bluuebunny/crossref_metadata_2025 huggingface dataset:
Infrared spectroscopy of model tropospheric aerosols as a function of relative humidity: Observation of deliquescence and crystallization
Authors: Cziczo D. J. , Nowak J. B. , Hu J. H. , Abbatt J. P. D.
Published: February 2004

The infrared extinction spectra of model tropospheric aerosols, (NH4)2SO4, NH4HSO4, NaCl, and artificial seawater, have been measured as a function of relative humidity. Experimentally, submicron‐sized aerosol particles are spectroscopically monitored as they flow at atmospheric pressure on a 30‐s timescale through a room temperature infrared absorption cell. By monitoring absorption features due to either constituent ions or water molecules, we infer both the physical phase and, to some degree, the chemical composition of the aerosol particles. It is observed that (1) solid (NH4) SO4 and NaCl aerosol particles exhibit deliquescence at 79±1% and 75±1% relative humidity, respectively, very close to their thermodynamic values; (2) (NH4)2SO4 and NaCl liquid particles exhibit crystallization at relative humidities of 33±2% and 43±2%, respectively, well below their deliquescence points; (3) NH4HSO4 aqueous aerosol particles remain in the liquid state to relative humidities as low as 2%, far below the thermodynamic deliquescence humidity of 39%; and (4) artificial seawater aerosol particles show strong H2O absorption features at low relative humidities, arising either because the particle has not crystallized or because solid hydrates of Mg2+ salts have formed. These observations illustrate the extent to which water will be present in the aerosol condensed phase in both laboratory experiments and in the atmosphere. Specifically, for (NH4)2SO4 and NaCl particles, the water content is expected to be low at relative humidities below the crystallization point, whereas the aerosol particles will be liquid at higher relative humidities. NH4HSO4 and artificial seawater aerosols will both contain significant quantities of water down to very low relative humidities, present either as a liquid or possibly as hydrates of Mg2+ in the case of artificial seawater. By adding gas‐phase D2O to NH4HSO4 and artificial seawater aerosols at low relative humidity, condensed phase D2O features appear in the spectra, indicating facile exchange of water between the gas‐phase and the particles. Conversely, aerosols with low water content, such as solid NaCl, do not exhibit condensed‐phase D2O features in the presence of gas‐phase D2O.
Simulation of Stratospheric Aerosol and Gas Experiment (SAGE) II cloud measurements using airborne lidar data
Authors: Kent G. S. , Winker D. M. , Vaughan M. A. , Wang P.‐H. , Skeens K. M.
Published: February 2004

The Stratospheric Aerosol and Gas Experiment (SAGE) II satellite instrument measures the attenuation due to stratospheric aerosols, gases, and clouds along a tangential path through the atmosphere from the Sun to the satellite instrument. Data from SAGE II and its predecessor, SAGE I, have been used to study and develop climatologies for high altitude cloud. Because these instruments measure the total attenuation on a long optical path and the data are inverted under the assumption of spherical homogeneity in the region of the measurement, interpretation in terms of individual cloud characteristics is difficult. Airborne lidar data, taken on approximately 8000 km of flight path over the tropical Pacific Ocean, have been used to simulate high altitude SAGE II cloud measurements and their inversion. The lidar data set, not necessarily typical for the region flown over, showed the general presence of patchy cloud with a mean horizontal cloud dimension of 20–25 km and a vertical thickness of 0.6–0.8 km. Use of the lidar data to simulate SAGE II measurements produces cloud extinction values similar in magnitude and distribution to those obtained from SAGE II. These simulations also show the existence of three possible error conditions that may occur as a result of cloud inhomogeneities along the viewing path. In the first error condition, the true altitude of a cloud may be higher than that found as a result of the SAGE II inversion, errors of 1 km or greater occurring in just under 40% of the simulations, mainly at the lower cloud altitudes. In the second error condition, the inverted cloud extinction may differ from the volume averaged extinction along the horizontal ray path, the former often being biased slightly low. In the third error condition, the presence of nonuniform or isolated cloud patches can result in an apparent negative inverted extinction value just below the cloud. Such values were observed in about one third of the simulations. The first two of these errors have been statistically quantified for application to SAGE II cloud data.
Retrieval of ozone mixing ratio profiles from ground‐based millimeter wave measurements disturbed by standing waves
Authors: Kuntz M. , Hochschild G. , Krupa R.
Published: February 2004

We present middle‐atmospheric ozone mixing ratio profiles obtained from measurements around 142 GHz using the ground‐based millimeter wave radiometer (MIRA 1) developed at the Forschungszentrum Karlsruhe. Data were recorded during spring season of 1994 from the summit of Hornisgrinde (1165 m, 48.61°N, 8.20°E), Germany. They were severely disturbed by standing waves caused by reflections in the internal transmission path. The profiles are retrieved by using a new extension of the well‐known constrained linear inversion and optimal estimation method, respectively. With this new extension both techniques are capable of fitting the amplitudes and phases of standing waves in the inversion algorithm itself rather than during preprocessing. In the present paper, analysis of the retrieval technique is given, including a short analysis of sources of error. The new retrieval technique proves to be a reliable tool for the inversion of measured spectra disturbed by standing waves.
Photochemical box modeling of long‐range transport from North America to Tenerife during the North Atlantic Regional Experiment (NARE) 1993
Authors: Schultz M. , Schmitt R. , Thomas K. , Volz‐Thomas A.
Published: February 2004

