bluuebunny/crossref_metadata_2025 huggingface dataset:
D region electron number density limits for the existence of polar mesosphere summer echoes
Authors: Rapp M. , Gumbel J. , Lübken F.‐J. , Latteck R.
Published: October 2002

We investigate the dependence of polar mesosphere summer echoes (PMSE) and mesosphere summer echoes (MSE) on the background electron number density. Both a lower and upper limit are quantified below and above which PMSE cannot exist. The result is that PMSE occur for a very wide range of electron number densities between ∼300–500/cm3 and ∼105/cm3. A comparison of the diurnal variation of MSE observed at Kühlungsborn (54°N) with current model estimates of the electron number density at midlatitudes shows that at least ∼300–500 electrons /cm3 are necessary for PMSE to exist. This lower limit is consistent with all available electron number density measurements obtained from sounding rockets in the vicinity of PMSE. It is shown that the existence of a lower electron number density limit can be understood in terms of the standard theory of the scattering of VHF waves in the D region. We have then analyzed PMSE observations during the major solar proton event on 14 July 2000. We have estimated the electron number density at PMSE altitudes based on proton and electron flux measurements obtained with detectors on board the GOES‐8 and ACE spacecrafts in combination with an ion‐chemical model. Comparing the electron number densities at 87 km altitude with the average PMSE signal to noise observations (SNR) we find a significant negative correlation between SNR and the electron number densities for densities on the order of ∼105/cm3. We propose that this negative correlation is due to a limited amount of aerosol particles: current PMSE theories assume that electron irregularities in the VHF band can only exist if more than ∼50% of the free electrons are bound to aerosol particles which thus reduce the electron diffusivity due to ambipolar forces. If, however, the electron number density increases significantly above the aerosol number density, this condition can no longer be fulfilled. On the other hand, the fact that PMSE is present up to electron number densities of ∼105/cm3 raises several important questions on our current understanding of aerosol particles around the polar summer mesopause and their role in the creation of PMSE: either many more aerosol particles exist than has been anticipated so far (with corresponding implications for the nucleation of these particles) or our current understanding of the role of aerosol particles in the creation of PMSE is not complete.
Accuracy of analyzed stratospheric temperatures in the winter Arctic vortex from infrared Montgolfier long‐duration balloon flights 2. Results
Authors: Knudsen B. M. , Pommereau J.‐P. , Garnier A. , Nunes‐Pinharanda M. , Denis L. , Newman P. , Letrenne G. , Durand M.
Published: October 2002

Five long‐duration flights with the Mongolfier infrared (MIR) balloon lasting 15 days, on average, have been conducted in the Arctic winter stratospheric vortex in 1997, 1999, and 2000. Temperatures from the European Centre for Medium‐Range Weather Forecasts (ECMWF), Met Office (MO), National Centers for Environmental Prediction (NCEP), Data Assimilation Office (DAO), and NCEP/NCAR reanalysis (REA) have been compared to the observations from 4 to 146 hPa. Occasional large errors (>14 K) occur in each analysis, mainly above 30 hPa. In 2000 the standard deviations of ECMWF, MO, and DAO with respect to the measured temperatures range from 1.0 to 1.3 K, whereas NCEP and REA have substantially larger errors. In 1999 the flights took place during a major warming, and all operational models had large standard deviations and substantial biases. Preoperational versions of the new ECMWF model with increased stratospheric resolution and assimilation of the advanced microwave sounding unit, which none of the other models assimilated, show small biases and standard deviations.
Alkaline rains on the Tibetan Plateau and their implication for the original pH of natural rainfall
Authors: Zhang D. D. , Peart M. R. , Jim C. Y. , La Jia
Published: October 2002

Natural rains are generally considered as weakly acidic. Long‐term measurement in the Tibetan capital city, Lhasa, reveals that alkaline rain is also natural. For the last 3 years the volume‐weighted mean pH of rainwater is 7.5. Earlier observation shows even higher average pH values, such as 8.36 in the 1987–1988 period. The major cause of alkaline rain is the alkaline and soil‐borne continental dusts in this semiarid area. Bicarbonate is the dominant anion in the water samples. The analysis also shows that the rainwater in this city contains few pollutants, compared with other places in the world. Measurements carried out in two additional industrial cities on the northern and northeastern Tibetan Plateau, Xining and Germu, demonstrate how fast human activities such as industrial development may increase rainwater acidity. In a period of 13 years the rainfall pH value of Germu has dropped from 8.03 to 6.8, representing a manyfold increase of the H+ concentration. Such an increase was caused by rising contents of NO−3 and SO−24 in the atmosphere. On the basis of the measurements on the Tibetan Plateau, evidence from other places around the world, and the experiments and calculation, the authors believe that the original pH of natural rainwater in arid and semiarid areas on this planet should be weakly alkaline because of the influence of alkaline dusts.
Noctilucent clouds getting brighter
Authors: Klostermeyer J.
Published: October 2002

