bluuebunny/crossref_metadata_2025 huggingface dataset:
Composition of the troposphere over the Indian Ocean during the monsoonal transition
Authors: Rhoads Kevin P. , Kelley Paul , Dickerson Russell R. , Carsey Thomas P. , Farmer Michael , Savoie Dennis L. , Prospero Joseph M.
Published: February 2004

The atmosphere over the equatorial Indian Ocean is a unique environment in which to study the chemical and radiative effects of an intense source of anthropogenic emissions from the northern hemisphere directly coupled to the relatively pristine background conditions present in the southern hemisphere. As an initial investigation into the role of the intertropical convergence zone (ITCZ) on interhemispheric transport of pollutants, a number of trace atmospheric species were measured aboard the National Oceanic and Atmospheric Administration (NOAA) R/V Malcolm Baldrige between Durban, South Africa, and Colombo, Sri Lanka, from March 12 to April 22, 1995. Sharp increases in the concentrations of carbon monoxide (CO), carbon dioxide (CO2), and aerosols were associated with four distinct meteorological regimes transected by the cruise track from 33°S to 9°N. Across the ITCZ, aerosol concentrations, including non‐sea‐salt sulfate, nitrate and ammonium, increased by a factor of 4. Surface ozone measurements showed a latitudinal gradient with a minimum near the equator and a strong diurnal variation in the equatorial regions. The latitudinal profile of gas‐phase reactive nitrogen paralleled ozone and was higher in the remote southern hemisphere than in the remote northern hemisphere. Evidence of direct anthropogenic impact on the region was observed more than 1500 km from the southern tip of India. Back trajectories, calculated with NOAA's medium range forecast data using the Hybrid Single‐Particle Lagrangian Integrated Trajectory (HY‐SPLIT) program, identified the origin of the air mass regimes characterized by the trace gas and aerosol data. Continental emissions in the northern hemisphere were shown to have a major impact on the radiative properties and oxidizing capacity of the marine atmosphere.
Radiosonde boundary layer budgets above a boreal forest
Authors: Barr Alan G. , Betts Alan K.
Published: February 2004

Daytime surface sensible and latent heat fluxes were estimated above boreal forest from the boundary layer budgets of heat and water vapor. Two budgets were integrated from serial soundings in the atmospheric boundary layer: one for the mixed layer and one for the entrainment layer. Horizontal and vertical advection were derived from synoptic analyses. The surface fluxes were then calculated as a budget residual. The mean entrainment parameter AR above the boreal forest was estimated to be 0.21. The dependence of entrainment on buoyant and shear forcings agreed with earlier studies.
Monitoring fire activities in the boreal ecosystem
Authors: Li Zhanqing , Cihlar Josef , Moreau Louis , Huang Fengting , Lee Bryan
Published: February 2004

Forest fire is a major disturbance to the boreal ecosystem and may interact with climate change. Unfortunately, we have relatively little knowledge regarding fire activities in the boreal ecosystem. This study investigates the extent and dynamics of the forest fires occurred in and around the Boreal Ecosystem‐Atmosphere Study (BOREAS) region during summer 1994, an active fire season on record. The statistics of fire activities were obtained from advanced very high resolution radiometer (AVHRR) (aboard NOAA 11) data employing two satellite‐based remote sensing techniques that were designed particularly for monitoring boreal forest fires. Active fires and burned area are estimated using single‐day images and 10‐day clear composites. Such basic fire attributes as the area and period of burning extracted from the satellite data are compared against the ground reports made by the fire management agencies in Saskatchewan and Manitoba, Canada. Overall, there were 99 fires of a total burning area of approximately 2 million ha found over an area of 800×700 km2around the BOREAS study region in summer 1994. Agreement in the area of burning is good between the surface observations and satellite‐based estimation using single‐day images but poor using the composite data that suffer from various uncertainties. The majority (87%) of the ground‐reported fires were detected by satellite; the satellite also identified some fires missed by the ground observers. Most fires in 1994 occurred in the transitional forest to the north and northwest of the BOREAS region. Regarding to the monitoring of fire evolution, the daily satellite detection approach can be as effective as or even more effective than ground observations, provided that cloud cover does not occur persistently. The smoke of the fires had an impact on some BOREAS flux measurements.
Observations of aerosols in the free troposphere and marine boundary layer of the subtropical Northeast Atlantic: Discussion of processes determining their size distribution
Authors: Raes Frank , Van Dingenen Rita , Cuevas Emilio , Van Velthoven Peter F. J. , Prospero Joseph M.
Published: February 2004

