bluuebunny/crossref_metadata_2025 huggingface dataset:
Methane emission from rice fields in China: Measurements and influencing factors
Authors: Cai Z. C. , Tsuruta H. , Minami K.
Published: February 2004

Methane emissions from rice fields in China were measured at eight sites in five provinces under conditions representative of local practices for rice cultivation. Methane emission rates during the rice growth period varied greatly from site to site and with treatments at the same site, ranging from 0.3 to 205 g CH4/m2. Flooded or waterlogged rice fields in the nonrice growth season continuously emitted CH4 substantially. The average CH4 emission rate from a rice field in Chongqing was as high as 36.2 g CH4/m2 in the nonrice growing season. Furthermore, flooding in the nonrice growth season also significantly stimulated CH4 emission during the rice growth period in the next year. Increases in the rate of CH4 flux after rice transplanting were less when the number of consecutive upland crops grown before rice transplanting was greater. CH4 emissions from rice fields located on downslope was larger than from those on midslope and upslope in hilly areas due to poor drainage of the former. Application of rice straw in fall when winter wheat was sown did not increase CH4 emission significantly during the following rice growth period. CH4 emission was depressed by the application of ammonium sulfate but was, in general, not significantly affected by urea application.
Model of tropospheric ion composition: A first attempt
Authors: Beig G. , Brasseur Guy P.
Published: February 2004

Recent atmospheric ion composition measurements in the troposphere have revealed the presence of several new families of ions below the tropopause, which had not been observed above this level. In a chemical model of tropospheric positive ions, several new channels are proposed to explain this observation, and the presence of very heavy clustered aerosol ions (charged ultrafine particles) are considered. Parent neutral compounds, that are responsible for the formation of positive cluster ions in the troposphere, include ammonia, pyridine, picoline, lutidine, acetone, etc. Model results show that the clustered aerosol ions are dominant near the surface, above which pyridinated cluster ions are most abundant up to about 5km altitude. Above 7km, ions having acetone as parent neutral species are in majority. Ammonia and methyl cyanide cluster ions are found to be less abundant as compared to the above. Above 13km the relative abundance of methyl cyanide cluster ions is rapidly increasing, which suggests that the present model results conform with stratospheric ion models. Experimental data suggest, however, that the concentration of pyridinated compounds is highly variable from one location to another and that the relative abundance of ammonia cluster ions could be high in some remote environments. Similarly, our model shows that NO3−.HNO3 (H2O)n, HSO4−, and NO3−‐core families of ions are the most abundant negative ions in the troposphere during nighttime. The first family dominates below 6km, whereas the second type dominates between 6 and 10km. NO3−‐core ions are the most abundant ions above 10km. However, some observational data suggest a dramatic increase in the concentration of sulfuric acid vapor, malonic acid, and methane sulfonic acid during daytime with a related change in the negative ion composition. Our model suggests that under these conditions HSO4−‐core ions are the dominant ions below 10km altitude.
Effect of meteorology and air pollutant transport on ozone episodes at a subtropical coastal Asian city, Hong Kong
Authors: Chan C. Y. , Chan L. Y.
Published: February 2004

Hong Kong is a subtropical coastal city situated at the rapid urbanizing and industrializing region of South China. High frequencies of elevated ozone are found in various remote, rural, and metropolitan districts of Hong Kong. Meteorological conditions related to synoptic‐scale flow pattern, mesoscale weather phenomenon, and local micrometeorology have strong effects on the pollutant transport to South China and the occurrence of ozone episodes in Hong Kong. The transport processes and the source regions of ozone are analyzed with the combined use of surface pressure patterns, streamline charts and prevalent winds, and with representative episodes. The synoptic meteorological conditions can be classified into nine patterns. The patterns associated with winter monsoon and traveling cyclones are found to be most conducive to the occurrence of ozone episodes. Long‐range transport of ozone with enhanced CO level from the aged continental air accompanying northeast monsoon contributes to substantial cases of episodes, which are especially noticeable in the upwind remote areas. These episodes feature consistently high ozone at night in the remote station and early morning peaks in the metropolitan stations. The possible source regions of this ozone may include Asian continent, the coastal areas of Mainland China, southern Japan, and Taiwan. This high inflow of background ozone together with elevated ozone enhanced by the channeling effect, and local transport causes the highest frequencies of ozone episodes in the downwind western side of the territory. The short‐range transport of ozone and pollutants from the industrial and urban centers in the Pearl River Delta region and Guangdong Province in southern China is another source of ozone. When transported ozone is being superimposed on ozone formed from local sources, territory‐wide and extremely high concentration episodes are found. Our meteorological and ozone episode diagnosis have also demonstrated that the ‘background’ outflow of Asian pollutants can be significantly modified on its way to the Pacific Ocean when passing through the fast growing Pearl River Delta cities such as Hong Kong.
Evaluation of radar retrieval algorithms in stratiform clouds using large‐eddy simulations
Authors: Ovtchinnikov Mikhail , Kogan Yefim L.
Published: February 2004

