bluuebunny/crossref_metadata_2025 huggingface dataset:
Isosbestic point: An application for aerosol spectrometry
Authors: Tavker Shubhangi , Kumar P Pradeep , Carlon Hugh R. , Milham Merrill E.
Published: February 2004

When only two substances, in chemical equilibrium with each other, are responsible for all absorption in a wavelength region, there must exist at least one wavelength in the spectrum of this region where the absorption coefficient will be independent of the ratio of the concentrations of the two substances. This wavelength is called as the isosbestic point, or point of equal absorbance. Such points can also exist in the extinction spectra where at a particular wavelength the extinction coefficient becomes independent of solution concentration. Mie theory has been applied to compute such extinction coefficients. This paper shows the occurrence of isosbestic points in the extinction spectra of liquid droplet aerosols of sizes ranging from 0.1 μm to 10 μm and comprising several concentrations each of hydrochloric, nitric, sulfuric, and orthophosphoric acids in water. Such isosbestic points can serve as reference wavelengths for aerosol spectrometry and also for estimation of liquid content in haze, fog, and nonraining clouds.
Elevational changes in meteorological variables along a midlatitude glacier during summer
Authors: Greuell Wouter , Knap Wouter H. , Smeets Paul C.
Published: February 2004

During the summer of 1994 a glaciometeorological experiment was carried out on the Pasterze (a glacier in Austria). This paper reports on the data from six energy‐balance stations ranging in altitude from 2075 to 3225 m above sea level (asl). The wind regime was dominated by the glacier wind. On the tongue, directional constancies ranged between 0.94 and 0.97. Mean 2 m wind speed and specifie humidity were almost constant along the glacier. The variation in the 2 m temperature along the glacier cannot be described by the usually assumed constant decrease with elevation. On the tongue the 2 m temperature even increased with elevation. A much better description of the temperature distribution is given by a linear relation between the potential temperature and the distance along the flow line. This can be understood from a simple thermodynamic analysis of the glacier‐wind layer. It is further shown that changes in clear‐sky global radiation with elevation are due mainly to changes in local albedo and relief and hardly at all to changes in absolute optical path length and atmospheric water vapor and aerosol content. On the tongue the laterally averaged ice albedo is almost constant with elevation. The flux of incoming long‐wave radiation during clear‐sky conditions at U2 (2310 m asl) was 48 W/m2 higher than the flux at U5 (3225 m asl), on average. More than half of the difference was due to systematic differences in the shape of the temperature profile (probably the inversion depth is larger at U2). The rest can be ascribed to higher 2 m temperatures and larger amounts of upper hemisphere slopes at U2. The distributions of the meteorological variables and the parameterizations described in this paper might be incorporated in surface energy‐balance models designed to simulate the surface mass balance.
Transport of 222radon to the remote troposphere using the Model of Atmospheric Transport and Chemistry and assimilated winds from ECMWF and the National Center for Environmental Prediction/NCAR
Authors: Mahowald Natalie M. , Rasch Philip J. , Eaton Brian E. , Whittlestone Stewart , Prinn Ronald G.
Published: February 2004

The Model of Atmospheric Transport and Chemistry (MATCH) is used to simulate the transport of 222Rn using both European Centre for Medium‐Range Weather Forecasts (ECMWF) winds and National Center for Environmental Prediction/National Center for Atmospheric Research (hereafter referred to as NCEP) reanalysis winds. These winds have the advantage of being based on observed winds but have the disadvantage that the subgrid‐scale transport processes are not routinely archived. MATCH derives subgrid‐scale mixing rates for the boundary layer using a nonlocal scheme and for moist convective mixing using one of two parameterizations (Tiedtke [1989] or Pan and Wu [1997]). This paper describes the ability of the model to recreate mixing rates of 222Rn using the forecast center winds. Radon 222 is a species with a continental crust source and a simple sink involving radioactive decay with an e‐folding timescale of 5.5 days. This atmospheric constituent is therefore a good tracer for testing the vertical transport in the chemical transport model, as well as the horizontal transport from continental regions to remote oceanic regions. The various simulations of 222Rn are compared with observations as well as with each other, allowing an estimate of the uncertainty in transport due to uncertainties in the winds and subgrid‐scale processes. The calculated vertical profiles over the western United States are somewhat similar to observed, and the upper tropospheric concentrations compare reasonably well in their spatial distribution with data collected during Tropospheric Ozone II (TROPOZ II), although the model values tend to be higher than observed values, especially in the upper troposphere. The model successfully simulates specific observed pollution events at Cape Grim. It has more difficulty at sites farther from continental source regions, although the model captures the seasonal structure of the pollution events at these sites (Macquarie Island, Amsterdam Island, Kerguelen Island, and Crozet Island). Inclusion of a moist convective mixing scheme in MATCH increases 222Rn concentrations in the upper troposphere by 50% compared to not having moist convective mixing, while surface concentrations do not appear to be very sensitive to moist convection. In addition, differences between the upper tropospheric concentrations of radon predicted using the ECMWF and NCEP winds can be 30% for large areas of the globe, due to either differences in the forecast center winds themselves or the moist convective mixing schemes used in conjunction with them. This has implications for model simulations of radiatively and chemically important trace species in the atmosphere.
Deducing CCl3F emissions using an inverse method and chemical transport models with assimilated winds
Authors: Mahowald Natalie M. , Prinn Ronald G. , Rasch Philip J.
Published: February 2004

