30 cm−3 in the control run, while it varies from 30 cm−3 below cloud base to a peak of 250 cm−3 at the inversion in the sensitivity run. Results from the sensitivity run show that droplet concentrations increase about twofold, effective radii decrease by 9–15% from cloud top to cloud base, liquid water content increases about 21%, and no drizzle reaches the ground in comparison with results from the control run. The dynamic response becomes significant by the end of the 5 hour simulation, as reflected in more vigorous eddies in the sensitivity run. The response of the cloud optical properties to entrainment occurs from the beginning of the simulations. Cloud albedo increases 12%, while cloud optical depth increases 33%. These results are consistent with both observations and modeling studies. It is stressed that knowledge of boundary layer deepening is critical to prediction of cloud optical properties, both from the thermodynamical perspective, because the properties of the entrained air affect bulk cloud features such as liquid water path, and from the microphysical perspective because aerosol gradients across the top of the boundary layer can alter microphysical properties and, in turn, cloud optical properties.
Tropospheric ozone simulation with a chemistry‐general circulation model: Influence of higher hydrocarbon chemistry
Authors: Roelofs Geert‐Jan , Lelieveld Jos
Published: February 2004

We present an improved version of the global chemistry‐general circulation model of Roelofs and Lelieveld [1997]. The major model improvement is the representation of higher hydrocarbon chemistry, implemented by means of the Carbon Bond Mechanism 4 (CBM‐4). Simulated tropospheric ozone concentrations at remote locations, which agreed well with observations in the previous model version, are not affected much by the chemistry of higher hydrocarbons. However, ozone formation in the polluted boundary layer is significantly enhanced, resulting in a more realistic simulation of surface ozone in regions such as North America, Europe, and Southeast Asia. Our model simulates a net global tropospheric ozone production of 73 Tg yr−1 when higher hydrocarbon chemistry is considered, and ‐36 Tg yr−1 without higher hydrocarbon chemistry. The simulated seasonality of surface CO agrees well with observations. However, the southern hemispheric maximum for O3 and CO associated with biomass burning emissions is delayed by 1 month compared to the observations, which demonstrates the need for a better representation of biomass burning emissions. Simulated peroxyacetyl nitrate (PAN) concentrations agree well with observed values, although the variability is underestimated. OH decreases strongly in the continental boundary layer due to its reaction with higher hydrocarbons. However, this is almost compensated by an increase of OH over oceans in the lower half of the troposphere. Consideration of higher hydrocarbon chemistry decreases the global annual tropospheric OH concentration by about 8% compared to a background tropospheric chemistry scheme. Further, the radiative forcing by anthropogenically increased tropospheric ozone on the northern hemisphere increases, especially in July. The forcing also increases on the southern hemisphere where biomass burning emissions produce tropospheric ozone, except between December and June, that is, outside the biomass burning season, when ozone formation is suppressed due to formation of PAN and MPAN from isoprene oxidation. Globally and annually averaged, the forcing increases only by a few percent due to higher hydrocarbon chemistry.
Optical properties and direct radiative effect of Saharan dust: A case study of two Saharan dust outbreaks using aircraft data
Authors: Haywood James M. , Francis Peter N. , Glew Martin D. , Taylor Jonathan P.
Published: February 2004

The radiative effects of Saharan dust are measured during two flights by the Met Office C‐130 aircraft off the west coast of Africa. Data from the broadband radiometers suggests that the perturbation to the top of the atmosphere net solar irradiance is as strong as −60 W m−2 ±5 W m−2 during the dust events. In situ measurements with the nephelometer and particle soot absorption photometer suggest that the single scattering albedo is approximately 0.87 at a wavelength of 0.55 μm. This is in agreement with the optical parameters calculated from independent measurements of the particle size distributions combined with suitable refractive indices and Mie‐scattering theory. The wavelength dependence of the extinction coefficient derived from measurements of the scattering coefficient by the nephelometer is also in excellent agreement with the calculations. Independent surface‐based measurements from Cape Verde suggest that the wavelength dependence of the aerosol optical depth appears reasonable. Calculations of the downward solar irradiances within the aerosol layer are generally in good agreement with the measurements demonstrating consistency between the measurements and the modeling efforts. The terrestrial radiative effect is not detectable by the current instrumentation, though it cannot be considered negligible. These measurements suggest that satellite retrieval algorithms may misclassify the aerosol outbreak as cloud because the aerosol optical depth at 0.55 μm is as high as 1.15, which is in excess of the thresholds used in some cloud detection algorithms. The measurements demonstrate that this method could be used to provide an accurate benchmark for satellite‐based estimates of the radiative effect of aerosols.
Photochemical production of gas phase NOx from ice crystal NO3−
Authors: Honrath R. E. , Guo S. , Peterson M. C. , Dziobak M. P. , Dibb J. E. , Arsenault M. A.
Published: February 2004

