bluuebunny/crossref_metadata_2025 huggingface dataset:
Assimilation of satellite observations of long‐lived chemical species in global chemistry transport models
Authors: Khattatov Boris V. , Lamarque Jean‐Francois , Lyjak Lawrence V. , Menard Richard , Levelt Pieternel , Tie Xuexi , Brasseur Guy P. , Gille John C.
Published: February 2004

Use of data assimilation techniques such as optimal interpolation or the Kaiman filter in global chemistry transport models (CTM) is becoming more common. However, owing to high computational requirements, it is often difficult to apply these techniques to multidimensional models containing extensive photochemical schemes. We present a sequential assimilation approach developed for use with general global chemistry transport models. It allows fast assimilation and mapping of satellite observations and provides estimates of analysis errors. The suggested data assimilation scheme evolved from the one described by Levelt et al. [1998]. It is a variant of the suboptimal Kaiman filter and is based on ideas described by Menard et al. [2000] and Menard and Chang [200O]. One of the most important features of the developed scheme is its ability to routinely estimate variance of the analysis and to predict variance evolution in the model. The developed technique (or its variants) has been successfully interfaced with a number of different global models and used for assimilation of several types of measurements, including aerosol extinction ratios. Some of these experiments are described by Lamarque et al. [1999] and W. D. Collins et al. (Forecasting aerosols using a chemical transport model with assimilation of satellite aerosol retrievals: Methodology for INDOEX, submitted to Journal of Geophysical Research, 2000, hereinafter referred to as Collins et al., submitted manuscript, 2000). We illustrate the method using assimilation of ozone observations made by the Upper Atmosphere Research Satellite/Microwave Limb Sounder in the three‐dimensional chemistry transport model ROSE [Research for Ozone in the Stratosphere and its Evolution; Rose and Brasseur, 1989].
Cooling trend of the tropical cold point tropopause temperatures and its implications
Authors: Zhou Xue‐Long , Geller Marvin A. , Zhang Minghua
Published: February 2004

Operational sounding data (1973–1998) were used to determine cold point tropopause (CPT) characteristics. A cooling trend (−0.57±0.06 K/Decade during 1973–1998) in tropical CPT temperatures has been found, which is opposite to what has been hypothesized to explain the trend in stratospheric water vapor. Given this trend, the annual averages of the CPT saturation mixing ratios (SMRs) inferred from the analysis of the 1994–1997 data in the work of Dessler [1998] are substantially smaller than those from data before the mid‐1990s. This implies that while Newell and Gould‐Stewart's “stratospheric fountain” might not have been necessary to explain stratospheric water vapor during the mid‐1990s, it was necessary in most years before that. Changes in tropical convection occurrence frequency and/or strength are suggested to explain the cooling trend in the CPT temperatures. It is suggested that the observed positive trend in stratospheric water vapor is probably due to changes in the residual circulation.
Observation of gravity wave generation and breaking in the lowermost stratosphere
Authors: Pavelin Edward , Whiteway James A. , Vaughan Geraint
Published: February 2004

Measurements with the Aberystwyth VHF radar have revealed a striking illustration of gravity wave generation and breaking in the lowermost stratosphere. Horizontal wind measurements show an inertia‐gravity wave that was essentially monochromatic and persisted for longer than 5 days, with a maximum perturbation amplitude of approximately 10 ms−1. Radiosonde measurements show that this wave induced shear instability that led to intense turbulence close to the tropopause. The event was associated with a highly curved jet stream over Europe, suggesting that geostrophic adjustment was a likely source mechanism.
Variability of sea ice emissivity estimated from airborne passive microwave measurements during FIRE SHEBA
Authors: Haggerty Julie A. , Curry Judith A.
Published: February 2004

Passive microwave radiometers with frequencies ranging from 37 GHz to 220 GHz were flown over the Surface Heat Budget of the Arctic (SHEBA) experimental site in May and July 1998. These measurements were motivated by the possibility of determining cloud liquid water path, ice water path, and precipitation over sea ice from these frequencies. The comprehensive cloud data set collected in this experiment offers a unique opportunity for improving and adapting passive microwave retrieval methods for application to arctic clouds. However, retrieval of cloud properties from a downward looking radiometer requires an estimate of the surface emissivity and its spectral, spatial, and temporal variation. In this study, brightness temperature measurements are used to calculate sea ice emissivity at each frequency using ancillary aircraft data to characterize the atmosphere and obtain surface temperature. Surface emissivities on clear sky days during the FIRE Arctic Cloud Experiment (FIRE ACE) aircraft campaign have been calculated and compared with previous estimates cited in the literature. Average emissivity at nadir for snow‐covered sea ice in the experimental region during late May is estimated as 0.89 at 37 GHz, 0.74 at 89 GHz, 0.72 at 90 GHz, 0.73 at 150 GHz, and 0.84 at 220 GHz. In early July the average nadir emissivity for melting sea ice is 0.86 for 37 GHz and 0.84 for 90 GHz. Estimates of emissivity at 50° off nadir are compared with previous satellite and ground‐based measurements of dual polarized emissivities at 37 GHz and 90 GHz. Significant variability exists in published emissivity values due to variations in dielectric and physical properties of snow and ice, but our results fall within previously observed ranges. Uncertainties in the emissivity calculations are estimated, and the accuracy required for use of surface emissivity estimates in cloud retrieval methods is discussed.
Measured profiles of aerosol particle absorption and its influence on clear‐sky solar radiative forcing
Authors: Keil Andreas , Wendisch Manfred , Brüggemann Erika
Published: February 2004

