bluuebunny/crossref_metadata_2025 huggingface dataset:
Characteristics of the nocturnal boundary layer: Okaukuejo, Namibia, during SAFARI‐92
Authors: Zunckel M. , Hong Y. , Brassel K. , O'Beirne S.
Published: February 2004

During the Southern African Fire‐Atmosphere Research Initiative (SAFARI) in September and October 1992, nocturnal tethersonde soundings were carried out at Okaukuejo, measuring dry‐bulb and dew‐point temperature, ozone concentration, wind speed, and wind direction. These showed the existence of a surface temperature inversion and a low‐level wind maximum on most nights. The inversion formed shortly after sunset, reached a maximum depth and strength at sunrise, and then gradually eroded from the surface upwards. The low‐level jet (LLJ) started developing after the onset of inversion formation. Speeds in excess of 11 m s−1 were not unusual. The height of the core of the jet ranged between 50 and 200 m above ground level and corresponded fairly closely with the level of the top of the inversion. While daytime thermal eddy convection prevailed, ozone in the boundary layer was well mixed, and no vertical structure to ozone concentration was evident. With the onset of the surface inversion ozone levels increased systematically with height from less than 10 ppbv near ground level to more than 30 ppbv at the LLJ core.
On the phase propagation of extratropical ozone quasi‐biennial oscillation in observational data
Authors: Yang Hu , Tung Ka Kit
Published: February 2004

Global column ozone data from total ozone mapping spectrometer (TOMS), backscattered ultraviolet (BUV) and Dobson stations are analyzed to determine the pattern and phase property of the ozone quasi‐biennial oscillation (QBO) signal. It is found that the ozone QBO signal is strongest in middle and high latitudes and is present mainly in the winter‐spring season in both hemispheres. The extratropical ozone QBO signal is out of phase with the equatorial ozone QBO, which is itself in phase with the QBO in equatorial zonal wind. There are three distinctive regions, namely tropical, midlatitudinal, and polar regions, in each of which the ozone QBO signal has a fairly constant phase with respect to latitude. There is a phase reversal (sign change) between the equatorial and the extratropical regions associated with the return branch of the equatorial QBO secondary circulation, and this sign reversal occurs at ±12° of latitude symmetric about the equator. In the northern hemisphere between the midlatitudinal and polar regions, there is another possible phase reversal in some (but not all) years possibly in connection with the presence or absence of midwinter sudden warming, which creates a positive or negative anomaly relative to the region outside the polar vortex. In the southern hemisphere polar latitudes, the ozone QBO signal is usually delayed until spring in connection with the final warming. These properties are found in all data sets analyzed by the same method. Evidence does not support a gradual phase propagation from the subtropical region to the high‐latitude region. Previous reported evidence for phase propagation is reexamined and is found to be artifacts of data processing.
Latitude and seasonal dependence of the semidiurnal tide observed by the high‐resolution Doppler imager
Authors: Burrage M. D. , Wu D. L. , Skinner W. R. , Ortland D. A. , Hays P. B.
Published: February 2004

The high resolution Doppler imager (HRDI) on the upper atmosphere research satellite (UARS) has provided measurements of the horizontal wind field in the mesosphere and lower thermosphere (MLT) since November 1991. The observed dynamical patterns are dominated by the (1,1) diurnal tide, but semidiurnal perturbations are also very prominent in the HRDI data, particularly at latitudes greater than 40°. This paper presents a preliminary study of the semidiurnal tide and discusses latitudinal and seasonal variations in the phenomenon. Regular seasonal changes are clearly detected, and significant interhemispheric asymmetries have been identified. In general, the observed features are consistent with previous measurements and model results, but some discrepancies have been found. This new technique, which brings a global perspective to the observation of MLT dynamics, is complementary to the very detailed but localized radar data sets currently available and should provide important constraints for numerical models.
Effects of longwave cloud radiative forcing anomalies on the atmospheric response to equatorial Pacific sea surface temperature anomalies
Authors: Chen Minghang , Cess Robert D. , Zhang Ming‐Hua
Published: February 2004

The latest version of the National Center for Atmospheric Research community climate model (CCM2) has been used to investigate cloud radiative forcing (CRF) anomalies associated with equatorial Pacific sea surface temperature (SST) anomalies and the effects of the longwave CRF (LWCRF) anomalies on the atmospheric response to the SST anomalies. The SST anomalies cause large CRF anomalies, both longwave and shortwave, as well as latent heat anomalies at low latitudes on a global scale. The relative magnitude of the simulated longwave and shortwave CRF anomalies is consistent with the result of the Earth Radiation Budget Experiment (ERBE), implying that cloud height and cloud radiative properties such as emissivity and reflectivity are well simulated by the model. The LWCRF anomaly strongly enhances the precipitation anomaly in the whole tropical belt. The positive (negative) LWCRF anomaly warms (cools) the troposphere and destabilizes (stabilizes) the upper troposphere. The LWCRF anomaly enhances the Southern Oscillation and the related Walker circulation anomaly. The effects of the LWCRF anomaly are essential to the northern hemispheric extratropical circulation anomaly, the Pacific/North American pattern.
Measurements and estimation of CO2 fluxes from aircraft and satellite‐based systems
Authors: Desjardins R. L. , Pelletier R. , Schuepp P. H. , MacPherson J. I. , Hayhoe H. , Cihlar J.
Published: February 2004