During July 1994, submicron aerosol size distributions were measured at two sites on Tenerife, Canary Islands. One station was located in the free troposphere (FT), the other in the marine boundary layer (MBL). Transport toward these two sites was strongly decoupled: the FT was first affected by dust and sulfate‐laden air masses advecting from North Africa and later by clean air masses originating over the North Atlantic, whereas the MBL was always subject to the northeasterly trade wind circulation. In the FT the submicron aerosol distribution was predominantly monomodal with a geometric mean diameter of 120 nm and 55 nm during dusty and clean conditions, respectively. The relatively small diameter during the clean conditions indicates that the aerosol originated in the upper troposphere rather than over continental areas or in the lower stratosphere. During dusty conditions the physical and chemical properties of the submicron aerosol suggest that it has an anthropogenic origin over southern Europe and that it remains largely externally mixed with the supermicron mineral dust particles during its transport over North Africa to Tenerife. Apart from synoptic variations, a strong diurnal variation in the aerosol size distribution is observed at the FT site, characterized by a strong daytime mode of ultrafine particles. This is interpreted as being the result of photoinduced nucleation in the upslope winds, which are perturbed by anthropogenic and biogenic emissions on the island. No evidence was found for nucleation occurring in the undisturbed FT. The MBL site was not strongly affected by European pollution during the period of the measurements. The MBL aerosol size distribution was bimodal, but the relative concentration of Aitken and accumulation mode varied strongly. The accumulation mode can be related to cloud processing of the Aitken mode but also to pollution aerosol which was advected within the MBL or entrained from the FT. No bursts of nucleation were observed within the MBL.
The susceptibility of ice formation in upper tropospheric clouds to insoluble aerosol components
Authors: DeMott Paul J. , Rogers David C. , Kreidenweis Sonia M.
Published: February 2004

Ice may form by both homogeneous and heterogeneous freezing nucleation processes in clouds at temperatures below −35°C. Most investigations have focused on the former process. This paper presents results from adiabatic parcel model calculations that include the effects of both freezing processes in unactivated solution droplets. Uncertainties in predicting the homogeneous freezing rates are discussed and used to select solution drop composition and freezing characteristics that bracket those expected in the upper troposphere. The heterogeneous freezing rates of insoluble atmospheric aerosols are parameterized based on published freezing rates of carbonaceous particles. Process model simulations show that the potential variability in ice formation in cirrus clouds is much greater if heterogeneous freezing nucleation is considered in addition to homogeneous freezing. The impact of insoluble aerosols on ice formation is inferred to increase with insoluble particle size and with the fraction of soluble aerosols containing insoluble components. The maximum impact of heterogeneous nucleation is indicated for vertical motions less than 0.2 m s−1 and for insoluble components being associated with at least 10% of all soluble aerosols. The wide range of ice crystal concentrations observed in cirrus is most consistent with the occurrence of both heterogeneous and homogeneous ice formation processes. These conclusions are partially supported by existing observations of aerosols and cloud microphysical characteristics in upper tropospheric clouds but require new measurements for confirmation.
CO2and CH4flux between a boreal beaver pond and the atmosphere
Authors: Roulet Nigel T. , Crill P. M. , Comer N. T. , Dove A. , Boubonniere R. A.
Published: February 2004

The surface‐atmospheric exchange of CO2and CH4was measured continuously using the flux gradient approach from a beaver pond in the northern study area of the Boreal Ecosystem‐Atmosphere Study between May 22 and September 19, 1994. The beaver pond was a large source of CO2and CH4for the entire study period. The half‐hourly mean flux of CO2and CH4ranged from −0.498 to 1.135 mg CO2m−2s−1and from −0.805 to 37.5 μg CH4m−2s−1, respectively, while the seasonal mean fluxes were 0.072±0.095 mg CO2m−2s−1and 1.26±1.87 μg CH4m−2s−1. The beaver pond rarely took up CO2. There was a large flux of both gases during the daytime. This increase is related to the transfer of the gases rather than to specific controls on production. The total efflux of CO2and CH4for the 120 days of the study was 678 g CO2m−2and 11.3 g CH4m−2, or 183 and 8.4 g C m−2, respectively. When the measurements ceased, the sediment temperatures were >10°C, so it is reasonable to expect that the fluxes of CO2and CH4continued into the late fall. This indicates that the beaver pond released more than 200 g C m−2yr−1.
An airborne study of air quality around the Hong Kong Territory
Authors: Kok Gregory L. , Lind John A. , Fang Ming
Published: February 2004

An airborne study of air quality was made around the Hong Kong Territory during October and November of 1994. The air quality measurements included NO, NOy, O3, CO, SO2, condensation nuclei, black carbon, aerosol light scattering, and aerosol particle size and number. Complex wind patterns are present, mixing the emissions from Hong Kong with aged pollution transported into the region from the People's Republic of China. This leads to poor air quality on the west and north sides of the Hong Kong Territory. CO mixing ratios were 200–800 parts per billion by volume (ppbv), and black carbon was present in the range of 5–7 μg/m3. Ozone mixing ratios of 100–120 ppbv were typical. The CO/NOy ratio in the heavily polluted region was 10, similar to the CO/NOy emission ratio for pollution emitted from the People's Republic of China immediately upwind of Hong Kong.
Experimental line strengths of the ClO fundamental
Authors: Birk Manfred , Wagner Georg
Published: February 2004

