bluuebunny/crossref_metadata_2025 huggingface dataset:
High concentrations of black carbon over middle latitudes in the North Pacific Ocean
Authors: Kaneyasu Naoki , Murayama Shohei
Published: February 2004

During 4 years of research cruises over the central Pacific Ocean from 1993 through 1996, a region of extremely high concentration of black carbon aerosol (>150 ng C m−3) was routinely observed in the middle‐latitude zone (27°–48°N) along the 175°E line of longitude (near the International Date Line). In the cruises along the latitude lines of 30°N and 35°N, a strong west‐to‐east gradient of black carbon was also observed. Backward trajectory analysis shows that the air parcels containing high concentration of black carbon had passed over the Asian continent during the previous 10 days. The concentration of black carbon was relatively well correlated with non‐sea‐salt (nss.) SO42− in the aerosols. This indicates that the high concentration of black carbon observed originated from the combustion of sulfur‐containing materials, presumably coal, on the Asian continent. Mass size distributions of the black carbon exhibit a mode at 0.5–0.7 μm in aerodynamic diameter, which is unexpectedly large and coincides with the mode of SO42− aerosol. This suggests that black carbon and SO42− were internally mixed, and additional growth in particle size occurred through cloud processing. This study shows that anthropogenic aerosols are transported over greater distances than previously indicated by global dispersion models. The trajectory analysis indicates that this is due to the high‐altitude transport of materials which reduces the possibility of encountering precipitating clouds; the aerosols are lofted over the continent initially, transported at relatively high altitudes over the ocean, and then brought to the surface in the central part of the North Pacific Ocean by mesoscale weather systems. The measured mixing ratio of black carbon/nss.SO42− and the state of the mixture (internal) suggest that the aerosols transported from Asia over the North Pacific Ocean have a low single‐scattering albedo.
A laboratory study of graupel charging
Authors: Pereyra Rodolfo G. , Avila Eldo E. , Castellano Nesvit E. , Saunders Clive P. R.
Published: February 2004

Measurements have been made of charge transfer when vapor grown ice crystals rebound from a riming target representing a graupel pellet falling in a thunderstorm. Earlier studies in the laboratory in Córdoba of charge transfer between an individual falling ice sphere and a riming target noted that the sign of the charge transfer was dependent upon temperature and effective liquid water content (EW). The new work uses a similar experimental technique; however, a cloud of ice crystals is grown in order to study multiple interactions with the riming target. The results also show charge sign dependence on temperature and EW; positive rimer charging is observed at high temperatures and for low and high values of EW at low temperature, while negative rimer charging is noted at low temperatures for intermediate values of EW. These results are similar to those obtained by Takahashi (1978) and, as has been reported before, are rather different from those obtained in Manchester by Jayaratne et al. (1983), Saunders et al. (1991), and Saunders and Peck (1998). Significant differences between the two types of data sets are attributed to the experimental techniques used in the various studies. In the present work the ice crystal cloud and the cloud of supercooled droplets responsible for riming the target are grown in separate chambers and then mixed shortly before the crystals and droplets encounter the riming target, so that the droplet cloud is not depleted by the growing ice crystals. In the Manchester experiments, the ice crystals grow in the same supercooled droplet cloud used to rime the target. It is possible that the mixing process provides an undepleted droplet cloud and a transient enhanced vapor supply that affects both the ice crystal and graupel vapor depositional growth rates, leading to the present results.
Bimodal size distribution influences on the variation of Angstrom derivatives in spectral and optical depth space
Authors: O'Neill N. T. , Eck T. F. , Holben B. N. , Smirnov A. , Dubovik O. , Royer A.
Published: February 2004

The variation of the aerosol optical depth and its first and second spectral derivatives (α and α′) can be largely described in terms of the spectral interaction between the individual optical components of a bimodal size distribution. Simple analytical expressions involving the separate optical components of each mode explain virtually all the features seen in spectra of the aerosol optical depth and its derivatives. Illustrations are given for a variety of measured optical depth spectra; these include comparative simulations of the diurnal behavior of α and α′ spectra as well as the diurnal and general statistical behavior of α and α′ as a function of optical depth (optical depth space). Each mode acts as a fixed “basis vector” from which much of the behavior in spectral and optical depth space can be generated by varying the extensive (number density dependent) contributions of fine and coarse mode optical depths. Departures from these basis vectors are caused by changes in aerosol type (average size and refractive index) and thus are associated with differing synoptical air masses, source trajectories or humidity conditions. Spectral parameters are very sensitive to interband errors in measured optical depth data. Third‐order polynomial fits within the visible‐NIR spectral region effectively filter such errors while representing the limit of useful extractable information.
Strong downslope winds and application of hydraulic‐like theory
Authors: Helmis C. G. , Flocas H. A. , Kalogiros J. A. , Asimakopoulos D. N.
Published: February 2004