A photochemical trajectory model was used to reproduce the concentrations of several trace gases measured at Izaña/Tenerife during a selected episode of August 1993 with long‐range transport from the North American continent across the Atlantic Ocean. The set of initial conditions necessary to match the observations is analyzed and compared with available data in the literature. Initial and final concentrations of several hydrocarbons are in reasonable agreement with other measurements made at similar latitudes, and the CO/NOy and NMHC/NOy ratios are within the range of those observed from airborne measurements over the eastern United States and Canada. Important exceptions are the light olefines ethene and propene which may originate from oceanic emissions. The simulated concentration of ozone at Izaña is very similar to the initial concentration used, and there is no indication for photochemical net ozone production during long‐range transport. The abundance of NOx at the end of the simulation is largely due to decomposition of peroxyacetyl nitrate during the descent of the air before reaching Izaña. The results differ significantly from those of a steady state calculation which was made for comparison, and the height evolution of the trajectory was found to be a critical parameter. A parameter variation study revealed that significant net ozone production only occurs for very low initial ozone concentrations and/or NOy levels that are well above those observed during the period at Izaña at night. This study also emphasizes the important role of water vapor for the tropospheric ozone budget and the role ozone itself plays in its photochemical balance.
Trace gas measurements during the Oxidizing Capacity of the Tropospheric Atmosphere campaign 1993 at Izaña
Authors: Fischer H. , Nikitas C. , Parchatka U. , Zenker T. , Harris G. W. , Matuska P. , Schmitt R. , Mihelcic D. , Muesgen P. , Paetz H.‐W. , Schultz M. , Volz‐Thomas A.
Published: February 2004

As part of the Oxidizing Capacity of the Tropospheric Atmosphere (OCTA) project, an intensive measurement campaign was conducted in July/August 1993 at the high‐altitude observatory Izaña, Tenerife. Measurements of NO, NO2, NOy, PAN, JNO2, CO, VOC, HCHO, H2O2, O3, and ROx were made to study the photochemical processes which control the oxidizing capacity of the remote troposphere. Special attention was paid to the processes controlling the budget of ozone. Diurnal changes in the concentration of the species resulted primarily from the transition between downslope flow (usually free tropospheric air) and upslope flow (a mixture of marine boundary layer air and free tropospheric air modified by island emissions). Median concentrations for downslope and upslope conditions were NOx (47/76 parts per trillion by volume (pptv)), NOy (392/519 pptv), peroxyacetylnitrate (PAN) (10/23 pptv), CO (89/92 parts per billion by volume (ppbv), ethane (499/486 pptv), propane (35/40 pptv), ethene (25/31 pptv), isoprene (0/60 pptv), HCHO (1.1/1.4 ppbv), H2O2 (2.4/2.1 ppbv), and O3 (40/38 ppbv). Maximum amounts of ROx were measured around noon and reached values up to 70 pptv with no observable signal in the night during downslope conditions.
Response of summertime odd nitrogen and ozone at 17 mbar to Mount Pinatubo aerosol over the southern midlatitudes: Observations from the Halogen Occultation Experiment
Authors: Mickley L. J. , Abbatt J. P. D. , Frederick J. E. , Russell J. M.
Published: February 2004

Observations of stratospheric NO, NO2, and O3 from the Halogen Occultation Experiment (HALOE) are examined over the southern midlatitudes (35°–45°S) in the summers following the eruption of Mount Pinatubo, from early 1992 through mid‐1995. The tracers HF and CH4, also observed by HALOE, are used to distinguish between the effects of chemistry and those of transport processes on the distribution of total reactive nitrogen (NOy) and ozone. Results show that at 17 mbar (about 28 km), in parcels of similar photochemical age, the abundances of summertime NO + NO2 (NOx.) increased dramatically between 1992 and 1993 and then leveled off in subsequent years. The 1992–1993 increase is coincident in time with a sharp drop in aerosol surface area density as the Pinatubo aerosol cleared the atmosphere and gives evidence of the sensitivity of NOx to sulfate aerosol even at these altitudes, where the cycling of NOy through HNO3 is rapid. Results also show that summertime ozone abundances at 17 mbar declined by nearly 10% between 1992 and 1993 and then, like NOx, remained about constant for the next three summers. The trend in ozone is opposite in sign to that observed at lower altitudes in the aftermath of the Pinatubo eruption and demonstrates the importance of the NOx catalytic cycles in the ozone loss budget above 26 km.
Assessment of the record of the 1982 El Chichón eruption as preserved in Greenland snow
Authors: Zielinski Gregory A. , Dibb Jack E. , Yang Qinzhao , Mayewski Paul A. , Whitlow Sallie , Twickler Mark S. , Germani Mark S.
Published: February 2004