The ability of inverse modeling to deduce the sources of CCl3F using a chemical transport model based on assimilated winds is examined. The sources of CCl3F are relatively well known and thus offer an opportunity to test methodologies that can be used to estimate the source strengths of trace gases whose sources are less well constrained. The Model of Atmospheric Transport and Chemistry (MATCH) is used in combination with assimilated winds from the European Center for Medium‐Range Weather Forecasts (ECMWF) operational analysis and National Center for Environmental Prediction (NCEP)/National Center for Atmospheric Research reanalysis. Using our best a priori information about the emissions, comparisons are made between model simulations and observations at nine observing stations from the Atmospheric Lifetime Experiment/Global Atmospheric Gases Experiment and the Climate Monitoring and Diagnostics Laboratory networks. The model simulates many features of pollution events and seasonal variability with both wind data sets. However, the interhemispheric gradient is too strong in the simulations with the ECMWF winds, although it is accurate with the NCEP winds. A recursive weighted least squares inverse method is used to determine the magnitude of emissions from five regions. The total magnitude as well as the hemispheric distribution of the sources of CCl3F are correctly estimated using the combination of the observations, model transport, and assumed a priori emission distribution. However, longitudinal source information is more difficult to estimate from observations. A sensitivity study suggests that locating the observing stations closer to the source regions would improve the ability of the inverse method to deduce longitudinal information about the sources.
Modified Lagrangian‐mean diagnostics of the stratospheric polar vortices: 2. Nitrous oxide and seasonal barrier migration in the cryogenic limb array etalon spectrometer and SKYHI general circulation model
Authors: Nakamura Noboru , Ma Jun
Published: February 2004

The Lagrangian‐mean transport and mixing properties associated with the life cycle of the stratospheric polar vortices are analyzed using nitrous oxide (N2O) as a tracer, both observed by the cryogenic limb array etalon spectrometer (CLAES) and simulated by the Geophysical Fluid Dynamics Laboratory SKYHI general circulation model. Based on the modified Lagrangian‐mean (MLM) diagnostic formalism, the area equivalent latitude‐potential temperature cross sections are constructed for the N2O mixing ratio and the squared equivalent length, a Lagrangian equivalence of the eddy diffusion coefficient Kyy. The robustness of the analysis is tested and confirmed by subsampling the SKYHI tracer field at a CLAES‐equivalent resolution and comparing the resultwith the full‐resolution analysis. The seasonal and interhemispheric variabilities of the MLM cross sections are examined in detail. Both CLAES and SKYHI identify two major barriers to horizontal mixing (minimal equivalent length) in both hemispheres: a perennial subtropical barrier and an annual polar barrier. The polar barrier splits from the subtropical barrier in fall, migrates poleward in winter, and disappears at the demise of the polar vortex. The barriers are in general collocated with a tracer edge (concentrated gradients), but larger gradients do not necessarily translate to a greater barrier. For example, at the Arctic vortex edge, where the tracer gradients are greater than at the Antarctic vortex edge, the barrier is actually weaker. A significant difference is found in the structure of the barriers between the two northern hemisphere winters examined.
MIPAS used as a nonlocal thermodynamic equilibrium sounder for NO 5.3 μm band
Authors: Kostsov V. S. , Fischer H. , Timofeyev Y. M. , Stiller G.
Published: February 2004

The spaceborne MIPAS experiment has been numerically simulated in the 5.3 μm spectral domain, and the possible nonlocal thermodynamic equilibrium (non‐LTE) emissions from NO have been investigated. The interfering non‐LTE contributions from H2O, CO2, and O3 to atmospheric limb radiances have been estimated. The results have shown a negligibly small non‐LTE interference from H2O, CO2, and O3 but a considerable non‐LTE effect from NO in the stratosphere. The inverse problem has been formulated for the retrieval of the NO vertical profile and the vibrational temperatures of the NO lower vibrational states. The optimal estimation technique has been selected as a tool for the evaluation of the retrieval accuracy of NO number density and vibrational temperature vertical profiles. The error matrix calculations have shown that the retrieval accuracy for NO concentration is 30–40% in the altitude region 10–50 km in case of simultaneous retrieval of NO and vibrational temperature vertical profiles but can be improved by 10% if averaging of spectra is performed to achieve low noise. The retrieval accuracy for the vibrational temperatures of the X1/2–1 and X3/2–1 states was estimated to be better than 10 K in the stratosphere, but for the states X1/2–2 and X3/2–2, it appeared to be rather poor. The problem of NO vertical profile retrieval in the non‐LTE atmosphere is discussed. It was shown that in order to obtain a reasonable accuracy of NO retrieval of 20% up to 50 km altitude, it is necessary that the a priori uncertainty of the vibrational temperature of the state X1/2–1 is not higher than 5 K.
A new chemical ionization mass spectrometer technique for the fast measurement of gas phase nitric acid in the atmosphere
Authors: Mauldin R. L. , Tanner D. J. , Eisele F. L.
Published: February 2004

