bluuebunny/crossref_metadata_2025 huggingface dataset:
The feasibility of monitoring CO2 from high‐resolution infrared sounders
Authors: Chédin A. , Saunders R. , Hollingsworth A. , Scott N. , Matricardi M. , Etcheto J. , Clerbaux C. , Armante R. , Crevoisier C.
Published: May 2003

Satellite instruments specifically designed to monitor atmospheric carbon dioxide concentrations have not been flown to date but, high‐resolution infrared sounders, being launched in the next few years, may offer the possibility of at least a basic carbon dioxide monitoring capability. This paper explores the sensitivity of this new generation of advanced infrared sounders to changing carbon dioxide concentrations and also compares this with uncertainties due to the atmospheric temperature, water vapor, and minor constituent concentrations using the current background errors in numerical weather prediction models as a baseline. The sensitivity results shown are computed for the Infrared Atmospheric Sounding Interferometer (IASI), which is due to fly on the European METOP platform from 2005. We show that although the carbon dioxide signal is below or at the instrument noise for IASI and that uncertainties in temperature and water vapor errors can dominate, a careful averaging of the retrieved carbon dioxide fields over areas of 500 × 500 km2 and 2 weeks should be able to extract changes at the level of 1% or less in the total column carbon dioxide amount. We also show results of an information content study for the Atmospheric InfraRed Sounder, AIRS, data, which suggests 50 channels are adequate for inferring the tropospheric carbon dioxide amounts but that they are not sensitive to CO2 changes in the boundary layer.
Latitudinal, vertical, and seasonal variations of C1‐C4 alkyl nitrates in the troposphere over the Pacific Ocean during PEM‐Tropics A and B: Oceanic and continental sources
Authors: Blake Nicola J. , Blake Donald R. , Swanson Aaron L. , Atlas Elliot , Flocke Frank , Rowland F. Sherwood
Published: May 2003

We present concentration distributions of C1‐C4 alkyl nitrates observed during the NASA airborne campaigns Pacific Exploratory Mission (PEM) ‐Tropics A (September–October 1996) and PEM‐Tropics B (March–April 1999). The total geographic range for PEM‐Tropics A was 45°N–72°S latitude and 153°E–75°W longitude, and for PEM‐Tropics B was 40°N–36°S latitude and 149°E–75°W longitude. The maximum altitude for these missions was 12 km. These experiments provide the most extensive set of tropospheric measurements collected to date over the tropical Pacific Ocean. We observed high methyl nitrate (MeONO2, CH3ONO2) mixing ratios (approximately 50 pptv) at low altitudes in a latitude band between 8°N to 13°S stretching across the equatorial Pacific, illustrating the oceanic source of MeONO2. This source may be associated with the high‐nutrient, low‐chlorophyll character of equatorial Pacific waters. We discuss MeONO2 and ethyl nitrate (EtONO2, C2H5ONO2), whose abundance is dominated by equatorial oceanic sources, 2‐Propyl nitrate (2‐PrONO2, 2‐C3H7ONO2), which has significant oceanic and northern hemispheric (NH) sources associated with urban/industrial hydrocarbon emissions, and 2‐butyl nitrate (2‐BuONO2 2‐C4H8ONO2), which has mostly NH sources. PEM‐Tropics A and B resulted in remarkably similar equatorial mixing ratios. The excellent correlations between MeONO2 and the other alkyl nitrates in this region produced comparable correlation slopes between the two expeditions. By contrast, NH air masses influenced by urban/industrial emissions typically exhibited much lower MeONO2:EtONO2, MeONO2:2‐PrONO2, and MeONO2:2‐BuONO2 ratios. These relationships can be useful as a diagnostic of air mass origin. North of 10°N, the springtime PEM‐Tropics B mixing ratios of C2‐C4 alkyl nitrates were many‐fold higher at low‐mid altitudes than for late summer PEM‐Tropics A, consistent with strong continental outflow of NMHC precursors during spring.
Mesoscale spatial variation of rainfall through a hidden semi‐Markov model of breakpoint data
Authors: Sansom John , Thompson Craig S.
Published: May 2003

Spatially, rainfall is very variable, and a common problem is that for a particular point of interest no observations are available. So a spatial model is required to provide suitable estimates at such points. The problem is further compounded by the many questions that can be asked regarding rainfall, with each question generally needing a particular spatial model for a particular data type. The approach here is through a temporal model of high temporal resolution rain rate data such that the spatial variability of rainfall is then, in part, that of the parameters of the model. Other aspects entailing contemporary differences in rainfall from point to point are not dealt with; only those based on long‐term point statistics are considered. The breakpoint format of rainfall data, which records the rain rate and the times when it changes, implicitly provides high temporal resolution rainfall information in a highly compressed form. A Markov model was chosen so that its states could be aligned with the different physical processes that occur in the atmosphere and are associated with rainfall. However, the data consist only of rain rates and durations with no labels indicating the prevailing process for each datum; thus the states in the model are “hidden.” The model was fitted to data from 20 locations in central New Zealand, and its parameters were then mapped using a thin‐plate smoothing spline and verified through simulations of artificial breakpoint data sets and the subsequent extraction of a few statistics for which independent spatial variations were available. Thus, through similar simulations at any arbitrary point a set of model parameters is available, and many rainfall questions can be answered. The model, being physically based, also provides some insights into rainfall processes.
Long‐range transport of continental plumes over the Pacific Basin: Aerosol physiochemistry and optical properties during PEM‐Tropics A and B
Authors: Moore K. G. , Clarke A. D. , Kapustin V. N. , Howell S. G.
Published: May 2003