Using a particle/soot absorption photometer (PSAP), airborne measurements of aerosol volume absorption coefficients were done in differently polluted air masses over a rural area in Germany under cloudless conditions in December 1997. Together with measurements of aerosol particle size distribution, particle chemical composition, and meteorological parameters, the PSAP‐measurements were used (1) to estimate vertical profiles of black carbon (BC) volume fraction and (2) to quantify the influence of particle absorption on solar radiative transfer by calculating their solar radiative forcing at the top of the atmosphere (TOA). Three case studies with different levels of absorption were investigated in detail. The vertical profiles of the volume absorption coefficient showed maximum values close to the ground (>6×10−5 m−1 in the most absorbing case) and a strong drop‐off toward higher altitudes. The estimated BC volume fractions were typically largest near the ground and decreased rapidly with increasing altitude. Radiative transfer calculations were carried out with and without consideration of the measured absorption of the particles. In the polluted cases and assuming a low surface albedo, the negative TOA solar radiative forcing of the aerosol particles decreased significantly when the measured particle absorption profiles were included in the calculations. Assuming a snow‐covered ground in these cases, the aerosol forcing changed from a cooling without aerosol particle absorption into a heating if the measured absorption of the particles was considered. An investigation of the forcing dependence on the solar zenith angle showed an increasing influence of the particle absorption for higher sun elevations.
Smoke aerosol from biomass burning in Mexico: Hygroscopic smoke optical model
Authors: Kreidenweis Sonia M. , Remer Lorraine A. , Bruintjes Roelof , Dubovik Oleg
Published: February 2004

The May 1998 transport of smoke from fires in Mexico and Central America into the United States is examined. We combine data from ground‐based Interagency Monitoring of Protected Visual Environments aerosol chemical sampling sites with in situ airborne and Sun photometer measurements to develop a consistent picture of the transported smoke‐impacted aerosol optical and chemical properties. The aerosol observed in Mexico and the southern United States is found to have a higher sulfate mass fraction, higher single‐scattering albedo, and larger accumulation mode radius than biomass burning aerosols observed by similar instrumentation in South America and Africa. We postulate that the smoke‐impacted aerosol in the 1998 event was more hygroscopic than that observed in the other locations, because of the higher mass fractions of sulfate, and show that a simple model of corresponding changes in aerosol water content yields agreement with the observed variations in refractive index and radii. We further show that the single‐scattering albedo cannot be fully explained by hygroscopic growth alone. Modifications to the model invoking variations in aerosol light‐absorbing carbon content, which are consistent with differences in observed composition among the various smoke‐impacted aerosols, bring the predictions of single‐scattering albedo into alignment with our observations. The model demonstrates that the particle size, single‐scattering albedo, and real refractive index of smoke‐impacted aerosols are not independent but vary in tandem with variations in particle hygroscopicity and with variations in black carbon content. This relationship is an important consideration in the assessment of the effects of biomass burning aerosols, particularly those subject to long‐range transport, on radiative forcing and climate.
Evaluation of aerosol indirect radiative forcing in MIRAGE
Authors: Ghan Steven , Easter Richard , Hudson James , Bréon Francois‐Marie
Published: February 2004

We evaluate aerosol indirect radiative forcing simulated by the Model for Integrated Research on Atmospheric Global Exchange (MIRAGE). Although explicit measurements of aerosol indirect radiative forcing do not exist, measurements of many of the links between aerosols and indirect radiative forcing are available and can be used for evaluation. These links include the cloud condensation nuclei concentration, the ratio of droplet number to aerosol number, the droplet number concentration, the column droplet number, the column cloud water, the droplet effective radius, the cloud optical depth, the correlation between cloud albedo and droplet effective radius, and the cloud radiative forcing. The CCN concentration simulated by MIRAGE agrees with measurements for supersaturations larger than 0.1% but not for smaller supersaturations. Simulated droplet number concentrations are too low in most but not all locations with available measurements, even when normalized by aerosol number. MIRAGE correctly simulates the higher droplet numbers and smaller droplet sizes over continents and in the Northern Hemisphere. Biases in column cloud water, cloud optical depth, and shortwave cloud radiative forcing are evident in the Intertropical Convergence Zone and in the subtropical oceans. MIRAGE correctly simulates a negative correlation between cloud albedo and droplet size over remote oceans for cloud optical depths greater than 15 and a positive correlation for cloud optical depths less than 15 but fails to simulate a negative correlation over land.
Simulating aerosols using a chemical transport model with assimilation of satellite aerosol retrievals: Methodology for INDOEX
Authors: Collins William D. , Rasch Phillip J. , Eaton Brian E. , Khattatov Boris V. , Lamarque Jean‐Francois , Zender Charles S.
Published: February 2004