Recent measurements have demonstrated that sunlight irradiation of snow results in the release of significant amounts of gas phase NOx (NO+NO2). We report here the results of a series of experiments designed to test the hypothesis that the observed NOx production is the result of nitrate photolysis. Snow produced from deionized water with and without the addition of nitrate was exposed to natural sunlight in an outdoor flow chamber. While NOx release from snow produced without added NO−3 was minimal, the addition of 100 µM NO−3resulted in the release of >500 pptv NOx in a 9 standard liter per minute (sLpm) flow of synthetic air exposed to the snow for 10–20 s; the rate of release was highly correlated with solar radiation. Further addition of radical trap reagents resulted in greatly increased NOx production (to >8 ppbv in a flow of 20 sLpm). In snow produced from deionized water plus sodium nitrate, production of NO2 dominated that of NO. The reverse was true in the presence of radical trap reagents; this suggests sensitivity of the NOx release mechanism to pH, as a basic compound was added, or to the presence of free radical scavengers. A mechanism for NOx release from NO−3photolysis consistent with these observations is presented. These results support previous suggestions that surface NOx release may have a significant impact on boundary layer photochemistry in snow‐covered regions and that nitrate photolysis on cirrus cloud particles may result in the release of gas phase NOx. A potential for pH‐dependent impacts on ice core records of oxidants and oxidized compounds is also suggested.
Effects of synoptic patterns on atmospheric chemistry and aerosols during the Arctic Ocean Expedition 1996
Authors: Nilsson E. Douglas , Barr Sumner
Published: February 2004

The atmospheric program on the Arctic Ocean Expedition of July through September 1996 (AOE‐96) was focused on aerosol climate feedback. The expedition took place close to the saddle point between a semipersistent anticyclonic ridge from near Scandinavia to the Arctic coast of eastern Siberia and a trough from the Canadian archipelago across the pole to north central Siberia. The weather varied from anticyclonic clear‐sky conditions to cyclonic cloudy conditions, and 13 identifiable migratory features (frontal bands, wave disturbances) clearly influenced local weather, clouds, atmospheric transport, and chemistry. This includes an explosive polar cyclone, born at the lateral heat gradient between Greenland and the pack ice rather than between open sea and the pack ice. The synoptic scale weather systems caused the strongest variability in trace gases (O3 in particular) and aerosols, and also strong variability in the cloud cover. The formation of air masses over the pack ice primarily depends on if there is cyclonic (convergent) or anticyclonic (divergent) flow. Cyclonic flow resulted in a modified marine air mass loaded with vapor, but with low aerosol number concentrations owing to frequent clouds and fogs and efficient cloud scavenging of the aerosol. Anticyclonic flow resulted in almost continental air masses with clear sky, long residence time over the pack ice and subsidence slowly replacing the boundary layer with free tropospheric air, low vapor concentrations, but large aerosol number in lack of efficient cloud scavenging. The synoptic variability and advection from south of the ice edge were weaker than during the predecessor International Arctic Ocean Expedition in 1991 (IAOE‐91), when on average the sampled air spent 55 hours over the pack ice compared to more than 120 hours during AOE‐96, owing to exceptionally high cyclone activity in 1991. This caused a large difference in atmospheric transport, chemistry, and aerosols between the two expeditions.
Bidirectional reflectance distribution function signatures of major biomes observed from space
Authors: Bicheron Patrice , Leroy Marc
Published: February 2004

Land surface bidirectional reflectance distribution function (BRDF) measurements were acquired from November 1996 to June 1997 at global scale and 6km spatial resolution with the POLDER instrument onboard the ADEOS‐I satellite. We selected 395 BRDF data sets on areas distributed on the 17 biomes of the IGBP 1‐km land cover classification (DISCover data set) at 443, 670, and 865nm, at several periods (November and December 1996, May and June 1997). The selected BRDF data are characterized by a low noise level, a sufficient number of clear days during the month, and a roughly even sampling of directional space. The data show large differences of the directional and spectral signatures of the various land cover classes, both in shape and in magnitude. Except for the desert and ice classes, all signatures present a peak in the backscattering direction, with sometimes an additional strong peak in the specular direction for wetlands. The data permit an assessment of the BRDF temporal evolution due to changes of surface state or Sun elevation, as well as a quantification of the BRDF variability within a land cover class. The maximum error level of the BRDF database is estimated to be of the order of 0.01 and 0.03 (in units of reflectance) in the red and near infrared, respectively. The BRDF database is available to the science community through the Internet. It should be helpful for the prototyping of various science applications, including the test of radiative transfer models and algorithmic schemes of corrections of angular effects on remote sensing data. As an example of application of the database, various semiempirical BRDF models published in the literature are tested and intercompared. Whereas all tested models catch reasonably well the overall shape of the BRDF, some differences appear between the red and the near infrared, between classes, and between models, which the use of the database permits to quantify.
Atmospheric sulfur cycle simulated in the global model GOCART: Comparison with field observations and regional budgets
Authors: Chin Mian , Savoie Dennis L. , Huebert Barry J. , Bandy Alan R. , Thornton Donald C. , Bates Timothy S. , Quinn Patricia K. , Saltzman Eric S. , De Bruyn Warren J.
Published: February 2004