This study investigates the relationships between airborne observations of CO2 fluxes and satellite‐based vegetation indices using data from the First International Land Surface Climatology Project Field Experiment (FIFE) 1989. The CO2 flux data were obtained by flying a grid pattern at a mean altitude of 90 m over a 225 km2 grassland ecosystem, on six occasions during a 2‐week period. Two satellite‐based data sets were obtained from NOAA 10 advanced very high resolution radiometer (AVHRR) and one from Landsat 5 thematic mapper (TM). Linear relationships Fc = b SR* + a were found between CO2 flux data (Fc) and footprint‐weighted simple ratios (SR*) from the various satellites. These relationships improved significantly when the footprint was adjusted with the continuously changing wind conditions and with an empirical parameter that accounts for effects of surface roughness and stability on the diffusion pattern between the surface and the airborne sampling locations. The AVHRR data collected on July 28 accounted for 53 to 81% of the variations in CO2 flux measured during the six grid flights over the site and the Landsat TM data collected on August 4 accounted for 68 to 83% of the same CO2 variations. The regression coefficient b, which is related to the photosynthetic efficiency, appears to be an excellent way to characterize the dynamic response of complex ecosystems, and the flux (Fc) at SR* = 1.5 provides an estimate of soil respiration as shown through a comparison with enclosure measurements.
Atmospheric humidity variations associated with westerly wind bursts during Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean Atmosphere Response Experiment (COARE)
Authors: Sheu Rong‐Shyang , Liu Guosheng
Published: February 2004

Precipitable water determined from special sensor microwave/imager brightness temperatures is used to examine westerly wind bursts (WWBs) in the equatorial western Pacific. Six microwave satellite water vapor retrieval algorithms are tested for their performance in the tropics. The study of two major WWB events and several localized and short‐lived wind perturbations in the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE) domain unveils the relationship between precipitable water field and anomalous wind regimes. WWBs are found to be accompanied by a drastic drop in the precipitable water amount. The decrease is partly explained by the transition between easterly and westerly wind regimes. A few days before the westerly wind is established, the transitional equatorward wind introduces the dry air from the subtropics. When the westerly wind eventually weakens, a transitional equatorward wind regime once again occurs and lasts for several days before the easterlies take over. The strongest WWB event during TOGA COARE is found to be consistent with midlatitude cold surge. This provides evidence for the midlatitude forcing of WWBs. While the interaction with higher latitudes explains the reduction in precipitable water in WWB events, it is not clear what region is responsible for feeding extra moisture to the western Pacific.
The design and testing of a relaxed eddy accumulation system
Authors: Nie D. , Kleindienst T. E. , Arnts R. R. , Sickles J. E.
Published: February 2004

A flux measuring system has been developed based on relaxed eddy accumulation (REA, or conditional sampling) methodology. The system uses sampling tubes packed with solid substrates to collect air samples for determining the average concentration of compounds of interest. It has been designed for measuring fluxes of various trace gas species by using appropriate solid substrates in the tubes. The chief advantages to this design are the following: (1) the placement of pump and flow control apparatus are located downstream of the sampling tubes to eliminate sample heating and reduce contamination of the sample stream; (2) the addition of a zero gas channel available to the sampling tubes improves the flow stability without diluting the sample; (3) the design permits the use of multiple sample tube magazines to allow sequential runs without interruption; and (4) the use of compact, easily transportable sampling tubes permits analyses to be conducted at an off‐site laboratory where analytical conditions are better controlled. The REA system was tested by measuring water vapor flux under field conditions and the resulting measurements are compared with those made using a Bowen ratio system. The two methods gave similar results with fluxes values that agreed within 12%, although the measurements from the REA system were generally higher than those from the Bowen ratio method.
Stratospheric Aerosol and Gas Experiment II‐Umkehr ozone profile comparisons
Authors: Newchurch M. J. , Cunnold D. M. , Wang H. J.
Published: February 2004