Noctilucent cloud (NLC) observations in northwest Europe show a significant secular increase in the occurrence frequency between 1964 and 1994. Although model computations predicted a brightness increase due to increasing stratospheric methane concentrations and a consequent increase in the number of observed NLCs, earlier investigations of the histograms of the solar depression angles at the first and last NLC sightings did not reveal any noticeable brightness change. However, a reinvestigation of the histograms based on statistical model predictions, χ2 tests, and least squares fitting clearly indicates that the NLC brightness increased by a factor of the order of 5 between the periods 1964–1974 and 1985–1994. The brightness increase goes along with a significant lengthening of the NLC season. Considering recently published trends in the upper mesospheric water vapor mixing ratio and temperature strongly suggests that the brightness increase is caused by an increase of the water vapor concentration rather than a decrease in the temperature. For 1985–1994 the analysis also predicts at least a doubling of the NLC brightness between dusk and dawn, in good agreement with lidar observations.
Tropospheric ozone information from satellite‐based polarization measurements
Authors: Hasekamp Otto P. , Landgraf Jochen
Published: October 2002

Many current and planned future satellite missions aim at the retrieval of tropospheric ozone from reflectance spectra. However, it is difficult to distinguish between stratospheric and tropospheric ozone on the basis of reflectance spectra only. In this paper we show that satellite‐based polarization measurements of backscattered light contain valuable additional information on the tropospheric ozone profile. In contrast to the reflectance the polarization properties of backscattered light show higher sensitivity to ozone in the troposphere than to ozone in the stratosphere. This is because the polarization is most affected by ozone at that altitude where most scattering takes place, which is in the troposphere for wavelengths greater than ∼300 nm. Retrievals performed on synthetic Global Ozone Monitoring Experiment 2 data show that the vertical resolution of the tropospheric ozone profile is significantly improved if polarization measurements are used in addition to the reflectance spectrum. Problems that are currently encountered in tropospheric ozone retrieval from reflectance spectra may be solved by using additional polarization measurements.
A study of the leakage of the Antarctic polar vortex in late austral winter and spring using isentropic and 3‐D trajectories
Authors: Öllers Michel C. , van Velthoven Peter F. J. , Kelder Hennie M. , Kamp Leon P. J.
Published: October 2002

The permeability of the Antarctic polar vortex is investigated in late austral winter and spring by comparing isentropic and three‐dimensional (3‐D) trajectories. Trajectory computations were performed with the help of the Royal Dutch Meteorological Institute (KNMI) trajectory model, using data from the European Centre for Medium‐Range Weather Forecasts (ECMWF) from August to November 1998. Large numbers of air parcels were initially released inside and outside the polar vortex on the 350, 450, and 550 K isentropic surfaces. They were integrated 4 months forward in time in an isentropic mode, as well as in a 3‐D mode that uses all three wind components from the ECMWF and takes into account diabatic heating and cooling effects. For the isentropic trajectory calculations, very little transport (0.37%/week) was found for August and September, while October and November gave somewhat higher transport rates (1.95%/week). The 3‐D trajectory calculations for October gave much more exchange between the vortex and midlatitudes than the isentropic ones owing to a significant number of parcels that descended inside the vortex. Descent rates were calculated for 350 K (October), 450 K (August–October) and 550 K (October). Overall, the results show that 3‐D trajectories will provide more accurate leakage rates than the isentropic ones. Also, despite the large‐scale mixing in the polar vortex or in midlatitudes, little ozone‐depleted air leaks from the ozone hole into the midlatitude stratosphere.
Accuracy of analyzed stratospheric temperatures in the winter Arctic vortex from infrared Montgolfier long‐duration balloon flights 1. Measurements
Authors: Pommereau Jean‐Pierre , Garnier Anne , Knudsen Bjoern M. , Letrenne Gérard , Durand Marc , Nunes‐Pinharanda Manuel , Denis Laurent , Vial Francois , Hertzog Albert , Cairo Francesco
Published: October 2002

Long series of temperature measurements in the winter Arctic stratosphere have been obtained from infrared Montgolfier (MIR) long‐duration balloon flights in 1997, 1999, and 2000, from which the performance of a variety of meteorological analyses has been investigated. In this first paper of a series of two, the experimental setup and the measurements are described. First flown in the winter Arctic in 1997, the MIR platform appears well adapted to perform flights for several weeks in the vortex. Flights of 12 and 22 days (1997), 7 and 17 days (1999), and 3 and 18 days (2000) have been achieved along which altitude, pressure, and temperature have been sampled every 9–10 min. Comparisons between various independent sensors on the same balloon, and to Vaisala and Russian radiosondes, have made it possible to evaluate the performances of the instruments. The accuracy of the altitude/location of the Global Positioning System proved to be ±100 m but that of the pressure sensors only ±2 hPa. The most accurate method for deriving pressure appears to be the use of GPS altitude together with the ECMWF (European Centre for Medium‐Range Weather Forecasts) geopotential height. Finally, the temperature was demonstrated to be measured with a precision of ±0.4 K and an average bias of less than ±0.5 K, but during nighttime and at an altitude below only 28 km.
Identification of an organic coating on marine aerosol particles by TOF‐SIMS
Authors: Tervahattu Heikki , Juhanoja Jyrki , Kupiainen Kaarle
Published: October 2002