Previously published band strengths of the ClO fundamental (800–880 cm−1) show large uncertainties and inconsistencies, mainly caused by the problem of accurate particle density determination of the instable radical ClO. A new procedure has been elaborated, utilizing additional far‐infrared (FIR) measurements of pure rotational transitions. ClO was prepared in a flow reaction of dichlorine monoxide with atomic chlorine and measured in a 72 m path White‐type absorption cell. A series of subsequent FIR, midinfrared (MIR) and FIR spectra was recorded with a Bruker IFS 120 HR Fourier transform spectrometer within the same experiment. From the FIR transitions and their calculated line strengths the particle density could be determined. Drifts of the ClO concentration during the flow experiment were monitored by the analysis of both FIR spectra bracketing the MIR measurement. The line strengths of 72 transitions in the 2II3/2 state were evaluated allowing the determination of the vibrational transition dipole moment (−0.03224(53) D) as well as the linear (+0.00684(80)) and quadratic (+1.56(64)×10−4) Herman‐Wallis coefficients (1σ uncertainties given).
Soil‐atmosphere fluxes of carbon monoxide during early stages of postfire succession in upland Canadian boreal forests
Authors: Zepp Richard G. , Miller William L. , Tarr Matthew A. , Burke Roger A. , Stocks Brian J.
Published: February 2004

Soil‐atmosphere fluxes of carbon monoxide (CO) were investigated during BOREAS 1994 (June to September 1994) in forest sites near the northern study area (NSA) of the Boreal Ecosystem‐Atmosphere Study (BOREAS). Fluxes and related ancillary data were measured for both upland black spruce (located on poorly drained clay‐textured soils) and jack pine sites (well‐drained sandy soils) that were in early stages of succession following stand replacement fires that occurred within 7 years of BOREAS 1994. Nearby control stands that had not burned in the past 80 years were studied for comparison. Net fluxes measured by using transparent closed chambers were generally positive at the warmer, sunlit burn sites but negative (sink activity) in the shaded, cooler control sites. Carbon monoxide uptake in controls, which was first order with respect to CO concentration, was little affected by covering the sampling chambers to exclude light. Median deposition velocities calculated from the uptake fluxes were 0.015 cm s−1at the black spruce control site and 0.0085 cm s−1at the jack pine control site, at the lower end of the range of values observed by others in tropical and temperate ecosystems. Daytime CO fluxes at the burn sites were generally positive (1011‐1012molecules cm−2s−1) and were lowered when solar irradiance was excluded from the chambers by covering or when cloudiness or smoke reduced the light intensity. Net fluxes at the burn sites were controlled by competition between abiotic production, mainly at the surface, and by oxidation deeper in the soil. Abiotic production, which was attributable to photoproduction and thermal decomposition of the surface organic layer and charcoal, strongly correlated with incident solar irradiance, and thus the greatest fluxes were observed during midday. Results of these studies indicate that the locally dependent changes in boreal fire return intervals that are linked to global climate change represent an important biospheric/physical feedback that is likely to alter the biosphere‐atmosphere exchange of CO.
Thermal band selection for the PRISM instrument: 2. Analysis and comparison of existing atmospheric and emissivity correction methods for land surface temperature recovery
Authors: Caselles V. , Coll C. , Valor E. , Rubio E.
Published: February 2004

This is the second paper in a series which discusses the thermal band selection for the Processes Research by Imaging Space Mission (PRISM) planned by the European Space Agency (ESA). In part 1, algorithms for emissivity‐temperature decoupling were analyzed with the aim of identifying optimum methods and attainable accuracies for extracting emissivity values in the PRISM thermal bands. In this paper, we have focused on studying the existing methods for land surface temperature (LST) recovery. The preliminary configuration of the PRISM instrument includes four thermal channels: A (∼3.7 μm ), B (∼8.8 μm ), C (∼10.8 μm ), and D (∼11.9 μm) and along‐track multipointing capability. Single‐channel, split‐window, and dual‐angle algorithms have been tested and compared using a database of simulated brightness temperatures. Although focused on PRISM, the study is relevant to present instruments with similar features, such as the advanced very high resolution radiometer (AVHRR) and the along‐track scanning radiometer (ATSR). In particular, we have shown that (1) atmospheric effects dominate errors in single‐channel LST recovery methods, (2) spectral emissivity dominates errors in the dual‐angle and split‐window methods, and (3) current AVHRR split‐window channel combination (channels C and D) provide the best results. It does not need auxiliary atmospheric information and therefore it can work in an operational scheme using only satellite data. It can be applied to any type of surface, and accuracies around ±1 K can be obtained for most natural surfaces. For the single‐channel method the use of coincident radiosoundings is necessary, because of the high sensitivity to the atmospheric variability. Dual‐angle methods are constrained to homogeneous surfaces, since in heterogeneous, rough surfaces, the temperatures measured at the two observation angles may present directional effects caused by the surface structure and the differences between vegetation and background temperatures. We have also shown that the utility of channel B to LST recovery is marginal. Combinations including channel A are only applicable in nighttime conditions owing to reflected radiation. For these reasons the split‐window algorithm, using channels within the 10–12.5 μm window, has been suggested for operational determination of LST.
Interaction of extratropical Rossby waves with westerly quasi‐biennial oscillation winds
Authors: O'Sullivan Donal
Published: February 2004