This study compares 1789 pairs of ozone profiles derived from 1384 Umkehr observations at 14 different stations and 1163 Stratospheric Aerosol and Gas Experiment (SAGE) II profiles coincident within 1000 km and 14 hours between October 1984 and April 1989. The comparison indicates the following significant percentage differences (SAGE II‐Umkehr)/Umkehr with 2 × standard errors of the mean: Umkehr layer 4, (18.3±0.8)%; layer 5, (−1.6±0.4)%; layer 6, (−6.2±0.5)%; layer 7, (0.8±0.6)%; layer 8, (7.7±0.6)%; and layer 9, (12.0±1.1)%. Differences in layers 4 and 6 are due, at least in part, to inaccurate Umkehr climatologies. Average SAGE II/Umkehr differences in layers 5 through 9 at individual stations are generally less than 10%. While the Umkehr retrievals are known to be sensitive to aerosol interference, the mean layer 8 correction during the period of this study is estimated to be only 2% with large station‐to‐station variability. The correction in lower layers is smaller. We have chosen to ignore the small Umkehr aerosol correction in this study. The mean difference would decrease if Umkehr profiles were corrected for a priori profile effects calculated by DeLuisi et al. (1989a). However, using the newer Bass and Paur (1985) ozone absorption cross sections would tend to increase the differences at most levels. The profile of mean differences is similar to previously observed differences between Umkehr and solar backscattered ultraviolet (SBUV) observations. Comparing SAGE II/Umkehr differences to SAGE I/Umkehr differences at seven common stations shows a bias of −4% at the ozone peak (layer 4). This bias increases with altitude to 8% in layer 8 and 15% in layer 9, with SAGE II ozone partial pressures higher than or equal to those of SAGE I (version 6.1) relative to Umkehr in all layers above 4. A systematic upward reference‐altitude shift between 0.25 and 0.50 km for SAGE I, similar to the quoted uncertainty, would increase SAGE I ozone 4% to 8% in layer 8 and would result in similar SAGE and Umkehr ozone trends during the 1980s. Cross correlations of numerous variables associated with the Umkehr and SAGE II data sets show a minimum correlation between SAGE II and Umkehr ozone partial pressures in layers 5, 8, and 9. This correlation is a result similar to the one previously noted in other comparisons against Umkehr data. We discuss these minimum correlations in relationship to the seasonal cycle in ozone and synoptic scale variations at midlatitudes based on model results. Substantial differences between SAGE II and Umkehr exist in both the mean and the variability of ozone in layers 8 and 9. Substantial differences also exist in layer 6 where the Umkehr algorithm does not retrieve the low ozone values periodically observed by SAGE II during winter.
Trajectory mapping and applications to data from the Upper Atmosphere Research Satellite
Authors: Morris Gary A. , Schoeberl Mark R. , Sparling Lynn C. , Newman Paul A. , Lait Leslie R. , Elson Lee , Waters Joe , Suttie Robert A. , Roche Aidan , Kumer Jack , Russell James M.
Published: February 2004

The problem of creating synoptic maps from asynoptically gathered trace gas data has prompted the development of a number of schemes. Most notable among these schemes are the Kalman filter, the Salby‐Fourier technique, and constituent reconstruction. This paper explores a new technique called “trajectory mapping.” Trajectory mapping creates synoptic maps from asynoptically gathered data by advecting measurements backward or forward in time using analyzed wind fields. A significant portion of this work is devoted to an analysis of errors in synoptic trajectory maps associated with the calculation of individual parcel trajectories. In particular, we have considered (1) calculational errors; (2) uncertainties in the values and locations of constituent measurements, (3) errors incurred by neglecting diabatic effects, and (4) sensitivity to differences in wind field analyses. These studies reveal that the global fields derived from the advection of large numbers of measurements are relatively insensitive to the errors in the individual trajectories. The trajectory mapping technique has been successfully applied to a variety of problems. In this paper, the following two applications demonstrate the usefulness of the technique: an analysis of dynamical wave‐breaking events and an examination of Upper Atmosphere Research Satellite data accuracy.
Derivation of contributions of sulfate and carbonaceous aerosols to cloud condensation nuclei from mass size distributions
Authors: Rivera‐Carpio C. A. , Corrigan C. E. , Novakov T. , Penner J. E. , Rogers C. F. , Chow J. C.
Published: February 2004

We have estimated the contributions of mass concentrations of major aerosol species (sulfate, carbonaceous material, and sea salt) to cloud condensation nuclei (CCN) number concentrations from simultaneous measurements of aerosol number size distributions, impactor‐derived mass size distributions of aerosol species, and CCN number concentrations (measured at 0.5% supersaturation). Our approach involves the derivation of number size distributions from the impactor data by numerical data inversion from which the mass contributions of sulfate, organic material and sea salt to CCN number concentrations are estimated. The utility of this approach is demonstrated by the results obtained at Point Reyes, California, where we compared the derived number size distributions with directly measured number size distributions. Direct measurements of the CCN concentrations for 0.5% supersaturation showed agreement of ±25% with CCN derived from impactor data. The contribution of sulfate to CCN concentrations was found to be between 20 and 65% versus contribution of organic particles between 4 and 80%. Thus sometimes sulfate can contribute most of the CCN, while at other times organic aerosols dominate the CCN concentrations.
A 10‐century comparison of prominent bipolar volcanic events in ice cores
Authors: Langway C. C. , Osada K. , Clausen H. B. , Hammer C. U. , Shoji H.
Published: February 2004

Measurements of key chemical and physical parameters made along continuous and selected long sections of polar ice cores provide reliable past snow accumulation rates and other environmental records. The prime accumulation indicators include variations found in the stable isotopes, ionic constituents, and acidity concentration levels; and physical changes in the strata and structure. Cross correlations of the time series curves resulting from multiparameter analyses of ice cores from Antarctica (NBY89, SP78, BS68) and Greenland (CR74, D3 81, D3 18C) have been made. The results permit construction of a bipolar chemical stratigraphy chronology of volcanic events that is coherent with the δ 18O and Electrical Conductivity Method data sets and consistent back‐in‐time with historically recorded volcanic activity. A bihemispheric comparison of high non‐sea‐salt SO42− peaks shows strong correlation at 1885, 1836, 1816, 1810, 1459, 1346, 1287, 1278, 1259, 1228, and 1168 A.D. at time of snow deposit. The prominent 1259 A.D. volcanic signal appears to be a significant bipolar ice core index horizon.
Ozone oxidation of sulfur in sea‐salt aerosol particles during the Azores Marine Aerosol and Gas Exchange experiment
Authors: Sievering H. , Gorman E. , Ley T. , Pszenny A. , Springer‐Young M. , Boatman J. , Kim Y. , Nagamoto C. , Wellman D.
Published: February 2004