We present a detailed evaluation of the atmospheric sulfur cycle simulated in the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model. The model simulations of SO2, sulfate, dimethylsulfide (DMS), and methanesulfonic acid (MSA) are compared with observations from different regions on various timescales. The model agrees within 30% with the regionally averaged sulfate concentrations measured over North America and Europe but overestimates the SO2 concentrations by more than a factor of 2 there. This suggests that either the emission rates are too high, or an additional loss of SO2 which does not lead to a significant sulfate production is needed. The average wintertime sulfate concentrations over Europe in the model are nearly a factor of 2 lower than measured values, a discrepancy which may be attributed largely to the sea‐salt sulfate collected in the data. The model reproduces the sulfur distributions observed over the oceans in both long‐term surface measurements and short‐term aircraft campaigns. Regional budget analyses show that sulfate production from SO2 oxidation is 2 to 3 times more efficient and the lifetimes of SO2 and sulfate are nearly a factor of 2 longer over the ocean than over the land. This is due to a larger free tropospheric fraction of SO2 column over the ocean than over the land, hence less loss to the surface. The North Atlantic and northwestern Pacific regions are heavily influenced by anthropogenic activities, with more than 60% of the total SO2 originating from anthropogenic sources. The average production efficiency of SO2 from DMS oxidation is estimated at 0.87 to 0.91 in most oceanic regions.
Photo‐induced isotopic fractionation
Authors: Miller Charles E. , Yung Yuk L.
Published: February 2004

This paper presents a systematic method for the analysis of photo‐induced isotopic fractionation. The physical basis for this fractionation mechanism centers on the fact that isotopic substitution alters the energy levels, molecular symmetries, spin statistical weights and other fundamental molecular properties, producing spectroscopic signatures distinguishable from that of the parent isotopomer. These mass‐dependent physical properties are identical to those invoked by Urey to explain stable isotope fractionation in chemical systems subject to thermodynamic equilibrium. Photo‐induced isotopic fractionation is a completely general phenomenon and should be observable in virtually all gas phase photochemical systems. Water photo‐induced isotopic fractionation has been examined in detail using experimental and theoretical data. These results illustrate the salient features of this fractionation mechanism for molecules possessing continuous UV absorption spectra and unit photodissociation quantum yields. Using the photo‐induced isotopic fractionation methodology in conjunction with standard photochemical models, we predict substantial deuterium enrichment of water vapor in the planetary atmospheres of Earth and Mars.
An efficient photolysis system for fast‐response NO2 measurements
Authors: Ryerson T. B. , Williams E. J. , Fehsenfeld F. C.
Published: February 2004

A new photolytic converter for NO2 measurement is described and its performance assessed using laboratory, ground‐, and aircraft‐based field data. Focusing the output of a 200‐W short‐arc Hg lamp into a photolytic cell attains conversion fractions of NO2 to NO greater than 0.70 in cell residence times of less than a second. Limiting lamp output to wavelengths greater than 350 nm by means of optical filters increases specificity for NO2, affording a peroxyacetyl nitrate conversion fraction of less than 0.006 and negligible conversion of HNO3. Unwanted (artifact) signal in clean synthetic air is also greatly minimized through the use of optical filters. Fast instrument response is achieved by minimizing NO2 inlet line and photolysis cell residence times. NO and NO2 sample residence times are matched in a multichannel instrument so that signal from ambient NO may be easily subtracted from the total signal and ambient NO2 calculated by difference at high time resolution. Induced change in the ambient ratio of NO to NO2, due to reaction of ozone and other oxidants with NO during sampling, is minimized in the new design. This configuration permits simple and accurate retrieval of NO2 concentrations in aircraft transects of power plant plumes, where ambient NO concentrations can change over several orders of magnitude in seconds. At lower concentrations found in the planetary boundary layer, agreement between calculated and observed NO2 is within ±(40 pptv+7%) for a 10‐s average. The new converter consumes less power, is more efficient, and is less expensive to operate than previous photolysis designs.
Excitation of the primary tropospheric chemical mode in a global three‐dimensional model
Authors: Wild Oliver , Prather Michael J.
Published: February 2004