The role of laterally radiating stratospheric Rossby waves from the winter hemisphere in the momentum balance of the quasi‐biennial oscillation (QBO) remains an unresolved issue. Observations of extratropical Rossby waves penetrating close to the equator suggest that Rossby wave absorption in the tropics must occur at least occasionally. This paper presents numerical simulations using a global shallow water model to examine the pattern of wave absorption at low latitude during the westerly wind phase of the QBO. Earlier results indicating that extratropical Rossby waves radiating to the equator do not break there when the QBO phase is westerly are reproduced. It is shown that the westerly QBO jet can persist, undiminished at its core, over seasonal length integrations, because the laterally radiating Rossby waves are absorbed along the QBO jet's flanks, where easterly acceleration occurs. The net effect of the Rossby waves on the westerly QBO jet is a reduction in the width of its meridional profile but not its core strength. This behavior is interpreted by considering the filtering of equatorward radiating Rossby waves by critical layer absorption, a process that depends on the background zonal mean wind and the waves' phase speed and amplitude. The simulations also show that adequate horizontal resolution is needed to capture the suppression of wave breaking at the QBO jet core and persistence of the jet core strength over seasonal timescales. The implications of this result for simulations of the QBO in general circulation models are considered.
Evaluation of model‐simulated upper troposphere humidity using 6.7 μm satellite observations
Authors: Spangenberg Douglas A. , Mace Gerald G. , Ackerman Thomas P. , Seaman Nelson L. , Soden Brian J.
Published: February 2004

Use of mesoscale models to simulate details of upper tropospheric relative humidity (UTRH) fields represents an important step toward understanding the evolution of small‐scale water vapor structures that are responsible for cirrus growth and dissipation. Because mesoscale model UTRH simulations require initialization and verification and since radiosonde measurements of relative humidity are unreliable in the upper troposphere, we use GOES 6.7 μm water vapor observations to validate the Pennsylvania State University/National Center for Atmospheric Research nonhydrostatic mesoscale model (MM5) simulations of UTRH. To accomplish this task, MM5 temperature and moisture profiles are used in a forward calculation of the clear‐sky 6.7 μm brightness temperature (T6.7), which is converted into UTRH. A statistical analysis is done to evaluate MM5 simulations of T6.7 and UTRH against the GOES 7 observations. For the simulations, an average correlation coefficient of 0.80 was found with a dry bias of 1.6 K. In terms of UTRH, the average correlation coefficient was 0.65 with a dry bias of 3.3%. We also found that MM5 fails to simulate accurately extrema in the UTRH field.
Atmospheric nuclei and related aerosol fields over the Atlantic: Clean subsiding air and continental pollution during ASTEX
Authors: Clarke A. D. , Uehara T. , Porter J. N.
Published: February 2004

Meteorological conditions in the Atlantic Stratocumulus Transition Experiment (ASTEX) region favored advection of clean air from the central Atlantic during the early part of the experiment that was replaced by polluted air of European origin during the latter part of the experiment. Marked differences in the aerosol size distribution, composition, and state of mixing existed in these air masses. Pronounced differences in their vertical structure also demonstrated that surface measurements often do not represent average boundary layer or column concentrations. Clean subsiding air from the free troposphere had concentrations of condensation nuclei that significantly exceeded concentrations in the boundary layer and had very low mass concentrations and volatility consistent with homogeneous nucleation aloft, supporting the hypothesis that these nuclei can provide a source for “new” nuclei into the marine surface layer. This finding was in contrast to polluted air characterized by high concentrations of aged aerosol but having no evidence for significant recent nuclei formation. Particles in polluted air consisted of more than 90% volatile mass (mostly sulfate) and a refractory residual remaining at 300°C. The refractory mass varied with the concurrently measured light absorption coefficient, associated with combustion‐derived soot. In spite of 2 orders of magnitude more volatile sulfate in polluted air, most of the particle number in polluted air remained after volatilization at 300°C, in contrast to less than 10% in clean air. This finding suggests that the number of particles in polluted air may reflect the combustion process producing the primary soot aerosol more than the SO2 emissions responsible for much of the sulfate aerosol mass. The accumulation mode mass mean diameter was found to increase with total accumulation mode mass, suggesting that a size parameterization based upon the more commonly measured sulfate mass could be included in current aerosol models.
A temporal interpolation method to obtain hourly atmospheric surface pressure tides in Reanalysis 1979–1995
Authors: van den Dool H. M. , Saha S. , Schemm J. , Huang J.
Published: February 2004

The diurnal cycle in climatology as revealed in National Centers for Environmental Prediction's Reanalysis 1979–1995 has been studied for global gridded data at 0000, 0600, 1200 and 1800 UT. Climatologies have been prepared for each of the four time levels separately. There are substantial differences in, for instance, the 00002md 0600 UT climatologies, owing to a diurnal cycle and/or what is referred to commonly as atmospheric tides. The semidiurnal tide is quite strong in the mass fields, but with sampling every 6 hours at the Nyquist frequency, some aspects cannot be studied properly. However, a method of interpolation based on spatial harmonic waves moving at an empirically determined speed can be called upon to make an educated guess about the atmospheric tides at any times in between. This interpolation technique is similar to the one published by Van den Dool and Qin [1996], but the wave speeds are not like the slow Rossby modes but roughly 15° westward per hour. We present the global tides in surface pressure thus obtained for every hour of the day for January.
Sensitivity of sulphur dioxide oxidation in sea salt to nitric acid and ammonia gas phase concentrations
Authors: Clegg Nicola A. , Toumi Ralf
Published: February 2004