A system for simulating aerosols has been developed using a chemical transport model together with an assimilation of satellite aerosol retrievals. The methodology and model components are described in this paper, and the modeled distribution of aerosols for the Indian Ocean Experiment (INDOEX) is presented by Rasch et al. [this issue]. The system generated aerosol forecasts to guide deployment of ships and aircraft during INDOEX. The system consists of the Model of Atmospheric Transport and Chemistry (MATCH) combined with an assimilation package developed for applications in atmospheric chemistry. MATCH predicts the evolution of sulfate, carbonaceous, and mineral dust aerosols, and it diagnoses the distribution of sea salt aerosols. The model includes a detailed treatment of the sources, chemical transformation, transport, and deposition of the aerosol species. The aerosol forecasts involve a two‐stage process. During the assimilation phase the total column aerosol optical depth (AOD) is estimated from the model aerosol fields. The model state is then adjusted to improve the agreement between the simulated AOD and satellite retrievals of AOD. During the subsequent integration phase the aerosol fields are evolved using meteorological fields from an external model. Comparison of the modeled AOD against estimates of the AOD from INDOEX Sun photometer data show that the differences in daily means are −0.03±0.06. Although the initial application is limited to the Indian Ocean, the methodology could be extended to derive global aerosol analyses combining in situ and remotely sensed aerosol observations.
Measurement and deduction of emissions of trichloroethene, tetrachloroethene, and trichloromethane (chloroform) in the northeastern United States and southeastern Canada
Authors: Kleiman G. , Prinn R. G.
Published: February 2004

We present optimal estimates of the emission patterns for trichloroethene (TCE, CHCl = CCl2), tetrachloroethene (perchloroethylene, CCl2 = CCl2), and trichloromethane (chloroform, CHCl3) utilizing hourly gas chromatographie measurements at Nahant, Massachusetts (approximately 10 km northeast of Boston). Our analysis combines the measurements with back trajectory information obtained from the Hybrid Single‐Particle Lagrangian Integrated Trajectory (HYSPLIT‐4) model (National Oceanic and Atmospheric Administration Air Resources Laboratory, Silver Spring, Maryland). Using a Kaiman filter inverse method and an analytical solution of the continuity equation to estimate the effects of eddy diffusion, we calculate the surface emissions on a 1°×1° grid for the selected species necessary to optimally match the observations. These emissions are compared with the estimates determined by the Reactive Chlorine Emissions Inventory (RCEI) working group of the International Global Atmospheric Chemistry Program Global Emissions Inventory Activity (GEIA). The new emissions scenarios computed here provide an observation‐based assessment for comparison with the RCEI emissions inventories for the northeastern United States and southeastern Canada. Results indicate that the RCEI estimates of the anthropogenic emissions of these chemicals in the geographical domain studied differ from our optimal estimates but generally lie within the estimated error of these optimal estimates. Results are sensitive to the assumed vertical distributions and hence to the assumed vertical mixing rates. The current accuracy achievable through this observation‐based technique (±45%) is limited in large part by the uncertainty in the vertical distribution of these compounds in the troposphere over highly emitting regions. Within this accuracy the optimal estimates of emissions presented here indicate that the emissions for many grid cells do not need to be corrected significantly from the 1990 estimates presented in the RCEI. However, for trichloroethene and tetrachloroethene we calculate large statistically significant decreases in emissions for some highly populated urban locations on the East Coast and correspondingly large increases for some less populous grid cells in Pennsylvania and New York State. Only anthropogenic sources of trichloromethane (representing roughly 11% of estimated global emissions) were gridded in the RCEI inventory and included in the initial inventory used here. We find that these anthropogenic emissions are, as expected, too low to explain the observations and that most grid cells require small increases and several cells require substantial increases (∼5–10 nmoles m−2 h−1) to produce emissions estimates that are consistent with observations. This is reasonable given current knowledge of natural land sources (e.g., soil emissions) and a large oceanic source for this compound inferred from previous oceanic observations.
A differential technique to retrieve column water vapor using sun radiometry
Authors: Michalsky Joseph J. , Min Qilong , Kiedron Piotr W. , Slater Donald W. , Barnard James C.
Published: February 2004

Techniques for retrieving column water vapor from Sun radiometer measurements involving the 940‐nm water vapor absorption band have been around for the better part of a century. Arguably, the best method to use for this retrieval is the modified Langley technique. However, to apply this method one must obtain the instrument response at the top of the atmosphere using modified Langley plots on clear days with a very stable water vapor column. Using subsequent measurements in this filter, ratioed to the top‐of‐the‐atmosphere response allows one to determine the transmission in the 940‐nm water band. In this paper, we present an approach that does not require an absolute knowledge of the extraterrestrial instrument response. The method discussed here relies, instead, on relative measurements of a calibration lamp and the extraterrestrial spectral irradiance within and just outside the 940‐nm absorption band. We execute these retrievals for the rotating shadowband spectroradiometer (RSS) on 3 days during the Department of Energy's Atmospheric Radiation Measurement program's 1997 Water Vapor Intensive Observation Period. We compare the results to those retrieved from a colocated multifilter rotating shadowband radiometer (MFRSR) that uses an empirical calibration and from a colocated microwave radiometer. Since our optical method of retrieving column water vapor from RSS measurements does not depend on a calibration performed against another water vapor measurement, it contributes an independent estimate in the search for absolute accuracy. The major contributors to the uncertainty of this retrieval are the water vapor band strength calculations, the difference in aerosol extinction in and near the water vapor band, the relative spectral irradiance output of the calibration lamp and the Sun at the nonabsorbing and band‐centered wavelengths, and the stability of the spectral response of the instrument, which will be discussed in detail.
Mineral dust concentrations, deposition fluxes and deposition velocities in dust episodes over Israel
Authors: Ganor E. , Foner H. A.
Published: February 2004