An experimental campaign was conducted at the lee side of a 1 km high steep mountain in order to study the development of strong downslope winds under favorable conditions, using combined remote and insitu instrumentation. The examination of the upstream atmospheric conditions reveals that the development of strong downslope winds is favored by a mean state critical layer or a significant decrease in static stability (such as at the top of a temperature inversion) at the proper height above the mountain top. Strong downslope winds could occur even for wind directions with a deviation of 60° off the axis perpendicular to the ridgeline, as long as the cross‐mountain wind has a significant value (at least 7 ms−1). The developed disturbances are associated with intense downdrafts of the order of 4–5 ms−1 within the first 600 m above ground and characteristic vertical turbulent structures that were observed by sodar. The same phenomenon is observed not only during nighttime but also under unstable and neutral conditions of the atmospheric boundary layer (ABL) albeit with weaker intensity. Fourier analysis of the vertical velocity field demonstrated that the typical time period of intense disturbances was about 5 min. Further evidence is also provided for the application of the hydraulic‐like theory under real atmospheric conditions.
Profile measurements of blowing snow at Halley, Antarctica
Authors: Mann G. W. , Anderson P. S. , Mobbs S. D.
Published: February 2004

Measurements of blowing snow particle concentration from the second Stable Antarctic Boundary Layer Experiment (STABLE 2) are presented. The measurements, made at Halley Station (75.6°S, 26.7°W) throughout the 1991 austral winter, are supplemented with profile measurements of wind speed, air temperature, and humidity. Threshold wind speeds for blowing snow are shown to be distinctly different for various episodes, often depending strongly on the the availability of loose snow. Blowing snow is measured to occur at Halley between 27 and 37% of the time during winter. Total winter (June, July, August) blowing snow sublimation at Halley is calculated to be around 4.7 mm water equivalent, 3.7% of total accumulation over the same period. Total winter blowing snow mass transport is calculated to be around 5.5×105 kg per metre width. Measured humidity profiles at Halley show that during winter blowing snow conditions, a layer of near‐saturated air forms, causing total sublimation to be less than that for seasonal or limited fetch snow covers. The extent of sublimation is shown to be strongly dependent on wind speed, temperature and fetch.
Flux of dimethylsulfide from the oceans: A comparison of updated data sets and flux models
Authors: Kettle A. J. , Andreae M. O.
Published: February 2004

Estimates of the fluxes of dimethylsulfide (DMS) from the global oceans are calculated using the parameterizations of Lisps and Merlivat [1986], Wanninkhof [1992], and Erickson [1993]; the wind fields of Trenberth et al. [1989], da Silva and Young [1994], and Gibson et al. [1996]; and the maps of sea surface DMS concentration of Kettle et al. [1999]. The Liss and Merlivat [1986], Wanninkhof [1992], and Erickson [1993] parameterizations gave global oceanic DMS fluxes of approximately 15, 33, and 21 Tg S yr−1, respectively. The different data sets for wind speed, sea surface temperature, and DMS sea surface concentration resulted in relatively small variations in these calculated fluxes (≤25%). The large uncertainty in the flux models prevents the estimation of the global flux of DMS with an error limit of better than about 50%. Interannual variation in the wind speeds and sea surface temperatures from the 15‐year European Centre for Medium‐Range Weather Forecasts (ECMWF) Reanalysis Project results in an interannual variation of DMS flux of approximately 10%. Possibly, there is greater interannual variation in the DMS flux due to variation in the DMS concentration fields, which was not taken into account.
A 4100‐year record of explosive volcanism from an East Antarctica ice core
Authors: Cole‐Dai Jihong , Mosley‐Thompson Ellen , Wight Shawn P. , Thompson Lonnie G.
Published: February 2004

Extensive archives of volcanic history are available from ice cores recovered from the Antarctic and Greenland ice sheets that receive and preserve sulfuric acid fallout from explosive volcanic eruptions. The continuous, detailed (average 1.2 samples per year) sulfate measurements of a 200‐m ice core from a remote East Antarctica site (Plateau Remote) provide a record of Southern Hemisphere volcanism over the last 4100 years. This extends the volcanic record beyond the last 1000 years covered by previous Antarctic ice cores. An average of 1.3 eruptions per century is recorded in East Antarctic snow during the last 4100 years. The record shows that on average eruptions have been more frequent and more explosive during the most recent 2000 years than in the previous 2100 years. Intervals up to 500 years are observed in which few explosive volcanic signals are detected. These periods include 2000–1500 B.C. (no eruptions), 500–1 B.C. (two eruptions), and 700–1200 A.D. (two eruptions). This new Plateau Remote volcanic record is compared with those from previous Antarctic ice cores covering the last 1000 years. In terms of dates for volcanic events, the new record is in excellent agreement with the earlier records. However, significant discrepancies are found between these records in relative signal magnitude (volcanic flux) of several well‐known events. The discrepancies among the records may be explained by the differences in the glaciology at the ice core sites, analytical techniques used for sulfate and sulfuric acid measurement, and the selection of detection thresholds for volcanic signals. Comparison with Greenland ice core volcanic records indicates that during the last millennium, nine large, low‐latitude eruptions contributed significant amounts of volcanic aerosols to the atmosphere of both hemispheres, potentially affecting global climate. In contrast, only one or possibly two such eruptions are found in the first millennium A.D.
Ensemble regional and global climate prediction for the 1997/1998 winter
Authors: Mo Kingtse , Kanamitsu Masao , Juang Hann Ming Henry , Hong Song‐You
Published: February 2004