The role of sea‐salt aerosol in the sulphur dioxide budget is investigated using a steady state box model. The model simulates, for certain species, the condensation onto, and chemical reactions within, deliquescent sea‐salt particles. The chemical reactions are controlled by the pH of the solution, and results show that the presence of gas‐phase nitric acid, HNO3, and ammonia, NH3, can have a significant effect on calculated concentrations of non‐sea‐salt (nss) sulphate, [nss‐SO42−], formed within the solution. For example, in the case of high [HNO3(g)] (100 pptv) the calculated [nss‐S042−] can be decreased by ∼40% after 2 days (with a 75% decrease at HNO3 concentrations of 1 ppbv). When NH3 is added, a decrease in calculated [nss‐SO42−] of 20% is observed for 500 pptv NH3, 50 pptv HNO3. Thus nitric acid is more effective than ammonia. In conditions appropriate for the marine boundary layer (50 pptv of SO2, and HNO3, and 200 pptv of NH3), nitric acid is still the governing factor, and the [nss‐SO42−], after 2 days, is decreased by approximately 25% compared to the concentration formed when conditions show no HNO3 or NH3. Results also show that comparisons with nss‐SO42− observations are nearly 1 order of magnitude lower, with observations giving a [nss‐SO42−]:Na+ ratio of 0.01–0.04 compared to predicted values of 0.005 or lower. Other mechanisms of nss‐sulphate production may have to be used to explain all the nss‐sulphate found in sea‐salt aerosol. Although in cases where [SO2(g)] is 10–100 pptv the decrease in [nss‐SO42−] is 15–30%, at values of [SO2(g)] > 100 pptv the effects of both nitric acid and ammonia on calculated [nss‐SO42−] are small. In a future scenario of increasing NOx (HNO3) emissions and decreasing SO2 emissions the role of HNO3 on the global sulphur budget could be very significant.
POAM II ozone observations in the Antarctic ozone hole in 1994, 1995, and 1996
Authors: Bevilacqua R. M. , Aellig C. P. , Debrestian D. J. , Fromm M. D. , Hoppel K. , Lumpe J. D. , Shettle E. P. , Hornstein J. S. , Randall C. E. , Rusch D. W. , Rosenfield J. E.
Published: February 2004

We present an overview of Polar Ozone and Aerosol Measurement (POAM) II satellite‐based observations of ozone in the Antarctic ozone hole in 1994, 1995, and 1996. The POAM II observations are consistent with previous observations suggesting that ozone loss in the ozone hole is confined to the polar vortex. Ozone concentrations are observed to decrease by nearly a factor of 10 near 20 km during the ozone hole formation period, and a reduction in ozone was observed up to 24 km. The timing of ozone loss and recovery was similar in each year. Ozone concentrations begin to decrease in July, and the period of largest depletion observed by POAM II occurs between early September and early October, when the observations are obtained at high southern latitudes (82°–88°S) near the vortex center. However, ozone concentrations were consistently lower (by about 10%) in 1996, throughout the ozone hole altitude region and time period, than in the other two years. We have also used the POAM II observations to compute vertical profiles of monthly averaged ozone photochemical loss rates as a function of potential temperature in August (450–800 K) and September (450–700 K) of each year, incorporating a correction for diabatic descent. We find that the ozone loss rates are not significantly different from zero in August 1994 at any potential temperature level. However, we do find significant chemical loss in August 1995 below 600 K, and in August 1996 at all levels up to 700 K. Maximum monthly averaged ozone chemical loss rates occurred in September near 500 K in each year (1994: 0.1±0.004 parts per million by volume per day (ppmv/d); 1995 and 1996: 0.08±0.004 ppmv/d). Generally, in September, loss rates were larger in 1994 than in 1995 and 1996 below 550 K, and above 550 K the largest loss rates occurred in 1996. We find significant chemical loss up to at least 700 K in September in all three years. Finally, the POAM II observations show that in late spring, after the ozone hole chemical processing has been completed, ozone mixing ratios are lower inside the Antarctic vortex (relative to outside the vortex) at all levels between at least 450 K and 1500 K, presumably resulting from a combination of dynamical and chemical effects.
An efficient method for parametric uncertainty analysis of numerical geophysical models
Authors: Tatang Menner A. , Pan Wenwei , Prinn Ronald G. , McRae Gregory J.
Published: February 2004

A new method for parametric uncertainty analysis of numerical geophysical models is presented. It approximates model response surfaces, which are functions of model input parameters, using orthogonal polynomials, whose weighting functions are the probabilistic density functions (PDFs) of the input uncertain parameters. This approach has been applied to the uncertainty analysis of an analytical model of the direct radiative forcing by anthropogenic sulfate aerosols which has nine uncertain parameters. This method is shown to generate PDFs of the radiative forcing which are very similar to the exact analytical PDF. Compared with the Monte Carlo method for this problem, the new method is a factor of 25 to 60 times faster, depending on the error tolerance, and exhibits an exponential decrease of error with increasing order of the approximation.
Heterogeneous BrONO2 hydrolysis: Effect on NO2 columns and ozone at high latitudes in summer
Authors: Randeniya L. K. , Vohralik P. F. , Plumb I. C. , Ryan K. R.
Published: February 2004