Total suspended particles (TSP) and dustfall were simultaneously sampled in Jerusalem (1969–1973) and in Tel Aviv (1976–1995) to determine the deposition flux, particle concentration, and deposition velocity of the mineral dust. In addition, the deposition flux of dustfall was measured throughout Israel and in the Sinai Desert during the years 1970–1973. TSP concentrations during dust pulses (“episodes”) were from 10 to 120 times higher than the background values of ∼100 μg m−3; the maximum concentration measured was 24,000 μg m−3. The annual deposition flux (from pulses only) decreased from 90 g m−2 yr−1 in the south to 30 g m−2 yr−1 in the north. The local contribution to the annual deposition flux was ∼50% in the south and near the Judean Desert in the east but decreased to 25% in the north. Deposition velocities of the dustfall during months without dust episodes were on the order of 1.0 cm s−1, while during months when dust storms occurred, deposition velocities are from 2 to 6 times greater than the background, depending on the location. Monthly mean deposition velocities during dust episodes varied between 2.6 cm s−1 and 9.0 cm s−1; short term (1 hour) values up to 27 cm s−1 have been measured.
Northern hemisphere winter climate response to greenhouse gas, ozone, solar, and volcanic forcing
Authors: Shindell Drew T. , Schmidt Gavin A. , Miller Ron L. , Rind David
Published: February 2004

The Goddard Institute for Space Studies (GISS) climate‐middle atmosphere model has been used to study the impacts of increasing greenhouse gases, polar ozone depletion, volcanic eruptions, and solar cycle variability. We focus on the projection of the induced responses onto Northern Hemisphere winter surface climate. Changes in the model's surface climate take place largely through enhancement of existing variability patterns, with greenhouse gases, polar ozone depletion, and volcanic eruptions primarily affecting the Arctic Oscillation (AO) pattern. Perturbations descend from the stratosphere to the surface in the model by altering the propagation of planetary waves coming up from the surface, in accord with observational evidence. Models lacking realistic stratospheric dynamics fail to capture these wave flux changes. The results support the conclusion that the stratosphere plays a crucial role in recent AO trends. We show that in our climate model, while ozone depletion has a significant effect, greenhouse gas forcing is the only one capable of causing the large, sustained increase in the AO observed over recent decades. This suggests that the AO trend, and a concurrent strengthening of the stratospheric vortex over the Arctic, are very likely anthropogenic in origin.
Ground‐based network observation of Asian dust events of April 1998 in east Asia
Authors: Murayama Toshiyuki , Sugimoto Nobuo , Uno Itsushi , Kinoshita Kisei , Aoki Kazuma , Hagiwara Naseru , Liu Zhaoyan , Matsui Ichiro , Sakai Tetsu , Shibata Takashi , Arao Kimio , Sohn Byung‐Ju , Won Jae‐Gwang , Yoon Soon‐Chang , Li Tao , Zhou Jun , Hu Huanling , Abo Makoto , Iokibe Kengo , Koga Ryuji , Iwasaka Yasunobu
Published: February 2004

We coordinated a ground‐based network that has been in use since 1997 to observe Asian dust during springtime. Huge Asian dust events that occurred in the middle of April 1998 were captured by this network. In this paper we present the organization of the network; a description of the instruments, including the lidar, sky radiometer, and optical particle counter; and the results of the observation, and offer discussions regarding the transport mechanism of Asian dust in east Asia using an on‐line tracer model. We discussed the time series of the surface concentration and the height distribution of the dust. A cutoff cyclone generated during the dust episode was responsible for trapping and sedimentation during the transportation of the Asian dust, particularly in the southern parts of China and Japan. Horizontal dust images derived from NOAA/AVHRR clearly revealed the structure of the vortex. The lidar network observation confirmed the general pattern of dust height distribution in this event; the height of the major dust layer was about 3 km over Japan but was higher (4 to 5 km) in Seoul and Hefei. A thin dust layer in the upper troposphere was also commonly observed in Hefei and Japan. Evidence of the coexistence of dust and cirrus was shown by the polarization lidar. The lidar network observation of Asian dust and satellite remote sensing provide key information for the study of the transport mechanism of Asian dust. Further extension of the lidar network toward the interior of the continent and the Pacific Rim would reveal the greater global mechanism of the transportation.
UV 380 nm reflectivity of the Earth's surface, clouds and aerosols
Authors: Herman J. R. , Celarier E. , Larko D.
Published: February 2004