A new chemical ionization mass spectrometer (CIMS) technique has been developed for the measurement of HNO3. The technique has been field tested and informally intercompared with a standard filter pack method, and a new CIMS technique developed by the National Oceanic and Atmospheric Administration Aeronomy Laboratory. The technique has been shown under lab conditions to have sensitivities as high as 0.2 parts per trillion volume (pptv) HNO3 for a 1 s integration of each the reagent and product ions, with a typical clean air field detection limit of 1–3 pptv for the same integration times. The new technique also responds quickly to changes in HNO3 concentrations. The high sensitivity and fast response time make this technique well suited for airborne measurements.
Distribution, flux, and photochemical production of carbon monoxide in a boreal beaver impoundment
Authors: Bourbonniere Richard A. , Miller William L. , Zepp Richard G.
Published: February 2004

Photochemical transformations are important processes in the production and distribution of carbon monoxide (CO) in lakes and oceans. Initial studies of dissolved CO concentrations, CO fluxes, and in situ photochemical production were conducted at a boreal forest beaver pond near Thompson, Manitoba. Dissolved CO concentration profiles are characterized by stronger vertical gradients during the summer months. Midday integrated CO content of the upper water column ranges from 5–6 μmol/m2 in the summer to 3–4 μmol/m2 in the autumn. Surface concentrations of dissolved CO increase after sunrise in both summer and fall, attaining up to 120 nmol/L by late afternoon in the summer compared with 50 nmol/L for the same time in the autumn. The CO supersaturation ratio for this pond ranges from 37 to 875, which is 4–7 times higher than other freshwater systems studied. CO flux across the air‐water interface, measured in static floating quartz chambers, tracks well with UV‐A, UV‐B, and full spectrum solar irradiance in the summer. The maximum flux measured at solar noon of 1.6 nmol/m2/s on Julian day 168 in June 1994 was 5 times higher than that measured at the same time of day and location on Julian day 269 in September 1995. The depth to which photochemical production of CO occurs is limited in these dark waters because of the strong absorbance of solar radiation by dissolved organic matter. In situ exposure experiments, conducted over 24 daylight hours, under autumn light conditions, indicated that up to 20 nmol of CO can be produced photochemically at 5 cm depth from 1 mg of dissolved organic carbon, rapidly decreasing to less than one fourth that amount below 7 cm depth. In situ photoproduction rates compare favorably with those from a theoretical model (GCSOLAR). Measured static fluxes are similar to what would be expected from a simple laminar film diffusive flux model given the measured supersaturation ratios or the in situ photoproduction rates as input values.
Estimation of surface net radiation in the boreal forest and northern prairie from shortwave flux measurements
Authors: Kaminsky Keith Z. , Dubayah Ralph
Published: February 2004

The relationships between net radiation and surface shortwave fluxes were explored by using one year of 15 min surface flux data for nine sites in central Canada. The sites were located throughout a 50,000 km2 area, in conjunction with the BOREAS project, across a gradient in surface cover from prairie grassland to boreal forest. When insolation alone was used to estimate instantaneous net radiation, the r2 values at the nine sites ranged from 0.93 to 0.98. The use of the net shortwave flux as a predictor improved the range in r2 values to between 0.96 and 0.99. The root‐mean‐square error across sites varied from 26 to 41 W m−2 without albedo to a range of 18–36 W m−2 when albedo was included. The data for all sites were combined to produce a single linear relationship with an r2 value of 0.97 and a root‐mean‐square error (RMSE) of 29 W m−2 when the surface albedo is included. Cross validations were performed where the data for eight sites were used to predict net radiation at a ninth (excluded) site, revealing little site‐specific bias. Classifications of the data by season and by transmittance provided little improvement in estimated net radiation at any site. Our study suggests that a single equation using net shortwave flux alone can be used to estimate net radiation for large portions of the boreal forest and northern prairie, regardless of spatial and temporal variation in surface cover and atmospheric conditions.
Retrieval of vertical profiles of ozone from high‐resolution infrared solar spectra at Rikubetsu, Japan
Authors: Nakajima H. , Liu X. , Murata I. , Kondo Y. , Murcray F. J. , Koike M. , Zhao Y. , Nakane H.
Published: February 2004

Vertical profiles of volume mixing ratios (VMRs) of O3 were retrieved from high‐resolution, infrared solar absorption spectra taken at Rikubetsu, Japan (43.5°N), by a ground‐based Fourier transform infrared spectrometer (FTIR). The VMR profile retrieval is based on an iterative inversion algorithm, which utilizes line‐by‐line, layer‐by‐layer calculations. Two different microwindow regions (1146 and 1163 cm−1) were used for the retrieval calculations. Eighty ozone vertical profiles were retrieved from the two spectral regions from 40 spectra and were compared with 17 ozonesonde measurements from a nearby station. Average differences between 18 and 34 km were less than ±12% for both 1146 and 1163 cm−1 retrievals. The standard deviation of the average differences between 18 and 34 km were less than 19% and 14% for respective windows. Effects of several different factors on errors of profile retrievals, such as initial guess, temperature profiles, and instrument line shape functions, were evaluated by numerical simulation. The estimated errors were extremely consistent with the observed errors between 18 and 34 km, suggesting that the FTIR was optically well adjusted. In general, retrieval of the O3 profile gives VMRs in an accuracy better than 40% at 14 km, improving to 10–15% between 18 and 30 km, with precision of 30% and 20% for these altitude ranges, respectively.
Simultaneous retrievals of cloud optical depth and droplet concentration from solar irradiance and microwave liquid water path
Authors: Boers Reinout
Published: February 2004