The performance of algorithms retrieving cloud liquid water content W from radar reflectivity factor Z is evaluated using data from a three‐dimensional large‐eddy simulation model with detailed size‐resolving microphysics. On the basis of case studies of a marine stratocumulus cloud observed during the Atlantic Stratocumulus Transition Experiment in June of 1992 and a continental stratus cloud observed over north central Oklahoma on April 30, 1994, it is shown that retrieval algorithms based only on Z are very sensitive to cloud microphysics and do not reproduce adequately the horizontally averaged vertical profile of W. Additional independent measurements are needed to constrain the retrieved W profile. Surprisingly, algorithms show little improvement when the exact concentrations of cloud droplets obtained from the model are included in the calculations. Vertically integrated liquid water path P is found to be a robust constraint that ensures a more accurate retrieval. In the two studied cases an algorithm based on the Z‐W relation of the form Z = aWb gives an optimal performance when a is inversely proportional to P and b = 1.32. The horizontal averaging of P over a computational domain of several kilometers does not affect the accuracy of the retrieval of the horizontally averaged profile of cloud liquid water content, which adds to the versatility of the algorithm.
Comparison of in situ N2O and CH4 measurements in the upper troposphere and lower stratosphere during STRAT and POLARIS
Authors: Hurst D. F. , Dutton G. S. , Romashkin P. A. , Elkins J. W. , Herman R. L. , Moyer E. J. , Scott D. C. , May R. D. , Webster C. R. , Grecu J. , Loewenstein M. , Podolske J. R.
Published: February 2004

Nitrous oxide (N2O) and methane (CH4) were measured in the upper troposphere and lower stratosphere by multiple instruments aboard the NASA ER‐2 aircraft during the 1995–1996 Stratospheric Tracers of Atmospheric Transport (STRAT) and 1997 Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaigns. Differences between coincidental, in situ measurements are examined to evaluate the agreement and variability in the agreement between these instruments during each flight. Mean N2O measurement differences for each flight were much smaller than limits calculated from quoted values of N2O measurement accuracy and for all but two flights were ≤8.7 ppb (3.5%). Mean CH4 measurement differences for flights were similarly much smaller than calculated limits and for all but three flights were ≤65 ppb (4.4%). Typical agreement between instruments during flights averaged 6.2 ppb (2.5%) for N2O and 43 ppb (2.9%) for CH4. In contrast, for about half of the flights, the variability of N2O and CH4 measurement differences exceeded limits calculated from quoted values of measurement precision. The typical measurement difference variability (1σ) during a flight averaged ±8.0 ppb (3.2%) for N2O and ±43 ppb (2.9%) for CH4. For some flights, large differences or variations in differences are attributable to the poor measurement accuracy or precision of one instrument. It is demonstrated that small offsets between the computer clocks of these instruments can result in significant differences between their “coincidental” N2O and CH4 data, especially when there is high spatial variability in tracer abundance along a flight track.
Effect of cloud inhomogeneity on direct radiative forcing due to aerosols
Authors: Highwood E. J.
Published: February 2004

The effect of including horizontal cloud inhomogeneity on the direct radiative forcing due to sulphate and soot aerosols is explored. Cloud inhomogeneity is represented using the gamma independent pixel approximation for optical depth. Using a two‐stream radiation model, the assumption of plane‐parallel clouds normally used in climate models is shown to systematically underestimate the magnitude of the negative radiative forcing due to sulphates and systematically overestimate the positive forcing due to soot aerosol. For overcast skies and a Northern Hemisphere mean aerosol profile, these biases can reach as much as 0.1 W m−2, representing up to 30% of the forcing for sulphates and 5% for soot. The bias introduced in forcing due to sulphate aerosol is much larger than would be expected from the effect of the different albedo produced by altering the treatment of cloud optical depth. For “global mean” conditions, considering both clear and cloudy regions, the biases are between 2 and 4% for all three quantities (around 0.04 W m−2 in radiative forcing). This bias can generally be reduced further by constraining the albedo to a fixed (observed) value, thereby using the plane parallel homogeneous approximation with a different (but incorrect) cloud optical depth. The sensitivity of these results to solar zenith angle, cloud properties, surface reflectance, the number of streams used in the radiative transfer model, and the relative humidity is also investigated. For regions with coincident overcast marine stratocumulus, high aerosol loading and high relative humidity, and an external mixture of soot and sulphate aerosol, absolute values of the bias in forcing due to sulphates could reach as much as 1.5 W m−2 for a solar zenith angle of 60°, this being around 15% of the total forcing.
General characteristics of tropospheric trace constituent layers observed in the MOZAIC program
Authors: Thouret Valérie , Cho John Y. N. , Newell Reginald E. , Marenco Alain , Smit Herman G. J.
Published: February 2004