The 380 nm radiance measurements of the Total Ozone Mapping Spectrometer (TOMS) have been converted into a global data set of daily (1979–1992) Lambert equivalent reflectivities R of the Earth's surface and boundary layer (clouds, aerosols, surface haze, and snow/ice) and then corrected to RPC for the presence of partly clouded scenes. Since UV surface reflectivity is between 2 and 8% for both land and water during all seasons of the year (except for ice and snow cover), reflectivities larger than the surface value indicate the presence of clouds, haze, or aerosols in the satellite field of view. A statistical analysis of 14 years of daily reflectivity data shows that most snow‐/ice‐free scenes observed by TOMS have a reflectivity less than 10% for the majority of days during a year. The 380 nm reflectivity data show that the true surface reflectivity is 2–3% lower than the most frequently occurring reflectivity value for each TOMS scene as seen from space. Most likely the cause is a combination of frequently occurring boundary layer water and/or aerosol haze. For most regions the observation of extremely clear conditions needed to estimate the surface reflectivity from space is a comparatively rare occurrence. Certain areas (e.g., Australia, southern Africa, portions of northern Africa) are cloud‐free more than 80% of the year, which exposes these regions to larger amounts of UV radiation than at comparable latitudes in the Northern Hemisphere. Regions over rain forests, jungle areas, Europe and Russia, the bands surrounding the Arctic and Antarctic regions, and many ocean areas have significant cloud cover (R>15%) more than half of each year. In the low to middle latitudes the areas with the heaviest cloud cover (highest reflectivity for most of the year) are the forest areas of northern South America, southern Central America, the jungle areas of equatorial Africa, and high mountain regions such as the Himalayas or the Andes. The TOMS reflectivity data show both the presence of large nearly clear ocean areas and the effects of the major ocean currents on cloud production.
Annual cycle of CO2 exchange at a bog peatland
Authors: Lafleur P. M. , Roulet N. T. , Admiral S. W.
Published: February 2004

Eddy covariance measurements of the carbon dioxide flux from an ombrotrophic bog near Ottawa, Canada, were made between June 1, 1998, and May 31, 1999. Net ecosystem exchange of CO2 (NEE) showed a distinct annual cycle, with net daily uptake increasing rapidly after snowmelt, peaking in midsummer and declining toward the fall. Summer (June to September) mean daily NEE flux was an uptake of −2.8±0.23 (standard error) g CO2 m−2 d−1, but daily values ranged considerably from a loss of 4.8 g CO2 m−2 d−1 to a maximum uptake of −8.3 g CO2 m−2 d−1. Daytime fluxes of CO2 were closely related to the photosynthetically active radiation flux, with derived relationships varying monthly. A curvilinear relationship developed between nighttime NEE and soil temperature produced a Q10 value of 3.0. Throughout the late fall and the snow‐covered periods (November 5 to April 6), mean daily fluxes showed a fairly constant efflux of ∼1.1±0.003 g CO2 m−2 d−1. The integrated non‐growing season CO2 loss was 183 g CO2 m−2. However, this was offset by gains during the long growing period resulting in an integrated annual NEE net uptake of 248±68 g CO2 m−2 yr−1 for this peatland. The growing season measurements of CO2 flux at this site were similar to those reported in the few previous studies on northern peatland ecosystems, but no previous annual estimates of NEE based on year‐round measurements have been published for other peatlands. Compared to annual measurements at forest sites in North America, the net exchange at this site falls between that of a small annual loss recorded at a northern boreal spruce forest and the substantial uptakes measured at a temperate mixed forest and boreal aspen forest sites.
State space analysis of changing seasonal ozone cycles (1988–1997) at Jungfraujoch (3580 m above sea level) in Switzerland
Authors: Schuepbach Evi , Friedli Thomas K. , Zanis Prodromos , Monks Paul S. , Penkett Stuart A.
Published: February 2004

The 10‐year (1988–1997) surface ozone record at Jungfraujoch (JFJ; 46°33′N, 07°59′E) in the Swiss Alps is analyzed with regard to changes in the seasonal cycle. For comparison, the surface ozone time series (1988–1996) at Zugspitze (ZUG; 47°25′N, 10°59′E, 2960 m above sea level) on the northern fringe of the Alps in Germany is also examined. The applied structural model in state space form is flexible enough to isolate a statistically significant decrease in the estimated seasonal component for May ozone concentrations over the last 10 years, in both the original JFJ and ZUG ozone records. A negative trend in May is also displayed by ordinary linear regression but is not statistically significant, demonstrating the advantage of the employed methodology over simple regression approaches in the analysis of changing seasonal cycles of tropospheric ozone records. We attempt to isolate potential mechanisms underlying the observed changes in the seasonal ozone cycles at JFJ by analyzing filtered ozone records. Sieving was carried out with wind direction, wind speed, and time windows to exclude the confounding influences from the atmospheric boundary layer and outflow from southern Europe. Taking only nighttime (2400–0500 central European time (CET)) ozone values into account, the seasonal ozone cycle at JFJ is characterized by a broad spring‐summer maximum. This filter is believed to represent the lower free troposphere over the Alps and also shows a significant decrease in May ozone concentrations. It is suggested that the results may reflect ozone changes in the free troposphere over continental Europe.
Measuring system for the long‐term monitoring of biosphere/atmosphere exchange of carbon dioxide
Authors: Haszpra L. , Barcza Z. , Bakwin P. S. , Berger B. W. , Davis K. J. , Weidinger T.
Published: February 2004