Coupling of local chemical processes over the globe by atmospheric transport leads to the existence of chemical modes that are a fundamental characterization of global atmospheric chemistry and provide a true description of the atmospheric response to small changes in trace‐gas emissions. Such coupled chemistry‐transport modes in global tropospheric chemistry are an inherent feature of three‐dimensional chemical transport models (CTMs). In CTMs these modes cannot be solved for explicitly, as they have been for the case of low‐order, fully linearized systems, but they are investigated here through a series of perturbation experiments. When using meteorological fields that recycle every year, the long‐lived modes are readily seen as seasonal decay patterns that e‐fold each year. An important application of chemical modes is the study of how emissions of CO and NO excite perturbations to the CH4‐like mode, the longest‐lived (primary) mode found in tropospheric chemistry (i.e., with fixed stratospheric composition). Perturbation experiments are conducted with the University of California, Irvine, three‐dimensional tropospheric CTM to identify this primary tropospheric mode and to determine its five‐dimensional structure. The previous demonstrations of a long‐lived chemical mode with 1.5 times the lifetime of CH4 are corroborated. The ability of emissions of CO and NO to excite this mode is then demonstrated, and a quantitative evaluation of the indirect effect of CO emissions on the greenhouse gases CH4 and tropospheric O3 is made, showing that 100 kg of CO is equivalent to 5–6 kg of CH4 emissions.
Full year cycle of desert dust spectral optical thickness and precipitable water vapor over Alexandria, Egypt
Authors: Sabbah I. , Ichoku Charles , Kaufman Yoram J. , Remer Lorraine
Published: February 2004

We study the annual cycle of dust loading in Alexandria, Egypt. Observations were performed from December 1997 to November 1998, including during the Kamaseen storms of March 1998. A ground‐based, manual Sun photometer was used to measure aerosol optical thickness (τa) at four spectral bands in the 340–870 nm wavelength range (namely 340, 440, 675, 870). Total precipitable water vapor (W) was also measured simultaneously, based on the 936 nm channel measurements and extrapolations of the aerosol optical thickness from the neighboring 675 and 870 nm channels. Aerosol optical thickness τa at all the four spectral bands were very high (averaging 2.0–4.0) during the Kamaseen storms of late March 1998. Correspondingly, the Ångström wavelength exponent α of the optical thickness fell close to zero during that time, implying a substantial increase in dust. Overall, the monthly mean and median aerosol optical thickness were highest during January‐May and lowest in June‐October. During the January‐May period total precipitable water vapor and Ångström exponent were lower than during the June‐October period. There is a high correlation between the Ångström exponent and the optical thickness (r = 0.63), with a ranging from 0.0 to 0.5 for τa > 1.0, indicating high dust concentration. Trajectory analysis shows that the presence of dust was associated with air masses arriving predominantly from the Sahara or North Africa. No significant correlation was found between the optical thickness and the precipitable water vapor. These basic systematic observations are vital for assessing dust climatology in this important part of the world and also for validating satellite observations and dust transport models.
Simulated response of the marine atmospheric boundary layer in the western Pacific warm pool region to surface flux forcing
Authors: Levy Gad , Ek Michael
Published: February 2004

The atmospheric boundary layer (ABL) response to surface fluxes and prescribed advective and radiative forcings is tested in a column ABL model. Observations are used to run an ABL model in a marine tropical convective regime. The influence of surface fluxes and prescribed advective and radiative forcings potential temperature and specific humidity throughout the ABL is examined. The manner by which the different ABL processes interact with each other is investigated in light of recently demonstrated sensitivity of the simulated general circulation to flux parameterizations. Simulated near‐surface temperature and sensible heat fluxes are in close agreement with observations, with both the advection and the sensible heat flux contributing approximately equally to ABL warming. Simulated near‐surface humidity and humidity profiles are dryer than observed. The model exhibits very vigorous nonlocal ABL mixing that self‐regulates the flux response in its surface flux formulation by reducing humidity gradients and subsequent evaporation. The total time‐averaged simulated heat flux for two surface flux parameterizations tested is within the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Experiment (TOGA‐COARE) required accuracy. When decoupled from model response, the ABL model formulation overpredicts (underpredicts) latent heat fluxes modestly. ABL depth is dominated by nonlinear terms involving interaction between different surface fluxes and the ABL turbulent mixing and between these fluxes and advective and radiative forcing. Simulations in coarse vertical resolution typical of General Circulation Models (GCMs) result in significant ABL deepening and drying over observed values but also in better agreement between the time‐averaged simulated and observed surface heat fluxes.
Biogenic coastal aerosol production and its influence on aerosol radiative properties
Authors: Dowd Colin D. O.
Published: February 2004