We present a statistical study on tropospheric layers as allowed by the most extensive ozone and water vapor database currently available. Considering O3 and H2O deviations from an automatically calculated background, we define four types of layers. These tropospheric layers are a common feature, with the percentage of the troposphere occupied by such layers varying from 7% to 33% depending on the region and the season. Most of the layers are found between 4 and 8 km altitude, and the median thickness is about 500 m. At northern midlatitudes we find 4 times more layers in summer than in winter, while in tropical Asia we observe a spring maximum in the occurrence of the layers. The most abundant layer type everywhere is O3+H2O− and corresponds to the signature of stratospheric intrusions or continental pollution. This suggests that stratosphere‐troposphere exchanges or at least their influence are not negligible in summer at midlatitudes or in the tropics. A complete understanding of the layers could lead to a better empirical assessment of the different tropospheric ozone sources and to an assessment of the potential vorticity fluxes in the troposphere.
Global distribution of CO2 in the upper mesosphere as derived from UARS/ISAMS measurements
Authors: Zaragoza G. , López‐Puertas M. , López‐Valverde M.Á. , Taylor F. W.
Published: February 2004

The latitudinal and seasonal variations of the CO2 concentration between 70 and 100 km are investigated using the 4.6 μm daytime measurements made by the Improved Stratospheric and Mesospheric Sounder (ISAMS) on the Upper Atmospheric Research Satellite (UARS). The analysis uses a detailed nonlocal thermodynamic equilibrium model of the CO2 states emitting in the 4.3 μm spectral region and the GENLN2 line‐by‐line radiance code to update an earlier study by López‐Puertas et al., [1998b]. The differences in the CO2 volume mixing ratio between the equator and the illuminated pole in the upper mesosphere are shown to be smaller than the detection limit established by the uncertainties in the data (approximately 10% at 85 km, 14% at 90 km, 23% at 95 km, and 28% at 100 km).
Ice crystal shapes in cirrus clouds derived from POLDER/ADEOS‐1
Authors: Chepfer H. , Goloub P. , Riedi J. , De Haan J. F. , Hovenier J. W. , Flamant P. H.
Published: February 2004

This paper discusses the retrieval of ice crystal shapes of cirrus clouds on a global scale using observations collected with POLDER‐1 (POLarization and Directionality of the Earth Reflectance) onboard the ADEOS‐1 platform. The retrieval is based on polarized bidirectional observations made by POLDER. First, normalized polarized radiances are simulated for cirrus clouds composed of ice crystals that differ in shape and are randomly oriented in space. Different values of cloud optical depths, viewing geometries and solar zenith angles are used in the simulations. This sensitivity study shows that the normalized polarized radiance is highly sensitive to the shape of the scatterers for specific viewing geometries, and that it saturates after a few scattering events, which makes it rapidly independent of the optical depth of the cirrus clouds. Next, normalized polarized radiance observations obtained by POLDER have been selected, based on suitable viewing geometries and on the occurrence of thick cirrus clouds composed of particles randomly oriented in space. For various ice crystal shapes these observations are compared with calculated values pertaining to the same geometry, in order to determine the shape that best reproduces the measurements. The method is tested fully for the POLDER data collected on January 12, 1997. Thereafter, it is applied to six periods of 6 days of observations obtained in January, February, March, April, May, and June 1997. This study shows that the particle shape is highly variable with location and season, and that polycrystals and hexagonal columns are dominant at low latitudes, whereas hexagonal plates occur more frequently at high latitudes.
Quantitative constraints on the atmospheric chemistry of nitrogen oxides: An analysis along chemical coordinates
Authors: Cohen R. C. , Perkins K. K. , Koch L. C. , Stimpfle R. M. , Wennberg P. O. , Hanisco T. F. , Lanzendorf E. J. , Bonne G. P. , Voss P. B. , Salawitch R. J. , Del Negro L. A. , Wilson J. C. , McElroy C. T. , Bui T. P.
Published: February 2004