We describe an ongoing program being carried out in Hungary to investigate the role of the temperate continental region in the global carbon cycle. Carbon dioxide mixing ratios are continuously monitored at 10, 48, 82, and 115 m above the ground on a television transmitter tower, and the atmosphere/surface exchange of CO2 is measured by eddy covariance at 82 m. The region surrounding the tower is typical of rural areas of central Europe, with agricultural fields, forest patches, and small villages. We first describe the layout and the operation of the measuring system designed for the continuous, unattended monitoring of the vertical distribution of CO2 mixing ratio in the lowest 115 m of the atmosphere based on a Li‐Cor model 6251 infrared gas analyzer (IRGA). It provides vertical profile data with a temporal resolution of 8 min. Next, we discuss the measuring system for long‐term, continuous monitoring of the biosphere/atmosphere exchange of CO2. The eddy correlation system is based on a Li‐Cor model 6262 fast‐response IRGA and a Gill ultrasonic anemometer running at 4 Hz sampling frequency. Results are to illustrate the performance of the systems. Among others, they show the occasional accumulation of CO2 in the boundary layer in the Carpathian Basin during winter and the diurnal variation of the vertical distribution of CO2 mixing ratio in summer. A simple method based on similarity theory to calculate vertical fluxes from vertical gradients is presented, which can be used to fill the data gaps that inevitably occur during long‐term eddy correlation measurements. The present study confirms the feasibility of the long‐term tall‐tower CO2 flux and mixing ratio measurements.
Turbulent aerosol fluxes over the Arctic Ocean: 1. Dry deposition over sea and pack ice
Authors: Nilsson E. D. , Rannik Ü.
Published: February 2004

An eddy‐covariance flux measurement system was applied successfully to measure aerosol number dry deposition over open sea, ice floes, and over the leads in between the ice floes during the Arctic Ocean Expedition 1996. The aerosol number dry deposition velocity geometric mean was 1.9×10−3 m s−1 for the open sea. The dry deposition velocity to the ice floes averaged approximately 3×l0−4 m s−1 with no significant difference between the melting summer ice and frozen, partly snow‐covered ice in the early freeze‐up. Over the leads the dry deposition velocity was 3.4×10−4 and 9.1×10−4 m s−1 for the summer and the freeze‐up period, respectively. For neutral and stable stratification the geometric averages were 6.5×10−4 and 2.9×10−4 m s−1, respectively. The dry deposition changed mainly with friction velocity and particle size, as expected from theory, and with static stability, and we are able to recommend an empirical parameterization that works reasonable well over the Arctic Ocean pack ice. The resistance through the quasi‐laminar sublayer dominated over the aerodynamic resistance and the ultrafine and Aitken mode particles dominated the dry deposition flux. The average turnover time for the submicometer aerosol in the boundary layer was only 4.6 days, but for cases when the ultrafine and Aitken modes dominated the aerosol size spectra, the turnover times were 1.3 and 3.5 days, respectively. The small turnover times were largely caused by the shallow boundary layer over the Arctic Ocean pack ice.
Comparisons between measurements and models of Antarctic ozone loss
Authors: Wu J. , Dessler A. E.
Published: February 2004

We present estimates of the chemical loss of lower stratospheric polar ozone during the Antarctic late winter directly from UARS Microwave Limb Sounder (MLS) ozone measurements in 1992, 1993, and 1994. These agree to within a few percent with estimates of the chemical loss inferred from UARS MLS ClO measurements. Thus, our results support the current theories of polar O3‐depleting chemistry, and we find no evidence for significant unknown chemistry during this period.
Nonmethane hydrocarbons in Southern Ocean boundary layer air
Authors: Lewis Alastair C. , Carpenter Lucy J. , Pilling Michael J.
Published: February 2004

Measurements at the remote marine boundary layer station of Cape Grim, Northwest Tasmania, allow study of the unperturbed background atmosphere. Here we present a continuous data series of nonmethane hydrocarbons (NMHCs), measured in situ and with high sensitivity during the Second Southern Ocean Photochemistry Experiment (SOAPEX 2) during austral summer 1999. Air masses arriving at Cape Grim originate from the Australian continent, Tasmania, and Southern Ocean and Antarctic regions. In Southern Ocean marine boundary layer (MBL) air, C2 and C3 alkanes show a highly uniform abundance (ethane 142±11.9 parts per trillion by volume (pptv), propane 8.9±1.7 pptv) at around 4 times lower concentrations than encountered in unpolluted Northern Hemisphere MBL air. The presence of shorter‐lived NMHCs in marine air indicates sources of both C4–C6 alkanes and several alkenes in the Southern Ocean. The alkane isomer distributions and abundances are in broad agreement with literature sea‐air flux rates. In dynamically stable maritime air, midday maxima in ethene, propene, and isoprene concentrations of a few pptv indicate photochemically driven sources very near to the sea surface. Despite their high reactivity, the impact of these oceanic alkenes on local OH concentrations in remote MBL air is not significant. However, the abundance of isoprene in marine air may significantly elevate formaldehyde above that generated by methane oxidation.
CloudSat instrument requirements as determined from ECMWF forecasts of global cloudiness
Authors: Miller S. D. , Stephens G. L.
Published: February 2004