The ensemble global and regional forecasts for the 1997/1998 winter were made in October 1997. The forecasted sea surface temperature anomalies (SSTA) in the tropical Pacific are obtained from the National Centers for Environmental Prediction (NCEP) coupled atmosphere‐ocean model. Outside of the tropical Pacific the observed SSTs at the initial date are relaxed to the SST climatology with the e‐folding time of 1 month. The global forecasts with the resolution of T62 are then downscaled using the NCEP regional spectral model (RSM) to enhance precipitation and temperature anomalies over the United States. The horizontal resolution of the RSM is 50km. The global forecasts over this warm El Niño‐Southern Oscillation (ENSO) winter have high skill over the Pacific North American region. Overall, the RSM downscaling improves the surface temperature and precipitation forecasts over the United States. However, it is more difficult to predict precipitation over a small region with certainty, because the spread among the members in the ensemble increases with the increase of horizontal resolution. The 50‐km RSM model is unable to forecast anomalies over high mountains and many centers of precipitation maxima along the California coast. The sensitivity test shows that to forecast basin scale precipitation, a regional model with 20 km or less is needed. Although the 20‐km RSM forecasts tend to overestimate precipitation maxima, it provides a more realistic distribution of precipitation. It can distinguish rainfall belts over high mountains and the relative dryness over the valley.
A study of internal variability of a regional climate model
Authors: Giorgi Filippo , Bi Xunqiang
Published: February 2004

We analyze sensitivity experiments in which random perturbations are applied to the initial conditions (IC) and lateral boundary conditions (LBC) of a set of seasonal regional climate model (RCM) simulations. The model response to these perturbations, which is a measure of the internal RCM variability, grows for the first 5–15 days of simulation and then reaches a dynamical equilibrium. The response is not sensitive to either the magnitude or the source of the perturbation, but it is mostly tied to synoptic conditions, season (maximum response in summer and minimum in winter), region, and model configuration. The model response does not affect significantly the domain‐wide average climatology, but it substantially influences the day‐to‐day model solution, especially precipitation, and aspects of the model climate such as the frequency of occurrence of heavy precipitation events. Results from the perturbation experiments are compared to those from an experiment in which some land surface conditions are modified. It is shown how some of the model responses to the physical forcing are within and some outside the internal model variability. Our results indicate that the internal model variability is a factor that should be considered in the design, analysis, and interpretation of RCM experiments.
Dust resuspension without saltation
Authors: Loosmore Gwen A. , Hunt James R.
Published: February 2004

Wind resuspension (or entrainment) provides a source of dust and contaminants for the atmosphere. Conventional wind erosion models parameterize dust resuspension flux with a threshold velocity or with a horizontal abrasion flux; in the absence of abrasion the models assume dust flux is transient only. Our experiments with an uncrusted, fine material at relative humidities exceeding 40% show a long‐term steady dust flux in the absence of abrasion, which fits the approximate form: Fd = 3.6(u*)3, where Fd is the dust flux (in μg/m2 s), and u* is the friction velocity (in m/s). These fluxes are generally too small to be significant sources of dust in most models of dust emission. However, they provide a potential route to transport contaminants into the atmosphere. In addition, dust release is substantial during the initial transient phase. Comparison with field data suggests that the particle friction Reynolds number may prove a better parameter than u* for correlating fluxes and understanding the potential for abrasion.
Measurements of excess O3, CO2, CO, CH4, C2H4, C2H2, HCN, NO, NH3, HCOOH, CH3COOH, HCHO, and CH3OH in 1997 Alaskan biomass burning plumes by airborne Fourier transform infrared spectroscopy (AFTIR)
Authors: Goode Jon G. , Yokelson Robert J. , Ward Darold E. , Susott Ronald A. , Babbitt Ronald E. , Davies Mary Ann , Hao Wei Min
Published: February 2004

We used an airborne Fourier transform infrared spectrometer (AFTIR), coupled to a flow‐through, air‐sampling cell, on a King Air B‐90 to make in situ trace gas measurements in isolated smoke plumes from four, large, boreal zone wildfires in interior Alaska during June 1997. AFTIR spectra acquired near the source of the smoke plumes yielded excess mixing ratios for 13 of the most common trace gases: water, carbon dioxide, carbon monoxide, methane, nitric oxide, formaldehyde, acetic acid, formic acid, methanol, ethylene, acetylene, ammonia and hydrogen cyanide. Emission ratios to carbon monoxide for formaldehyde, acetic acid, and methanol were 2.2±0.4%, 1.3±0.4%, and 1.4±0.1%, respectively. For each oxygenated organic compound, a single linear equation fits our emission factors from Alaska, North Carolina, and laboratory fires as a function of modified combustion efficiency (MCE). A linear equation for predicting the NH3/NOxemission ratio as a function of MCE fits our Alaskan AFTIR results and those from many other studies. AFTIR spectra collected in downwind smoke that had aged 2.2±1 hours in the upper, early plume yielded ΔO3/ΔCO ratios of 7.9±2.4% resulting from O3production rates of ∼50 ppbv h−1. The ΔNH3/ΔCO ratio in another plume decreased to 1/eof its initial value in ∼2.5 hours. A set of average emission ratios and emission factors for fires in Alaskan boreal forests is derived. We estimate that the 1997 Alaskan fires emitted 46±11 Tg of CO2.
A simple expression for wind erosion threshold friction velocity
Authors: Shao Yaping , Lu Hua
Published: February 2004