Variability in the SO42− and Cl− time series for the 1980s from 12 shallow snow pits across the Greenland ice sheet is used to evaluate the record of the 1982 El Chichón eruption and the potential for recording a moderate northern equatorial eruption in a single Greenland ice core. Composition of volcanic glass found in spring 1983 snow in one of the pits in the Summit region matches that from El Chichón glass, thereby verifying the deposition of material from the eruption. High Na+ and Cl− concentrations in this same layer probably represent deposition of the reaction products of halite and H2SO4 as observed in the stratosphere following the eruption. These findings and the presence of a Cl− signal in five of the other pits indicate that the Cl− aerosol component of some eruptions has the potential to remain aloft for at least 1 year after the eruption. Some of these aerosols may be adsorbed onto tephra particles. Distinct SO42− peaks that can be confidently linked to El Chichón were found in only 50% of the records developed through subseasonal sampling. However, in several other pits an elevated baseline, thought to represent more lengthy periods of El Chichón aerosol deposition as opposed to deposition from a single snowfall event, were observed. Smoothing of the original data by the calculation of annual SO42− flux resulted in the presence of high flux values between 1982 and 1984 (years thought to be affected by El Chichón aerosol deposition) in 9 of the 12 pits. These results suggest that a single ice core from anywhere in Greenland may record a signal from a northern equatorial eruption of magnitude similar to that of El Chichón about 75% of the time; this is despite the overall high levels of SO2−4 deposition from anthropogenic sources that now make identification and quantification of the volcanic SO2− portion of the record more difficult than obtaining the same data for preindustrial volcanism. Nevertheless, composite records from all the pits sampled yielded stratospheric loading (∼20 Mt) and optical depth (τ=0.13) estimates similar to stratospheric and satellite‐based measurements following the eruption. Equally high SO42− concentration and flux values in snow from 1980 to 1982 reflect deposition from the series of middle to high northern latitude volcanic eruptions 1979–1981.
Tropical mixing barriers in the lower stratosphere in the Geophysical Fluid Dynamics Laboratory SKYHI model
Authors: Bowman Kenneth P. , Hu Yongyun
Published: February 2004

Large‐scale isentropic mixing in the tropical lower stratosphere is investigated by computing Lagrangian trajectories using winds from the Geophysical Fluid Dynamics Laboratory SKYHI general circulation model. The use of model‐simulated winds avoids some problems with analyzed winds in the tropics. During the solstitial seasons a weak‐mixing zone is present in the tropics in the model simulations. The weak‐mixing zone is relatively isolated from the midlatitude surf zones in the northern and southern hemispheres by two mixing barriers: a strong one on the summer hemisphere side and a weak one on the winter side. The mixing barriers vary seasonally in response to seasonal variations of the tropical zonal‐mean zonal flow and wave spectrum. The roles of the zonal‐mean flow and of different parts of the wave spectrum in meridional transport are tested using filter techniques. Effective meridional diffusion coefficients are calculated by using filtered and antifiltered winds (i.e., winds with selected spatial scales either attenuated or amplified). The results suggest that mixing in the tropical easterlies occurs near the critical layers for the large‐scale (zonal wavenumber <10) westward moving waves, which is where the waves would be expected to break. Wave spectrum analysis indicates that during the solstitial seasons there is a zone in the tropics, exactly matching the weak‐mixing zone, where there is relatively little power in westward moving waves. The tropical mixing barriers are a result of the relative lack of such waves.
Geophysiological coupling of marine and terrestrial ecosystems
Authors: Klinger Lee F. , Erickson David J.
Published: February 2004

Areas of high ocean productivity tend to be coastal, suggesting that ocean and terrestrial ecosystems may be closely coupled. A geophysiological model proposes that coastal marine and peatland ecosystems are tightly coupled primarily via atmospheric pathways of sulfur from oceans to peatlands, which are mainly sulfur limited, and via the riverine and groundwater transport of soluble organic iron from peatlands to oceans, which are mainly iron limited. Findings from the literature, along with precipitationpH data from southeast Alaska, are presented in support of the proposed pathways. Nutrient limitation studies show increases in primary productivity of marine and peatiand ecosystems associated with additions of iron and sulfur, respectively. Mechanisms of nutrient transport commonly invoked to explain ocean productivity are not sufficient in accounting for regional and global patterns of chlorophyll concentration. Regional and global distributions of peatland cover shows remarkable correspondance with chlorophyll concentration data, consistent with model predictions.
Terminating the 100 kyr ice age cycle
Authors: Tarasov Lev , Peltier W. Richard
Published: February 2004

We report a simulation of the most recent 100,000‐year glaciationdéglaciation cycle of the late Pleistocene ice age, a simulation that delivers an ice sheet chronology that is in close accord with that inferred from the geological record. Our analyses are performed with a reduced model of the climate system that incorporates significant improvements to the representation of both climate forcing and mass balance response in a previously described theory based upon a coupled one‐level energy balance model (EBM) and vertically integrated ice sheet model (ISM). The theory fully incorporates the influences of orbital insolation forcing, glacial isostatic adjustment and variations in the atmospheric concentrations of greenhouse gases. It correctly predicts the main geographical regions of the northern hemisphere that were glaciated at last glacial maximum 21,000 years ago as well as the abrupt termination of the glacial epoch that occurred subsequently. The latter feature of the ice age cycle is obtained without the need to incorporate unconstrained and therefore controversial physical processes into the model, a limitation of all previous attempts to understand this global scale climate cycle. Our analyses suggest that the radiative impact on surface glaciation due to the changing atmospheric concentration of CO2 Is critical to the ability of the model to deliver a synthetic history of glaciation and déglaciation that is in accord with inferences based upon surface geological and geomorphological evidence. With the incorporation of this influence, model‐predicted ice thickness distributions at last glacial maximum (LGM) are very similar to those of the recently described ICE‐4G reconstruction that was based upon the inversion of postglacial relative sea level histories.
Dimethylsulfide chemistry in the remote marine atmosphere: Evaluation and sensitivity analysis of available mechanisms
Authors: Capaldo Kevin P. , Pandis Spyros N.
Published: February 2004