The Pacific Exploratory Mission Tropics (PEMT) A (1996) and B (1999) field campaigns occurred over a large area of the Pacific Basin and revealed the presence of “rivers” of continental outflow propagating into the remote marine atmosphere that were also supported by remote sensing and modeling efforts. Airborne measurements of both the coarse and fine mode aerosol during these campaigns provided assessment of the spatial variability in aerosol parameters (including optical properties and degree of internal versus external mixing) in continental plumes encountered over the Pacific Ocean. Large perturbations to the “pristine” marine atmosphere were observed. Most plumes were encountered in the Southern Hemisphere during PEMT A, while the opposite was observed during PEMT B. A variety of anthropogenic and natural sources for these continental plumes are suggested by the data, including biomass burning, urban/industrial emissions, and in the case of Asian outflow, dust storms. Aerosol size distributions (particularly for the refractory component) varied from one plume to another and most combustion‐derived aerosol appeared to be an internal mix of a refractory soot‐like constituent in a volatile matrix. Within the sampled plumes, size‐resolved volatility suggested that this volatile matrix was relatively well neutralized, implying the presence of ammonia in the particle phase. The radiatively important single scatter albedo (ω) obtained from measured “dry” scattering and absorption coefficients ranged from approximately 0.88 (pollution with no coarse particles) to 0.94 (pollution and dust) in the free troposphere (FT) to 0.98 (pollution and sea salt) within the marine boundary layer (MBL). Vertical profiles often revealed more concentrated plumes aloft, typically situated in dry air with ambient relative humidity (RH) <40%, and much lower values of ω than in the underlying MBL.
Overview of the 1999 Atlanta Supersite Project
Authors: Solomon Paul A. , Chameides William , Weber Rodney , Middlebrook Ann , Kiang C. S. , Russell Armistead G. , Butler Andre , Turpin Barbara , Mikel Dennis , Scheffe Richard , Cowling Ellis , Edgerton Eric , St. John James , Jansen John , McMurry Peter , Hering Susanne , Bahadori Tina
Published: May 2003

This paper presents an overview of the 1999 Atlanta Supersite Project coordinated through the Southern Oxidants Study and Georgia Institute of Technology (GIT) and funded by the U.S. Environmental Protection Agency (EPA) along with other sponsors who provided in‐kind support primarily through existing studies. The Atlanta Supersite Project was located at the existing Southeastern Aerosol Research Characterization Study (SEARCH)/Aerosol Research Inhalation Epidemiology Study (ARIES) site on Jefferson Street in NW Atlanta, Georgia. The primary objective of the Atlanta Supersite Project was to evaluate and compare advanced measurement methods for particulate matter mass and its components. Methods included filter‐ and denuder‐based time‐integrated or discrete samplers, a variety of semicontinuous methods measuring mass, its major components (sulfate, nitrate, ammonium, organic carbon, elemental carbon, trace elements) and gas‐phase precursors, and for the first time ever, a comparison among particle mass spectrometers; four in total. These data were complemented by meteorological data as well as gas‐phase criteria pollutant measurements and other supplemental data such as particle physical properties, volatile organic compounds (VOC), oxygenated VOC, and NOy. The primary and supplemental data also were used to better understand the formation and accumulation of particulate matter in Atlanta and to better understand source‐receptor relationships. This paper overviews the study, summarizing objectives, the site and measurements, and the relative reference data used for comparisons, and it overviews the meteorological and chemical characteristics of pollution in Atlanta during the study, puts the study in context of Atlanta and the southeast United States, and finally summarizes the key findings from the over 30 publications published, submitted, or in preparation. This paper also provides as complete a list as is currently available of those publications. Others certainly will be emerging over time. The comprehensive database is available through the Atlanta Supersite Project Web site sponsored by GIT (http://www-wlc.eas.gatech.edu/supersite/).
The seasonal evolution of NMHCs and light alkyl nitrates at middle to high northern latitudes during TOPSE
Authors: Blake Nicola J. , Blake Donald R. , Sive Barkley C. , Katzenstein Aaron S. , Meinardi Simone , Wingenter Oliver W. , Atlas Elliot L. , Flocke Frank , Ridley Brian A. , Rowland F. Sherwood
Published: May 2003