Marine aerosol particles play an important role in atmospheric processes. It has been suggested that as marine aerosol particles form, they acquire a coating of organic surfactants. This theory has been supported only by indirect evidence. Recently, we gave new morphological indication of such organic coating without however providing molecular speciation. Here we have studied the surface of marine aerosol particles by time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS), which is very suitable for surface research due to its unique combination of surface sensitivity and the detailed molecular information obtained. Spectra from the outermost surface gave high intensity for palmitic acid and lower peaks for other fatty acids. According to TOF‐SIMS images, palmitic acid was distributed on small particles, similar with the marine particles. Sputtering stripped palmitic acid and revealed the inner core of the sea‐salt particles. Our results show that fatty acids are important ingredients of the outermost surface layer of the studied aerosol particles.
A proposed mechanism for the regulation of minimum midtropospheric temperatures in the Arctic
Authors: Chase T. N. , Herman B. , Pielke R. A. , Zeng X. , Leuthold M.
Published: October 2002

This paper documents an observed lower extreme of midtropospheric (500 mbar) temperatures in the Arctic of approximately −45°C during the winter season in several data sets. Each data set shows that the coldest air masses in the Arctic reach −45°C during the fall months but seldom get much colder even into late winter despite a continued net radiative loss. We demonstrate that midtropospheric temperatures are significantly skewed toward warmer temperatures, indicating a regulatory mechanism at work. We further provide evidence that minimum Arctic midtropospheric temperatures are regulated by moist convective processes and that minimum 500 mbar temperatures are controlled to a large extent by high‐latitude sea surface temperatures. The temperature −45°C is the expected 500 mbar temperature in an atmosphere regulated by moist adiabatic ascent from a surface temperature of 1°–2° below 0°C, the approximate freezing point of seawater. This implies that Arctic air masses are regularly in contact with unfrozen seawater to the south, an easily verified observation. Climate model simulations of the effects of increased greenhouse gasses hypothesize that high, northern latitude regions should warm at a faster rate than the globe as a whole, a hypothesis which does not appear to have strong observational support. We discuss the implications of this result for the accelerated Arctic warming hypothesis.
Lidar validation of SAGE II aerosol measurements after the 1991 Mount Pinatubo eruption
Authors: Antuña Juan Carlos , Robock Alan , Stenchikov Georgiy L. , Thomason Larry W. , Barnes John E.
Published: October 2002

After the Mount Pinatubo volcanic eruption on 15 June 1991 the Stratospheric Aerosol and Gas Experiment (SAGE) II instrument made extensive aerosol extinction retrievals using the limb‐viewing technique. In regions of high‐aerosol loading, SAGE II was not able to make measurements, resulting in large information gaps both in latitudinal and in longitudinal coverage as well as in the vertical. Here we examine the possibility of filling the vertical gaps using lidar data. We compare every coincident backscattering measurement (at a wavelength of 0.694 μm) from two lidars, at Mauna Loa, Hawaii (19.5°N, 155.6°W), and at Hampton Virginia (37.1°N, 76.3°W), for the 2‐year period after the Pinatubo eruption with the SAGE II version 6.0 extinctions at 0.525 and 1.02 μm wavelengths. This is the most comprehensive comparison ever of lidar data with satellite data for the Pinatubo period. We convert backscattering to extinction at the above wavelengths. At altitudes and times with coincident coverage, the SAGE II extinction measurements agree well with the lidar data but less so during the first six months after the eruption, due to the heterogeneity of the aerosol cloud. This shows that lidar data can be combined with satellite data to give an improved stratospheric aerosol data set.
Forward modeling and retrieval of water vapor from the Global Ozone Monitoring Experiment: Treatment of narrowband absorption spectra
Authors: Lang Rüdiger , Maurellis Ahilleas N. , van der Zande Wim J. , Aben Ilse , Landgraf Jochen , Ubachs Wim
Published: October 2002

We present the algorithm and results for a new fast forward modeling technique applied to the retrieval of atmospheric water vapor from satellite measurements using a weak ro‐vibrational overtone band in the visible. The algorithm uses an Optical Absorption Coefficient Spectroscopy (OACS) method which is well suited to situations where line widths in the absorption spectrum are much narrower than the instrumental resolution and where efficient numerical solutions of the equation of radiative transfer are needed. We present examples of OACS forward modeled reflectivity which include the differential contribution of singly scattered photons and compare them with spectral measurements of the Global Ozone Monitoring Experiment (GOME). In particular, we apply OACS to the retrieval of water vapor column (WVC) densities from GOME spectral data between 585 and 600 nm. Method precisions are better than 0.7% for high and better than 3.4% for low WVC. The total accuracy of the retrieval method appears to be better than 0.8 and 4% for high and low WVC, respectively. The retrieval results are compared to WVC values given by European Centre for Medium‐Range Weather Forecasts and Special Sensor Microwave Imager and allow us to conclude that the OACS retrieval method is reliable except in cloud‐rich situations. In cloud‐free cases, an upper limit of 18% error on the retrieved WVC due to the impact of multiple and aerosol scattering as well as aerosol extinction is estimated. In addition, the accuracy of the retrieval is sufficient to permit the detection of systematic differential fit mismatches between modeled and real GOME spectra. The magnitude of such mismatches places an upper limit on the accuracy of HITRAN'96 water vapor line strength values of about 10–20%.
Chemical composition and sources of the high Arctic aerosol relevant for cloud formation
Authors: Leck Caroline , Norman Michael , Bigg E. Keith , Hillamo Risto
Published: October 2002