A box model of the marine boundary layer is used to simulate the oxidation products of dimethylsulfide, including non‐sea‐salt (nss) sulfate, sulfur dioxide (SO2), methane sulfonic acid (MSA), dimethylsulfoxide (DMSO), methane sulfinic acid (MSEA), and dimethylsulfone (DMSO2). The gas phase oxidation schemes of Yin et al. [1990], Koga and Tanaka [1993], Hertel et al. [1994], Pham et al. [1995], and Benkovitz et al. [1994] are compared with field measurements using nine scenarios. Heterogeneous oxidation of SO2 in cloud droplets and sea‐salt particles is also simulated. A sensitivity analysis is performed to evaluate which atmospheric parameters require the greatest attention in future field studies. Results indicate that the variations among the gas phase mechanisms are small with the parameterized mechanisms performing as accurately as the comprehensive ones. Among the nine scenarios tested, nss‐sulfate is predicted without bias. Predicted MSA and SO2 concentrations depend more on the gas phase mechanism, with the mechanisms tending to underpredict SO2 concentrations. Compared to differences in MSA and SO2 predictions, DMSO, MSEA, and DMSO2 predictions by the various mechanisms are similar. Sulfate predictions are sensitive to the uncertain parameterizations of heterogeneous processes. The interaction of the marine boundary layer with the free troposphere can explain much of the discrepancy between the model predictions and measurements.
Diabatic cross‐isentropic dispersion in the lower stratosphere
Authors: Sparling L. C. , Kettleborough J. A. , Haynes P. H. , McIntyre M. E. , Rosenfield J. E. , Schoeberl M. R. , Newman P. A.
Published: February 2004

A significant contribution to vertical dispersion of tracers in the stratosphere arises from the variability in the diabatic heating of air parcels. Air parcels starting on a given isentropic surface experience different time histories of diabatic heating, which causes vertical dispersion across isentropic surfaces. We refer to this process as “diabatic cross‐isentropic dispersion,” or “diabatic dispersion” for brevity. The present study investigates diabatic dispersion in the lower stratosphere by computing parcel trajectories initialized uniformly over the 500 K surface on January 1, 1993. Parcels are followed for 2 months using analyzed winds and diabatic heating rates computed from analyzed temperatures. Diabatic dispersion depends on the statistics of the large‐scale horizontal eddy motion as well as on the spatial structure of the diabatic heating field. The trajectory statistics suggest that the polar vortex, surf zone, tropics, and extratropical summer hemisphere are, to varying extents, isolated from each other and that the diabatic dispersion within each of these regions is different. In both the surf zone and southern hemisphere extratropics the dispersion is initially advective, with potential temperature variance 〈δθ(t)2〉 increasing as t2 as time t increases. After about 1 month, the dispersion becomes diffusive in the sense that 〈δθ(t)2〉 ∼2Kθθt and with a diffusivity Kθθ in the range 2–6 K2 d−1, roughly equivalent to Kzz ∼0.1–0.2 m2 s−1. The emergence of a diffusive regime is discussed in terms of loss of memory of diabatic heating along parcel paths, as measured by the decay of the Lagrangian autocorrelation function. Diabatic dispersion within the tropics and polar vortex over the 2‐month period is more than an order of magnitude smaller and is less clearly diffusive. The diabatic dispersion of parcels moving poleward out of the tropics into either hemisphere is faster than either differential advection or diffusion, and in this cas 〈δθ(t)2〉 increases as t3. For the total ensemble of all parcels, the potential temperature variance increases as t2, consistent with global‐scale differential advection by the mean diabatic circulation. This is inconsistent, at least on the 2‐month timescale considered here, with a one‐dimensional diffusive model of vertical dispersion.
A model description for cirrus cloud nucleation from homogeneous freezing of sulfate aerosols
Authors: Tabazadeh Azadeh , Jensen Eric J. , Toon Owen B.
Published: February 2004