In the years preceding the launch of CloudSat in 2003, important questions regarding instrument requirements sufficient to fulfilling the mission's science objectives must be addressed. Qualified and useful answers to these questions require in turn a careful simulation strategy whereupon the observing system and modeled environment are represented as realistically as possible. In this paper, we consider the W band (94 GHz) cloud radar minimum detectable signal (MDS) requirement in the context of specified boundary fluxes and in‐cloud heating rates. Realistic cloud distribution and water contents from short‐range forecasts produced by the European Centre for Medium‐Range Weather Forecasts (ECMWF) are converted to attenuated equivalent radar reflectivity to yield full‐orbit virtual CloudSat observations. The radiative implications of variable radar MDS are examined using a two‐stream radiative transfer model. Simulations show that a MDS of ∼−28 dBZ will detect a fraction of the true cloud field sufficient to reconstruct the instantaneous top‐of‐atmosphere and surface fluxes to within Clouds and the Earth's Radiant Energy System (CERES) requirements. The results of this analysis form collectively a statement on instrument engineering requirements that is predicated on and hence mappable directly to the physical parameters that define CloudSat science objectives.
Role of adjacency effect in the remote sensing of aerosol
Authors: Lyapustin A. I. , Kaufman Y. J.
Published: February 2004

Adjacency effect is known to reduce apparent surface contrast by decreasing the top of the atmosphere radiance over bright pixels and increasing the brightness of the dark pixels. This effect is systematic and thus becomes important for the land remote sensing applications developed for use with either dark or bright targets. In this paper we trace the error caused by adjacency effect in the data processing chain from aerosol retrieval over dark targets to the atmospheric correction of the top of the atmosphere radiance. A systematic overestimation of the retrieved aerosol optical thickness and underestimation of surface albedo is demonstrated in accurate numeric simulations for a variety of atmospheric conditions and surface models. Results obtained suggest that 3‐D radiation effects should be taken into account not only in the atmospheric correction algorithms but also in the aerosol retrieval over land at a high‐to‐medium (up to 1 km) resolution of sensor.
Secondary organic aerosol formation in northern Europe: A model study
Authors: Andersson‐Sköld Yvonne , Simpson David
Published: February 2004

This paper describes a set of studies which has attempted to make a first estimate of the contribution of biogenic emissions to secondary organic aerosol (SOA) formation over the Nordic countries. The studies comprise first simulations of smog‐chamber experiments and second atmospheric modeling. The α‐pinene chemical mechanism suggested by Kamens et al. [1999] has been extended for use in regional oxidant models. Inclusion of dimer‐formation in this scheme substantially improved performance for some experimental conditions. Although the modified scheme is necessarily tentative, it fits with experimental findings over a wide range of conditions, covering α‐pinene concentrations of between 20 and 900 ppb and NOx concentrations of between zero and 240 ppb. The EMEP oxidant model has been used to estimate SOA formation in northern Europe using this chemical scheme and modified gas/particle partitioning methods. The results suggest that biogenic volatile organic compounds (VOCs) contribute far more than anthropogenic VOC to SOA formation. SOA is calculated to be a very variable fraction, between 2 and 50%, of total organic carbon mass in aerosols over the Nordic countries. This fraction is least at coastal sites in southern Norway and Denmark, and much greater for inland sites and sites further north, reflecting the relative distribution of biogenic precursors. There are great uncertainties in these figures, however, and we have had to neglect important primary sources of organic carbon (OC), notably plant waxes, which probably also contribute significantly to total OC. Further progress will be difficult until more measurements are available to characterize the ambient organic aerosol.
Analysis of motor vehicle emissions during the Nashville/Middle Tennessee Ozone Study
Authors: Harley Robert A. , McKeen Stuart A. , Pearson James , Rodgers Michael O. , Lonneman William A.
Published: February 2004

On‐road gasoline and diesel‐powered vehicle emissions in Nashville, Tennessee, were characterized using fuel sales as a measure of vehicle activity, and emission factors derived from infrared remote sensing, ambient air concentration ratios, and roadway tunnel measurements. On‐road vehicle emissions of carbon monoxide (CO), nonmethane hydrocarbons (NMHC), and oxides of nitrogen (NOx) on weekdays during summer 1995 were estimated to be 270±60, 43±13, and 53±9 metric tons per day, respectively. Diesel engines were a minor source of CO and NMHC, but were responsible for ∼50% of NOx emissions from on‐road vehicles. The Environmental Protection Agency's MOBILE 5B emission model predictions were similar to fuel‐based estimates for all pollutants, except for NOx where the MOBILE model predicted a smaller contribution to total on‐road vehicle emissions from diesel engines. Chemical composition profiles for hydrocarbon emissions were developed based on tunnel air and fuel samples collected in Nashville during summers 1995 and 1999. More than half of the tunnel NMHC mass was liquid fuel that escaped combustion; the remaining mass came from products of incomplete combustion such as ethane, acetylene, C2‐C4 alkenes, and 1,3‐butadiene. No major changes in the composition of vehicle‐related NMHC emissions were observed between 1995 and 1999 in Nashville.
OH radicals in the boundary layer of the Atlantic Ocean: 1. Measurements by long‐path laser absorption spectroscopy
Authors: Brauers Theo , Hausmann Martin , Bister Arne , Kraus Alexander , Dorn Hans‐Peter
Published: February 2004