Sea‐salt aerosol particles in the lowest tens of meters above the ocean are, typically, more than three‐fourths water on a volume basis. Calculations herein indicate that aqueous‐phase conversion of sulfur dioxide dissolved in the water associated with sea‐salt particles (sea‐salt aerosol water) supported the production of 2–8 nmol m−3 of nonsea‐salt sulfate (nssSO4=) during the Marine Aerosol and Gas Exchange (MAGE) experiment intensives. This production is based on ozone oxidation of dissolved SO2 in sea‐salt aerosol water and accounts for sulfur gas and ozone mass transfer limitations as a function of sea‐salt particle size. Measurements showed that 1–15 nmol m−3 of nssSO4= was actually present in the sea‐salt particle mode except for four enhanced concentration cases due to continental sulfur input. The range in predicted, as well as observed, nssSO4= was primarily due to variability in sea‐salt aerosol water volume. The nssSO4= produced by ozone oxidation of sulfur dioxide, being in the sea‐salt particle mode with observed volume geometric median diameter of 3.5–5 μm, is dry deposited at a fairly rapid rate. The remainder, being large‐particle cloud condensation nuclei, may contribute little to cloud albedo over the global oceans. The two papers following this one, Kim et al. (this issue) on aerosol size distribution and water content and Parfai et al. (this issue) on compositional variations of sea‐salt‐mode aerosol particles observed by electron microscopy, complement and support results presented here. All three present results of the MAGE experiment and precede a forthcoming special issue of the Journal of Geophysical Research‐Atmospheres on MAGE (B. Huebert, guest editor).
Quantification of the transport of chemically activated air from the northern hemisphere polar vortex
Authors: Norton W. A. , Chipperfield M. P.
Published: February 2004

Output from a three‐dimensional off‐line chemical transport model and the technique of contour advection are used to give a high‐resolution view of the transport of PSC‐activated air in the northern hemisphere lower stratosphere. Three northern hemisphere winters are examined, 1991–1992, 1992–1993, and 1993–1994. The structure of the polar vortex and area of temperatures below 195 K on the 475 K isentrope are compared with 500 hPa geopotential height fields. These show how meridional excursions of the tropospheric jets can have a strong influence on the polar vortex. In contrast, approximately westerly flow in the troposphere can result in a relatively undisturbed vortex with a large area of temperatures below 195 K. Two tropospheric flow patterns are identified which frequently produce transport from the polar vortex into the midlatitudes. These are ridges over northern Europe and troughs over the North Pacific. By using a vortex‐following coordinate system, the amount of PSC‐activated air transported into midlatitudes is quantified for the periods of this study at isentropic levels 400 K, 475 K and 550 K. This calculation indicates that there is considerable interannual variation in the amount. On the 475 K isentrope the amount ranges from nearly 50% of the total vortex mass in 1991–1992 to around 10% in 1993–1994 or, when expressed as a percentage of the mass at midlatitudes, 25% and 5%, respectively. In 1992–1993, even though there was a long period with temperatures below the PSC threshold, PSC‐activated air was largely contained within the vortex, with only around 20% on the 475 K isentrope and less than 10% on the 550 K isentrope of the total vortex mass transported into midlatitudes. However in 1992–1993, unlike the other two years, there was much stronger transport of PSC‐activated air into midlatitudes on the 400 K isentrope (this transport increased right down to the 350 K isentrope), the figure being around 25% of the total vortex mass. These results are discussed in the context of the observed ozone decrease in midlatitudes during winter. In particular, the results indicate that in situ destruction of ozone in midlatitudes by PSC‐activated air transported from the vortex cannot account for the large ozone decrease observed during the winter of 1992–1993.
Characterization of submicron aerosol size distributions from time‐resolved measurements in the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange
Authors: Russell Lynn M. , Huebert Barry J. , Flagan Richard C. , Seinfeld John H.
Published: February 2004

As part of the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange (ASTEX/MAGE), ship‐based aerosol size distribution measurements have been carried out with a scanning electrical mobility spectrometer (SEMS) aboard the R/V Oceanus. The fast time resolution of these measurements illustrates some of the short‐timescale variability of both the marine background aerosol and the anthropogenically influenced continental air masses. The resulting total number concentrations are largely in agreement with those from a collocated condensation nucleus counter (CNC). The aerosol size distributions provide characteristic signatures for different atmospheric conditions, showing low‐concentration bimodal distributions in cleaner air masses and higher‐concentration single‐mode distributions in air masses with apparent recent continental influence.
Fine particulate chemical composition and light extinction at Canyonlands National Park using organic particulate material concentrations obtained with a multisystem, multichannel diffusion denuder sampler
Authors: Eatough Delbert J. , Eatough David A. , Lewis Laura , Lewis Edwin A.
Published: February 2004