In situ observations Of NO2, NO, NOy, ClONO2, OH, O3, aerosol surface area, spectrally resolved solar radiation, pressure and temperature obtained from the ER‐2 aircraft during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) experiments are used to examine the factors controlling the fast photochemistry connecting NO and NO2 and the slower chemistry connecting NOx and HNO3. Our analysis uses “chemical coordinates” to examine gradients of the difference between a model and precisely calibrated measurements to provide a quantitative assessment of the accuracy of current photochemical models. The NO/NO2 analysis suggests that reducing the activation energy for the NO+O3 reaction by 1.7 kJ/mol will improve model representation of the temperature dependence of the NO/NO2 ratio in the range 215–235 K. The NOx/HNO3 analysis shows that systematic errors in the relative rate coefficients used to describe NOx loss by the reaction OH + NO2 → HNO3 and by the reaction set NO2 + O3 → NO3; NO2 + NO3 → N2O5; N2O5 + H2O → 2HNO3 are in error by +8.4% (+30/−45%) (OH+NO2 too fast) in models using the Jet Propulsion Laboratory 1997 recommendations [DeMore et al., 1997]. Models that use recommendations for OH+NO2 and OH+HNO3 based on reanalysis of recent and past laboratory measurements are in error by 1.2% (+30/−45%) (OH+NO2 too slow). The +30%/−45% error limit reflects systematic uncertainties, while the statistical uncertainty is 0.65%. This analysis also shows that the POLARIS observations only modestly constrain the relative rates of the major NOx production reactions HNO3 + OH → H2O + NO3 and HNO3 + hν → OH + NO2. Even under the assumption that all other aspects of the model are perfect, the POLARIS observations only constrain the rate coefficient for OH+HNO3 to a range of 65% around the currently recommended value.
The 2.5 THz heterodyne spectrometer THOMAS: Measurement of OH in the middle atmosphere and comparison with photochemical model results
Authors: Englert Christoph R. , Schimpf Birger , Birk Manfred , Schreier Franz , Krocka Michael , Nitsche Roland G. , Titz Ruth U. , Summers Michael E.
Published: February 2004

The interpretation of recent odd hydrogen measurements in the stratosphere from balloons and in the mesosphere from space indicates a serious lack of understanding in atmospheric HOx chemistry. In order to resolve these persisting problems, coincident measurements of HOx molecules and/or measurements that cover both altitude regions are desirable. In this work, the airborne 2.5THz heterodyne spectrometer Terahertz OH Measurement Airborne Sounder (THOMAS) is introduced. Since the first THOMAS measurements in 1994/1995, the spectrometer was significantly improved by modification or replacement of individual components. The THOMAS instrumental setup and properties are presented together with a retrieval algorithm for atmospheric parameters based on a Phillips‐Tikhonov regularization scheme. Furthermore, the results of a complete error assessment are given. In August 1997, during the second CRISTA/MAHRSI campaign (Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere / Middle Atmosphere High Resolution Spectrograph Investigation), OH observations were performed by THOMAS covering altitudes between about 30 and 90 km over a full diurnal cycle. Hydroxyl column densities derived from THOMAS measurements are presented and compared to photochemical model results. The model calculations using the standard HOx chemistry systematically show higher values by about 15% for the 40–90 km and 50–90 km OH columns. Moreover, a recently proposed change of an HOx chemistry reaction rate is included into the comparison which, for the same altitude intervals, yields OH column densities that are about 10% lower than the THOMAS measurements. A detailed comparison of the THOMAS and MAHRSI measurements is presented in a seperate publication [Englert et al., 2000].
Can new particle formation occur in the clean marine boundary layer?
Authors: Pirjola Liisa , O'Dowd Colin D. , Brooks Ian M. , Kulmala Markku
Published: February 2004