Coastal zones have been shown to provide a massive source of new, tidal‐related, aerosol particles in the atmospheric boundary layer with concentrations exceeding 1,000,000 cm−3 during nucleation bursts sustained over many hours [O Dowd, 200O]. While coastal regions are very strong sources of natural aerosol particles, hithertofore, it has not been demonstrated that these particles contribute to aerosol‐related radiative flux. In this brief report, evidence is presented for growth of these new particles, following tidal‐related particle formation, into radiatively active particle sizes (i.e. radius>50 nm) where an increase in concentration of more than 300% can be seen. This increase of radiatively active particles concomitantly results in more than a threefold enhancement in both aerosol scattering ability (thereby influencing direct radiative flux) and cloud condensation nuclei (CCN) availability (thereby influencing indirect radiative flux). These results provide direct evidence for coastal biogenic emissions significantly enhancing the radiative flux potential of atmospheric aerosols.
Field determination of drag forces and shear stress partitioning effects for a desert shrub (Sarcobatus vermiculatus, greasewood)
Authors: Gillies J. A. , Lancaster N. , Nickling W. G. , Crawley D. M.
Published: February 2004

Drag coefficients (Cd) for the desert shrub greasewood (Sarcobatus vermiculatus) were developed from force versus wind speed data collected with an omnidirectional force balance. The average Cd for a small (0.6 m high, 0.5 m wide) shrub and a larger (1.6 m high, 1.3 m wide) shrub were 1.425 (±0.103) and 0.435 (±0.200), respectively. These values are much larger than similarly shaped solid elements and previously reported values for creosote bush (Larrea tridentata, Cd = 0.485) and an artificial tree (0.4). The greater Cd value for greasewood probably results from factors related to porosity and vegetation structure that gives this shrub‐type greater momentum extracting potential. The drag coefficients for the greasewood shrubs were found to show dependence upon flow Reynolds numbers >6×105, corresponding to wind speeds greater than 18 m s−1 at 10 m. The developed greasewood Cd values were used in a shear stress partitioning model that indicated they would be extremely effective at reducing wind‐generated sediment transport at low‐percent coverage.
Assessment of contemporary Arctic river runoff based on observational discharge records
Authors: Lammers Richard B. , Shiklomanov Alexander I. , Vörösmarty Charles J. , Fekete Balázs M. , Peterson Bruce J.
Published: February 2004

We describe the contemporary hydrography of the pan‐Arctic land area draining into the Arctic Ocean, northern Bering Sea, and Hudson Bay on the basis of observational records of river discharge and computed runoff. The Regional Arctic Hydrographic Network data set, R‐ArcticNET, is presented, which is based on 3754 recording stations drawn from Russian, Canadian, European, and U.S. archives. R‐ArcticNET represents the single largest data compendium of observed discharge in the Arctic. Approximately 73% of the nonglaciated area of the pan‐Arctic is monitored by at least one river discharge gage giving a mean gage density of 168 gages per 106 km2. Average annual runoff is 212 mm yr−1 with approximately 60% of the river discharge occurring from April to July. Gridded runoff surfaces are generated for the gaged portion of the pan‐Arctic region to investigate global change signals. Siberia and Alaska showed increases in winter runoff during the 1980s relative to the 1960s and 1970s during annual and seasonal periods. These changes are consistent with observations of change in the climatology of the region. Western Canada experienced decreased spring and summer runoff.
Sources and sinks of ozone in savanna and forest areas during EXPRESSO: Airborne turbulent flux measurements
Authors: Cros B. , Delon C. , Affre C. , Marion T. , Druilhet A. , Perros P. E. , Lopez A.
Published: February 2004

An airborne study of ozone concentrations and fluxes in the lower layers of the atmosphere was conducted over the Central African Republic (CAR) and northern Congo in November/December 1996, within the framework of the Experiment of Regional Sources and Sinks of Oxidants (EXPRESSO). The first 4 km of the atmosphere above savanna, rain forest, and the transitional area between them, were investigated with the French research aircraft Avion de Recherche Atmosphérique et de Télédétection (ARAT). Turbulent fluxes and deposition velocities of ozone were determined using the Eddy Correlation (EC) method. A specific methodology was developed to obtain accurate airborne turbulent flux measurements. This methodology is linked to the turbulence stationarity. The average values of ozone fluxes and ozone deposition velocities in the Atmospheric Boundary Layer (ABL) increase appreciably from savanna to forest. Near the ground, the ozone fluxes range between −0.115 +/−0.073 ppbv m/s above savanna and −0.350 +/−0.115 ppbv m/s above forest; for the deposition, the ranges are 0.0042 +/−0.0018 m/s and 0.015 +/−0.004 m/s. A simple empirical relationship between deposition velocity and Leaf Area Index (LAI) is proposed, giving an estimation of the deposition velocity for a whole latitudinal band. Vertical inputs of ozone to the ABL are estimated according to entrainment fluxes. The role of advection is neglected for horizontal transport of ozone in the ABL. The photochemical ozone production is deduced from the photo‐stationary state deviation, and compared to the net ozone increase in the ABL during the flights performed above the forest. A tentative ozone budget based on the aircraft measurements is proposed in the ABL of the rain forest. Around noon, the photochemical production dominates with a net production of about 10 ppbv/h.
Monte Carlo simulations of enhanced visible radiance in clear‐air satellite fields of view near clouds
Authors: Kobayashi Takahisa , Masuda Kazuhiko , Sasaki Masayuki , Mueller Johannes
Published: February 2004