A 20‐month time series of continuous observations of microwave radiation and solar irradiance was used to estimate the cloud optical depth and droplet number concentration at the Cape Grim Baseline Air Pollution Station, Tasmania, (40°41′S, 144°41′E). The data were selected by wind directions. When the air was from “baseline” origin, i.e., it had travelled over long oceanic distances and was mostly devoid of anthropogenic influences, the retrieved droplet concentration and optical depth were lower than when the air was from “nonbaseline” i.e., continental origin. Therefore the observed variation in cloud microphysical properties reflects the difference between the natural background conditions over the Southern Ocean and continental conditions with elevated droplet counts. Under baseline conditions the retrieved cloud optical depth exhibits a weak but perceptible seasonal cycle that has been previously observed from satellite data with a minimum in the austral winter, and a maximum in the austral summer. The results demonstrate that routine retrievals of cloud microphysical properties are possible using only a pyranometer and a microwave liquid water radiometer.
Mesospheric bores
Authors: Dewan E. M. , Picard R. H.
Published: February 2004

A “spectacular gravity wave event” was observed from Haleakala on a four‐color all‐sky imager, as well as in OH Meinel‐band intensity and temperature, on October 10, 1993, during the Airborne Lidar and Observations of Hawaiian Airglow (ALOHA‐93) campaign [Taylor et al. 1995]. The event consisted of a sharp frontal passage in airglow brightness propagating at 76 m/s across the field of view and accompanied by small‐scale waves behind the front. The purpose of this paper is to propose an explanation for the frontal passage in terms of an internal mesospheric undular bore. Although there have been a number of observations of internal bores in the troposphere and in the ocean, we believe this is the first reported observation of such a bore in the mesosphere. A simple two‐layer model, constructed by analogy with the theory of bores in river channels, explains the observations quantitatively. Several predictions are advanced which could serve to test the model.
A meteorological overview for the Pacific Exploratory Mission‐West Phase B
Authors: Merrill John T. , Newell Reginald E. , Bachmeier A. Scott
Published: February 2004

The large‐scale meteorology of the western North Pacific region during the Global Tropospheric Experiment (GTE) Pacific Exploratory Mission‐West Phase B (PEM‐West B) study period, February 7 to March 15, 1994, is described. Mean streamline/isotach charts and both zonal and meridional cross sections are presented which contrast the wind conditions encountered during PEM‐West B and the earlier Pacific Exploratory Mission‐West Phase A (PEM‐West A) expedition (September 17 to October 21, 1991). Further, vertical profiles of wind and thermodynamic variables are presented to show the differing air mass structures during the two periods, using operational rawinsonde data from stations at Guam, Hong Kong, Tateno (Japan), and Yan An (China). Finally, five case studies indicating the variety of chemical source signatures and transport patterns studied in the campaign are displayed and discussed.
Methods for measuring changes in atmospheric O2 concentration and their application in southern hemisphere air
Authors: Keeling Ralph F. , Manning Andrew C. , McEvoy Elizabeth M. , Shertz Stephen R.
Published: February 2004

Methods are described for measuring changes in atmospheric O2 concentration with emphasis on gas handling procedures. Cryogenically dried air samples are collected in 5 L glass flasks at ambient pressure and analyzed against reference gases derived from high‐pressure aluminum tanks. Fractionation effects are minimized by avoiding pressure and flow variations throughout the gas‐handling system. The overall external reproducibility is approximately ±3.3 per meg, with systematic errors associated with collecting samples and with storing them for 1 year reduced to the level of 3 per meg or smaller, The demonstrated short‐term reproducibly of air delivered from high‐presure tanks is ±1.5 per meg, with the composition changing by at most 5 per meg by surface desorption reactions as the tank is depleted to below 3500 kPa. A 9‐year survey of a suite of six reference gases showed no systematic long‐term trends in relative O2 concentrations to the level of 5 per meg. Results are presented from samples collected at Cape Grim (41°S), Macquarie Island (54°S) and the South Pole Station (90°S). From measurements spanning 1991–1995 it is found that the O2 concentrations at the South Pole are on average 3.6±1.2 per meg higher than at Cape Grim. This result runs contrary to the expectation that the air at high southern latitudes should be depleted in O2 as a result of O2 uptake from the Southern Ocean and may require the existence of unknown O2 sources near Antarctica or unexpected atmospheric transport patterns.
A Monte Carlo study of upper tropospheric reactive nitrogen during the Pacific Exploratory Mission in the Western Pacific Ocean (PEM‐West B)
Authors: Thompson Anne M. , Singh Hanwant B. , Stewart Richard W. , Kucsera Tom L. , Kondo Yutaka
Published: February 2004