An analysis of new particle formation probability in the marine boundary layer (MBL) is conducted using a detailed aerosol dynamics and gas‐phase chemistry model, thermodynamically correct classical binary (H2O‐H2SO4) nucleation theory, and recently developed ternary (H2O‐H2SO4‐NH3) nucleation theory. Additionally, the effect of boundary‐layer meteorology (i.e., adiabatic cooling, small scale fluctuations, and entrainment) in enhancing nucleation is also examined. The results indicate that for typical marine conditions, binary nucleation does not occur for any realistic conditions regardless of adiabatic cooling, turbulent fluctuations, or entrainment. For polar marine conditions, binary nucleation does occur due to lower temperatures, and is enhanced due to turbulent fluctuations. An increase in detectable particle sizes (N3>3 nm), is only seen after multiple boundary layer circulations for conditions of high dimethyl sulphide (DMS) concentrations (400 ppt). Under extreme conditions of entrainment of free‐troposphere layers containing very low aerosol condensation sinks and extraordinary high sulphuric acid concentrations (>108 molecules cm−3), increases in detectable particles up to 10,000 cm−3 are predicted only in polar marine air, but are viewed as unlikely to occur in reality. Comparison of model simulations with observed values of DMS and sulphuric acid in polar marine air masses suggest that binary nucleation may lead to an enhancement of ≈ 1000 cm−3 in N3 particle concentration, but not to enhancements of ≈ 10,000 cm−3. Ternary nucleation is predicted to occur under realistic sulphuric acid (1.2×107 molecules cm−3) and ammonia (>5 ppt) concentrations; however, significant growth to detectable sizes (N3) only occurs for DMS concentrations of the order of 400 ppt and very low aerosol condensation sinks, but these conditions are thought to be very infrequent in the MBL and are unlikely to make a significant contribution to the general MBL aerosol concentration. It is plausible that the background MBL aerosol concentration could be maintained by a slow, almost undetectable production rate, and not by noticeable nucleation events where large enhancements in N3 concentrations are observed. The former requires sustained DMS concentrations of the order of 100 ppt which seems unlikely. In summary, the occurrence of new particles in the unperturbed MBL would be difficult to explain by DMS emissions alone. DMS emissions can explain the occurrence of thermodynamically stable sulphate clusters, but under most conditions, to grow these clusters to detectable sizes before they are scavenged by coagulation, an additional condensable species other than DMS‐derived sulphuric acid would be required. In the event, however, of significant removal of the preexisting aerosol due to precipitation, the MBL aerosol can be replenished through growth of new particle formed through ternary nucleation under moderately high DMS concentrations.
Validation of visible/near‐IR atmospheric absorption and solar emission spectroscopic models at 1 cm−1 resolution
Authors: Lubin Dan , Vogelmann Andrew , Lehr Pamela J. , Kressin Ann , Ehramjian James , Ramanathan V.
Published: February 2004

A Fourier transform infrared (FTIR) spectrometer, operating at 1 cm−1 resolution between 9000 and 24,669 cm−1 (0.405–1.111 μm) has been used to check the spectral composition of databases that form the basis for most atmospheric absorption parameterizations used in climate models, remote sensing, and other radiative transfer simulations. The spectrometer, operating near sea level under clear skies, obtained relative atmospheric transmission measurements of the direct solar beam by means of a heliostat. The spectroscopic data were compared with a line‐by‐line radiative transfer model (LBLRTM) calculation of direct solar beam flux, which used a input data a monochromatic model extraterrestrial solar flux spectrum currently in common use. This intercomparison revealed that the extraterrestrial solar flux spectrum contains 266 solar absorption features that do not appear in the data, resulting in an excess of approximately 1.92 W m−2 in the model's solar constant. The intercomparison also revealed 97 absorption features in the data that do not appear in the HITRAN‐96 database as used by LBLRTM, resulting in a model underestimate of shortwave absorption of ∼0.23 W m−2 for a solar zenith angle of 42°. These small discrepancies revealed by the intercomparison indicate that current extraterrestrial solar irradiance models and spectroscopic databases used by shortwave atmospheric radiative transfer models are nearly entirely complete for purposes of atmospheric energy budget calculation. Thus clear or cloudy sky ‘excess absorption’ is unlikely to be related to an incomplete identification of atmospheric absorbing gases and their spectroscopic features, at 1 cm−1 resolution, for a clean troposphere of normal composition.
Comparison of mesospheric and lower thermospheric residual wind with High Resolution Doppler Imager, medium frequency, and meteor radar winds
Authors: Lieberman R. S. , Smith A. K. , Franke S. J. , Vincent R. A. , Isler J. R. , Manson A. H. , Meek C. E. , Fraser G. J. , Fahrutdinova A. , Thayaparan T. , Hocking W. , MacDougall J. , Nakamura T. , Tsuda T.
Published: February 2004