Threshold friction velocity u*t is the friction velocity at which wind erosion is initiated. While u*t is affected by a range of surface and soil properties, it is a function of particle size only for idealized soils. In this paper we present a simple expression for u*t for spherical particles loosely spread over a dry and bare surface. In this expression we consider the balance between the driving forces (aerodynamic drag and lift) and the retarding forces (cohesion and gravity) and assume that the cohesive force is proportional to particle size. It is found that u*t can be expressed as
, with Y1 and Y2 being empirical constants. The new expression is both simple and effective.
Linking local air pollution to global chemistry and climate
Authors: Mayer Monika , Wang Chien , Webster Mort , Prinn Ronald G.
Published: February 2004

We have incorporated a reduced‐form urban air chemistry model in the Massachusetts Institute of Technology's two‐dimensional land and ocean resolving coupled chemistry‐climate model. The computationally efficient reduced‐form urban model was derived from the California Institute of Technology–Carnegie Institute of Technology (at Carnegie Mellon University) Urban Airshed Model by employing the probabilistic collocation method. To study the impact of urban air pollution on global chemistry and climate, we carried out three simulations, each including or excluding the reduced‐form urban model for the time period from 1977 to 2100. All three runs use identical total emissions; however, in the two runs involving the reduced‐form urban model, the emissions assigned to urban areas are allocated in different ways, depending on the scenario assumed for the future development of polluted urban areas. These two simulations are compared to the reference, which does not utilize the reduced‐form urban model. We find that the incorporation of urban air chemistry processes leads to lower global tropospheric NOx, ozone, and OH concentrations, but a higher methane mole fraction, than in the reference. The tropospheric mole fraction of CO is altered either up or down depending on the projections of urban emissions. The global mean surface temperature is affected very little by implementation of the reduced‐form urban model because predicted increases in CH4 are offset in part by decreases in O3, leading to only small changes in overall radiative forcing.
Daytime HO2 concentrations at Oki Island, Japan, in summer 1998: Comparison between measurement and theory
Authors: Kanaya Yugo , Sadanaga Yasuhiro , Matsumoto Jun , Sharma Usha K. , Hirokawa Jun , Kajii Yoshizumi , Akimoto Hajime
Published: February 2004

The daytime variation of hydroperoxy (HO2) radical concentration was observed by an instrument based on laser‐induced fluorescence with NO addition at Oki Island, Japan, in July/August 1998. Although OH was not detected due to the high detection limit of the instrument, HO2 was determined with the detection limit of 0.8 parts per trillion by volume (pptv) (S/N = 2, integration time of 1 min). On average, HO2 showed a maximum concentration of around 9 pptv in the early afternoon hours. During the field campaign, chemical species and meteorological parameters such as O3, CO, nonmethane hydrocarbons(C2‐C6), NO/NO2, HCHO/CH3CHO, SO2, HNO3 and J(NO2) were also observed. Unfortunately, the absolute value of J(O1D) was not measured. Model calculations for radical concentrations were performed and they were compared to the observed hourly HO2 concentrations. On August 9 the calculated HO2 matched the observation very well within the uncertainties of observations (±26%, 1σ) and the model (±24%, 1σ). This indicates a good performance of model calculations in estimating HO2 under certain conditions with plenty of isoprene. On other days, however, model usually overestimated HO2 by a factor of 2, especially in the hours around noon. It is deduced that some important HOx loss chemistry is missing in the model. Although the cause of the discrepancy is not fully understood, possible mechanisms to explain the overestimation are studied. A hypothesis with additional loss of HO2 would be more plausible than that with additional OH loss. The additional loss rate for HO2 that can reduce the calculated HO2 to the measured level was computed for each hour. Its variation correlated well with those of H2O and some photochemical products. The possibilities of HO2 loss on aerosol surface, unknown acceleration of HO2 reactions in the presence of high H2O, and HO2 reactions with carbonyl species are discussed.
A generalized algorithm for retrieving cloudy sky skin temperature from satellite thermal infrared radiances
Authors: Jin Menglin , Dickinson Robert E.
Published: February 2004

A physical algorithm for calculating cloudy pixel skin temperature from its neighboring clear pixels is proposed generalizing that of Jin [2000]. Two neighboring pixels over the same land cover have a difference in temperature largely explained by surface insolation. Hence this algorithm starts from the surface energy balance equation (SEB) and expresses each energy term of SEB as a function of skin temperature. Then SEB is solved to derive cloudy pixel skin temperature from neighboring clear skin temperature plus a correction term determined by surface insolation, air temperature, and wind speed. This algorithm can be used for nights and winter hemisphere high latitudes where there is no surface insolation and is applicable to any surface where the principle of SEB is applicable. The algorithm is evaluated by using FIFE and BOREAS field experiments. Its global application has been examined through simulations with the NCAR Community Climate Model (CCM) coupled with the Biosphere‐Atmosphere Transfer Scheme (BATS) simulations. The accuracy of the algorithm is encouraging: the rms error at a monthly pixel level is 1–2 K. Observed errors are greater when there is precipitation.
Effect of the 1997 El Niño on the distribution of upper tropospheric cirrus
Authors: Massie Steven , Lowe Paul , Tie XueXi , Hervig Mark , Thomas Gary , Russell James
Published: February 2004