The chemical composition, sources, and methods of production of cloud condensation nuclei (CCN) were studied over the pack ice covered Arctic Ocean in summer. After ∼2 days' travel of air over the pack ice, two local CCN sources were identified: particles derived from the bursting bubbles on open water between ice floes (film and jet drops) and particles reaching CCN size by acquisition of oxidation products of dimethyl sulfide. The bubble‐derived particle modes were found to have an exponential dependence on prevailing wind speed. The film drop generation depended less on wind speed and jet drop production depended more on wind speed than particles over open water did. However, the enhanced presence of film drop particles at wind speeds <5 m s−1 suggested a source of bubbles, other than wind, that is most active during clear sky days and ice melting. The film drop particles were mainly organic in nature, but their sea‐salt component increased at high wind speeds >12 m s−1 to dominate both mass and number. The jet drop mode was composed largely of sea salt, making a significant mass contribution already for wind speeds >5 m s−1. However, the smaller‐sized jet drops were not pure sodium chloride but contained a varied and appreciable organic component. The sulfur‐containing particles dominated the CCN population and mass for the most common wind speeds of 5–12 m s−1. Measured CCN concentrations were on average less than would have been expected from either a sulfate or a sea‐salt composition and the observed particle‐size distribution. It is concluded that other components, probably organic, depressed the nucleating ability of the particles. However, on clear sky days, there were a majority of occasions on which CCN concentrations were more than predicted from a sulfate composition and the measured size distribution.
Meteorological influences on coastal new particle formation
Authors: de Leeuw Gerrit , Kunz Gerard J. , Buzorius Gintautas , O'Dowd Colin D.
Published: October 2002

The meteorological situation at the midlatitude coastal station of Mace Head, Ireland, is described based on observations during the New Particle Formation and Fate in the Coastal Environment (PARFORCE) experiments in September 1998 and June 1999. Micrometeorological sensors were mounted near the shore line on a small mast with a height of 3 m and on a 22 m high tower at about 100 m away from the sea. Turbulent fields of wind speed, air temperature, and water vapor were measured. Parameters such as the friction velocity, drag coefficient, kinematic fluxes of heat and water vapor, and various variances were derived. The influence of meteorological parameters on coastal nucleation events is examined, and it is found that the occurrence of nucleation is, more or less, independent of air mass origin and is primarily driven by the occurrence of exposed shore areas during low tide and solar radiation. Micrometeorological influences were also examined in terms of promoting particle production events in this environment. A positive correlation was found between kinematic heat flux and particle production probability. In contrast, a strong negative correlation was found between production probability and both kinematic water vapor fluxes and relative humidity. These results indicate that the occurrence of new particle production events in the coastal zone are most probable during conditions when the shore area containing coastal biota has dried out and the biota are exposed directly to the solar radiation flux and increased shore, or surface, temperatures. These conditions correspond to drying and stressing of the biota, which is known to increase the emissions of biogenic vapors.
Interpretation of TOMS observations of tropical tropospheric ozone with a global model and in situ observations
Authors: Martin Randall V. , Jacob Daniel J. , Logan Jennifer A. , Bey Isabelle , Yantosca Robert M. , Staudt Amanda C. , Li Qinbin , Fiore Arlene M. , Duncan Bryan N. , Liu Hongyu , Ginoux Paul , Thouret Valerie
Published: October 2002

We interpret the distribution of tropical tropospheric ozone columns (TTOCs) from the Total Ozone Mapping Spectrometer (TOMS) by using a global three‐dimensional model of tropospheric chemistry (GEOS‐CHEM) and additional information from in situ observations. The GEOS‐CHEM TTOCs capture 44% of the variance of monthly mean TOMS TTOCs from the convective cloud differential method (CCD) with no global bias. Major discrepancies are found over northern Africa and south Asia where the TOMS TTOCs do not capture the seasonal enhancements from biomass burning found in the model and in aircraft observations. A characteristic feature of these northern tropical enhancements, in contrast to southern tropical enhancements, is that they are driven by the lower troposphere where the sensitivity of TOMS is poor due to Rayleigh scattering. We develop an efficiency correction to the TOMS retrieval algorithm that accounts for the variability of ozone in the lower troposphere. This efficiency correction increases TTOCs over biomass burning regions by 3–5 Dobson units (DU) and decreases them by 2–5 DU over oceanic regions, improving the agreement between CCD TTOCs and in situ observations. Applying the correction to CCD TTOCs reduces by ∼5 DU the magnitude of the “tropical Atlantic paradox” [Thompson et al., 2000], i.e. the presence of a TTOC enhancement over the southern tropical Atlantic during the northern African biomass burning season in December–February. We reproduce the remainder of the paradox in the model and explain it by the combination of upper tropospheric ozone production from lightning NOx, persistent subsidence over the southern tropical Atlantic as part of the Walker circulation, and cross‐equatorial transport of upper tropospheric ozone from northern midlatitudes in the African “westerly duct.” These processes in the model can also account for the observed 13–17 DU persistent wave‐1 pattern in TTOCs with a maximum over the tropical Atlantic and a minimum over the tropical Pacific during all seasons. The photochemical effects of mineral dust have only a minor role on the modeled distribution of TTOCs, including over northern Africa, due to multiple competing effects. The photochemical effects of mineral dust globally decrease annual mean OH concentrations by 9%. A global lightning NOx source of 6 Tg N yr−1 in the model produces a simulation that is most consistent with TOMS and in situ observations.
Observational constraints on the contribution of isoprene oxidation to ozone production on the western slope of the Sierra Nevada, California
Authors: Dreyfus Gabrielle B. , Schade Gunnar W. , Goldstein Allen H.
Published: October 2002