The heterogeneous reaction, N2O5 + H2Oaerosol → → 2HNO3, is responsible for increasing [HOx] and repartitioning of active NOx into HNO3. Throughout much of the atmosphere, N2O5 is formed predominantly at night owing to the rapid photolysis of its precursor, NO3, in sunlit hours. Laboratory measurements have shown that BrONO2 + H2Oaerosol → HOBr + HNO3 (reaction (3)) also has the potential to cause repartitioning of ozone‐depleting species, although better determination of γ, the reaction probability, is still required for some stratospheric conditions. The diurnal behavior of N2O5 and BrONO2 are entirely different. In contrast to N2O5, BrONO2 is formed predominantly during the daytime. The result of (3) is to increase [HOBr] at the expense of [BrONO2]. Photolysis of HOBr then leads to increased [OH] and increased O3 loss. In this work two‐dimensional calculations show clearly that the impact of (3) is greatest for high aerosol levels and for high latitudes in summer. The calculations have been used to determine the effects of increased aerosol loading on calculated NO2 columns in the Antarctic during summer and autumn of 1990, 1991, 1992 and 1993. It is shown that (3) could be responsible for reductions in NO2 columns during polar day comparable to those measured in 1992 and 1993 following the eruption of Mount Pinatubo. Reaction (3) results in only marginal changes to ozone catalytic loss cycles in 1990. However, for the high aerosol levels of 1992, the inclusion of this reaction results in up to 50% higher ozone loss rates in the 12 to 20 km range. This is caused predominantly by a large increase in [HOx] tempered by a reduction in loss due to NOx. Calculations in which transport terms were switched off showed that, between 12 and 20 km at 77.5°N, local chemistry removes about 30% of the ozone between April and September compared with 20% when the effects of (3) are not included.
Extinction efficiency and single‐scattering albedo for laboratory and natural cirrus clouds
Authors: Yang Ping , Liou K. N. , Arnott W. P.
Published: February 2004

A combination of the finite‐difference time domain technique and a ray‐by‐ray integration method has been applied to compute the extinction efficiency and single‐scattering albedo for various size distributions associated with nonspherical ice crystals in laboratory and natural cirrus clouds. The two methods are applicable to small and large size parameters, respectively. The results obtained by the two methods converge when effective size parameters are larger than about 6. For laboratory ice crystals the overall features of the computed extinction efficiency are in general agreement with those determined from measurements. In particular, significant extinction windows at 2.85 and 10.5 μm, associated with the Christiansen effect, are observed in both theoretical and experimental results. These extinction minima appear because the real part of the refractive index approaches unity, so that absorption dominates light attenuation. The single‐scattering albedos at the two Christiansen spectral regions are found to be smaller than 0.5 for the laboratory ice crystals. The contours of extinction efficiency and single‐scattering albedo versus wavelength and particle size show that the magnitude of the Christiansen effect is dependent on particle size. For large ice crystals, the extinction windows are not significant because the extinction efficiency converges to its asymptotic value of 2, regardless of size parameters. For a number of size distributions observed during FIRE II IFO, the Christiansen effect is small. However, for cold cirrus, the extinction efficiencies in the Christiansen bands are approximately one half of the values at nearby wavelengths due to a significant number of small ice crystals that are present in cold cirrus clouds. It is concluded that the Christiansen effect must be accounted for in the determination of the extinction efficiency and the single‐scattering albedo for small ice particles in order to obtain a reliable optical depth and emissivity for cirrus clouds at infrared wavelengths. Finally, we show that using spherical particles with Mie theory is inadequate to explain the extinction measurements.
Association between ionospheric maximum electron density and lower stratospheric temperature
Authors: de Adler Nieves Ortiz , Elías Ana G. , Manzano Jose R.
Published: February 2004

A correlation has been found between the peak electron density of the ionospheric F2 layer, Nmax, and the temperature of the lower stratosphere. The electron density anomaly, NmaxA, and the 12‐month running mean of the lower stratospheric temperature anomaly (LSTA) are correlated. NmaxA is the residual of the linear regression of Nmax on the sunspot number, Rz; and LSTA is the difference between the lower stratospheric temperature and its mean value. Monthly median values at 1200 LT of Nmax measured at Yamagawa, Kodaikanal, Okinawa, and Manila ionospheric stations are considered, covering the period 1979–1994. The lower stratospheric temperature corresponds to monthly mean values of the period January 1979 to December 1993. Since solar EUV and UV radiation affect the F2 layer ionization and the stratospheric heating respectively, the part of the EUV and UV flux time variation that is not dependent on Rz has also been analyzed. This has been done using the photoelectron current (Ipe) measured with a Langmuir Probe on the Pioneer Venus Orbiter as indicator of the EUV flux, and the Mg II core‐to‐wing ratio as indicator of the UV flux. The temporal variations of Nmax independent of Rz are found to be similar to LSTA variations. The temporal variations, not dependent on Rz, of the solar EUV and UV energy output, are also similar.
Tropospheric chemical ozone tendencies in CO‐CH4‐NOy‐H2O system: Their sensitivity to variations in environmental parameters and their application to a global chemistry transport model study
Authors: Klonecki A. , Levy H.
Published: February 2004