The concentration of fine particulate carbonaceous material has been measured over a 1‐year period at the Interagency Monitoring of Protected Visual Environments (IMPROVE) Canyonlands National Park, Utah sampling site using a Brigham Young University organic sampling system (BOSS) multisystem, multichannel diffusion denuder sampler. Samples were collected on the IMPROVE schedule of a 24‐hour sample every Wednesday and Saturday. The concentrations of particulate C, determined using only a quartz filter pack sampling system, were low by an average of 39%, as a result of the loss of semi‐volatile organic compounds from the particles collected on quartz filters during sampling. The loss was higher during the summer than during the winter sampling periods. The BOSS and IMPROVE quartz filter carbon measurements were in agreement except for a few samples collected during the summer. The fine particulate carbonaceous material concentrations determined using the BOSS have been combined with concentrations of particulate elemental C (soot), sulfate, nitrate, crustal material, and fine and coarse particulate mass from the IMPROVE sampling system, as well as relative humidity, light absorption, and transmissometer measurements of light extinction from IMPROVE. Extinction budgets have been calculated using multilinear regression analyses of the data set. Literature data were used to estimate the change in the mass extinction coefficients for the measured species as a function of relative humidity. The results show carbonaceous material to be the principal contributor to light extinction due to particles during the study period, with the major contributor to light extinction being light‐absorbing carbonaceous material. However, the periods of maximum light extinction are associated with high humidity and the associated increased scattering of light due to particulate sulfate during the winter. The effect of particulate organic compounds on light extinction is greatest in the summer and smallest in the winter.
Line‐by‐line calculation of atmospheric fluxes and cooling rates: 2. Application to carbon dioxide, ozone, methane, nitrous oxide and the halocarbons
Authors: Clough S. A. , Iacono M. J.
Published: February 2004

A line‐by‐line model (LBLRTM) has been applied to the calculation of clear‐sky longwave fluxes and cooling rates for atmospheres including CO2, O3, CH4, N2O, CCl4, CFC‐11, CFC‐12, and CFC‐22 in addition to water vapor. The present paper continues the approach developed in an earlier article in which the radiative properties of atmospheres with water vapor alone were reported. Tropospheric water vapor continues to be of principal importance for the longwave region due to the spectral extent of its absorbing properties, while the absorption bands of other trace species have influence over limited spectral domains. The principal effects of adding carbon dioxide are to reduce the role of the water vapor in the lower troposphere and to provide 72% of the 13.0 K d−1 cooling rate at the stratopause. In general, the introduction of uniformly mixed trace species into atmospheres with significant amounts of water vapor has the effect of reducing the cooling associated with water vapor, providing an apparent net atmospheric heating. The radiative consequences of doubling carbon dioxide from the present level are consistent with these results. For the midlatitude summer atmosphere the heating associated with ozone that occurs from 500 to 20 mbar reaches a maximum of 0.25 K d−1 at 50 mbar and partially offsets the cooling of 1.0 K d−1 contributed by H2O and CO2 at this level. In the stratosphere the 704 cm−1 band of ozone, not included in many radiation models, contributes 25% of the ozone cooling rate. Radiative effects associated with anticipated 10‐year constituent profile changes, 1990–2000, are presented from both a spectral and spectrally integrated perspective. The effect of the trace gases has been studied for three atmospheres: tropical, midlatitude summer, and midlatitude winter. Using these results and making a reasonable approximation for the polar regions, we obtain a value for the longwave flux at the top of the atmosphere of 265.5 W m−2, in close agreement with the clear‐sky Earth Radiation Budget Experiment (ERBE) observations. This agreement provides strong support for the present approach as a reference method for the study of radiative effects resulting from changes in the distributions of trace species on global radiative forcing. Many of the results from the spectral calculations reported here are archived at the Carbon Dioxide Information and Analysis Center for use by the community.
Corrections to the Nimbus 7 solar backscatter ultraviolet data in the “nonsync” period (February 1987 to June 1990)
Authors: Gleason James F. , McPeters Richard D.
Published: February 2004

The solar backscatter ultraviolet (SBUV) radiance data in the period February 1987 to June 1990 were affected by chopper synchronization errors. The synchronization errors have been characterized and a method was developed for correcting the individual radiance measurements. The profile and total ozone values retrieved using the corrected radiance data have much less variance than the uncorrected data. The total and profile ozone values were compared with external ozone measurements, and no apparent bias was introduced by the correction method.
Factors influencing the concentration of gas phase hydrogen peroxide during the summer at Kinterbish, Alabama
Authors: Watkins B. A. , Parrish D. D. , Buhr S. , Norton R. B. , Trainer M. , Yee J. E. , Fehsenfeld F. C.
Published: February 2004