The Tropospheric Ozone Production about the Spring Equinox (TOPSE) experiment was designed to follow the role of photochemistry in the evolution of the springtime maximum of tropospheric ozone (O3) in the Northern Hemisphere (NH) at high latitudes. Determination of the composition and seasonal evolution of volatile organic carbon (VOC) species, which take part in and are good indicators for photochemical processes in the troposphere, was an important part of this study. We report measurements of a large number of C2–C10 nonmethane hydrocarbons (NMHCs), selected C1–C2 halocarbons, and C1–C4 alkyl nitrates. These gases were quantified in whole air samples collected aboard the National Center for Atmospheric Research (NCAR) C‐130 aircraft at altitudes between 30 m and 8 km. Seven TOPSE sampling trips were flown between early February and mid‐May 2000 covering the region from Colorado (40°N) to Churchill (in Manitoba, Canada), Thule (in northern Greenland), and as far north as 85°N. These measurements represent the most comprehensive spatial characterization of the North American Arctic to date and revealed strong latitudinal, vertical, and temporal NMHC gradients. In the midtroposphere north of Churchill (58°N), ΣNMHCs decreased by ∼6.2 ppbC between February and May (1.6 ppbC month−1) and the magnitude of this change diminished with altitude. Over the same period, midtropospheric O3 levels increased by ∼16 ppbv (4.2 ppbv month−1). Free tropospheric NMHC decreases were consistent with removal by hydroxyl (OH) radicals at an average mixing ratio for mid‐March to mid‐May of 4.1 × 105 mol cm−3. The alkyl nitrates, which are a reservoir species for tropospheric reactive odd nitrogen (NOY), revealed similar latitudinal, vertical, and temporal gradients to their parent NMHCs. Their total decreased by ∼4 pptv month−1, representing 10% or less of NOY. In conjunction with meteorological trajectory analysis, different trace gas signatures provided significant clues to the origins of individual polluted air masses. Several of these air masses were rapidly advected over the Pole from source regions in northeastern and western Europe as well as an air mass that originated over the southwestern United States/Baja California that contained unusually high levels of alkanes. In addition, episodes of low boundary layer (BL) O3 associated with low NMHC mixing ratios and trajectories from over the Arctic Ocean were frequently sampled toward the latter part of the experiment. The TOPSE data described here provide a unique picture of NH trace gas evolution from winter to summer that will be invaluable to models investigating the role that anthropogenic emissions play in high latitude O3 chemistry.
An assessment of ozone photochemistry in the central/eastern North Pacific as determined from multiyear airborne field studies
Authors: DiNunno B. , Davis D. , Chen G. , Crawford J. , Olson J. , Liu S.
Published: May 2003

In this study, the photochemistry of ozone is examined for the central/eastern North Pacific (CENP), a region bounded by 180°–120°W and 0°–45°N. Measurements of ozone precursors have been made in this region during 10 previous airborne studies covering the last two decades. The two seasons for which the most extensive analysis has been possible are spring (March–May) and fall (September–November). Box model results have been displayed in the form of latitudinal/altitudinal plots for ozone formation, destruction, and net tendency. They indicate that for both seasons of the year, significant net destruction is found for altitudes in the 0–4 km range; whereas, at higher altitudes, the ozone tendency calculations lead to small values of net production. For both seasons, however, the total column integrated ozone trend is one of net destruction. The largest difference between seasons was in the value for ozone destruction. During springtime, this value was significantly larger than for fall. This trend coincides with higher average springtime ozone levels. The trends in CENP ozone were also compared to those for the western Pacific using measurements recorded during PEM‐West A (fall 1991) and PEM‐West B (spring 1994). While the PEM‐West A results revealed a neutral ozone column tendency, those from PEM‐West B showed net production at all altitudes. Both results are in sharp contrast to those reported here for the CENP region showing net destruction. This suggests that while lightning and surface emissions of NOx from the northwestern Pacific Rim have a strong influence on the ozone tendency in the surrounding region, due to the combined effects of dispersion and chemical loss most of this NOx does not reach the CENP region. Hence, ozone in the central Pacific is typically destroyed by photochemistry.
Rainfall downscaling in an alpine watershed applying a multiresolution approach
Authors: Ahrens B.
Published: May 2003

Distributed hydrological modeling in alpine watersheds needs high‐resolution (∼1 km in space, 1 h in time) rainfall input. Spatial resolution of operational numerical weather forecast or regional climate models is coarser by at least one order of magnitude. Stochastic downscaling of meteorological model output is the appropriate way to bridge this scale gap. This study examines radar data in an orographically complex area in the southern European Alps by multiplicative multiresolution decomposition. Multiresolution rainfall fluctuations exhibit simple scaling characteristics. This result motivates the implementation of a spatial downscaling model, an example of an anisotropic microcanonical multiplicative random cascade, that is applied and evaluated. Necessity and skill of downscaling is illustrated by hydrological modeling of four heavy precipitation events in an Alpine watershed (total area of 2627 km2) with the grid‐based model WaSiM‐ETH. Downscaling of rainfall fields with 16 km grid spacing to fields with 1 km grid spacing improves the simulated event‐mean runoff by 5–10% and the peak runoff by up to 20%.
Ozone depletion events observed in the high latitude surface layer during the TOPSE aircraft program
Authors: Ridley B. A. , Atlas E. L. , Montzka D. D. , Browell E. V. , Cantrell C. A. , Blake D. R. , Blake N. J. , Cinquini L. , Coffey M. T. , Emmons L. K. , Cohen R. C. , DeYoung R. J. , Dibb J. E. , Eisele F. L. , Flocke F. M. , Fried A. , Grahek F. E. , Grant W. B. , Hair J. W. , Hannigan J. W. , Heikes B. J. , Lefer B. L. , Mauldin R. L. , Moody J. L. , Shetter R. E. , Snow J. A. , Talbot R. W. , Thornton J. A. , Walega J. G. , Weinheimer A. J. , Wert B. P. , Wimmers A. J.
Published: May 2003