Geographical distributions of Halogen Occultation Experiment (HALOE) aerosol extinction data for 1993–1998 are analyzed in the troposphere and stratosphere at pressures between 121 and 46 hPa. The El Niño conditions of 1997 increased upper tropospheric cirrus over the mid‐Pacific and decreased cirrus over Indonesia. Longitudinal centroids of cirrus in the Pacific and over Indonesia shifted eastward by 25° in the troposphere in 1997. Longitudinal centroids of aerosol in the lower stratosphere do not exhibit longitudinal shifts in 1997, indicating that the effects of El Niño upon equatorial particle distributions are confined to the troposphere. The correlation of the longitudinal centroids of outgoing longwave radiation and HALOE extinction confirms the spatial relationship between deep convective clouds and upper tropospheric cirrus. The number of cirrus events observed each year in 1993–1998 in the upper troposphere are quite similar for the region from the Indian Ocean to the mid‐Pacific (30°S to 30°N, 50° to 240°E).
A catchment‐based approach to modeling land surface processes in a general circulation model: 1. Model structure
Authors: Koster Randal D. , Suarez Max J. , Ducharne Agnès , Stieglitz Marc , Kumar Praveen
Published: February 2004

A new strategy for modeling the land surface component of the climate system is described. The strategy is motivated by an arguable deficiency in most state‐of‐the‐art land surface models, namely, the disproportionately higher emphasis given to the formulation of one‐dimensional, vertical physics relative to the treatment of horizontal heterogeneity in surface properties, particularly subgrid soil moisture variability and its effects on runoff generation. The new strategy calls for the partitioning of the continental surface into a mosaic of hydrologic catchments, delineated through analysis of high‐resolution surface elevation data. The effective “grid” used for the land surface is therefore not specified by the overlying atmospheric grid. Within each catchment, the variability of soil moisture is related to characteristics of the topography and to three bulk soil moisture variables through a well‐established model of catchment processes. This modeled variability allows the partitioning of the catchment into several areas representing distinct hydrological regimes, wherein distinct (regime specific) evaporation and runoff parameterizations are applied. Care is taken to ensure that the deficiencies of the catchment model in regions of little to moderate topography are minimized.
Kinetic studies of OH‐initiated reactions of isoprene
Authors: Zhang Renyi , Suh Inseon , Lei Wenfang , Clinkenbeard Angela D. , North Simon W.
Published: February 2004

We report laboratory kinetic studies of isoprene reactions initiated by the hydroxyl radical OH, using a turbulent flow reactor coupled to chemical ionization mass spectrometry (CIMS) detection. The rate constants for the reaction of isoprene with OH have been measured in the pressure range of 70 to 120 torr at 298±2 K and are found to be independent of pressure with an averaged value of (10.1±0.8)×10−11 cm3 molecule−1 s−1. The error limit given is within 1 standard deviation; a systematic error is estimated to be ±15%. We also describe direct observation of the OH‐isoprene adduct based on ion‐molecule reactions by using the CIMS method. The formation of the OH‐isoprene adduct was used to extract the rate constant between OH and isoprene; within the uncertainty of the experiments the results were consistent with those obtained from the observed disappearance of OH. By monitoring the formation of the OH‐isoprene adduct in the presence of oxygen molecules, an overall rate constant between OH‐isoprene adduct and O2 has been first determined, with an averaged value of (2.8±0.7)×10−15 cm3 molecule−1 s−1 at 76 torr and an estimated systematic error of ±50%. Atmospheric implications of the present results to the photochemical oxidation of isoprene are discussed.
Vertical photon transport in cloud remote sensing problems
Authors: Platnick S.
Published: February 2004

Photon transport in plane‐parallel, vertically inhomogeneous clouds is investigated and applied to cloud remote sensing techniques that use solar reflectance or transmittance measurements for retrieving droplet effective radius. Transport is couched in terms of weighting functions which approximate the relative contribution of individual layers to the overall retrieval. Two vertical weightings are investigated, including one based on the average number of scatterings encountered by reflected and transmitted photons in any given layer. A simpler vertical weighting, based on the maximum penetration of reflected photons, proves useful for solar reflectance measurements. These weighting functions are highly dependent on droplet absorption and solar/viewing geometry. A superposition technique, using adding/doubling radiative transfer procedures, is used to accurately determine both weightings, avoiding time‐consuming Monte Carlo methods. Effective radius retrievals from modeled vertically structured liquid water clouds are then made using the standard near‐infrared bands and compared with size estimates based on the proposed weighting functions. Agreement between the two methods is generally within several tenths of a micrometer, much better than expected retrieval accuracy. Though the emphasis is on photon transport in clouds, the derived weightings can be applied to any multiple‐scattering plane‐parallel radiative transfer problem, including arbitrary combinations of cloud, aerosol, and gas layers.
A modeling study of the nighttime radical chemistry in the lower continental troposphere: 1. Development of a detailed chemical mechanism including nighttime chemistry
Authors: Bey Isabelle , Aumont Bernard , Toupance Gérard
Published: February 2004