Measurements of H2O2 along with a variety of other photochemically active compounds were made during the summers of 1990 and 1992 at a rural site in Alabama. The chemical composition in the air sampled at this site provides an interesting contrast to earlier measurements made at a site in the Colorado mountains. Even though the ozone levels were slightly smaller, the H2O2 levels were a factor of 2 to 3 higher at the Alabama site. These measurements serve to further test our understanding of the meteorological and chemical parameters that determine the concentration of H2O2 in the rural troposphere. Deposition beneath a shallow nocturnal inversion layer as well as precipitation events played a major role in the loss of H2O2 and had a strong effect on the diurnal trend. When these major influences were eliminated through selection of dry, high sunlight and midmorning to late afternoon periods only, several relationships were apparent. A strong positive correlation between H2O2 and temperature was probably due to an increase in the concentration of isoprene with temperature. Isoprene serves as a major source of HO2 radicals, the precursors of H2O2. At the Alabama site, higher temperatures lead to much higher isoprene levels and, consequently, the higher H2O2 levels compared to the Colorado site. At the Alabama site the relative humidity was anticorrelated with the concentration of H2O2, probably due to the effect of the humidity on deposition, evidently the dominant loss process. H2O2 exhibited only a weak positive correlation with O3 and no correlation with H2O. The degree of photochemical processing of the air mass was positively related to the concentration of H2O2. An increase in the concentration of NOx had a small but significant negative effect on the level of H2O2, and the diurnal correlation of H2O2 closely reflected the variation of the ratio of the concentrations of isoprene to NOx, which again reflects the important role isoprene may play as a photochemical precursor for H2O2 and the inverse role that NOx plays in curtailing the photochemical production of H2O2. Finally, the H2O2 to HNO3 ratio indicates that the photochemistry is strongly NOx limited at this Alabama site.
Seasonal measurements of nonmethane hydrocarbons and carbon monoxide at the Mauna Loa Observatory during the Mauna Loa Observatory Photochemistry Experiment 2
Authors: Greenberg J. P. , Helmig D. , Zimmerman P. R.
Published: February 2004

Measurements of carbon monoxide and nonmethane hydrocarbons were made in situ at the Mauna Loa Observatory from September 1991 through August 1992. A distinct seasonal cycle in concentrations was observed for CO and most NMHCs, with higher average concentrations from January through April and much lower average concentrations from mid‐May through October. The sources of CO and NMHCs appeared to vary with season and appeared more homogeneous in winter (urban/industrial) and spring (unidentified). The ocean around Hawaii did not appear to be a major source of CO and most NMHCs, but was the major source of ethane and propene. Ethene and propene were always present in free tropospheric air sampled, indicating daily exchange between the free troposphere and the marine boundary layer. Air arriving at MLO during free tropospheric flow periods had seasonally similar photochemical age (approximately 20 days), with no significant fresh source inputs of NMHCs (except ethene and propene) within at least 10 days. The influence of atmospheric dilution processes on atmospheric concentrations was weakest in the spring period, when the influence of chemical loss processes was greatest. Aircraft measurements made during the spring agreed in trends and absolute concentrations with ground‐based measurement made at MLO. The temporal trends in concentrations of NMHCs within altitude ranges were similar. These trends varied with altitude and characterized a layered atmosphere with differing chemical composition. Acetone and methyl ethyl ketone were detected in most samples. N‐alkyl aldehydes were also detected but may be artifacts formed from organic aerosols collected in the sampling process.
Comparison of airborne and surface spectral bidirectional reflectance factors, spectral hemispherical reflectance and spectral vegetation indices
Authors: Russell C. A. , Walthall C. L. , Irons J. R. , de Colstoun E. C. Brown
Published: February 2004

Six sets of airborne advanced solid‐state array spectroradiometer (ASAS) and ground modular multiband radiometer (MMR) remotely sensed bidirectional measurements acquired over one First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment site in 1987 and 1989 were compared for the following parameters: bidirectional spectral reflectance factor, spectral hemispherical reflectance, simple ratio, and normalized difference vegetation index. ASAS at‐sensor radiances were atmospherically corrected and converted to surface reflectance factors. Selected MMR bands were simulated with ASAS data, and hemispherical reflectance for both ground and airborne data sets was estimated using data collected only in or close to the solar principal plane. The shapes of the reflectance factor response curves (as a function of view zenith angle) were strongly affected by solar zenith angle and compared well between instruments. Off‐nadir anisotropy in reflectance factors was comparable between sensors, while actual values of reflectance factors differed 2–35% relative between instruments in the green channel and 0–38% relative in the red channel. For the data set giving the overall closest agreement, ASAS reflectance factors differed from MMR values by 5–17% relative (0.3–1.8% absolute) in the green and 5–10% relative (0.2–0.5% absolute) in the red. These differences showed no correlation with solar zenith angle. Some of the differences were arbitrarily introduced by the 5° offset in view zenith angles (except nadir) between the two instruments. Other differences were caused by the discrepancy in solar zenith angle for some of the sets, variable deviations from the solar principal plane, hotspot and associated shadowing effects not consistently recorded by both instruments, and sampled ground area variations. Estimates of hemispherical reflectance compared very well between sensors, with differences of only 3–14% relative (0.3–1.6% absolute) in the green and 11–27% relative (0.8–2.4% absolute) in the red. For the ASAS data, using atmospherically corrected reflectance factors (instead of at‐sensor radiances) significantly increased the values of the spectral vegetation indices (SVIs). Off‐nadir anisotropy for the SVIs derived from both sensors' data sets was less than that observed for the bidirectional reflectance factors, and NDVI off‐nadir deviations were much less than those of the simple ratio (SR). Large differences in the values of SR and NDVI between sensors indicate SVIs calculated from broadband (MMR) versus narrowband (ASAS) data are not comparable.
Mixing entropy analysis of dispersal of aircraft emissions in the lower stratosphere
Authors: Sparling L. C. , Schoeberl M. R.
Published: February 2004