During the Tropospheric Ozone Production about the Spring Equinox (TOPSE) aircraft program, ozone depletion events (ODEs) in the high latitude surface layer were investigated using lidar and in situ instruments. Flight legs of 100 km or longer distance were flown 32 times at 30 m altitude over a variety of regions north of 58° between early February and late May 2000. ODEs were found on each flight over the Arctic Ocean but their occurrence was rare at more southern latitudes. However, large area events with depletion to over 2 km altitude in one case were found as far south as Baffin Bay and Hudson Bay and as late as 22 May. There is good evidence that these more southern events did not form in situ but were the result of export of ozone‐depleted air from the surface layer of the Arctic Ocean. Surprisingly, relatively intact transport of ODEs occurred over distances of 900–2000 km and in some cases over rough terrain. Accumulation of constituents in the frozen surface over the dark winter period cannot be a strong prerequisite of ozone depletion since latitudes south of the Arctic Ocean would also experience a long dark period. Some process unique to the Arctic Ocean surface or its coastal regions remains unidentified for the release of ozone‐depleting halogens. There was no correspondence between coarse surface features such as solid ice/snow, open leads, or polynyas with the occurrence of or intensity of ozone depletion over the Arctic or subarctic regions. Depletion events also occurred in the absence of long‐range transport of relatively fresh “pollution” within the high latitude surface layer, at least in spring 2000. Direct measurements of halogen radicals were not made. However, the flights do provide detailed information on the vertical structure of the surface layer and, during the constant 30 m altitude legs, measurements of a variety of constituents including hydroxyl and peroxy radicals. A summary of the behavior of these constituents is made. The measurements were consistent with a source of formaldehyde from the snow/ice surface. Median NOx in the surface layer was 15 pptv or less, suggesting that surface emissions were substantially converted to reservoir constituents by 30 m altitude and that ozone production rates were small (0.15–1.5 ppbv/d) at this altitude. Peroxyacetylnitrate (PAN) was by far the major constituent of NOy in the surface layer independent of the ozone mixing ratio.
Effect of sulfate aerosol on tropospheric NOx and ozone budgets: Model simulations and TOPSE evidence
Authors: Tie Xuexi , Emmons Louisa , Horowitz Larry , Brasseur Guy , Ridley Brian , Atlas Elliot , Stround Craig , Hess Peter , Klonecki Andrzej , Madronich Sasha , Talbot Robert , Dibb Jack
Published: May 2003

The distributions of NOx and O3 are analyzed during TOPSE (Tropospheric Ozone Production about the Spring Equinox). In this study these data are compared with the calculations of a global chemical/transport model (Model for OZone And Related chemical Tracers (MOZART)). Specifically, the effect that hydrolysis of N2O5 on sulfate aerosols has on tropospheric NOx and O3 budgets is studied. The results show that without this heterogeneous reaction, the model significantly overestimates NOx concentrations at high latitudes of the Northern Hemisphere (NH) in winter and spring in comparison to the observations during TOPSE; with this reaction, modeled NOx concentrations are close to the measured values. This comparison provides evidence that the hydrolysis of N2O5 on sulfate aerosol plays an important role in controlling the tropospheric NOx and O3 budgets. The calculated reduction of NOx attributed to this reaction is 80 to 90% in winter at high latitudes over North America. Because of the reduction of NOx, O3 concentrations are also decreased. The maximum O3 reduction occurs in spring although the maximum NOx reduction occurs in winter when photochemical O3 production is relatively low. The uncertainties related to uptake coefficient and aerosol loading in the model is analyzed. The analysis indicates that the changes in NOx due to these uncertainties are much smaller than the impact of hydrolysis of N2O5 on sulfate aerosol. The effect that hydrolysis of N2O5 on global NOx and O3 budgets are also assessed by the model. The results suggest that in the Northern Hemisphere, the average NOx budget decreases 50% due to this reaction in winter and 5% in summer. The average O3 budget is reduced by 8% in winter and 6% in summer. In the Southern Hemisphere (SH), the sulfate aerosol loading is significantly smaller than in the Northern Hemisphere. As a result, sulfate aerosol has little impact on NOx and O3 budgets of the Southern Hemisphere.
Short‐ to medium‐range superensemble precipitation forecasts using satellite products: 1. Deterministic forecasting
Authors: Shin D. W. , Krishnamurti T. N.
Published: May 2003

As a comprehensive extension of the previous multimodel/multianalysis superensemble (SE) studies of rainfall forecasts, the benefits and prospects of the SE precipitation forecasts are explored using satellite products. Three different precipitation ensemble configurations are first established from a great number of numerical experiments. These configurations are multianalysis (MA), multicumulus‐scheme (MC), and multimodel (MM) configurations. A set of MA ensemble comes from the use of several different satellite‐derived rain rates through the physical initialization procedure within the Florida State University Global Spectral Model (FSUGSM) system. Six different state‐of‐the art cumulus parameterization schemes are incorporated into the FSUGSM in order to introduce the MC ensemble configuration. The MM configuration is composed of an FSU control forecast and those provided by five operational numerical weather prediction centers. In addition to the original technique, a possible deterministic SE enhancement technique (regression dynamic linear model) is then proposed and applied to the above three configurations of ensemble members as well as all of them together. The impact of a higher resolution family of models on the performance of SE forecasts is extensively investigated by repeating the above procedure with T170 resolution precipitation forecasts. Results show that short‐ to medium‐range SE forecasts are generally superior in skill to various conventional forecasts. A notably improved (∼20%) quantitative precipitation forecast is exhibited by the newly proposed SE technique. The MM configuration proved to be the most effective ensemble prediction system. Although a higher‐resolution SE forecast requires a large amount of computing time, it turns out that the impact is significant not only in skill scores but also in resolving mesoscale‐based convective disturbances.
Short‐ to medium‐range superensemble precipitation forecasts using satellite products: 2. Probabilistic forecasting
Authors: Shin D. W. , Krishnamurti T. N.
Published: May 2003