A subset of Pacific Exploratory Mission in the Western Pacific Ocean (PEM‐West B) data (northwestern Pacific, March 1994), selected to represent upper troposphere (UT) midlatitude conditions, is analyzed to answer the following questions: (1) Is there a shortfall in total reactive nitrogen (NOy) as measured during PEM‐West B in this region? (2) If so, what are the likely contributions of interfering constituents like HCN or of other reactive nitrogen species? For question 1 our analyses show that 87% of total reactive nitrogen measured in the UT is accounted for by NOx + HNO3 + PAN (similar to Kondo et al. [this issue (a, b)], Talbot et al. [this issue], and Singh et al. [1997a, b]). For question 2 we find that less than 20 pptv (<5% of mean NOy) is possibly HCN. A one‐dimensional model that simulates mean mixing ratios of this PEM‐West B data is used with a Monte Carlo approach to explore other candidates for unmeasured nitrogen species and NOy partitioning. Using standard gas‐phase reactions with varying rate coefficients [Thompson and Stewart, 1991; Stewart and Thompson, 1996], it is found, on average, that NOx + HNO3 + PAN = 399 pptv (observed mean equal to 432±97 pptv). A more complete inventory for total reactive nitrogen (Σ NOi = NOx + HNO3 + PAN + HNO4 + CH3O2NO2 + alkyl nitrates + C2H5O2NO2) is 494±91 pptv, with 19% consisting of HNO4 + CH3O2NO2 + alkyl nitrates. Thus, whether unmeasured forms of reactive nitrogen are present or not, total reactive nitrogen as measured at midlatitude UT during PEM‐West B is accounted for within the measurement uncertainty. The greatest kinetics uncertainties are in thermolytic losses for HNO4, PAN, and CH3O2NO2 (120–150% by the method of Stewart and Thompson [1996]). Nonetheless, comparison with PEM‐West B data shows that panel‐recommended kinetics expressions [Demore et al., 1994] can explain reactive nitrogen observations without invoking extreme rates or heterogeneous processes. In summary, large concentrations of unmeasured reactive nitrogen species were not prevalent during midlatitude UT PEM‐West B sampling although the observed shortfall (13%) can be explained by HNO4 + alkyl nitrates + CH3O2NO2. Agreement between theory and observations may also reflect improved instrument capabilities for measuring reactive nitrogen.
Footprint considerations in BOREAS
Authors: Kaharabata S. K. , Schuepp P. H. , Ogunjemiyo S. , Shen S. , Leclerc M. Y. , Desjardins R. L. , MacPherson J. I.
Published: February 2004

Comparisons of observations of concentration or flux from platforms at various heights, such as tower and aircraft, must take into account differences in the location and extent of upwind surface source or sink areas which affect the individual observations, with their physical and biological characteristics. Such “footprint” estimates are based on solutions of the diffusion/advection equation which have not previously been evaluated over a boreal ecosystem. In order to adjust an analytical footprint model within the surface layer above forest canopies typical for the Boreal Ecosystem‐Atmosphere Study (BOREAS) sites, 29 tracer gas release experiments were carried out between August 30 and September 9, 1994, at three tower sites in the northern study area (NSA). Sulphur hexafluoride (SF6) was released from point sources at various upwind distances from the towers under various meteorological, terrain, and release conditions. Wind, temperature, and stability parameters, during each release period, were used as input into calculations of vertical concentration profiles sampled at the towers, based on a three‐dimensional diffusion model. Predictions of concentration profiles, or back calculation of source strength from observed profiles, were in good agreement with observed concentrations or actual release rates, respectively. The diffusion model was then used to compute footprint estimates for stable to unstable conditions, for tower and aircraft‐based observation platforms. They show spatially constrained footprints in the surface layer, due to effective vertical coupling, so observations from towers and low‐flying aircraft must be expected to be very site specific, and scaling up to larger areas will have to be done with careful consideration of surface mosaics. Our study also included footprint estimates made for airborne observations above the surface layer, based on large‐eddy simulations over “BOREAS‐like” terrain, for boundary layer structures comparable to those observed in BOREAS. They document the progressive decoupling of airborne observations from the surface features at these heights.
The potential for atmospheric mixing processes to enhance the binary nucleation rate
Authors: Nilsson E. Douglas , Kulmala Markku
Published: February 2004

The formation of sulfate aerosol particles due to atmospheric mixing processes is investigated using a classical model for binary nucleation. The nucleation rate is seen to be enhanced when two air parcels with different temperature and relative humidity mix with each other. This is due to the curvature on the vapor pressure diagram, and the whole process is more enhanced in the binary H2SO4 ‐ H2O system when compared with the unary case. If the differences are, for example, 8 K and 60%, the nucleation rate can increase by 2 to 3 orders of magnitude if they are mixed. A brief survey of atmospheric situations that could favor this process is included. The negative feedback effects of coagulation and condensation on the new aerosol particles are found to decelerate the particle formation, but not to prevent it. The damping effect of condensation on preexisting aerosols is examined. It is found that the mixing process is most likely to be important at background conditions, which are the most difficult for explaining nucleation. The mixing effect is also compared with the effect of fluctuations in temperature and relative humidity. In some cases the mixing effect seems to be the most important of the effects. The differences in the effect of mixing on nucleation rate and the limitations of its validity are given for different initial conditions. A simple parameterization of the effect of atmospheric mixing on the binary homogeneous nucleation rate of H2SO4 and H2O is also given.
Large‐scale enhancements in NO/NOy from subsonic aircraft emissions: Comparisons with observations
Authors: Witte Jacquelyn C. , Folkins I. A. , Neima J. , Ridley B. A. , Walega J. G. , Weinheimer A. J.
Published: February 2004