The objective of this study is to compare observed mean meridional winds with those deduced from theory. The diabatic circulation is computed from High Resolution Dopper Imager (HRDI) mesospheric and lower thermospheric temperatures during January and July conditions. The meridional wind component is compared with HRDI Eulerian mean meridional winds near 95 km and with seasonal averages of meridional winds at a number of radar medium‐frequency (MF) and meteor wind (MW) sites. The diabatic wind is directed from the summer toward the winter hemisphere. Peak values exceed 20 m s−1 and are observed at 105 km near 20° in the summer hemisphere. A secondary maximum of about 10 m s−1 is observed in the wintertime lower mesosphere during the July case. The diabatic wind is qualitatively consistent with HRDI 95‐km mean meridional winds at latitudes equatorward of 50°. Time‐averaged summertime radar winds are consistent with HRDI and diabatic winds between 50°S and 20°N. At winter midlatitudes, MF radar winds are directed oppositely to the diabatic wind, while one available MW measurement is directed with the diabatic wind. The zonal acceleration implied by the diabatic wind is about 150–200 m s−1 d−1 in the midlatitude summer lower thermosphere.
Atmospheric sulfur cycle simulated in the global model GOCART: Model description and global properties
Authors: Chin Mian , Rood Richard B. , Lin Shian‐Jiann , Müller Jean‐Francois , Thompson Anne M.
Published: February 2004

The Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model is used to simulate the atmospheric sulfur cycle. The model uses the assimilated meteorological data from the Goddard Earth Observing System Data Assimilation System (GEOS DAS). Global sulfur budgets from a 6‐year simulation for SO2, sulfate, dimethylsulfide (DMS), and methanesulfonic acid (MSA) are presented in this paper. In a normal year without major volcanic perturbations, about 20% of the sulfate precursor emission is from natural sources (biogenic and volcanic), and 80% is anthropogenic; the same sources contribute 33% and 67%, respectively, to the total sulfate burden. A sulfate production efficiency of 0.41–0.42 is estimated in the model, an efficiency which is defined as a ratio of the amount of sulfate produced to the total amount of SO2 emitted and produced in the atmosphere. This value indicates that less than half of the SO2 entering the atmosphere contributes to the sulfate production, the rest being removed by dry and wet depositions. In a simulation for 1990 we estimate a total sulfate production of 39 Tg S yr−1, with 36% and 64% from in‐air and in‐cloud oxidation, respectively, of SO2. We also demonstrate that major volcanic eruptions, such as the Mount Pinatubo eruption in 1991, can significantly change the sulfate formation pathways, distributions, abundance, and lifetime. Comparison with other models shows that the parameterizations for wet removal or wet production of sulfate are the most critical factors in determining the burdens of SO2 and sulfate. Therefore a priority for future research should be to reduce the large uncertainties associated with the wet physical and chemical processes.
The impact of the mixing properties within the Antarctic stratospheric vortex on ozone loss in spring
Authors: Lee Adrian M. , Roscoe Howard K. , Jones Anna E. , Haynes Peter H. , Shuckburgh Emily F. , Morrey Martin W. , Pumphrey Hugh C.
Published: February 2004

Calculations of equivalent length from an artificial advected tracer provide new insight into the isentropic transport processes occurring within the Antarctic stratospheric vortex. These calculations show two distinct regions of approximately equal area: a strongly mixed vortex core and a broad ring of weakly mixed air extending out to the vortex boundary. This broad ring of vortex air remains isolated from the core between late winter and midspring. Satellite measurements of stratospheric H2O confirm that the isolation lasts until at least mid‐October. A three‐dimensional chemical transport model simulation of the Antarctic ozone hole quantifies the ozone loss within this ring and demonstrates its isolation. In contrast to the vortex core, ozone loss in the weakly mixed broad ring is not complete. The reasons are twofold. First, warmer temperatures in the broad ring prevent continuous polar stratospheric cloud (PSC) formation and the associated chemical processing (i.e., the conversion of unreactive chlorine into reactive forms). Second, the isolation prevents ozone‐rich air from the broad ring mixing with chemically processed air from the vortex core. If the stratosphere continues to cool, this will lead to increased PSC formation and more complete chemical processing in the broad ring. Despite the expected decline in halocarbons, sensitivity studies suggest that this mechanism will lead to enhanced ozone loss in the weakly mixed region, delaying the future recovery of the ozone hole.
A constrained algorithm for retrieval of stratocumulus cloud properties using solar radiation, microwave radiometer, and millimeter cloud radar data
Authors: Mace Gerald G. , Sassen Kenneth
Published: February 2004