A trajectory calculation is used to compare the characteristic time for northern midlatitude aircraft emissions to disperse during both summer and winter. The mixing process is summarized by the time evolution of the “mixing entropy,” which is proportional to the number of ways of partitioning N parcels over M equal area subregions of the hemisphere. The mixing entropy is a measure of the disorder in the parcel distribution and quantifies the approach to a spatially uniform state as time increases. We show that the time for a single release of exhaust to become mixed down to length scales of the order of 800 km is about 2 months in winter and at least 4 months in summer. We also examine the time evolution of the mixing entropy of emissions from a continuous source over two 6‐month periods starting in January and June. For a single continuous source, which represents a persistent and highly asymmetric perturbation, we find that the horizontal distribution of the total emissions reaches a mixing steady state in 6–8 months. On this timescale the mixing entropy is close to the maximum value, and the likelihood of large temporal fluctuations becomes small. The dependence of the mixing on the location of the source and the effect of multiple flight paths on the overall uniformity of the spatial distribution is also considered.
Comparison of HF and HCl vertical profiles from ground‐based high‐resolution infrared solar spectra with Halogen Occultation Experiment observations
Authors: Liu X. , Murcray F. J. , Murcray D. G. , Russell J. M.
Published: February 2004

HCl and HF vertical mixing ratio (VMR) profiles have been retrieved from very high resolution, ground‐based infrared solar absorption spectra taken from Denver, Colorado. The VMR profile retrieval is based on an iterative inversion algorithm, which incorporates a line‐by‐line layer‐by‐layer calculation for the forward model. The method has been successfully applied to both simulated and measured solar spectra. The HCl and HF profiles retrieved from the ground‐based solar spectra are compared with the profiles measured by the UARS Halogen Occultation Experiment (HALOE) instrument for the same day near 40°N latitude. The ground‐based HF profile agrees very well with the HALOE results. The HCl profiles agree with HALOE within the combined of error limits but are higher by 5 to 20%.
Validation of aerosol measurements from the improved stratospheric and mesospheric sounder
Authors: Lambert A. , Grainger R. G. , Remedios J. J. , Reburn W. J. , Rodgers C. D. , Taylor F. W. , Roche A. E. , Kumer J. B. , Massie S. T. , Deshler T.
Published: February 2004

The retrieval and validation of the infrared measurements of the stratospheric aerosol layer derived from the improved stratospheric and mesospheric sounder (ISAMS) onboard the Upper Atmsophere Research Satellite (UARS) are discussed in detail. The retrieval method is presented and an error analysis and sensitivity study are used to provide an error budget. The validation involves internal consistency checks, comparisons with coincident aerosol observations from other satellite‐based instruments (cryogenic limb array etalon spectrometer (CLAES) and Stratospheric Aerosol and Gas Experiment II (SAGE II)) and in situ particle counters. The internal comparisons show that the precision of the 6.20‐μm retrievals are better than 30% for the data on the 32‐hPa and 22‐hPa pressure surfaces and that the 12.11‐μm retrievals are better than 20% and 25% for the data on pressure surfaces at 46 hPa and 32 hPa, respectively. Comparison of the ISAMS aerosol extinctions with the simultaneous CLAES measurements show systematic biases for both the 6‐μm and the 12‐μm retrievals with ISAMS generally measuring lower extinction values than CLAES. The comparison with calculated extinctions from balloon dustsonde data at 41°N shows that the aerosol extinctions are systematically too high for pressures less than 40 hPa.
Validation of UARS microwave limb sounder 183 GHz H2O measurements
Authors: Lahoz W. A. , Suttie M. R. , Froidevaux L. , Harwood R. S. , Lau C. L. , Lungu T. A. , Peckham G. E. , Pumphrey H. C. , Read W. G. , Shippony Z. , Suttie R. A. , Waters J. W. , Nedoluha G. E. , Oltmans S. J. , Russell J. M. , Traub W. A.
Published: February 2004

The Upper Atmosphere Research Satellite (UARS) microwave limb sounder (MLS) makes measurements of thermal emission at 183.3 GHz which are used to infer the concentration of water vapor over a pressure range of 46 – 0.2 hPa (∼20 to ∼60 km). We provide a validation of MLS H2O by analyzing the integrity of the measurements, by providing an error characterization, and by comparison with data from other instruments. It is estimated that version 3 MLS H2O retrievals are accurate to within 20–25% in the lower stratosphere and to within 8–13 % in the upper stratosphere and lower mesosphere. The precision of a single profile is estimated to be ∼0.15 parts per million by volume (ppmv) in the midstratosphere and 0.2 ppmv in the lower and upper stratosphere. In the lower mesosphere the estimate of a single profile precision is 0.25–0.45 ppmv. During polar winter conditions, H2O retrievals at 46 hPa can have a substantial contribution from climatology. The vertical resolution of MLS H2O retrievals is ∼5 km.
Regional estimates of lightning production of nitrogen oxides
Authors: Biazar Arastoo P. , McNider Richard T.
Published: February 2004