Observations of isoprene and its oxidation products methacrolein (MACR) and methyl vinyl ketone (MVK) are used to quantify the impact of isoprene oxidation on ozone production along the western slope of the Sierra Nevada mountains. Regular daytime up‐slope wind flow patterns transport anthropogenic volatile organic compounds (VOC) and NOxemissions from the Central Valley toward the Sierra Nevada. A north–south band of oak forests stretching along the foothills and located approximately halfway between Sacramento and our measurement site (Blodgett Forest Research Station; elevation 1315 m) injects isoprene into this mixture. Subsequently, high ozone levels are encountered in these air masses. At Blodgett, daytime mixing ratios of isoprene's oxidation products and ozone were highly correlated. The observed daytime MVK/MACR ratio was used to estimate a mean [OH] of 9 (±4) × 106molec. cm−3between the measurement site and the Sierra foothills. The slope of the correlation between ozone and MVK was analyzed and compared to theoretical yield ratios for the photooxidation of isoprene to estimate the fraction of total ozone production due to isoprene oxidation. On average, over 40% of the observed midday ozone formation in this region was attributable to isoprene oxidation. On ozone episode days (maximum [O3] > 90 ppb), the mean isoprene contribution was over 70%. The calculated isoprene contribution to ozone production was variable from day to day but tended to increase exponentially with both isoprene input and air temperature. NOxconditions in the up‐slope air masses were very important in determining the ozone formation potential of isoprene, and the general dominance of isoprene as an ozone precursor suggests that summertime ozone abatement strategies for the region must focus on anthropogenic NOxrather than VOC reductions.
Impact of satellite‐derived effective cloud fraction on calculated actinic flux, photodissociation rates, and OH production
Authors: Tuinder O. N. E. , de Winter‐Sorkina R. , Builtjes P. J. H.
Published: October 2002

Clouds are a large error source in the retrieval of tropospheric column densities and concentration profiles of trace gas species from satellites and in the calculation of their photodissociation rates. The Global Ozone Monitoring Experiment instrument on the ERS‐2 satellite is capable of detecting clouds, and a number of methods have been developed to retrieve their properties. In our previous study it was shown that these cloud retrieval methods calculate an effective cloud fraction that is related to a cloud with a high optical thickness. Furthermore, the retrieved solutions are nonunique, as different combinations of optical thickness, cloud fraction, and cloud top height can give the same albedo at the top of the atmosphere. The impact of the scaling of various cloud solutions to an effective cloud fraction on the vertical photodissociation profiles of ozone (O3) and nitrogen dioxide (NO2) and on the tropospheric column production of the hydroxyl radical (OH) is studied in this paper, using the tropospheric ultraviolet‐visible radiative transfer model. Results show that for selected cloud solutions, local differences in the vertical OH primary production profile can be from +7% for small solar zenith angles to −14% for large angles with a cloud between 2 and 3 km. Column‐integrated OH primary production differences can be between +9% and −7% depending on the solar zenith angle when a cloud is located between 2 and 3 km but can increase to +23% and −81% for a cloud at higher altitudes. NO2photodissociation was shown to be locally more influenced by the scaling of clouds to an effective cloud fraction, but for the tropospheric column difference this influence is less prominent.
Intramolecular 15N and 18O fractionation in the reaction of N2O with O(1D) and its implications for the stratospheric N2O isotope signature
Authors: Kaiser Jan , Brenninkmeijer Carl A. M. , Röckmann Thomas
Published: October 2002

Atmospheric nitrous oxide (N2O) is enriched in heavy oxygen and nitrogen isotopes relative to its tropospheric sources. This enrichment is traced back to kinetic isotope effects in the two stratospheric N2O sink mechanisms, i.e., photolysis and reaction with O(1D). Most of the previous studies on the cause of isotopic enrichment in N2O have focused on photolysis. Here we present results on the 18O and the position‐resolved 15N kinetic isotope effects in the reaction of nitrous oxide with O(1D) obtained by recently developed mass spectrometric techniques. Just as in the photolysis sink, a heavy isotope enrichment in the residual N2O was found, but of smaller magnitude. However, the fractionation pattern of nitrogen isotopes at the two nonequivalent positions in the molecule is clearly distinct from that in photolytic N2O destruction. The fractionation constant for the terminal nitrogen atom, 15ε1 = k(14N2O)/k(15N14NO) − 1 = (8.87 ± 0.15) %, is larger than for the central nitrogen atom, 15ε2 = k(14N2O)/k(14N15NO) − 1 = (2.22 ± 0.12) % (all errors are 2σ). The fractionation constant for oxygen, 18ε = k(N216O)/k(N218O) − 1 = (12.38 ± 0.14) %, was found to be larger than for nitrogen and amounts to about twice the value from the single previous determination. The larger influence of the O(1D) sink at lower stratospheric altitudes could probably explain the lower ratio of 15ε2/15ε1 (≡η) observed there, which is shown to be only marginally influenced by transport. The published data on stratospheric η values suggest that, if there are no other chemical reactions involved, up to 60% of the overall N2O loss at lower altitudes could be from the reaction with O(1D).
Black carbon and organic carbon in aerosol particles from crown fires in the Canadian boreal forest
Authors: Conny Joseph M. , Slater John F.
Published: October 2002