Classical nucleation theory for homogeneous freezing as well as recent laboratory data are used to formulate an algorithm for ice nucleation from an aqueous sulfuric acid solution droplet A new parameterized function is derived from recent thermodynamic data to express the variation of sulfuric acid solution composition with temperature and relative humidity. This function is then used to derive critical ice nucleation parameters from recent laboratory data. The critical nucleation parameters are used in a classical nucleation theory to derive the diffusion activation energy of water molecules in a sulfuric acid solution from the measurements. The derived diffusion activation energy of water molecules in a sulfuric acid solution does not agree with the diffusion activation energy calculated from a viscous flow formulation that is commonly used in classical nucleation rate calculations. Our results show that ice nucleation in a sulfate droplet occurs when the interface energy of ice against the sulfate solution is approximately 17 dyn cm−1. We calculate that a supersaturation ratio of about 1.3 to 1.5 is required to nucleate ice from an aqueous sulfuric acid solution droplet in the temperature range of about 185 to 240 K. This supersaturation corresponds to a supercooling of a sulfate solution to about 2 to 3 K below the equilibrium condensation point of ice. Simple functions are given for estimating the nucleation point of ice in the upper troposphere. The differences between previous parameterizations and this work are discussed.
Electric fields of initial breakdown in positive ground flash
Authors: Ushio Tomo‐o , Kawasaki Zen‐Ichiro , Matsu‐ura Kenji , Wang Daohong
Published: February 2004

Pulse trains superimposed on the initial part of the electric field change of positive ground flashes are investigated. The characteristics of these pulse trains are found to be different from those of negative ground flashes and cloud flashes. Each pulse included in a pulse train appears as a bipolar waveform with pulse width from 5 to 52 μs and a mean of 18 μs. The time intervals between successive pulses are 10 to 180 μs with a mean of 54 μs. For each bipolar pulse, the ratio of the peak amplitude of the initial polarity to the maximum overshoot amplitude ranges from 0.4 to 4.7 with a mean value of 1.3. The ratio of maximum peak amplitude of successive pulses to the amplitude of the following return‐stroke peak ranges from 0.02 to 1.9 with a mean of 0.27.
A hotspot function in a simple bidirectional reflectance model for satellite applications
Authors: Chen J. M. , Cihlar J.
Published: February 2004

The model presented here is an improvement over the semiempirical model of Roujean et al. [1992] for estimating the bidirectional reflectance from vegetation. Roujean's model has been considered for global applications because of its simplicity and the underlying physics. However, the model does not adequately describe the hotspot near the Sun's illumination direction. In this paper, a hotspot kernel based on a canopy gap size distribution theory developed by Chen and Leblanc [1997] is used to modify Roujean's model. The modified model requires two additional coefficients for controlling the hotspot magnitude and width, respectively. It is found that the hotspot magnitude coefficient is only weakly dependent on cover type and can be treated as a constant at a given geographical location. The hotspot width parameter is determined by the ratio of the characteristic foliage clump size and canopy height. The ratio varies in a small range across different cover types because the foliage clump size and canopy height are usually correlated. For example, the ratio of leaf size to crop height is similar to the ratio of crown size to tree height. Because of the small variabilities of these parameters, the modified model can be a substantial improvement over the original model by just using best estimates for the parameters. With this hotspot adjustment the simple form of the semiempirical model is preserved for remote sensing applications without additional input requirements. The performance of the modified model is shown using data from the advanced very high resolution radiometers (AVHRR). The results show that the patterns of reflectance distribution with the view angle are similar among all cover types investigated, suggesting that one simple model may be sufficient for global applications. The modified model based on simplified physics with four adjustable coefficients may be adequate for this purpose. The model can be further improved to consider the noncircular hotspot shape. Formulae for this purpose are suggested.
Bayesian estimation of uncertainty in land surface‐atmosphere flux predictions
Authors: Franks Stewart W. , Beven Keith J.
Published: February 2004

This study addresses the assessment of uncertainty associated with predictions of land surface‐atmosphere fluxes using Bayesian Monte Carlo simulation within the generalized likelihood uncertainty estimation (GLUE) methodology. Even a simple soil vegetation‐atmosphere transfer (SVAT) scheme is shown to lead to multiple acceptable parameterizations when calibration data are limited to timescales of typical intensive field campaigns. The GLUE methodology assigns a likelihood weight to each acceptable simulation. As more data become available, these likelihood weights may be updated by using Bayes equation. Application of the GLUE methodology can be shown to reveal deficiencies in model structure and the benefit of additional calibration data. The method is demonstrated with data sets taken from FIFE sites in Kansas, and ABRACOS data from the Amazon. Estimates of uncertainty are propagated for each data set revealing significant predictive uncertainty. The value of additional periods of data is then evaluated through comparing updated uncertainty estimates with previous estimates using the Shannon entropy measure.
On the relationship between cloud‐to‐ground lightning and convective rainfall
Authors: Petersen Walter A. , Rutledge Steven A.
Published: February 2004

Ratios of area mean rainfall and cloud‐to‐ground lightning flash count (termed “rain yields”) were computed for several different locations around the globe, over temporal and spatial scales of 1 month and 104–105 km2, respectively. Values of the rain yield clustered near 108 kg/fl for a large portion of the midcontinental United States. Rain yields were slightly lower over the arid southwestern United States, averaging ∼6×107 kg/fl. In tropical locations the rain yields increased systematically from a tropical continental value of 4×108 kg/fl to a value of 1010 kg/fl for the tropical western Pacific Ocean. The observed stability of the rain yield, coupled with demonstrated positive correlations between cloud‐to‐ground flash density and rainfall amount, suggests that cloud‐to‐ground lightning data may be useful for inferring monthly convective rainfall statistics in certain rainfall regimes.
Biennial, quasi‐biennial, and decadal oscillations of potential vorticity in the northern stratosphere
Authors: Baldwin Mark P. , Dunkerton Timothy J.
Published: February 2004