We describe a detailed chemical mechanism (140 species, 440 reactions) representing atmospheric chemistry of the lower continental troposphere. We focus on chemical reactions potentially important for nighttime chemistry. The mechanism is based on the methods and concepts proposed by Carter [1990] for the development of the Statewide Air Pollution Research Center (SAPRC) mechanism to reduce the number of species and reactions. Peroxy radical chemistry is represented with chemical operators and secondary VOCs are lumped into a limited number of surrogate species. To account for new experimental data provided since the first formulation of the SAPRC mechanism, the whole set of reactions has been updated. Representation of peroxy radical chemistry has been modified to account for variability of organic peroxy radical‐peroxy radical reaction rates. New chemical operators and new secondary species have been added to the mechanism to introduce reactions of alkenes with NO3. Reactions of alkenes with O3 have been largely revised to represent largest radical yields suggested by recent experimental studies. Chemistry of three biogenic compounds (isoprene, α‐pinene and β‐pinene) has been included. The mechanism is coupled to a two‐layer box model (including anthropogenic and biogenic emissions of VOCs and NOx and dry deposition) in order to represent different chemical environments (from urban to rural cases). Comparisons with the SAPRC‐99 mechanism show good agreements especially for high NOx regime. Results from the model are evaluated with typical observed concentrations in the different environmental cases with a special focus on the representation of radical species concentations. For both daytime and nighttime, the calculated OH and peroxy radical concentrations are within the range of available observed values although the model tends to overestimate HO2 concentrations in daytime. At night, OH concentrations are found to vary from 1 to 6×106 molecules cm−3 in the urban environment, 5×104 to 2×105 molecules cm−3 in polluted rural environments and 4×104 molecules cm−3 in remote environments.
A modeling study of the nighttime radical chemistry in the lower continental troposphere: 2. Origin and evolution of HOx
Authors: Bey Isabelle , Aumont Bernard , Toupance Gérard
Published: February 2004

Nighttime chemistry of OH, HO2, and organic peroxy radicals is examined in different environmental cases of the lower continental troposphere. The scenarios cover a large range of NOx concentrations and various mixtures of anthropogenic and biogenic VOCs. For each scenario the primary formation rate of radicals species is calculated and the evolution of the radical pool is investigated by calculating the efficiency of the propagation reactions versus the termination reactions in the RO2 → H02 → OH cycle. Finally, the net OH formation rates as well as the relative contributions of the NO3− and O3‐initiated mechanisms in the OH production are calculated. We found that the radical initiation is driven by ozonolysis of VOC in urban and environments with low NOx concentrations, while both NO3 and O3 contribute to the radical initiation in rural sites with moderated NOx levels. The nighttime formation of OH radicals exhibits similarities between the different scenarios. In most of the cases, OH formation at night results mainly from VOC + O3 reactions in the mixing layer as well as the upper layer of the troposphere. The VOC + NO3 reactions were found to account for a maximum of 25% in OH production in scenarios with high levels of biogenic compounds and moderated levels of NOx. In the mixing layer the interconversion among radicals (RO2 → HO2 and HO2 → OH conversions), when it happens at night, is essentially driven by NO. The resulting OH fluxes vary from 0.2–1×107 to 1×106 molecule cm−3 s−1 in the urban scenario in the mixing and top layer, respectively. In rural/remote situations, nighttime fluxes range from 1×106 molecule cm−3 s−1 in the mixing layer to 1×105 molecule cm−3 s−1 in the upper layer.
Evidence of H2O nonlocal thermodynamic equilibrium emission near 6.4 μm as measured by cryogenic infrared spectrometers and telescopes for the atmosphere (CRISTA 1)
Authors: Edwards D. P. , Zaragoza G. , Riese M. , López‐Puertas M.
Published: February 2004

The spectral region from 6.2 to 6.6 μm contains the strongest infrared bands of water vapor and is often used by satellite remote sounding instruments to obtain the middle atmosphere concentration of this gas. However, the diurnal variation in mesospheric H2O emission is not well understood owing to nonlocal thermodynamic equilibrium (non‐LTE) processes. This has led to H2O retrieval problems with past satellite instruments. Spectral measurements taken by the Space Shuttle cryogenic infrared spectrometers and telescopes for the atmosphere (CRISTA 1) instrument have been used to study these infrared limb emissions. The H2O radiance shows a daytime enhancement in the mesosphere due to nonthermal collisions and solar absorption. We use a combination of measurements and modeling to determine consistent values for the collisional rate constants and photolysis quantum yield that affect the non‐LTE processes. Our new estimates of these quantities should improve considerably the confidence in the mesospheric H2O retrievals that will be made by the EOS TIMED sounding of the atmosphere using broadband emission radiometry (SABER) and Aura high‐resolution dynamics limb sounder (HIRDLS) instruments.
A three‐dimensional modeling investigation of the evolution processes of dust and sea‐salt particles in east Asia
Authors: Song Chul H. , Carmichael Gregory R.
Published: February 2004