Summertime distribution of lightning over the United States and the potential importance of lightning‐generated NOx (NO + NO2) was investigated by using data from the National Lightning Detection Network (NLDN) for June, July, and August 1989 through 1992. The data were compiled and gridded to yield hourly and monthly flash densities. Without correcting the data for the network's detection efficiency, on the average, 10 million flashes occur over the United States each summer with 2.6 strokes occurring per flash. The densest concentration of flashes is over the Southeast. In 1989 the summertime lightning activity (9.4 million flashes) accounted for 70% of the annual flashes. To investigate the regional characteristics of lightning, the data were also compiled for the eastern United States and a smaller subdomain of the southeastern United States. NOx production rates of 0.36×1026, 4×1026, and 30×1026 molecules/flash were chosen to represent the low, median, and high end of estimates suggested by different investigators. Using these three production rates and hourly gridded flash densities, lightning‐generated NOx emissions were calculated. These estimates were compared to anthropogenic emissions derived from the 1985 National Acid Precipitation Assessment Program (NAPAP) inventory. Based on the high production rate, NOx emissions produced by lightning are comparable to monthly anthropogenic NOx emissions in the Southeast during the summer. Even for the low production rate, hourly emissions of lightning‐produced NOx frequently exceed anthropogenic emissions, with the highest frequencies in the Southeast. These results suggest that estimates of lightning‐generated NOx in the rural southeastern United States are not negligible and that this natural source of NOx could play a significant role in summertime tropospheric ozone production in the Southeast. Given the importance of NOx in ozone photochemistry, especially in NOx‐limited regimes, this natural source cannot be dismissed relative to anthropogenic sources even though the uncertainties in actual emission factors are large.
Observations of accumulation mode aerosol composition and soot carbon concentrations by means of a high‐temperature volatility technique
Authors: Smith Michael H. , O'Dowd Colin D.
Published: February 2004

A high‐temperature volatility system has been deployed for the measurement of the composition and concentration of the accumulation mode aerosol (0.05 μm < r < 1 μm) within the atmospheric boundary layer. This instrumentation comprises a volatility system based around a Particle Measuring Systems ASASP‐X optical particle counter, which was operated together with an aethalometer for the direct observation of soot carbon concentrations. By cycling the heater tube through a range of temperatures from near ambient to over 1000°C, size‐differentiated information upon aerosol composition may be obtained. Furthermore, by careful selection of analysis temperatures, discrimination is possible between elemental carbon and the more volatile fractions of the soot carbon aerosol. Observations made over the North Sea near the Dutch coast and in the central United Kingdom are presented for differing environmental conditions with soot carbon concentrations ranging from about 100 to over 6000 ng m−3. For polluted conditions over the North Sea the volatility technique clearly showed the dominance of soot carbon particles over other aerosol components with a narrow carbon particle distribution of mode radius around 0.06 μm accounting for about 80% of all particles with radii below 0.1 μm. Under polluted conditions, only about 25% of the total soot carbon aerosol comprised elemental carbon (with the remainder consisting of more volatile material), whereas this proportion rose to around 50% in the lower carbon loadings found in a cleaner maritime air mass. The use of soot carbon loadings as a tracer of anthropogenic aerosol inputs to oceanic regions is explored on the basis of measurements from a NE Atlantic cruise.
Multiyear present‐day and 2 × CO2 simulations of monsoon climate over eastern Asia and Japan with a regional climate model nested in a general circulation model
Authors: Hirakuchi Hiromaru , Giorgi Filippo
Published: February 2004

Two continuous 5‐year‐long simulations over eastern Asia and the Japan islands, one for present‐day climate (control) and one for climate under doubled carbon dioxide concentration (2×CO2) are completed with a regional climate model (RegCM) nested, in a one‐way mode, within a general circulation model (GCM). The GCM is run at R15 resolution (4.5×7.5° latitude × longitude), and the RegCM is run at 50‐km grid point spacing. In the control run, both the GCM and the RegCM reproduce the seasonal migration of the westerly jet but produce too strong a monsoonal circulation, which results in a significant overestimate of summer precipitation over the eastern Asian continent. The temporal evolution of the eastern Asia summer monsoon, with steady phases separated by more rapid transitions, is reproduced, but the monsoon rain belt reaches too far north, and the occurrence of tropical storms is underestimated. Regionally averaged surface air temperatures are mostly within 1–3°C of observations. Seasonal precipitation amounts over Japan are within 10–35% of observed ones. The narrow Korea and Japan land masses, which are not captured by the GCM grid, substantially affect the simulated surface precipitation climatology in the RegCM. Under 2×CO2 forcing, warming in the range of 4–11°C is simulated, greater in winter than in summer and increasing toward high latitudes. The strength of the monsoonal circulation increases in 2×CO2 conditions, leading to a general increase in precipitation over all regions by 10–30%. The simulated precipitation change shows significant regional and, within Japan, subregional structure. The Japan and Korea land masses substantially affect the summer precipitation changes, indicating the need to capture them in numerical simulations of climate change. Because of the uncertainties in the control simulations, our 2×CO2 results are intended not to provide climate change scenarios for impact assessments, but only to illustrate the model sensitivity to different forcings. Plans are under way to further test and improve the RegCM performance over the region and to use more recent GCM simulations to drive the RegCM.