Short‐ to medium‐range probabilistic precipitation forecasts over the global tropics are explored using satellite products, from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) and Special Sensor Microwave/Imager (SSM/I) instruments. In addition to the conventional probability of precipitation (POP) forecast, superensemble (SE) POP forecasts are introduced and applied to the multianalysis, multicumulus‐scheme, and multimodel ensemble configurations in two different horizontal resolution forecasts. It is shown that an ensemble system using a single model has a more consistent bias, which can at least partially be removed by a simple bias correction. With the aid of properly prepared ensemble members, meaningful POP forecasts have much longer forecast lead times. Results also show that a family of higher‐resolution forecasts has a greater ability in removing model biases. The advantage of the SE approach is found to be evident in making POP forecasts, compared to the conventional method. The skills of SE POP are 10 to 20 percent better than those of the bias‐corrected.
Atmospheric controls on soil moisture‐boundary layer interactions: Three‐dimensional wind effects
Authors: Findell Kirsten L. , Eltahir Elfatih A. B.
Published: May 2003

This paper expands the one‐dimensionally based CTP‐HIlow framework for describing atmospheric controls on soil moisture‐boundary layer interactions [Findell and Eltahir, 2003] to three dimensions by including low‐level wind effects in the analysis. The framework is based on two measures of atmospheric thermodynamic properties: the convective triggering potential (CTP), a measure of the temperature lapse rate between approximately 1 and 3 km above the ground surface, and a low‐level humidity index, HIlow. These two measures are used to distinguish between three types of early morning soundings: those favoring rainfall over dry soils, those favoring rainfall over wet soils, and those whose convective potential is unaffected by the partitioning of fluxes at the surface. The focus of this paper is the additional information gained by incorporating information about low‐level winds into the CTP‐HIlow framework. Three‐dimensional simulations using MM5 and an analysis of observations from the FIFE experiment within this framework highlight the importance of the winds in determining the sensitivity of convection to fluxes from the land surface. A very important impact of the 3D winds is the potential for low‐level backing or unidirectional winds with great shear to suppress convective potential. Because of this suppression of convection in certain wind conditions, far fewer simulations produced rain than would be anticipated based solely on the 1D framework of understanding. However, when the winds allowed, convection occurred in a manner consistent with the 1D‐based expectations. Generally speaking, in the regime where dry soils were expected to have an advantage, convection was triggered over dry soils more often than over wet; in the regime where wet soils were expected to have an advantage, convection was more frequently triggered over wet soils than over dry. Additionally, when rainfall occurred in both simulations with wet soils and simulations with dry soils for a given day, rainfall depths were typically greater in the simulations with wet soils. Similarly, the FIFE data showed numerous days with convective potential but no rainfall: each of these days had low‐level backing or strongly shearing winds. Four days with high humidity deficits and veering winds in the lowest 300 mbar did have rain, highlighting the enhanced buoyancy effects of low‐level veering winds.
Nitric acid emission from the F100 jet engine
Authors: Miller T. M. , Ballenthin J. O. , Hunton D. E. , Viggiano A. A. , Wey C. C. , Anderson B. E.
Published: May 2003

A chemical ionization mass spectrometer (CIMS) from the Air Force Research Laboratory at Hanscom AFB was used to measure levels of HNO3 in the exhaust stream of the F100 jet engine both in a test cell at NASA Glenn Research Center (GRC) at simulated altitudes 3–17 km and in flight behind F‐16 aircraft at 9–11 km altitude. The HNO3 concentrations have been expressed as emission indices (EI) by normalizing to engine‐produced CO2 measured by GRC instruments in the test cell and NASA‐Langley Research Center instruments in flight. The test cell data showed that EI(HNO3) is independent of fuel sulfur level, and is highly dependent upon altitude for altitudes at which most commercial aircraft fly (below 12 km), mainly because the emission is a strong function of combustor inlet temperature, which is lower at high altitudes. EI(HNO3) at military power was 0.003 g NO2 kg−1 fuel at low altitude (∼3 km), and increased to 0.02 g kg−1 for altitudes above 11 km. Operating the engine at 85% maximum turbine speed increased the figures above by a factor of about 2.5. At engine idle, EI(HNO3) was typically 0.14 g kg−1. The decrease in EI(HNO3) with combustor inlet temperature is attributed to the temperature dependence of the NO2 + OH reaction producing HNO3. Comparing HNO3 emission indices with those measured at GRC for NO and NOx allowed us to determine the fraction of NO2 which is converted into HNO3 in the combustion process as a function of inlet temperature. Conversion of NO2 to HNO3 is 0.9% at military power for altitudes ≥11 km, and twice that at 85% maximum turbine speed. The conversion fraction changes dramatically in the 8–11 km altitude range, and is only 0.02% at 3 km altitude, for military power, or 0.2% at 85% maximum turbine speed. At engine idle, this fraction was about 8% at all altitudes. The accuracy of the (average) figures quoted above is ±25%. The in situ data, obtained on 10 flights during the NASA Subsonic Near‐Interaction Field (SNIF) experiment, are much less definitive but are consistent with the test cell results.
Long‐term record of nss‐sulfate and nitrate in aerosols on Midway Island, 1981–2000: Evidence of increased (now decreasing?) anthropogenic emissions from Asia
Authors: Prospero Joseph M. , Savoie Dennis L. , Arimoto Richard
Published: May 2003