The evolution of sea‐salt and dust particles in East Asia is investigated using a three‐dimensional transport and chemistry model. A kinetic approach under thermodynamic constraint is utilized to model the condensation/evaporation processes, and other important aerosol processes and influential components (e.g., dust/sea‐salt generation, NH3 emissions, gravitational settling, nucleation) are taken into account in this analysis. The model is used to study the Pacific Exploratory Mission‐West B period (March 1–6, 1994). It is found that (1) during strong continental outflow, in general, the fine aerosol mode (<2 μm in aerodynamic diameter) accommodates sulfate and ammonium and the cation‐rich coarse mode (>2 μm in aerodynamic diameter) attracts nitrate. However, in the dust plume, sulfate preferentially resides in the coarse mode due to larger coarse mode mass loading; (2) particulate nitrate coupled with particulate ammonium in the fine mode is predicted over regions where high gaseous NH3 mixing ratios are present (lower courses of the Huang river); (3) dust and sea‐salt particles provide important reaction surfaces for sulfate production in the troposphere and increase sulfate production rates by 20–80%; and (4) soil dust and sea salt provide an important source of boundary layer and free troposhpere alkaline material, and these cations play an important role in controlling the partitioning of semivolatile HNO3 throughout large portions of the troposphere, increasing particulate nitrate levels 10–50%.
Long‐range transport of Asian dust to the Lower Fraser Valley, British Columbia, Canada
Authors: McKendry I. G. , Hacker J. P. , Stull R. , Sakiyama S. , Mignacca D. , Reid K.
Published: February 2004

For the first time, long‐range transport of “Kosa” mineral aerosol from western China to southwestern British Columbia is documented. This late April 1998 event coincided with an episode of photochemical smog and reduced dispersion in the Lower Fraser Valley (LFV). Filter samples in the region show a massive injection of crustal elements (Si, Fe, Al, and Ca) with concentrations of Si approximately double those previously recorded. Ratios of these elements to Fe are shown to be statistically similar to ratios observed in mineral aerosol events in Hawaii and China. On the basis of the difference between observed and expected elemental concentrations and reconstructed soil mass in the episode, it is estimated that Asian dust contributed 38–55% to observed PM10 in the LFV, the remainder being attributed to local sources. Comparison of the April 1998 event with two spring meteorological analogs is consistent with this estimate. Mesoscale model simulations suggest that mineral dust was incorporated into the planetary boundary layer as a result of strong subsidence over the interior of southern British Columbia and Washington State which permitted interception of lower tropospheric elevated aerosol layers by surface‐based mixing processes over mountainous terrain. Surface easterly (“outflow”) winds then transported this material into the Lower Fraser Valley where it contributed significantly to total particulate loadings and an intense haze. This mechanism is consistent with the observed spatial and temporal distribution of PM10.
Remote sensing of aerosols over land surfaces from POLDER‐ADEOS‐1 polarized measurements
Authors: Deuzé J. L. , Bréon F. M. , Devaux C. , Goloub P. , Herman M. , Lafrance B. , Maignan F. , Marchand A. , Nadal F. , Perry G. , Tanré D.
Published: February 2004

The polarization measurements achieved by the POLDER instrument on ADEOS‐1 are used for the remote sensing of aerosols over land surfaces. The key advantage of using polarized observations is their ability to systematically correct for the ground contribution, whereas the classical approach using natural light fails. The estimation of land surface polarizing properties from POLDER has been examined in a previous paper. Here we consider how the optical thickness δ0 and Ångstrom exponent α of aerosols are derived from the polarized light backscattered by the particles. The inversion scheme is detailed, and illustrative results are presented. Maps of the retrieved optical thickness allow for detection of large aerosol features, and in the case of small aerosols, the δ0 and α retrievals are consistent with correlative ground‐based measurements. However, because polarized light stems mainly from small particles, the results are biased for aerosol distributions containing coarser modes of particles. To overcome this limitation, an aerosol index defined as the product AI = δ0α is proposed. Theoretical analysis and comparison with ground‐based measurements suggest that AI is approximately the same when using δ0, and α is related to the entire aerosol size distribution or derived from the polarized light originating from the small polarizing particles alone. This invariance is specially assessed by testing the continuity of AI across coastlines, given the unbiased properties of aerosol retrieval over ocean. Although reducing the information concerning the aerosols, this single parameter allows a link between the POLDER aerosol surveys over land and ocean. POLDER aerosol index global maps enable the monitoring of major aerosol sources over continental areas.
Free tropospheric CO, C2H6, and HCN above central Europe: Recent measurements from the Jungfraujoch station including the detection of elevated columns during 1998
Authors: Rinsland C. P. , Mahieu E. , Zander R. , Demoulin P. , Forrer J. , Buchmann B.
Published: February 2004