Seasonal and interannual variations of the northern hemisphere stratosphere on the 600 K isentropic surface (20–35 hPa) are investigated by using observations for 1964–1996. An area diagnostic is defined in terms of Ertel's potential vorticity (PV), which measures the area enclosed by PV contours as a function of equivalent latitude and is not tied to the spherical coordinate system. Data from all seasons are examined for oscillations with periods of 6 months to more than a decade. Spectral analysis of PV at equivalent latitudes from 20°N to the pole reveals several possible signals: (1) In addition to the well‐known annual cycle of the polar vortex there is a striking semiannual oscillation of PV in middle equivalent latitudes. At 34°–53°N the semiannual oscillation is larger than the annual cycle. This oscillation arises from the formation of a surf zone in winter, due to planetary wave breaking, superposed on the annual cycle. (2) A signal associated with the quasi‐biennial oscillation (QBO) of the equatorial stratosphere, with average period slightly less than 30 months, is strongest at low equivalent latitudes and is apparent in middle equivalent latitudes up to 67°N. Through interaction with the annual cycle, oscillations with periods of 20.2 and 8.5 months are generated. These three spectral peaks at approximately 30, 20, and 8.5 months have been observed in ozone and wind data and are seen here, for the first time, in PV. (3) At middle to high equivalent latitudes, on the other hand, we observe biennial and associated 8‐month oscillations related to the occurrence of stratospheric warmings and strong, undisturbed vortices in midwinter to late winter. The data record is remarkable in that, at high equivalent latitudes, strong and weak polar vortices alternated from year to year, producing the biennial signal and a significant negative lag‐1‐year autocorrelation. The addition of an 8‐month harmonic generates a much better fit to the sharp late winter anomalies. (4) South of 60°N a spectral peak at 10.6 years is observed, with the clearest signal near 20°N. This spectral peak may be caused by the solar cycle, or it may arise through interaction of biennial and quasi‐biennial oscillations.
Variability of boreal forest reflectances as measured from a helicopter platform
Authors: Loechel S. E. , Walthall C. L. , de Colstoun E. Brown , Chen J. , Markham B. L. , Miller J.
Published: February 2004

Helicopter and surface‐based radiometric measurements of forested sites acquired during the Boreal Ecosystem‐Atmosphere Study (BOREAS) were analyzed to assess reflectance variability and to address the relationships of reflectances and vegetation indices (VIs) with overstory leaf area index (LAI). Statistical measures of central tendency and linear regression analysis, including a multivariate regression which introduced the effects of understory reflectance, were used. The sensors were deployed during all three intensive field campaigns (IFCs) of 1994 and consisted of (1) a helicopter‐based eight‐channel Barnes modular multiband radiometer (MMR) and a ground‐based Spectron Engineering SE‐590 field spectroradiometer to measure spectral reflectance factors, (2) a ground‐based Sun photometer used for atmospheric correction, and (3) a LI‐COR LAI‐2000 plant canopy analyzer and the tracing radiation and architecture of canopies (TRAC) sensor for retrieval of canopy LAI. Of the eight MMR bands, those most strongly correlated to variations in surface LAI were the second and third middle infrared (IR) (1.57–1.80 μm and 2.08–2.37 μm, respectively) with r2=0.45 in both cases, and the red (0.63–0.68 μm) with r2=0.34. In general, linear regression analyses between the canopy LAI measurements and both the helicopter reflectances and VIs resulted in low r2 values commonly less than 0.3. When stratified by season and species, only among sites that were vegetated by aspen (Populus tremuloides) was a stronger relationship observed. In the aspen stands, LAI displayed a traditional trend that maximizes the utility of the VIs: a negative relationship with red reflectance (r2=0.62) and a positive relationship with the near IR (NIR) of r2 = 0.21. The absence of a positive LAI‐NIR reflectance trend degraded the information content of the VIs for the conifers. A multivariate regression confirmed the influence of the understory vegetation on the sparse canopies of the boreal forest and suggested that a more complex model‐based approach is necessary to better characterize variations in canopy reflectance. Atmospheric contamination of the radiometric signal by smoke from forest fires may also have contributed to the initial unsatisfactory results.
Moss and soil contributions to the annual net carbon flux of a maturing boreal forest
Authors: Harden J. W. , O'Neill K. P. , Trumbore S. E. , Veldhuis H. , Stocks B. J.
Published: February 2004