One of the DC‐8 flights from the 1992 AASE 2 campaign flew south from Maine over the Atlantic Ocean, sampling air downstream of areas in the eastern United States associated with heavy air traffic. We use a photochemical trajectory model to help interpret observed NO/NOy ratios from the stratospheric portions of this flight. The model is run with and without an additional in situ NOx source from the 1992 Boeing‐McDonnell Douglas (BMD) emissions climatology. During the northern section of this flight, the inclusion of this additional NOx source resulted in a significant improvement with observed large‐scale NO/NOy ratios. This comparison suggests that air traffic over the eastern United States is sufficiently dense to enhance NO/NOy ratios on a regional scale, even when the characteristic NO spikes from exhaust plumes are absent. During the southern portion of the flight, in which the DC‐8 flew at a higher altitude, observed NO/NOy ratios agreed much better with the no‐emissions scenario. This may be a reflection of the difficulty of using a climatological NOx emissions database to infer instantaneous NO/NOy ratios. It would be desirable to have a larger database of lower stratospheric NO and NOy measurements downstream of the eastern United States. This would enable more stringent statistical comparisons of observed NO/NOy ratios with the model‐predicted enhancements of this ratio arising from aircraft emissions.
Role of aerosol size distribution and source location in a three‐dimensional simulation of a Saharan dust episode tested against satellite‐derived optical thickness
Authors: Schulz Michael , Balkanski Yves J. , Guelle Walter , Dulac Francois
Published: February 2004

An off‐line global three‐dimensional tracer model based on analyzed wind fields was augmented to simulate the atmospheric transport of mineral dust. The model describes the evolution of the aerosol size distribution and hence allows to compute aerosol number and mass concentrations. In this study we describe the parameterization of the sedimentation process and include a preliminary source formulation but exclude wet deposition. Validation of the model is done during a 16‐day period in June‐July 1988 with very scarce precipitation. It is based on a comparison of every model grid box with daily satellite‐derived optical thickness observations of Saharan dust plumes over the North Atlantic and the Mediterranean. The model reproduces accurately the daily position of the dust plumes over the ocean, with the exception of Atlantic regions remote from the African coast. By systematic analysis of transport and aerosol components we show that the largest uncertainty in reproducing the position of the dust clouds is the correct localization of the source regions. The model simulation is also very sensitive to the inclusion of convection and to an accurate treatment of the sedimentation process. Only the combination of source activation, rapid transport of dust to higher altitudes by convective updraft and long‐range transport allows the simulation of the dust plumes position. This study shows that a mineral dust transport model is only constrained if both the source strength and the aerosol size distribution are known. The satellite observation of optical thickness over the Mediterranean and assumptions about the size distribution indicate that the dust emission flux was of the order of 17×106 t for the 16‐day period under investigation. The simulations suggest that a major aerosol mode initially around 2.5 μm with a standard deviation of 2.0 plays the dominant role in long‐range transport of mineral dust.
Particle shape and internal inhomogeneity effects on the optical properties of tropospheric aerosols of relevance to climate forcing
Authors: Pilinis Christodoulos , Li Xu
Published: February 2004

Direct climate forcing by atmospheric aerosols is a strong function of the aerosol scattering coefficient, single‐scatter albedo, and upscatter fraction. Recent estimates of forcing assume that atmospheric particles are homogeneous spheres. In this paper we attempt to quantify the uncertainty associated with this assumption. We have developed a model, NEPH3, which calculates all the optical properties relevant to direct forcing. The model is tested against measured scattering coefficients from Barbados, assuming that the aerosol particles are homogeneous spheres, stratified spheres, and spheroids. The correlation coefficient of model predictions with measurements is 0.98, for the April 5 to May 3 period, while it is 0.86 for the low dust period of April 14 to May 3. The model indicates that the scattering and extinction coefficients and as a result, the single‐scatter albedo are insensitive to the shape and the structure of the particles throughout the measurement period. On the other hand, our calculations show that volume equivalent spherical‐nonspherical differences in the backscatter fraction can be large, especially in cases of nonhygroscopic (dust) aerosol, or when the relative humidity is low enough so that the particles are dry. The shape of the particles seems to be important in upscatter fraction calculations too, especially for small solar zenith angles and super‐micron‐sized particles. Therefore for the size distributions measured in this study, the assumption that atmospheric aerosols are homogeneous spheres may introduce errors as high as 300% in direct forcing estimations, especially when the Sun is increasingly close to the zenith.
Size‐resolved and chemically resolved model of atmospheric aerosol dynamics
Authors: Meng Zhaoyue , Dabdub Donald , Seinfeld John H.
Published: February 2004