In the boreal forest, high‐intensity crown fires account for an overwhelming proportion of the area burned yearly. Quantifying the amount of black carbon (BC) from boreal crown fires in Canada is essential for assessing the effect on regional climate from natural wildfire aerosol emissions versus that from anthropogenic activities. This is particularly relevant because climate change will likely lead to increased wildfire activity in northern Canada. During 4–5 July 1998, two controlled fires in Northwest Territories, Canada, were conducted as part of the International Crown Fire Modeling Experiment. We report here the BC and organic carbon (OC) compositions of aerosols produced during the flaming and smoldering stages of burning. Particles were collected on back‐to‐back quartz‐fiber filters by helicopter with a hi‐vol sampler and at ground level with a dichotomous sampler to separate the fine (≤2.5 μm diameter) and coarse (2.5–10 μm diameter) particle fractions. An analysis of the back filter in relation to the front filter from the dichot sampler for both the fine and coarse fractions provided a means to correct for the adsorption of gas‐phase organic compounds on filters (positive artifact) and for the loss of particulate carbon from filters by volatilization (negative artifact). BC and OC masses, which combine here to give total carbon (TC), were determined by the thermal‐optical method. The BC to TC ratio for the flaming stage was 0.085 ± 0.032 (±ksn−1/2,k= 2,n= 2), based on aerial sampling of the dark plume 300–500 m above the flame front. BC/TC for the smoldering stage was 0.0087 ± 0.0046 from ground‐based sampling. Uncertainties consist of the combined variances in measurement and sampling and in emissions from different fires. These averages and uncertainties serve as important aerosol data input for predictions of climate change on both global and regional scales.
Budgets of O3 and CO in the upper troposphere: CARIBIC passenger aircraft results 1997–2001
Authors: Zahn A. , Brenninkmeijer C. A. M. , Asman W. A. H. , Crutzen P. J. , Heinrich G. , Fischer H. , Cuijpers J. W. M. , van Velthoven P. F. J.
Published: October 2002

In situ measurements of ozone (O3) and carbon monoxide (CO) were conducted using a passenger aircraft during 47 flights (November 1997 to April 2001) between Germany and the Indian Ocean region at 10–11 km altitude, project Civil Aircraft for Regular Investigation of the Atmosphere Based on an Instrument Container (CARIBIC). Here, emphasis is on a better understanding of the budgets of O3 and CO in the middle and upper northern hemispheric (NH) troposphere. Latitudinal and seasonal variations of the mean values and the local variability of O3 and CO are assessed. CO is moreover applied as tracer of polluted (surface) air, that is, as a measure of O3 precursors. This enables us to assign positive or negative O3–CO correlations and their slopes α = dO3/dCO to O3 produced by photochemistry or to O3 imported from the stratosphere. In the tropics, photochemical O3 production clearly dominates all year round, with mean O3–CO slopes of 0.45–0.53 in spring/summer and 0.20–0.27 in autumn/winter. Over the Arabian Sea and the Middle East, high annual net O3 production rates in the troposphere of 17.6 × 1010 O3 molecules cm−2 s−1 are estimated. In the extratropics, the well‐documented springtime O3 maximum emerged in the CARIBIC data set. Stratospheric O3 influx and photochemical O3 production contribute equally to this spring maximum. In summer, photochemical O3 formation clearly dominates, in winter, per contra, stratospheric O3 influx clearly dominates. In the extratropics, CO shows a sine seasonal variation between ∼81 ppbv in September and ∼111 ppbv in March, being delayed by ∼5 weeks compared to ground‐based background monitoring stations. In the tropics, however, no significant seasonal variations of CO were observed, mainly because of occasional high CO periods such as those associated with the 1997 Indonesian forest fires or the outflow of the Intertropical Convergence Zone, which pumps in summer immense amounts of pollutants emitted in South Asia (India, Bangladesh, Indochina) to the 10–11 km altitude range surveyed during CARIBIC.
Eastern Atlantic Spring Experiment 1997 (EASE97) 2. Comparisons of model concentrations of OH, HO2, and RO2 with measurements
Authors: Carslaw N. , Creasey D. J. , Heard D. E. , Jacobs P. J. , Lee J. D. , Lewis A. C. , McQuaid J. B. , Pilling M. J. , Bauguitte S. , Penkett S. A. , Monks P. S. , Salisbury G.
Published: October 2002