Monte Carlo techniques are used to examine a potential error in the interpretation of satellite measurements for broken cloud fields. Solar radiation reflected from clouds and scattered by the atmosphere will enhance observed visible radiances, when clouds lie on the antisolar side of a cloud‐free satellite field of view (FOV). Results of the simulation show that the assumption of a plane‐parallel, cloud‐free atmosphere in atmospheric correction in a common retrieval algorithm introduces a bias in the inferred surface reflectivity and biological concentrations in the ocean from space. The biases arise because the radiation scattered from the adjacent cloud into the FOV is not correctly determined by calculations with a one‐dimensional radiative transfer model assuming a cloud‐free atmosphere. It is a function of the optical properties of the atmosphere, the distance of the adjacent cloud from fields of view, the solar zenith angle, etc. Optically thicker and closer clouds to the fields of view lead to a larger enhanced radiance. The bias is up to more than 10% at a nadir viewing angle for a cloud optical thickness of 9, a solar zenith angle of 30°, and a surface albedo of 0.1. The enhanced radiance overestimates retrieved surface albedo by only 0.01, which may be negligible for a study of the surface radiation budget. The bias is, however, significant in the estimation of biological pigment concentration in the ocean. Because water‐leaving radiance is only a small part of radiance detected at a spaceborne sensor, at most 10–20%, a small error in measured radiance causes a large error in retrieved pigment concentrations. Accurate atmospheric correction therefore is needed for the retrieval of phytoplankton concentrations from space. Assuming an improvement of satellite sensor technology in the near future, the issue shown in the present study will become significant because real clouds are often broken.
Sea‐salt aerosol chemistry in coastal areas: A model study
Authors: Moldanová Jana , Ljungström Evert
Published: February 2004

A trajectory model of Coastal Chemistry and Deposition of Sea‐Salt Particles (CCDSSP) has been developed. Release of reactive Cl and Br species takes place under a variety of conditions. The expulsion of HCl by strong acids was found to be the most important mechanism of Cl release. In a clean marine environment, Br is released mainly through a catalytic mechanism involving HOBr, BrCl, and Br2, as described in earlier model studies. This pathway is ineffective for chlorine release, and formation of HOCl and Cl2 leads to further activation of Br2. In environments rich in oxides of nitrogen, reactive uptake of N2O5, ClONO2, and BrONO2 leads to release of chlorine and bromine from the sea‐salt particles. The importance of these processes is enhanced in the winter half year, when reactive uptake of N2O5 causes approximately 40% of the total Cl release and BrONO2 becomes the dominant pathway of bromine release. Simulated peak winter concentrations of reactive halogen species were similar to those in summer simulations, leading to an increasing relative importance of halogens in VOC oxidation in wintertime, when Cl can account for 9% of the VOC oxidation. The model simulated sea‐salt deposition along the trajectory quite satisfactorily when compared to measurements. With increasing time of transport from the coast, the sea‐salt deposition becomes less important, while deposition of HCl keeps almost constant. The Cl− deficiency in the deposited, aged sea salt is thus fully compensated for, and an excess of Cl− is found because of HCl deposition. The excess Cl− is coupled with acidity which is not accounted for in deposition measurements.
Hypothesized climate forcing time series for the last 500 years
Authors: Robertson A. , Overpeck J. , Rind D. , Mosley‐Thompson E. , Zielinski G. , Lean J. , Koch D. , Penner J. , Tegen I. , Healy R.
Published: February 2004

A new compilation of annually resolved time series of atmospheric trace gas concentrations, solar irradiance, tropospheric aerosol optical depth, and stratospheric (volcanic) aerosol optical depth is presented for use in climate modeling studies of the period 1500 to 1999 A.D. Atmospheric CO2, CH4, and N2O concentrations over this period are well established on the basis of fossil air trapped in ice cores and instrumental measurements over the last few decades. Estimates of solar irradiance, ranging between 1364.2 and 1368.2 W/m2, are presented using calibrated historical observations of the Sun back to 1610, along with cosmogenic isotope variations extending back to 1500. Tropospheric aerosol distributions are calculated by scaling the modern distribution of sulfate and carbonaceous aerosol optical depths back to 1860 using reconstructed regional CO2 emissions; prior to 1860 the anthropogenic tropospheric aerosol optical depths are assumed to be zero. Finally, the first continuous, annually dated record of zonally averaged stratospheric (volcanic) optical depths back to 1500 is constructed using sulfate flux data from multiple ice cores from both Greenland and Antarctica, in conjunction with historical and instrumental (satellite and pyrheliometric) observations. The climate forcings generated here are currently being used as input to a suite of transient (time dependent) paleoclimate model simulations of the past 500 years. These forcings are also available for comparison with instrumental and proxy paleoclimate data of the same period.
Model dynamics of summertime low‐level jets over northwestern Mexico
Authors: Anderson Bruce T. , Roads John O. , Chen Shyh‐Chin , Juang Hann‐Ming H.
Published: February 2004