Increasing anthropogenic emissions from Asia, especially from regions undergoing rapid industrial development, have raised interest in the outflow of chemically and radiatively important gases and aerosols. Previous studies have shown that high concentrations of Asian pollution spread over a broad region of the North Pacific every spring. Here we report on studies of aerosol concentrations at Midway Island (28°13′N, 177°22′W) in the central North Pacific over the period 1981–2000. Using a relatively simple procedure we estimate the natural and anthropogenic fractions of sulfate and nitrate aerosol and show that the estimated anthropogenic component almost doubled from 1981 to the mid‐1990s. This increase closely parallels estimates of increased emissions of SO2 from China. However, measurements in the late 1990s suggest that sulfate and nitrate concentrations have stabilized and perhaps decreased. Thus over the longer term pollution emissions from Asia and concentrations over the North Pacific may be less than earlier projections, a factor which has implications for the assessment of future climate trends.
Point and column aerosol radiative closure during ACE 1: Effects of particle shape and size
Authors: Fridlind A. M. , Jacobson M. Z.
Published: May 2003

We used data collected during the First Aerosol Characterization Experiment (ACE 1) to study point and column aerosol radiative closure over the remote ocean. To test point closure, total and hemispheric backscattering coefficients calculated with a Mie single‐scattering model were compared with measurements made by ship and aircraft at three wavelengths (400, 550, and 700 nm). On the ship, assuming spherical particles, calculated total scattering was usually within 10% of measurements (closure obtained in >80% of the cases) but calculated backscattering was usually 15–25% lower than measurements (closure obtained in <50% of the cases). When a model for particle nonsphericity was applied to the dried sea spray, assuming the particles to be ideal cubes or irregular convex and concave crystals resulted in overestimation of backscattering. However, when nonsphericity parameters were fit to the measurements, calculated backscattering was also usually within 10% of measurements (closure obtained in >80% of the cases). On the aircraft, however, calculated scattering and backscattering were usually lower than measurements by 20–45% regardless of assumed particle shape (closure obtained in <50% of the cases), likely owing to differences in the aerosol inlet penetration efficiencies to each instrument or unidentified uncertainties in the measured number size distributions or scattering coefficients. To test column closure, aerosol extinction profiles calculated from in situ observations (below 5.5 km) and satellite observations (above 5.5 km) were vertically integrated, and the resulting aerosol optical depth was compared with measurements made on the ship during two clear‐sky days at three wavelengths (500, 778, and 862 nm). Calculated spectral optical depths were usually within 25% of measurements (closure obtained at one or more wavelengths on both days), and agreement at longer wavelengths was improved when satellite measurements were spectrally scaled using in situ model results. On both days, large sea salt particles produced a spectrally neutral aerosol optical depth in the marine boundary layer whereas smaller ammonium sulfate particles contributed to greater optical depth at shorter wavelengths in the overlying upper atmosphere.
The deposition coefficient and its role for cirrus clouds
Authors: Gierens Klaus M. , Monier Marie , Gayet Jean‐Francois
Published: May 2003

The potential of homogeneous freezing nucleation to produce large numbers of ice crystals in regions of gentle updraft has been underestimated so far. Small but realistic deposition coefficients for small ice crystals lead to longer periods with relative humidity staying near the critical value for nucleation, which results in much larger numbers of ice crystals than with deposition coefficients close to unity. This mechanism allows to reproduce number densities of crystals in the 10–30 μm size range obtained from recent measurement campaigns. Potential reasons for small deposition coefficients are discussed, and new laboratory experiments to measure them are recommended.
Seasonal cycle and interannual variability of the Sahelian rainfall at hydrological scales
Authors: Lebel Thierry , Diedhiou Arona , Laurent Henri
Published: May 2003

Sahelian rainfall is characterized both by a strong interannual variability and by periods of long‐lasting droughts, such as the years 1970–1997. The controlling factors of this variability have been the subject of a significant amount of research, but most of this research is carried out using low‐resolution averages, typically, monthly to seasonal in time and over 5° × 5° grid boxes (or larger) in space. This paper is an attempt at characterizing the Sahelian rainfall regime at finer scales, with the objective of establishing links between the seasonal cycle and the interannual variability. To that end, high space‐time resolution data sets are analyzed. One is composed of around three hundred daily rain gauges covering a 1,700,000 km2 area for the period 1951–1990. The second is a set of full resolution Meteosat images covering the years 1989–1999, allowing for a systematic tracking of the mesoscale convective systems (MCSs). The third data set was produced from an experimental network of recording rain gauges covering 16,000 km2 in the region of Niamey, Niger, during the years 1990–2000. The analysis of the regional daily rainfall data set tends to revisit the common vision of the seasonal cycle of the Sahelian rainfall. It is shown that the average regime is in fact composed of two subregimes. One is an oceanic regime characterized by a progressive increase of the moist air flow from the ocean into the continent, associated with the seasonal migration of the ITCZ from its southern position in the boreal winter to its northern position in the boreal summer. The second regime is a continental regime in which rain is mostly produced by large convective systems embedded in the easterly circulation. This continental regime sets in abruptly during the second half of June, and 90% of the Sahelian rainfall is then produced by a small number (12% of the total number) of large and organized mesoscale convective systems. The mean event rainfall associated with these systems is larger than the mean event rainfall observed in the oceanic regime. The average proportion of the Sahelian rainfall occurring during the continental regime represents between 75% and 90% of the total annual rainfall. It is thus necessary to study this regime in order to understand the interannual rainfall variability of the region better. It is shown, for instance, that the main factor of interannual variability is the variability of the number of the large convective systems from year to year. It is also shown, using NCEP/NCAR reanalysis, that the easterly waves, which are a major synoptic feature of the region, are not systematically associated with rain‐efficient convective systems and that further studies are needed to understand the differences between wet and dry waves.
GCM greenhouse and high‐obliquity Solutions for early Proterozoic glaciation and middle Proterozoic warmth
Authors: Jenkins Gregory S.
Published: May 2003