Time series of free tropospheric carbon monoxide (CO), ethane (C2H6), and hydrogen cyanide (HCN) column abundances have been derived from observations at the International Scientific Station of the Jungfraujoch (ISSJ) at 3.58‐km altitude in the Swiss Alps (latitude 46.55°N, 7.98°E longitude). The free troposphere was assumed to extend from 3.58 to 11km altitude, and the related columns were derived for all three molecules from high spectral resolution infrared solar spectra recorded between January 1995 and October 1999. The three molecules show distinct seasonal cycles with maxima during winter for CO and C2H6, and during spring for HCN. These seasonal changes are superimposed on interannual variations. The tropospheric columns of all three molecules were elevated during 1998. Increases were most pronounced for HCN with enhanced values throughout the year, up to a factor of 2 in January 1998 when compared to averages of the other years. The increased tropospheric columns coincide with the period of widespread wildfires during the strong El Niño warm phase of 1997–1998. The emission enhancements above ISSJ are less pronounced, and they peaked after the increases measured above Mauna Loa (19.55°N, 155.6°W). Tropospheric trends for CO, C2H6, and HCN of (2.40±0.49), (0.47±0.64), and (7.00±1.61)% yr−1(1 sigma) were derived for January 1995 to October 1999. However, if 1998 measurements are excluded from the fit, CO and HCN trends that are not statistically significant, and a statistically significant decrease in the C2H6tropospheric column, are inferred. Comparisons of the infrared CO columns with CO in situ surface measurements suggest that the CO free tropospheric vertical volume mixing ratio profile generally decreases with altitude throughout the year.
Quasi‐Lagrangian investigation into dimethyl sulfide oxidation in maritime air using a combination of measurements and model
Authors: James J. D. , Harrison Roy M. , Savage N. H. , Allen A. G. , Grenfell J. L. , Allan B. J. , Plane J. M. C. , Hewitt C. N. , Davison B. , Robertson L.
Published: February 2004

Using a combination of field measurement data and a modified photochemical box model, strong evidence is presented to suggest that the rate of daytime oxidation of dimethyl sulfide (DMS) by OH radicals is insufficient to describe the measured conversion. Quasi‐Lagrangian measurements were made at two sites in the eastern Atlantic (Research Vessel and Mace Head Research Station, Ireland) as part of the Atmospheric Chemistry Studies in the Oceanic Environment (ACSOE) program. Periods of connected flow between the two sites were identified, air parcel transit times were estimated, and measurements of the main DMS oxidation products (MSA, SO2, and nss‐SO42−) were compared with model predictions to establish whether the model's chemical mechanism could account for observed changes. The main finding was that during daytime periods with maritime air masses, the model failed to predict a sufficient increase in DMS oxidation products during the estimated transit time. This was despite a tendency to overprediction of the progress of nitrogen chemistry during air mass advection, and independent checks on the model estimates of hydroxyl radical concentrations through measurements. In the light of this, the involvement of halogen species (most probably halogen oxides) or heterogeneous oxidation processes is tentatively suggested as the cause of enhanced daytime DMS oxidation in the marine boundary layer (MBL). Increasing the rate constant for the OH + DMS reaction by a factor of 3.3 (as a crude way of simulating parallel channels of DMS oxidation) permitted model results to reproduce the measurements very much more closely.
Estimates of cloud radiative forcing in contrail clusters using GOES imagery
Authors: Duda David P. , Minnis Patrick , Nguyen Louis
Published: February 2004

Using data from the Geostationary Operational Environmental Satellite (GOES), the evolution of solar and longwave radiative forcing in contrail clusters is presented in several case studies. The first study examines contrails developing over the midwestern United States in a region of upper tropospheric moisture enhanced by the remnants of Hurricane Nora on September 26, 1997. Two other cases involve contrail clusters that formed over the Chesapeake Bay and the Atlantic Ocean on February 11 and March 5, 1999, respectively. The last study includes contrails forming over the tropical Pacific near Hawaii. Observations of tropical contrails near Hawaii show that the contrail optical properties are similar to those measured from satellite in the midlatitudes, with visible optical depths between 0.3 and 0.5 and particle sizes between 30 and 60 μm as the contrails mature into diffuse cloudiness. Radiative transfer model simulations of the tropical contrail case suggest that ice crystal shape may have an important effect on radiative forcing in contrails. The magnitudes of the observed solar and longwave radiative forcings were 5.6 and 3.2 W m−2 less than those from the corresponding model simulations, and these differences are attributed to the subpixel scale low clouds and uncertainties in the anisotropic reflectance and limb‐darkening models used to estimate the observed forcing. Since the broadband radiative forcing in contrails often changes rapidly, contrail forcing estimates based only on the polar orbiting advanced very high resolution radiometer (AVHRR) data could be inaccurate due to the lack of sufficient temporal sampling.
Seasonal variation in methane fluxes from sedimentary basins to the atmosphere: Results from chamber measurements and modeling of transport from deep sources
Authors: Klusman Ronald W. , Leopold M. Emmelyn , LeRoy Michael P.
Published: February 2004

Results from modeling of microseepage are presented that suggest that under proper conditions some thermogenic CH4 is transported to the atmosphere without complete consumption by methanotrophic bacteria in soils. These computer simulations are supported by direct measurement of CH4 fluxes in seasonally dry and/or cold climate sedimentary basins where losses to the atmosphere were postulated to occur. Measurements in four semiarid sedimentary basins where a thermogenic CH4 source is expected found small positive fluxes during cooler/drier seasons and negative fluxes during warmer/wetter seasons. Small positive fluxes up to 43 mg CH4 m−2 d−1 were found. A similar result was found for two geothermal areas in a semiarid climate where CH4 derived from the Fischer‐Tropsch reaction operating in the reservoir provides a deep source of CH4. The results of the computer simulations, and direct field measurement of CH4 fluxes in dry and/or cold terrestrial environments suggest that a portion of the global atmospheric budget of CH4 may be from microseepage.