An observationally constrained box model has been used to investigate the chemistry of the marine boundary layer at the Mace Head Atmospheric Research Station, a remote site on the west coast of Ireland. The model aims to simulate concentrations of the hydroxyl (OH) and hydroperoxy (HO2) radicals measured by an in situ fluorescence assay by gas expansion instrument, and the sum of peroxy radicals ([HO2] + Σ[RO2]) as determined by a peroxy radical chemical amplification instrument. The model has been constructed based on observed concentrations of nonmethane hydrocarbons, measured in situ during the campaign by gas chromatography. The chemical mechanism for the model is a subset of a comprehensive master chemical mechanism. This paper details comparisons of the concentrations of modeled and measured radical species from a field campaign held at Mace Head during spring 1997. The air masses arriving at the site have been split into three categories depending on their origin and chemical characteristics and model‐measurement comparisons carried out for each air mass. The average model‐measurement ratios are 2.4 for [OH], 3.6 for [HO2], and 0.9 for ([HO2] + Σ[RO2]), respectively, between 1100 and 1500 hours: the level of agreement is better for all three sets of radicals in the cleanest air mass. Possible reasons for the observed discrepancies are discussed. In addition, a rate of production analysis is used to identify key OH and HO2 reactions in the three air masses. The rate of OH production from HO2 with NO exceeds that from ozone photolysis by factors of 2–6 in the polluted air masses studied. In cleaner air from the northern polar region, primary production from ozone photolysis exceeds that from HO2 + NO by a factor of 2.5. HO2 and CH3O2 dominate the peroxy radical composition in all air masses, but peroxy radicals derived from the oxidation of nonmethane hydrocarbons are more important in polluted air masses.
Aerosol distributions and cloud interactions at a mountaintop laboratory
Authors: Lowenthal Douglas H. , Borys Randolph D. , Wetzel Melanie A.
Published: October 2002

The first measurements of ultrafine (3–10 nm diameter) condensation nuclei concentrations and particle size distributions at Storm Peak Laboratory (3210 MSL) are presented. In‐cloud average hourly average ultrafine concentrations peaked at 1608 cm−3 in the 1300 hour, MST. Under clear conditions in April and May 2001, average hourly average ultrafine concentrations reached 8947 cm−3 during the 1400 hour, MST. The highest observed 5‐minute average ultrafine concentration was 32000 cm−3. A comparison of clear‐air and interstitial aerosol size distributions indicates that particles larger than 0.5 μm were efficiently removed by snowing, mixed‐phase clouds. Particles with diameters between 0.03–0.04 and 0.2 μm appeared to serve as cloud condensation nuclei. The observed loss of particles smaller than 16 nm in‐cloud may be explained by diffusion to cloud drops.
Regional simulation of interannual variability over South America
Authors: Misra V. , Dirmeyer P. A. , Kirtman B. P. , Juang H.‐M. Henry , Kanamitsu M.
Published: October 2002

Three regional climate simulations covering the austral summer season during three contrasting phases of the El Niño‐Southern Oscillation cycle were conducted with the Regional Spectral Model (RSM) developed at the National Centers for Environmental Prediction (NCEP). The simulated interannual variability of precipitation over the Amazon River Basin, the Intertropical Convergence Zone, the Pacific and Atlantic Ocean basins, and extratropical South America compare reasonably well with observations. The RSM optimally filters the peturbations about a time‐varying base field, thereby enhancing the information content of the global NCEP reanalysis. The model is better than the reanalysis in reproducing the observed interannual variability of outgoing longwave radiation at both high frequencies (3–30 days) and intraseasonal (30–60 days) scales. The low‐level jet shows a peak in its speed in 1998 and a minimum in the 1999 simulations. The lag correlation of the jet index with convection over various areas in continental South America indicates that the jet induces precipitation over the Pampas region downstream. A detailed moisture budget was conducted over various subregions. This budget reveals that moisture flux convergence determines most of the interannual variability of precipitation over the Amazon Basin, the Atlantic Intertropical Convergence Zone, and the Nordeste region of Brazil. However, both surface evaporation and surface moisture flux convergence were found to be critical in determining the interannual variability of precipitation over the southern Pampas, Gran Chaco area, and the South Atlantic Convergence Zone.
A study of the impact of the Intertropical Convergence Zone on aerosols during INDOEX
Authors: Andronache C. , Donner, L. J. , Seman, C. J. , Hemler R. S.
Published: October 2002

We report model simulations of the effect of deep convection on aerosol under typical Intertropical Convergence Zone (ITCZ) conditions in the tropical Indian Ocean as encountered during the Indian Ocean Experiment (INDOEX). Measurements taken during various phases of INDOEX showed significant aerosol mass concentrations of nss‐sulfate, carbonaceous, and mineral dust over the northern Indian Ocean. During the winter dry season these aerosol species accumulate and are transported long distances to the tropical regions. In contrast, aerosol measurements south of the ITCZ exhibit significantly lower aerosol concentrations, and the convective activity, mixing, and wet removal in the ITCZ are responsible for their depletion. Our results, based on a cloud‐resolving model, driven by National Centers for Environmental Prediction analysis, show that convection and precipitation can remove significant amounts of aerosol, as observed in the Indian Ocean ITCZ. The aerosol lifetime in the boundary layer (BL) is of the order of hours in intense convection with precipitation, but on average is in the range of 1–3 days for the case studied here. Since the convective events occur in a small fraction of the ITCZ area, the aerosol lifetime can vary significantly due to variability of precipitation. Our results show that the decay in concentration of various species of aerosols is comparable with in situ measurements and that the ITCZ can act to reduce the transport of polluted air masses into the Southern Hemisphere especially in cases with significant precipitation. Another finding is that aerosol loading typical to north of ITCZ tends to induce changes in cloud microphysical properties. We found that a difference between clean air masses as those encountered south of the ITCZ to aerosol polluted air masses as encountered north of the ITCZ is associated with a slight decrease of the cloud droplet effective radius (average changes of about 2 μm) and an increase in cloud droplet number concentration (average changes by about 40 to 100 cm−3) consistent with several in situ measurements. Thus polluted air masses from the northern Indian Ocean are associated with altered microphysics, and the extent of these effects is dependent on the efficiency of aerosol removal by ITCZ precipitation and dilution by mixing with pristine air masses from the Southern Hemisphere.
Modeling interannual variability of water isotopes in Greenland and Antarctica
Authors: Werner Martin , Heimann Martin
Published: October 2002