The first definitive evidence of low‐latitude glaciation occurs in the early Proterozoic approximately 2.3 billion years ago. This period of glaciation occurs near the time for a significant rise in oxygen levels. A reduction in the methane producing population through the oxidation of Earth's atmosphere has been proposed as the cause for generating cold climatic conditions. Another alternative is that cold climatic conditions could have been generated by the migration of a landmass into low latitudes if Earth's obliquity was considerably higher than present. These two hypotheses are tested with global climate model simulations using present‐day and high‐obliquity (70°) boundary conditions. Further, the solar constant is reduced by 17% and 10 × CO2 is used. An idealized super‐continent is used and methane mixing ratios vary from 0.1 ppmv to 25 ppmv in the simulations. For the present‐day obliquity, low‐latitude glaciation occurs when CH4 concentrations are reduced from 10 to 0.1 ppmv. However, the same result is produced when the oceanic heat transport is turned off. In high obliquity simulations, low‐latitude glaciation occurs when the idealized super‐continent is moved from middle into low latitudes. Finally, high‐obliquity may explain the warm middle Proterozoic, as warm simulated climatic conditions occur even for a low latitude super‐continent using a solar constant representative of the Middle Proterozoic and reduced atmospheric methane mixing ratios.
Variations of ozone in the marine boundary layer over the Arabian Sea and the Indian Ocean during the 1998 and 1999 INDOEX campaigns
Authors: Chand Duli , Lal Shyam , Naja Manish
Published: May 2003

Extensive measurements of surface ozone (O3) have been undertaken to study its transport from the continent to the marine region during the field cruises of Indian Ocean Experiment (INDOEX) in the winters of 1998 and 1999. The role of Intertropical Convergence Zone (ITCZ) in the distribution of O3 is investigated. These measurements show a large difference in ozone features during 1998 and 1999. Ozone mixing ratios, its latitudinal gradients, and amplitudes of diurnal variations are higher during INDOEX 1999 than in INDOEX 1998 over the Arabian Sea. The INDOEX 1999 period was characterized by many anticyclones and stronger inversions over the Indian region. The observed maximum latitudinal gradient and amplitude of diurnal variation are 7.5 ppbv lat−1 and 54%, respectively, during INDOEX 1999, which are more than the earlier levels observed over this region. There is almost no longitudinal gradient along the 20°S latitudinal transect over the Indian Ocean. The marine type of diurnal variations observed in the regions where gradients in ozone are very small suggest that the theory recently given by de Laat and Lelieveld [2000] could not fully explain these variations. Using exponential decrease of ozone with latitude, we have estimated an e‐fold time of 4.8 days for ozone, which is lower than that for aerosol (7 days) measured independently for this region. This indicates that in addition to mixing and deposition, which are common to both these species, chemical loss of ozone is also important over the marine environments of this region. Ozone levels over the Arabian Sea (∼65 ppbv) are found to be more than four times the levels of ozone over the remote Indian Ocean (∼14 ppbv), south of the ITCZ. These results suggest that anthropogenic activities in the Indian subcontinent can significantly affect its adjoining marine environments.
Canadian Aerosol Module: A size‐segregated simulation of atmospheric aerosol processes for climate and air quality models 1. Module development
Authors: Gong S. L. , Barrie L. A. , Blanchet J.‐P. , von Salzen K. , Lohmann U. , Lesins G. , Spacek L. , Zhang L. M. , Girard E. , Lin H. , Leaitch R. , Leighton H. , Chylek P. , Huang P.
Published: May 2003