Using a fine‐scale nested regional modeling system, the diurnal forcing of summertime low‐level winds over the Gulf of California and northwestern Mexico is investigated. On diurnal timescales, simulated nocturnal low‐level jets develop over the northern portion of the Gulf, the foothills of the Sierra Madre Occidental, and parts of southern Arizona. The southerly component of the nocturnal jet is the result of a geostrophic balance involving the Coriolis force and a cross‐gulf pressure gradient force associated with nighttime slope cooling over the elevated Sierra Madre Occidental. Additionally, horizontal temperature gradients over the sloped orography produce vertical variations in this cross‐gulf pressure gradient force, generating the jet‐like vertical shear in wind components above the nocturnal boundary layer; frictional effects are responsible for producing shear in the wind profiles below the boundary layer. This balance is distinctly different from the inertial balance that is believed to be responsible for the low‐level jet over the Great Plains region of the United States. Daytime winds are part of a directly driven wind field forced by horizontal pressure gradients associated with slope heating (up the Sierra Madre Occidental) and sea‐land temperature gradients (north of the Gulf). During synoptically forced surge events, a similar diurnal cycle in low‐level flow is still present; however, the local thermal forcing appears to be superimposed upon the large‐scale synoptic forcing, resulting in weaker up‐slope flow during the day and stronger along‐slope flow at night.
Detailed analysis of the isotopic composition of CO and characterization of the air masses arriving at Mount Sonnblick (Austrian Alps)
Authors: Gros Valérie , Bräunlich Maya , Röckmann T. , Jöckel P. , Bergamaschi P. , Brenninkmeijer C. A. M. , Rom W. , Kutschera W. , Kaiser A. , Scheel H. E. , Mandl M. , van der Plicht J. , Possnert G.
Published: February 2004

Air sampling for analysis of CO and its isotopic composition (13C, 18O, and 14C) has been performed at the alpine station Sonnblick (47°N, 13°E, 3106 m above sea level) since September 1996. A high degree of variability is observed, which is due to the wide variation in the origin of air masses sampled. On the basis of the CO and isotope results, a classification of the different samples is performed. Other data such as 7Be, O3, relative humidity, and back trajectories are used to give additional information about the air mass origin. Background values, representative of the NH midlatitudes free troposphere, are observed about 50% of the time and are used to define seasonal cycles. CO and its isotopes show a minimum in summer and a maximum in winter with extreme values of 90 and 160 ppb for CO, −30 and −25‰ for δ13C, 0 and 8‰ for δ18O, and 8 and 20 molecules cm−3 STP for 14CO. CO and stable isotope data are compared with results from a three‐dimensional model (TM2). Generally good agreement supports the CO, δ13CO, and δC18O source/sink distributions inferred by the model. According to model calculations, fossil fuel combustion contributes 35% in summer and 50% in winter of total CO for such a midlatitude location. Other categories of sampled air are “subtropical,” “polluted,” and “stratospheric” and are observed 24%, 18%, and 4% of the time. Corresponding signatures of CO and its isotopic variations are presented, and some specific events are discussed.
Amplitude enhancements in Antarctic MF radar echoes
Authors: Murphy D. J. , Vincent R. A.
Published: February 2004

Enhancements in the amplitude of returns from a medium‐frequency (MF) radar at Davis, Antarctica, have been identified and their potential use as a measure of polar mesosphere summer echoes (PMSE) has been explored. A method for finding these enhancements has been applied to data spanning the period from mid‐1995 to the end of 1997. The character of these enhancements on short and long timescales has been studied, and factors that may affect their detection have been considered. It has been found that they are short‐lived (2 min or less being most common) and largely limited to the months around summer. Apart from describing the character of these amplitude enhancements, this study illustrates the potential pitfalls associated with identifying a proxy measure of PMSE.
Characterization of the optical properties of biomass burning aerosols in Zambia during the 1997 ZIBBEE field campaign
Authors: Eck T. F. , Holben B. N. , Ward D. E. , Dubovik O. , Reid J. S. , Smirnov A. , Mukelabai M. M. , Hsu N. C. , O'Neill N. T. , Slutsker I.
Published: February 2004