A simulation with the Hamburg atmospheric general circulation model ECHAM‐4, forced with sea surface temperatures of the period 1950–1994 and with stable water isotopes H218O and HDO explicitly included in the water cycle, was performed to examine interannual to decadal variations of the isotopic composition of precipitation in Greenland and Antarctica. The analyses focus on the Summit region, central Greenland, and the Law Dome region, East Antarctica, respectively. Simulation results reveal that about one third of the simulated variability in H218O can be explained by simultaneous changes of the surface temperature at the precipitation sites. Other climate variables influencing the isotope signal are identified by multiple linear regression, and the results show that the H218O record in central Greenland integrates the climatic history of a broader region. For the Law Dome region in Antarctica the H218O record appears to be mainly related to climate changes at the precipitation site only. For the deuterium excess a clear influence of climate condition at two distinct regions of the Indian Ocean on the simulated deuterium excess record of the Law Dome region is found. On the contrary, a reconstruction of the year‐to‐year variability of the deuterium excess signal in central Greenland, by Atlantic Ocean surface parameters only, fails. Additional correlation analyses between the ECHAM‐4 isotope simulation and indices of the North Atlantic Oscillation (NAO) and the El Niño‐Southern Oscillation phenomenon (ENSO) enable the detection of influenced regions in Greenland and Antarctica: While NAO and ENSO are imprinted in the simulated H218O record of precipitation, they cannot be detected in the simulated deuterium excess record.
Simulation of stable water isotope variations by the GENESIS GCM for modern conditions
Authors: Mathieu Renaud , Pollard David , Cole Julia E. , White James W. C. , Webb Robert S. , Thompson Starley L.
Published: October 2002

Incorporating stable isotope physics in a general circulation model (GCM) provides a promising means to study isotopic variability in precipitation, including the processes that cause isotopic variability in paleoclimatic archives such as ice cores. This paper describes the implementation and validation of stable isotope tracers in the GENESIS 2.0 GCM. The model reproduces the main features of present‐day isotopic fields and the characteristic large‐scale isotope‐climate relationships. Global δ18O‐δD, temperature‐δ18O, and precipitation‐δ18O relationships are well simulated, and the modeled regional patterns associated with continental vapor recycling over Europe and vertical gradients agree well with observations. In GENESIS a more sophisticated parameterization of interactions between precipitation and atmospheric vapor contributes to a better simulation of isotopic variations in dry climates. The standard model underestimates the global mean deuterium excess (δD‐8δ18O) in precipitation, although a heuristic sensitivity test suggests this may be remedied by accounting for nonneutral stratification in isotopic evaporative fractionation over ocean. Errors in simulated isotopic fields are analyzed to determine whether they are caused by local climatic biases in the GCM or by inaccurate parameterizations of isotope physics. Using the results of sensitivity experiments and comparisons with other isotopic GCM results, we identify key isotopic and climatic processes at the origin of the main errors and suggest additional studies to improve isotope simulations.
Spatial and temporal variations of Venus haze properties obtained from Pioneer Venus Orbiter polarimetry
Authors: Braak C. J. , de Haan J. F. , Hovenier J. W. , Travis L. D.
Published: October 2002

The spatial and temporal variations of the polarization of light scattered by Venus, as observed by the Pioneer Venus Orbiter between 1978 and 1990, is analyzed in terms of spatial and temporal variations of Venus upper haze properties. Special attention is given to choosing maps with sufficiently accurate geometrical information. This selection involves multiple‐scattering calculations for a number of reasonable models for the Venus atmosphere. For a set of observations in four wavelength bands of the Venus disk as a whole or a particular region thereof, observed values of Stokes parameters I and Q were used to obtain the average observed relative Stokes parameter qobs. Subsequently, the haze particle column density and the cloud top pressure were iteratively adjusted until values of qobs at two wavelengths were reproduced. For the planet as a whole, it was found that the haze particle column density decreased gradually during the Pioneer Venus Orbiter mission, whereas little significant temporal variability was found for the cloud top pressure. Similar long‐term behavior was derived for selected regions of Venus. Regions at higher latitudes exhibited higher values of both the haze particle column density and the cloud top pressure than regions at lower latitudes. Something similar holds for regions with low solar elevations as compared to regions in which the Sun was close to the zenith.