A three‐dimensional, size‐resolved and chemically resolved aerosol model is developed. Gas‐to‐particle conversion is represented by dynamic mass transfer between the gas and aerosol phases. Particle‐phase thermodynamics is computed by a new thermodynamic model, Simulating Composition of Atmospheric Particles at Equilibrium 2. The aerosol model is applied to simulate gas and aerosol behavior in the August 27–29, 1987, episode in the South Coast Air Basin of California. The assumption that volatile inorganic species such as NH4NO3 are at instantaneous, local equilibrium is examined.
Particle production in the remote marine atmosphere: Cloud outflow and subsidence during ACE 1
Authors: Clarke A. D. , Varner J. L. , Eisele F. , Mauldin R. L. , Tanner D. , Litchy M.
Published: February 2004

During November and December 1995 the First Aerosol Characterization Experiment (ACE 1) was undertaken as part of the International Global Atmospheric Chemistry (IGAC) Program. A key objective of the aircraft component of this experiment included the identification of source regions for new particles in the remote marine atmosphere. No evidence was found for particle production in the marine boundary layer (MBL), but extensive observations of enhanced layers of “new” particles were found in the free troposphere (FT). These layers were generally found at altitudes that corresponded to nearby cloud top heights and exhibited concentrations that exceeded MBL air by about 1000 to 10,000 cm−3. Many layers were also associated with enhanced concentrations of water vapor and sulfuric acid. Focused cloud experiments demonstrated that these particles were recently formed and originated in the outflow region of clouds preferentially after late morning when photochemical processes had become sufficiently active. The production and growth of these particles were rapid, and they appeared to evolve and merge with a background nuclei spectra on the scale of hours to a day. These measurements in midlatitude postfrontal air undergoing shallow convection indicated that particles were produced in the trailing cloud outflow region as low as 2 km altitude and that the base of this layer extended down to the inversion in the region of postfrontal subsidence. Other ACE 1 measurements made in transit near equatorial convection also revealed small nuclei aloft at altitudes up to 6 km and a trend in decreasing concentrations, in conjunction with steadily increasing size, during descent toward the surface. The concentration and size distributions in this these regions indicate that significant numbers of new nuclei are formed aloft in various cloud outflow regions and that they can provide a source for the MBL aerosol via subsidence. This nucleation appears to be favored when existing surface areas approach or drop below about 5–10 μm2 cm−3.
Localized rapid ozone loss in the northern winter stratosphere: An analysis of UARS observations
Authors: Nair Hari , Allen Mark , Froidevaux Lucien , Zurek Richard W.
Published: February 2004

Observations of low‐ozone air pockets forming during northern winter in the middle stratosphere outside the polar vortex provide an opportunity to test models of the photochemistry of ozone at several altitudes, as the trajectories of the associated air parcels are well defined by Lagrangian transport codes over the periods of interest, and vertical profiles of key species, including ozone, are available from instruments aboard the Upper Atmosphere Research Satellite (UARS). We find that a Lagrangian photochemical model, where the chemistry within an isolated parcel of air is simulated as it travels along a specified trajectory, does reproduce the observed formation of low‐ozone pockets in the 6–10 mbar altitude range within the measurement uncertainties but overestimates the ozone loss rate at higher altitudes. The rapid ozone loss localized in these pockets is due to the isolation of air at high latitudes (and high solar zenith angles). Thus the low ozone levels are due to a decrease in the odd oxygen production rate and not to an increase in the loss rate by reaction with halogen species, as in the “classical” ozone hole.
A model of meteoric iron in the upper atmosphere
Authors: Helmer Magdalena , Plane John M. C. , Qian Jun , Gardner Chester S.
Published: February 2004

A one‐dimensional steady state model of meteoric iron in the Earth's upper atmosphere has been developed. The model covers the altitude range 70 to 110 km at latitude 40°N and allows for seasonal variations in the chemical composition, total density and temperature of the upper atmosphere. It addresses the three processes thought to be most influential in affecting the layer of free Fe atoms at approximately 85 km altitude, that is, the deposition of iron via meteor ablation, the vertical distribution of the ablated material, and the partitioning of the metal among 10 constituent species as a result of neutral and ion gas phase reactions in dynamical steady state. A comparison between calculated and observed average monthly Fe profiles reveals that many features displayed by the meteoric iron layer (such as the peak height and width, the small scale heights at the topside and underside, and the seasonal changes in these parameters) can be successfully reproduced by the model.
Some inferences on the propagation mechanisms of dart leaders and return strokes
Authors: Rakov Vladimir A.
Published: February 2004