A photochemical box model with CO‐CH4‐NCyH2O chemistry is used to calculate the diurnally averaged net photochemical rate of change of ozone (hereinafter called the chemical ozone tendency) in the troposphere for different values of parameters: NOx. and ozone concentration, temperature, humidity, CO concentration, and surface albedo. To understand the dependency of the chemical ozone tendency on the input parameters, a detailed sensitivity study is performed. Subsequently, the expected variations of the ozone tendencies with altitude, latitude, and season are analyzed. The magnitude of the tendency decreases rapidly with height mostly as a result of lower absolute humidity and temperature. In the upper troposphere (at 190 mbar) the maximum tendencies are below 2 parts per billion by volume/day. Lower temperature and specific humidity cause a shift of the value of NOx. at which the ozone production balances the destruction of ozone (balance point) to lower NOx values; these two parameters are also, to a large extent, responsible for lower magnitudes of the tendency at higher latitudes and in winter. In the upper troposphere we find that the net tendency is at least as sensitive to variations in H2O concentration as to NOx. This suggests a possible synergism between direct NOx pollution by aircraft and the indirect modification of H2O by climate change. In the second part of the paper the box model calculated rates are used as ozone's chemical tendency terms during a simulation conducted with the three‐dimensional global chemistry transport model (GCTM). The box model is used to calculate the tendencies as a function of NOx and ozone at all tropospheric levels of the GCTM, at nine latitudes and for four seasons using zonally and monthly averaged data: water vapor and temperature from observations and model CO. These tables together with the NOx fields obtained in an earlier GCTM simulation are used in the GCTM simulation of O3 if nonmethane hydrocarbon levels are low. The global monthly averaged chemical ozone tendency fields saved during the simulation are presented and analyzed for the present‐day and preindustrial conditions. The chemical tendency fields show a strong correlation with the NOx fields. In contrast with the lower and middle troposphere where the tendencies are negative in remote regions over the oceans, in the upper troposphere, where NOx is generally greater than 50 parts per trillion by volume and the balance point is low, the tendencies are generally small but positive. The GCTM simulations of the preindustrial ozone show that in the upper troposphere the present‐day ozone tendencies are greater than the simulated preindustrial tendencies. In the boundary layer and in the midtroposphere the present‐day tendencies are greater near anthropogenic NOx. sources and smaller (generally more negative), due to higher ozone levels, in regions not affected by these sources.
Prediction of cloud droplet number in a general circulation model
Authors: Ghan Steven J. , Leung L. Ruby , Easter Richard C. , Abdul‐Razzak Hayder
Published: February 2004

A predictive treatment of droplet number is applied to both a single‐column cloud model and a global circulation model. Droplet number is predicted from the droplet number balance, which accounts for droplet nucleation, mixing, and droplet loss due to autoconversion of droplets to rain and collection by rain, ice, and snow. Droplet nucleation is parameterized in terms of the parameters of a lognormal aerosol size distribution and in terms of a Gaussian probability distribution of vertical velocity within each grid cell. The predicted droplet number is found to be significantly less than observations unless vertical resolution provides at least 10 levels within the planetary boundary layer. When droplet number is simply diagnosed from the number nucleated, droplet concentrations are found to be consistently greater than with the predictive treatment. Predicted droplet number concentrations are found to be nonlinearly related to aerosol number concentration.
Real‐time kinetics of the uptake of HOBr and BrONO2 on ice and in the presence of HCl in the temperature range 190–200 K
Authors: Allanic A. , Oppliger R. , Rossi M. J.
Published: February 2004

The uptake kinetics of the title compounds on ice with and without HCl has been measured in a Teflon‐coated low‐pressure flow reactor (Knudsen cell) at temperatures of 190–200 K. At 190 K, values of the uptake coefficient for BrONO2 are of the order of 0.25±0.09 at limiting low doses and low flow rates of 1014 molecules per pulse and 1014 molecules s−1, respectively. The reaction leads to the formation of HOBr and also to the secondary product Br2O resulting from the reaction between BrONO2 and HOBr. The reaction of BrONO2 on HCl‐doped ice was studied under pulsed and continuous flow conditions. It appears that the formation of both BrCl and HOBr occurs promptly. In contrast to ClONO2 on ice, the reaction of BrONO2 with H2O does not involve the formation of a precursor releasing hypobromous acid. HOBr itself is found to interact with ice: at 190 and 200 K, values of the uptake coefficient are 0.27±0.04 and 0.15±0.01 at a residence time of 0.3 s, respectively. These values are higher than the ones previously published. The rate of interaction of HOBr on HCl‐doped ice is enhanced with respect to the one on pure ice, and BrCl is observed as a product. Possible atmospheric implications of this work are discussed.
Spatial and temporal variation of sublimation on Antarctica: Results of a high‐resolution general circulation model
Authors: van den Broeke Michiel R.
Published: February 2004

In this paper we use output of a high‐resolution general circulation model (ECHAM‐3 T106, resolution 1.1°×1.1°) to study the spatial and temporal variation of sublimation on Antarctica. First, we compare model results with available observations of sublimation rates. The yearly cycle, with small latent heat fluxes during the winter, is well reproduced, and the agreement with sparsely available spot observations is fair. The model results suggest that a significant 10–15% of the annual precipitation over Antarctica is lost through sublimation and that sublimation plays an important role in the formation of blue ice areas. A preliminary analysis of the atmospheric boundary layer moisture budget shows that the spatial variation of sublimation in the coastal zone of East Antarctica can be explained by variations of horizontal advection of dry air. Dry air advection, and thus surface sublimation, is enhanced in areas where katabatic winds are strong and have a large downslope component and where the Antarctic topography drops suddenly from the plateau to the coastal zone. In areas where horizontal advection is small, like the plateau and the large ice shelves, special conditions must be met to make significant sublimation at the surface possible.
Effect of black carbon and sulfate aerosols on the Global Radiation Budget
Authors: Schult Ingrid , Feichter Johann , Cooke William F.
Published: February 2004