We used input and decomposition data from 14C studies of soils to determine rates of vertical accumulation of moss combined with carbon storage inventories on a sequence of burns to model how carbon accumulates in soils and moss after a stand‐killing fire. We used soil drainage—moss associations and soil drainage maps of the old black spruce (OBS) site at the BOREAS northern study area (NSA) to areally weight the contributions of each moderately well drained, feathermoss areas; poorly drained sphagnum—feathermoss areas; and very poorly drained brown moss areas to the carbon storage and flux at the OBS NSA site. On this very old (117 years) complex of black spruce, sphagnum bog veneer, and fen systems we conclude that these systems are likely sequestering 0.01–0.03 kg C m−2 yr−1 at OBS‐NSA today. Soil drainage in boreal forests near Thompson, Manitoba, controls carbon storage and flux by controlling moss input and decomposition rates and by controlling through fire the amount and quality of carbon left after burning. On poorly drained soils rich in sphagnum moss, net accumulation and long‐term storage of carbon is higher than on better drained soils colonized by feathermosses. The carbon flux of these contrasting ecosystems is best characterized by soil drainage class and stand age, where stands recently burned are net sources of CO2, and maturing stands become increasingly stronger sinks of atmospheric CO2. This approach to measuring carbon storage and flux presents a method of scaling to larger areas using soil drainage, moss cover, and stand age information.
Measurement of total nonmethane organic carbon (Cy): Development and application at Chebogue Point, Nova Scotia, during the 1993 North Atlantic Regional Experiment campaign
Authors: Roberts James M. , Bertman Steven B. , Jobson Tom , Niki Hiromi , Tanner Roger
Published: February 2004

A technique was developed for the measurement of total nonmethane organic carbon (Cy) in the gas phase based on cryogenic collection, preseparation of CO, CH4 and CO2, conversion of all carbon to CH4, and measurement by flame ionization detector. Collection and recovery efficiencies were found to be essentially quantitative for C2‐C7 hydrocarbons and better than 95 and 93% for methanol and formaldehyde, respectively. CO2 prevented the measurement of the C2 hydrocarbons in atmospheric samples, and system response to ambient water vapor limited the precision of the measurement at very low Cy. The measurement system had an estimated uncertainty of 10% at ambient mixing ratios of 100 ppb C, and a detection limit of between 5 and 7 ppb C. Cy was measured in Boulder, Colorado, during June of 1993 and at Chebogue Point, Nova Scotia, during the North Atlantic Regional Experiment intensive campaign, August to September 1993. Cy values ranged from several hundred ppb C to 11 ppb C in Boulder, and from 177 to below 5 ppb C at Chebogue Point, with medians of 55 and 11 ppb C, respectively. Cy was found to correlate with both CO and O3 at higher levels of those two pollutants. The sum of Cy and the C2 hydrocarbons was virtually always equal to or greater than the sum of the hydrocarbons. Several exceptions to this occurred when high HC values were observed in cannister samples but not Cy measurements. These instances were attributed to short‐term perturbations from local sources. There was a net difference between total nonmethane organic carbon and the sum of hydrocarbons and carbonyls of 3.5±7.8 ppb C overall. This difference appears to be broadly correlated with anthropogenic pollution. There does not appear to be a significantly large reservoir of reactive carbon unaccounted for by hydrocarbon or carbonyl measurements.
Principal factors determining the 18O/16O ratio of atmospheric CO as derived from observations in the southern hemispheric troposphere and lowermost stratosphere
Authors: Brenninkmeijer Carl A. M. , Röckmann Thomas
Published: February 2004

Measurement of the 18O/16O ratio in atmospheric CO based on mass spectrometry after its oxidation to CO2, which is applied to separate it from air, is evaluated. A convenient method to correct for the isotope effect of the addition of an oxygen atom by the oxidant to the original CO molecule is given, results of which confirm earlier data. Measurement of CO extracted from air containing varying amounts of CO derived from transportation and other man‐made pollution sources reveals that the 18O/16O ratios of CO from these sources approach that of atmospheric oxygen. Analysis of upper tropospheric samples collected using aircraft flights between New Zealand and Antarctica allows a determination of the 18O/16O ratio of biomass burning derived CO after its transportation into the extratropical regions. The inferred δ18O value of 4.5‰ would indicate that the kinetic fractionation in the removal of CO by OH during transport has decreased the 18O content by as much as 10‰. Evaluation of the δ18O values for samples collected in the stratosphere allows a determination of this fractionation, and a value for kl6/k18 of 0.992 is derived, close to laboratory results. Further, by using a southern hemisphere CO budget it is possible to estimate that δ18O of CO produced via the reaction CH4 + OH has a value probably as low as 0‰.
Total column and surface densities of atmospheric carbon monoxide in Alaska, 1995
Authors: Yurganov Leonid N. , Jaffe Daniel A. , Pullman Eric , Novelli Paul C.
Published: February 2004

The results of correlated investigations of atmospheric carbon monoxide in Alaska during the spring‐summer of 1995 using three different techniques are presented. CO total column abundance was measured in Fairbanks using IR spectroscopy with the Sun as a light source. A new computer retrieval code was developed and compared with the previously used technique. Surface mixing ratios were determined in situ by gas filter correlation and by gas chromatography with a mercuric oxide reduction detector. Surface measurements were made at two uncontaminated sites: Poker Flat Research Range in interior Alaska and the National Oceanic and Atmospheric Administration Point Barrow Observatory. In spring, the measurements revealed considerably more CO in the surface layer as compared with the tropospheric mean values determined by spectroscopy. This suggests an accumulation of anthropogenic CO in the boundary atmospheric layer over vast areas of the northern hemisphere during the winter. Beginning in mid‐April, the CO concentration in the troposphere decreases, but the rate of decrease in the surface layer was 2–2.5 times greater than that for the troposphere as a whole. By June the surface mixing ratios and mean tropospheric values nearly converged, and the CO mixing ratio seemed to be almost constant with altitude. The July measurements revealed days with enhanced CO total column burden; these are most likely associated with lifted layers of air, polluted by forest fires.
Carbon distribution and aboveground net primary production in aspen, jack pine, and black spruce stands in Saskatchewan and Manitoba, Canada
Authors: Gower S. T. , Vogel J. G. , Norman J. M. , Kucharik C. J. , Steele S. J. , Stow T. K.
Published: February 2004