Inferences regarding the propagation mechanisms of dart leaders and return strokes are made from a comparison of the behavior of traveling waves on a lossy transmission line and the observed characteristics of these two lightning processes, in particular, their observed light profiles [Jordan and Uman, 1983; Jordan et al., 1997]. The bottom kilometer or so of the subsequent‐stroke lightning channel is modeled as an R‐L‐C transmission line whose resistance per unit length R is first reduced by the downward propagating dart leader and then further reduced by the upward propagating return stroke, while variations in the inductance per unit length L and the capacitance per unit length C during the dart‐leader and return‐stroke processes are neglected. The transmission line is assumed to be linear (R = const) but different ahead of and behind either the dart‐leader or the return‐stroke front with any nonlinear effects occurring at the front. The resistance per unit length is estimated to be as follows: (1) ahead of the dart‐leader front about 18 kΩ/m or greater; (2) behind the dart‐leader front (and ahead of the return‐stroke front) about 3.5 Ω/m; and (3) behind the return‐stroke front about 0.035 Ω/m. Comparison of the behavior of traveling waves on such a transmission line with the observed properties of luminosity waves associated with dart leaders and return strokes in conjunction with other observations suggests that there is a difference between these two lightning processes in terms of the dominant propagation mechanism. It appears that the return stroke is similar to a “classical” (linear) traveling wave that suffers appreciable attenuation and dispersion and whose advancement can be visualized as being due to the progressive discharging of elemental capacitors (previously charged by the leader process) of the equivalent R‐L‐C transmission line. Ionization does occur during the return‐stroke process but has a relatively small effect on the wave propagation characteristics which are primarily determined by the transmission‐line parameters ahead of the front, as opposed to being determined by the wave magnitude. On the other hand, the progression of the dart leader is apparently facilitated by sustained electrical breakdown at its front, so the downward propagation characteristics of the dart‐leader wave are primarily determined by the wave magnitude, which largely determines the front electric field, as opposed to being determined by the transmission‐line parameters of the channel ahead of the front. Thus the dart leader may be best described as a downward moving ionizing front which generates current waves that propagate upward along the dart‐leader channel. Finally, it can be argued that the subsequent return stroke can be viewed as a ground “reflection” of the dart leader.
Measurements of tropospheric NO3 at midlatitude
Authors: Aliwell S. R. , Jones R. L.
Published: February 2004

The column abundance of NO3 has been measured through sunrise at the midlatitude site of Cambridge, England. (52.2°N, 0.1°E) using zenith sky visible absorption spectroscopy. This technique allows distinction between the tropospheric and stratospheric components of the total NO3 vertical column during sunrise. Measurements of the tropospheric component during July to October 1995 are presented and show an average concentration of 89 ppt if it is assumed that all tropospheric NO3 is present in a 1 km thick atmospheric boundary layer. The day to day variability is considerable. The NO3 measurements together with point measurements of O3, NO2, NO and temperature show that the NO3 lifetime is significantly longer than reported in other studies. The very short lifetime expected as a result of the measured levels of NO suggests that the ground level measurements of O3, NO2 and NO are not representative of the whole boundary layer and therefore that the NO3 lifetime at the surface is not representative of the average boundary layer lifetime. Idealized model calculations confirm that a gradient in NO3 concentration would be expected in the boundary layer as a result of the different timescales for mixing and reaction with NO in a stable nocturnal situation. Any other rapid loss reaction near the surface, for example, with dimethyl sulphide in the marine environment or isoprene at a wooded rural site, may have a similar effect. In addition, such high concentrations of NO3 may alter the balance toward significant nighttime oxidation of hydrocarbons either directly or by production of OH.
Wet deposition of major ions and trace elements in the eastern Mediterranean basin
Authors: Al‐Momani Idrees F. , Aygun Sezer , Tuncel Gürdal
Published: February 2004

Elemental and ionic composition of the eastern Mediterranean precipitation was determined on the Mediterranean coast of Turkey between January 1992 and January 1994. Measured concentrations were comparable with the concentrations reported from other rural stations. Concentrations of elements and ions show strong short‐term and well‐defined long‐term variations. Short‐term variations are due to transport from different source areas whereas long‐term (seasonal) variations are governed by the seasonal changes in the wet removal of particles during their transport from source areas to the eastern Mediterranean region. The most important characteristic of the precipitation in the eastern Mediterranean region is the extensive neutralization of acidity by the airborne CaCO3 particles from soil. Wet deposition fluxes of elements and ions are smaller than the fluxes reported from other rural stations owing to small annual rainfall in the region. Except for soil‐related elements, wet deposition fluxes of all elements are significantly higher in the winter season. Annual wet deposition fluxes are highly episodic, with only 10–20 of the samples accounting more than 50% of annual fluxes for most of the measured elements and ions.
Lightning energetics: Estimates of energy dissipation in channels, channel radii, and channel‐heating risetimes
Authors: Borovsky Joseph E.
Published: February 2004

In this report, several lightning‐channel parameters are calculated with the aid of an electrodynamic model of lightning. The electrodynamic model describes dart leaders and return strokes as electromagnetic waves that are guided along conducting lightning channels. According to the model, electrostatic energy is delivered to the channel by a leader, where it is stored around the outside of the channel; subsequently, the return stroke dissipates this locally stored energy. In this report this lightning‐energy‐flow scenario is developed further. Then the energy dissipated per unit length in lightning channels is calculated, where this quantity is now related to the linear charge density on the channel, not to the cloud‐to‐ground electrostatic potential difference. Energy conservation is then used to calculate the radii of lightning channels: their initial radii at the onset of return strokes and their final radii after the channels have pressure expanded. Finally, the risetimes for channel heating during return strokes are calculated by defining an energy‐storage radius around the channel and by estimating the radial velocity of energy flow toward the channel during a return stroke. In three appendices, values for the linear charge densities on lightning channels are calculated, estimates of the total length of branch channels are obtained, and values for the cloud‐to‐ground electrostatic potential difference are estimated.