A size‐segregated multicomponent aerosol algorithm, the Canadian Aerosol Module (CAM), was developed for use with climate and air quality models. It includes major aerosol processes in the atmosphere: generation, hygroscopic growth, coagulation, nucleation, condensation, dry deposition/sedimentation, below‐cloud scavenging, aerosol activation, a cloud module with explicit microphysical processes to treat aerosol‐cloud interactions and chemical transformation of sulphur species in clear air and in clouds. The numerical solution was optimized to efficiently solve the complicated size‐segregated multicomponent aerosol system and make it feasible to be included in global and regional models. An internal mixture is assumed for all types of aerosols except for soil dust and black carbon which are assumed to be externally mixed close to sources. To test the algorithm, emissions to the atmosphere of anthropogenic and natural aerosols are simulated for two aerosol types: sea salt and sulphate. A comparison was made of two numerical solutions of the aerosol algorithm: process splitting and ordinary differential equation (ODE) solver. It was found that the process‐splitting method used for this model is within 15% of the more accurate ODE solution for the total sulphate mass concentration and <1% accurate for sea‐salt concentration. Furthermore, it is computationally more than 100 times faster. The sensitivity of the simulated size distributions to the number of size bins was also investigated. The diffusional behavior of each individual process was quantitatively characterized by the difference in the mode radius and standard deviation of a lognormal curve fit of distributions between the approximate solution and the 96‐bin reference solution. Both the number and mass size distributions were adequately predicted by a sectional model of 12 bins in many situations in the atmosphere where the sink for condensable matter on existing aerosol surface area is high enough that nucleation of new particles is negligible. Total mass concentration was adequately simulated using lower size resolution of 8 bins. However, to properly resolve nucleation mode size distributions and minimize the numerical diffusion, a sectional model of 18 size bins or greater is needed. The number of size bins is more important in resolving the nucleation mode peaks than in reducing the diffusional behavior of aerosol processes. Application of CAM in a study of the global cycling of sea‐salt mass accompanies this paper [Gong et al., 2002].
Retrieval of aerosol optical thickness over land surfaces from top‐of‐atmosphere radiance
Authors: von Hoyningen‐Huene W. , Freitag M. , Burrows J. B.
Published: May 2003

Aerosol remote sensing requires techniques enabling the determination of aerosol optical thickness (AOT) over land surfaces, because the most important sources (continental aerosols, anthropogenic aerosols, biomass burning, desert dust, volcano eruptions and others) are on continents. Here a retrieval method for the AOT over land surfaces from top‐of‐atmosphere (TOA) radiance using nadir looking instruments of the ocean color type (like Ocean Color and Temperature Sensor (OCTS), Sea viewing Wide Field Sensor (SeaWiFS), Moderate resolution Imaging Sensor (MODIS) or Medium Resolution Imaging Sensor (MERIS)) is presented. It is scheduled as an off‐line procedure for the ENVISAT radiometers SCIAMACHY and MERIS. The method is based on lookup tables (LUT) between the AOT and the aerosol reflectance for wavelength <0.67 μm. The aerosol reflectance is obtained from TOA reflectance accounting for Rayleigh path reflectance and the apparent spectral surface reflectance. Over land the surface reflectance is estimated by a mixing model of bare soil and green vegetation spectra, tuned by the normalized differential vegetation index (NDVI) of the satellite scene. The method has been tested and validated with SeaWiFS data and with aerosol properties of the closure experiment LACE‐98 (Lindenberg Aerosol Charactrization Experiment). For short wave channels (0.412–510 μm) an agreement between the retrieved and ground‐based data of 20% is achieved. Thus the method enables the investigation of AOT over land, yielding the regional turbidity situation as well as the identification of aerosol sources like large cities, large fire plumes, haze, small scale dynamical events and also thin cirrus clouds.
Dating of two nearby ice cores from the Illimani, Bolivia
Authors: Knüsel S. , Ginot P. , Schotterer U. , Schwikowski M. , Gäggeler H. W. , Francou B. , Petit J. R. , Simões J. C. , Taupin J. D.
Published: May 2003

In order to establish a chronology of two nearby ice cores from a glacier at Illimani (6438 m), Bolivia, a broad dating approach is presented here, which in particular makes use of the fast, simple, and nearly nondestructive electrical conductivity method (ECM) that provides a highly resolved record. Thus, ECM is suited for counting annual layers in the ice, especially for ice cores extracted from high‐mountain glaciers with a fast layer thinning. Furthermore, ECM can be used for detecting volcanic signals. Annual signals in the ECM record of the Illimani ice core were identified using the 1964 A.D. tritium reference horizon and were counted along 125 m or 90% of the core, representing the time period from 1200 ± 240 A.D. (estimated accumulated error) to 1999 A.D. The resulting age–depth relationship was supported by counting annual peaks in the microparticle record as well as by nuclear dating using the decay of 210Pb. The identification of volcanic signals originating from eruptions such as Pinatubo (1991 A.D.), El Chichón (1982 A.D.), Agung (1963 A.D.), Krakatoa (1883 A.D.), Tambora (1815 A.D.), and the Unknown 1258 A.D. significantly reduced the uncertainty of annual layer counting (ALC) to ±2 years in the vicinity of these events.
Introduction to Special Section: Photochemistry Experiment in BERLIOZ
Authors: Volz‐Thomas Andreas , Geiss Heiner , Hofzumahaus Andreas , Becker Karl‐Heinz
Published: May 2003

PHOEBE was conducted during the Berlin Ozone Experiment (BERLIOZ) between 5 July and 7 August 1998 near the city of Berlin. It aimed at a quantitative understanding of the fast radical chemistry in rural and suburban air by simultaneous measurements of the major free radicals (OH, HO2, RO2, and NO3) and the chemical compounds and physical parameters that control the radical concentrations. Interpretation of the measurements involved models of different complexity. The methods deployed and the results are discussed in the subsequent publications of this special section. This paper outlines the aims of PHOEBE, describes the observational site at Pabstthum, and briefly summarizes the different measurements and instrument comparisons as well as the main results of the accompanying publications.