The objectives of this study are to (1) characterize the carbon (C) content, leaf area index, and aboveground net primary production (ANPP) for mature aspen, black spruce, and young and mature jack pine stands at the southern and northern Boreal Ecosystem‐Atmosphere Study (BOREAS) areas and (2) compare net primary production and carbon allocation coefficients for the major boreal forest types of the world. Direct estimates of leaf area index, defined as one half of the total leaf surface area, range from a minimum of 1.8 for jack pine forests to a maximum of 5.6 for black spruce forests; stems comprise 5 to 15% of the total overstory plant area. In the BOREAS study, total ecosystem (vegetation plus detritus plus soil) carbon content is greatest in the black spruce forests (445,760–479,380 kg C ha−1), with 87 to 88% of the C in the soil, and is lowest in the jack pine stands (68,370–68,980 kg C ha−1) with a similar distribution of carbon in the vegetation and soil. Forest floor carbon content and mean residence time (MRT) also vary more among forest types in a study area than between study areas for a forest type; forest floor MRT range from 16 to 19 years for aspen stands to 28 to 39 years for jack pine stands. ANPP differs significantly among the mature forests at each of the BOREAS study areas, ranging from a maximum of 3490 to 3520 kg C ha−1 yr−1 for aspen stands to 1170 to 1220 kg C ha−1 yr−1 for jack pine stands. Both net primary production (NPP) and carbon allocation differ between boreal evergreen and deciduous forests in the world, suggesting global primary production models should distinguish between these two forest types. On average, 56% of NPP for boreal forests occurs as detritus and illustrates the need to better understand factors controlling aboveground and below‐ground detritus production in boreal forests.
Introduction to special section: Polar Ozone and Aerosol Measurement (POAM II)
Authors: Bevilacqua R. M.
Published: February 2004

The Polar Ozone and Aerosol Measurement instrument (POAM II) is a satellite‐based nine‐channel visible/near infrared photometer for making measurements of aerosol extinction, ozone, water vapor, and nitrogen dioxide in the polar stratosphere using the solar occultation technique. The instrument is similar in measurement technique and scientific capabilities to the NASA Stratosphere and Gas Experiment (SAGE) II [Mauldin et al., 1985]. The POAM II instrument concept originated at the Department of Physics and Astronomy at the University of Wyoming and was substantially redesigned by the Naval Research Laboratory and ThermoTrex Corporation (TTC) of San Diego, California. The flight instrument was fabricated by TTC.
Aerosol optical properties in the marine boundary layer during the First Aerosol Characterization Experiment (ACE 1) and the underlying chemical and physical aerosol properties
Authors: Quinn P. K. , Coffman D. J. , Kapustin V. N. , Bates T. S. , Covert D. S.
Published: February 2004

Measurements were made onboard the NOAA R/V Discoverer during the First Aerosol Characterization Experiment (ACE 1) to understand the optical properties of a minimally perturbed natural aerosol system in terms of its chemical and physical properties. ACE 1 took place during November and December of 1995 in the Southern Ocean region south of Australia. Reported here are observations at a wavelength of 550 nm of the submicron and supermicron aerosol scattering coefficient, σsp; single scattering albedo, ω0; and the hemispheric backscattered fraction and mass scattering efficiencies of non‐sea‐salt sulfate, sea salt, and the total aerosol. Also presented is the Ångström exponent, å, for the 450 and 700 nm wavelength pair. Variations in these parameters were found to be a strong function of the relative concentrations and size distributions of the dominant aerosol chemical components. Both the submicron and supermicron aerosol mass were composed primarily of water‐soluble ionic species. This is in agreement with an experiment‐average single scattering albedo of 0.99 (−0.4, +1.0%). Of the submicron ionic mass, 80±10% was sea salt, 16±8% was non‐sea‐salt sulfate, and 4±3% was methanesulfonate. Sea salt composed 99±0.7% of the supermicron ionic mass. The magnitude of scattering by both submicron and supermicron aerosol was controlled by sea salt. The backscattered fraction for the submicron aerosol averaged 0.11±0.02 and was controlled by the tailing of coarse mode sea‐salt mass into the submicron size range. Calculated mass scattering efficiencies for submicron non‐sea‐salt sulfate ion averaged 1.5±0.74 m2 g−1 (at 30 to 45% relative humidity) with the highest values corresponding to continentally influenced air masses where sulfate aerosol surface mean diameters and surface area concentrations were the largest. Mass scattering efficiencies for submicron sea salt were higher (averaging 4.2±0.96 m2 g−1) due to the tailing of coarse mode sea salt into the particle size range most efficient for light scattering. Given the similar lifetimes of submicron non‐sea‐salt sulfate and sea salt in the marine boundary layer, it is evident that sea salt controls the aerosol optical properties in this Southern Ocean region.