Knowing the concentration of hydroxyl (OH) radicals is most important for the understanding of the chemical processes in the troposphere. This paper describes the first direct measurements of OH in the boundary layer of the tropical Atlantic Ocean. The use of Differential Optical Absorption Spectroscopy provided direct measurements of OH with a calibration uncertainty of 6%. The 1‐σ precision of the OH data was in the range of (1–4) × 106 cm−3 because of the exceptional experimental conditions encountered on the ship. On 10 measurement days we collected a total of 483 OH concentration data between 5°N and 40°S. Careful analysis was applied to select data not affected by the ship and its exhaust. The selected data (N = 238) exhibit diurnal profiles with maxima around 7×106 cm−3 for overhead Sun and clean air conditions. On average the measured OH concentrations are 16% higher than corresponding box model calculations based on simultaneously measured trace gas concentrations and photolysis frequencies. The deviation from the 1:1 relation, however, is covered by the combined calibration errors of OH, CO, and the photolysis frequencies.
Equilibrium constant of the reaction ·OH + HNO3 ⇆ H2O + NO3. in aqueous solution
Authors: Poskrebyshev G. A. , Neta P. , Huie R. E.
Published: February 2004

The equilibrium reaction ·OH + HNO3 ⇆ H2O + NO3. was investigated by pulse radiolysis in aqueous solutions. An estimate of the equilibrium constant was derived from the dependence of the optical absorbance due to NO3. radicals upon the concentration of HNO3. In addition, estimates of the forward and reverse rate constants were obtained from the dependence of the formation rate constant on the ratio of the activities of nitric acid and water. Studies were carried out at different values of the dose per pulse and at pH values of 1 and 0. The observations were modeled, taking into account all relevant reactions for the formation and decay of the nitrate radical. From these modeling studies the estimated rate and equilibrium constants were refined. The rate constant of the forward reaction is found to be (8.6 ± 1.3) × 107 L mol−1 s−1, the rate constant of the reverse reaction is found to be (3 ± 1) × 102 L mol−1 s−1, and the equilibrium constant Keq = (2.8 ± 0.4) × 105, all at zero ionic strength. From the latter value of Keq and taking E0(H+, .OH/H2O) = 2.72 V versus NHE, we calculate the reduction potentials E0(H+, NO3./HNO3) = (2.40 ± 0.01) V and E0(NO3./NO3−) = (2.48 ± 0.01) V. This value of the reduction potential for this couple leads to a Henry's law coefficient of KH = 0.018 mol L−1 bar−1 at 298 K for NO3.. This value suggests that the impact of NO3. on atmospheric droplets will be due solely to reactive uptake.
Recent trends in weather and seasonal cycles: An analysis of daily data from Europe and China
Authors: Yan Zhongwei , Jones Philip D. , Moberg Anders , Bergström Hans , Davies Trevor D. , Yang Chi
Published: February 2004

A wavelet analysis is conducted to investigate daily variability (DV, timescales of less than 3 days), weekly (weather) variability (WV, timescales of 5 days up to 2 months), and seasonal variability (SV, timescales of 8 to 17 months) in five temperature series from Europe and China and two westerly indices for the European/North Atlantic sector back to the 18th century. DV exhibits local features so that it is sensitive to any inhomogeneity in each series. Analysis of DV shows the potential for further homogenization of the data and suggests that for the present study, daily series are only truly homogeneous back to the 19th century. WV is responsible for extremes of large‐scale cold/warm variations in the daily series and explains about 80% of the total variance. WV is found to be significantly weaker in northern Europe by 7–10% during warming periods, especially for winter and autumn, but summer temperature correlates positively with WV, with a maximum coefficient of 0.52 for central England. This indicates that for warming periods, WV is reduced in the cold season, but is potentially increased in the warm season. The principal timescale of weather, about 16 days in Europe and 11 days in China, does not exhibit significant trends. Changes in SV from cold to warm periods often result in weaker seasonal cycles, with an unprecedented reduction of up to 3°C at St. Petersburg during the warm period since 1988. The analysis of the westerly indices supports the recent unusually anomalous seasonal cycles, with stronger winter westerlies over the northern Atlantic and Europe. The trends in WV of the westerly indices coincide with the temperature data, implying they are responsible for the large‐scale changes over northern Europe.
A study of the sensitivity of simulated mineral dust production to model resolution
Authors: Liu Ming , Westphal Douglas L.
Published: February 2004

The dependence of dust production on model grid‐space resolution is investigated using the Navy's operational mesoscale meteorological model with an imbedded dust emission model. The study covers a 2‐week period of strong dust storms in April 1998 in the major dust source area of East Asia. The modeled surface winds at grid resolutions of 20, 40, 60, and 80 km are verified against observational data. At all resolutions the model has a positive bias in wind speed that decreases as resolution increases. Dust fluxes that are proportional to the fourth power of the friction velocity (u*) and the third power of the wind speed are calculated at all four resolutions and compared. Compared with the 20‐km resolution u*‐driven flux, which is deduced to be the most accurate, the u*‐driven flux on the coarser grids overestimate the flux, with the 80 km being 60% higher for individual events and nearly 20% higher in the total dust production for the entire study period. The wind‐driven flux misses the smaller events due to the lack of a dependence on stability and wind shear, when compared with the timing of surface dust observations, and has differences of up to 70%, when compared with the 20‐km u*‐driven flux. Averaging over space and time tends to reduce the differences among grids and might support modeling at coarse resolution.