The physical and optical properties of biomass burning aerosols in a savanna region in south central Africa (Zambia) were analyzed from measurements made during the Zambian International Biomass Burning Emissions Experiment (ZIBBEE) during August‐September 1997. Due to the large spatial extent of African savannas and the high frequency of occurrence of burning in the annual dry seasons, characterization of the optical properties of the resultant biomass burning aerosols is important for the study of atmospheric radiative processes and for remote sensing of both surface and atmospheric properties in these regions. Aerosol Robotic Network Sun‐sky radiometer spectral measurements of direct Sun observations and directional sky radiances were utilized to infer spectral aerosol optical depths (τa), aerosol size distributions, and single‐scattering albedos. During the primary ZIBBEE study period, which coincided with the peak period of biomass burning in the region, there was a high correlation between the measured τaand the total column water vapor or precipitable water vapor (PWV), suggesting transport of smoke aerosol from regions with higher PWV. Size distribution retrievals of the biomass burning smoke show that the accumulation mode dominated and a comparison with smoke from Amazonia (Bolivia) shows a shift toward smaller particles for African savanna smoke. This may be the result of differences in mode of combustion (flaming versus smoldering), fuel type and moisture content, and the aging processes of the aerosol. The single‐scattering albedo (ω0) of the aerosols were retrieved using several approaches, yielding average values of ω0at ∼550 nm during ZIBBEE varying from ∼0.82 to ∼0.85, thus showing good agreement within the retrieval uncertainty of ∼0.03 of these methods. In general, ω0was relatively constant as a function of aerosol loading, with very little change occurring for τaat 440 nm ranging from 0.7 to 1.7. African savanna smoke exhibits significantly higher absorption than smoke from Amazonian forested regions and also a greater rate of decrease of ω0with increasing wavelength. Variations in the spectral change of the Angström wavelength exponent were also investigated with respect to the degree of aerosol absorption and changes in the accumulation mode size distributions.
On the relation between size and shape of desert dust aerosol
Authors: Koren Ilan , Ganor Eliezer , Joseph Joachim H.
Published: February 2004

Mathematical image analysis techniques are implemented in a novel manner to analyze aerosol particles in scanning electron microscope (SEM) data. By evaluating and removing the variable background in a digital SEM image using dynamic thresholding and then calculating the area and perimeter of each particle, one can study the relationship between particle area and particle shape. It is possible to distinguish in this manner between at least two different particle populations, separated by their fractal dimension, in a set of samples collected during 1 day in a heavy dust storm in Israel.
Modeling seasonal and interannual variability in ecosystem carbon cycling for the Brazilian Amazon region
Authors: Potter Christopher , Klooster Steven , de Carvalho Claudio Reis , Genovese Vanessa Brooks , Torregrosa Alicia , Dungan Jennifer , Bobo Matthew , Coughlan Joseph
Published: February 2004

Previous field measurements have implied that undisturbed Amazon forests may represent a substantial terrestrial sink for atmospheric carbon dioxide. We investigated this hypothesis using a regional ecosystem model for net primary production (NPP) and soil biogeochemical cycling. Seasonal and interannual controls on net ecosystem production (NEP) were studied with integration of high‐resolution (8‐km) multiyear satellite data to characterize Amazon land surface properties over time. Background analysis of temporal and spatial relationships between regional rainfall patterns and satellite observations (for vegetation land cover, fire counts, and smoke aerosol effects) reveals several notable patterns in the model driver data. Autocorrelation analysis for monthly vegetation “greenness” index (normalized difference vegetation index, NDVI) from the advanced very high resolution radiometer (AVHRR) and monthly rainfall indicates a significant lag time correlation of up to 12 months. At lag times approaching 36 months, autocorrelation function (ACF) values did not exceed the 95% confidence interval at locations west of about 47°W, which is near the transition zone of seasonal tropical forest and other (nonforest) vegetation types. Even at lag times of 12 months or less, the location near Manaus (approximately 60°W) represents the farthest western point in the Amazon region where seasonality of rainfall accounts significantly for monthly variations in forest phenology, as observed using NDVI. Comparisons of NDVI seasonal profiles in areas of the eastern Amazon widely affected by fires (as observed from satellite) suggest that our adjusted AVHRR‐NDVI captures year‐to‐year variation in land cover greenness with minimal interference from small fires and smoke aerosols. Ecosystem model results using this newly generated combination of regional forcing data from satellite suggest that undisturbed Amazon forests can be strong net sinks for atmospheric carbon dioxide, particularly during wet (non El Niño) years. However, drought effects during El Niño years can reduce NPP in primary forests of the eastern Amazon by 10–20%, compared to long‐term average estimates of regional productivity. Annual NEP for the region is predicted to range from −0.4 Pg C yr−1 (net CO2 source) to 0.5 Pg C yr−1 (net CO2 sink), with large interannual variability over the states of Pará, Maranhao, and Amazonas. As in the case of predicted NPP, it appears that periods of relatively high solar surface irradiance combined with several months of adequate rainfall are required to sustain the forest carbon sink for positive yearly NEP estimates.