Global fields of radiative forcing due to natural and anthropogenic sulfate aerosols, black carbon aerosols, and an external mixture of the two have been calculated with a one‐dimensional radiative transfer model developed for estimates of the direct radiative forcing by aerosols. Estimates of solar radiative forcing by different aerosols are presented for January and July, based on calculated three‐dimensional, global distributions of sulfate and black carbon mass. We show that the radiative forcing by sulfate is negative, as already known, while the forcing due to black carbon aerosols is mainly positive. Considering both black carbon and sulfate together and assuming an external mixture, we calculate a globally averaged radiative forcing of approximately −0.2 W/m2, with a quite nonuniform geographical distribution. The radiative forcing due to aerosols is highly dependent upon the optical properties of the aerosol, while the surface reflectance and the Sun angle influence the direction of the forcing. Our results show that the presence of black carbon, the main absorbing component of anthropogenic aerosol, may reduce the cooling effect of aerosol, thus leading to an increase in the greenhouse warming.
The effect of uncertainties in kinetic and photochemical data on model predictions of stratospheric ozone depletion
Authors: Fish D. J. , Burton M. R.
Published: February 2004

During the past few years, three‐dimensional chemical transport models have matured to a point at which their results are often in good agreement with observations, lending confidence to model estimates of ozone loss. In this paper a Lagrangian photochemical model is used to assess the error in model predictions that results from errors in the kinetic and photochemical data used by the model. The results indicate that Arctic ozone destruction can be modeled with an uncertainty of ±25% (1 sigma) and that midlatitude ozone destruction can be modeled with an uncertainty of ±50%. It is also shown that two reactions, from a total of more than 100, are responsible for more than a third of the uncertainty in model calculations of Arctic ozone loss.
Comparisons of field measurements of carbon dioxide and nitrous oxide fluxes with model simulations for a legume pasture in southeast Australia
Authors: Wang Y.‐P. , Meyer C. P. , Galbally I. E. , Smith C. J.
Published: February 2004

We measured the fluxes of carbon dioxide (CC2) and nitrous oxide (N2O) and relevant environmental variables for two treatments in a legume pasture system on an acid soil in southeast Australia during 1993. The two treatments were control and liming (to reduce soil acidity). These trace gas fluxes were also simulated using a revised version of the process‐based model DNDC. Our version of the DNDC (Denitrification and Decomposition) model has been significantly modified by including a surface energy balance submodel; by using more comprehensive formulations for soil evaporation, plant transpiration, plant growth, and plant nitrogen uptake; by using an implicit difference scheme to solve the diffusion equations governing heat and water fluxes in the soil; and by initializing the soil organic carbon in different pools using their relative proportions at the steady state. The simulated average nighttime CO2 fluxes from the control plot were 0.07, 0.05, and 0.16 g C m−2 h−1 in March, August, and October of 1993, respectively, as compared with the measured average nighttime CO2 fluxes of 0.10, 0.07, and 0.14 g C m−2 h−1 for the corresponding periods. The simulated average daily N2O fluxes from the control plot were 0.09, 0.03, and 0.04 mg N m−2 d−1 in March, August, and October of 1993, respectively, as compared with the measured average daily N2O fluxes of 0.07, 0.03, and 0.16 mg N m−2 d−1 for the corresponding periods. Similar agreements between model simulations of CO2 and N2O fluxes and measurements were also obtained for the limed plot. We find that the simulations by model DNDC and field observations of gaseous N2O emissions agree well over a range of 3 orders of magnitude. We conclude that the daily and seasonal variations of CO2 and N2O fluxes on a plot scale can be reasonably simulated by this process‐based model. The model shows that the fraction of N2O produced aerobically from nitrification was 73% for the control plot and 55% for the limed plots in 1993; therefore mitigation strategies for reducing N2O emission from such temperate or semiarid nitrogen‐limited systems in Australia should be focused on the N2O loss from nitrification.
Experimental evidence of a general description for raindrop size distribution properties
Authors: Sempere‐Torres Daniel , Porrà Josep M. , Creutin Jean‐Dominique
Published: February 2004

A body of previous theoretical and experimental studies suggests that there is a general formulation for raindrop size distribution properties able to describe them in a wide variety of rain conditions. The formulation basically relates the evolution of the drop populations to a rain bulk variable, such as the rain rate. This paper proposes a simple methodology to carry out testing in order to seek experimental evidence of such a description. Four data sets from diverse devices, locations, and climates are used in an event‐by‐event analysis. The results show good general agreement between data and theory in regard to of the defined criteria: linear increase in the exponents of bulk variables relationships and self‐consistency of the fitting procedure, leading to an experimental confirmation of the general description and its implicit hypothesis. The question of the number of parameters and the shape of the function needed to identify drop size distribution are discussed at different levels of the study. Dimensional considerations about the general expression are given in the appendix.