Nonprecipitating liquid phase clouds of the planetary boundary layer are important components of the Earth's energy budget. Modern remote sensors are able to probe these clouds with increasing precision and detail, and extensive boundary layer cloud data sets are being generated at multiple sites around the world. In this paper we integrate several recent cloud property retrieval algorithms targeted at boundary layer stratocumulus clouds into a single framework that returns the vertical profile of microphysical properties. The new algorithm guarantees consistency among the radar reflectivity profile, the microwave radiometer‐derived liquid water path, and the surface solar flux transmission ratio. We demonstrate a 0.9 μm rootmean‐square difference and 0.89 correlation coefficient when results from the new approach are compared to aircraft‐observed volume mean cloud droplet radii. An additional stratocumulus case study observed at an Atmospheric Radiation Measurement site is briefly described.
PROPHET 1998 meteorological overview and air‐mass classification
Authors: Cooper O. R. , Moody J. L. , Thornberry T. D. , Town M. S. , Carroll M. A.
Published: February 2004

The Program for Research on Oxidants: Photochemistry, Emissions, and Transport (PROPHET) summer 1998 measurements intensive was conducted from a tower above a forested site in northern Michigan. This paper provides a brief overview of the meteorological conditions and establishes that the study period was moderately drier and warmer than the climatological mean. The paper also identifies and chemically characterizes the major air mass types influencing the site. Meteorological analyses and back trajectories establish that air mass origin oscillated between relatively clean Canadian regions (except for periods influenced by Canadian forest fires) and regions of greater anthropogenic emissions in the contiguous United States. Higher mixing ratios of ozone, CO, NOx, and NOz were generally associated with southerly transport, which occurred 24% of the time. Lower mixing ratios were observed under northerly transport, which occurred 44% of the time. The dominance of northerly transport was due to a stronger than normal Hudson Bay low. The remaining 32% of the time was occupied by transitional periods between distinct air masses. A positive slope exists between ozone and CO when photochemically aged air masses are selected. However, no meaningful relationship between ozone and CO was observed when northerly and southerly transport periods were considered separately. Comparison to other summertime rural locations suggests that rapid and frequent transitions between air masses of contrasting source regions play an important role in maintaining the 0.3 O3/CO slope commonly observed in eastern North America.
Radiative transfer in the summertime Arctic
Authors: Benner Timothy C. , Curry Judith A. , Pinto James O.
Published: February 2004

This study used data from FIRE ACE and SHEBA with an explicit radiative transfer model to explore the effects of inhomogeneous cloud and surface properties on solar and longwave radiative transfer in the summertime Arctic. Three representative cases were selected from three flights in May and July 1998, with a detailed specification of cloud, surface, atmospheric, and aerosol properties. Results show that the surface inhomogeneity affects the fluxes at small scales but not in the domain average. The independent pixel approximation (IPA) and plane‐parallel approximation (PPA) can produce accurate domain‐average fluxes for low Arctic clouds, although the IPA can create large local errors. Measured and modeled fluxes are reasonably well matched, within the bounds of uncertainty in the model inputs, of which surface albedo has the largest impact. The fluxes are also sensitive to cloud liquid water path, cloud droplet effective radius, and aerosol amount.
Observations of ozone and related species in the northeast Pacific during the PHOBEA campaigns: 2. Airborne observations
Authors: Kotchenruther Robert A. , Jaffe Daniel A. , Beine Harald J. , Anderson Theodore L. , Bottenheim Jan W. , Harris Joyce M. , Blake Donald R. , Schmitt Rainer
Published: February 2004

During late March and April of 1999 the University of Wyoming's King Air research aircraft measured atmospheric concentrations of NO, O3, peroxyacetyl nitrate (PAN), CO, CH4, VOCs, aerosols, and J(NO2) off the west coast of the United States. During 14 flights, measurements were made between 39°–48°N latitude, 125°–129°W longitude, and at altitudes from 0–8 km. These flights were part of the Photochemical Ozone Budget of the Eastern North Pacific Atmosphere (PHOBEA) experiment, which included both ground‐based and airborne measurements. Flights were scheduled when meteorological conditions minimized the impact of local pollution sources. The resulting measurements were segregated by air mass source region as indicated by back isentropic trajectory analysis. The chemical composition of marine air masses whose 5‐day back isentropic trajectories originated north of 40°N latitude or west of 180°W longitude (WNW) differed significantly from marine air masses whose 5‐day back isentropic trajectories originated south of 40°N latitude and east of 180°W longitude (SW). Trajectory and chemical analyses indicated that the majority of all encountered air masses, both WNW and SW, likely originated from the north‐western Pacific and have characteristics of emissions from the East Asian continental region. However, air masses with WNW back trajectories contained higher mixing ratios of NO, NOx, O3, PAN, CO, CH4, various VOC pollution tracers, and aerosol number concentration, compared to those air masses with SW back trajectories. Calculations of air mass age using two separate methods, photochemical and back trajectory, are consistent with transport from the northwestern Pacific in 8–10 days for air masses with WNW back trajectories and 16–20 days for air masses with SW back trajectories. Correlations, trajectory analysis, and comparisons with measurements made in the northwestern Pacific during NASA's Pacific Exploritory Mission‐West Phase B (PEM‐West B) experiment in 1994 are used to investigate the data. These analyses provide evidence that anthropogenically influenced air masses from the northwestern Pacific affect the overall chemical composition of the northeastern Pacific troposphere.
Cloud radiative forcing at the top of the atmosphere during FIRE ACE derived from AVHRR data
Authors: Doelling David R. , Minnis Patrick , Spangenberg Douglas A. , Chakrapani Venkatesan , Mahesh Ashwin , Pope Shelly K. , Valero Francisco P. J.
Published: February 2004

Cloud radiative forcing at the top of the atmosphere is derived from narrowband visible and infrared radiances from NOAA‐12 and NOAA‐14 advanced very high resolution radiometer (AVHRR) data taken over the Arctic Ocean during the First ISCCP Regional Experiment Arctic Cloud Experiment (FIRE ACE) during spring and summer 1998. Shortwave and longwave fluxes at the top of the atmosphere (TOA) were computed using narrowband‐to‐broadband conversion formulae based on coincident Earth Radiation Budget Experiment (ERBE) broadband and AVHRR narrowband radiances. The NOAA‐12/NOAA‐14 broadband data were validated using model calculations and coincident broadband flux radiometer data from the Surface Heat Budget of the Arctic Ocean experiment and from aircraft data. The AVHRR TOA albedos agreed with the surface‐ and aircraft‐based albedos to within one standard deviation of ±0.029 on an instantaneous basis. Mean differences ranged from −0.012 to 0.023 depending on the radiometer and platform. AVHRR‐derived longwave fluxes differed from the model calculations using aircraft‐ and surface‐based fluxes by −0.2 to −0.3 W m−2, on average, when the atmospheric profiles were adjusted to force agreement between the observed and the calculated downwelling fluxes. The standard deviations of the differences were less than 2%. Mean total TOA albedo for the domain between 72°N and 80°N and between 150°W and 180°W changed from 0.695 in May to 0.510 during July, while the longwave flux increased from 217 to 228 W m−2. Net radiation increased from −89 to −2 W m−2 for the same period. Net cloud forcing varied from −15 W m−2 in May to −31 W m−2 during July, while longwave cloud forcing was nearly constant at ∼8 W m−2. Shortwave cloud forcing dominated the cloud effect, ranging from −22 W m−2 during May to −40 W m−2 in July. The mean albedos and fluxes are consistent with previous measurements from the ERBE, except during May when the albedo and longwave flux were greater than the maximum ERBE values. The cloud‐forcing results, while similar to some earlier estimates, are the most accurate values hitherto obtained for regions in the Arctic. When no significant melting was present, the clear‐sky longwave flux showed a diurnal variation similar to that over land under clear skies. These data should be valuable for understanding the Arctic energy budget and for constraining models of atmosphere and ocean processes in the Arctic.
Laboratory oxygen isotopic study of sulfur (IV) oxidation: Origin of the mass‐independent oxygen isotopic anomaly in atmospheric sulfates and sulfate mineral deposits on Earth
Authors: Savarino Joël , Lee Charles C. W. , Thiemens Mark H.
Published: February 2004

The oxygen isotopic composition (16O, 17O, and 18O) of sulfate formed from different oxidative reactions has been investigated. In the aqueous phase, sulfur oxidation by H2O2, O3, and O2, catalyzed by Fe(III) and Mn(II) were studied. In the gas phase we have investigated the only relevant reaction for the atmosphere: SO2+OH and its chain termination reaction SO3+H2O. The results show that none of these reactions, gas or aqueous phase, produce a mass‐independent oxygen isotopic composition in sulfate. Since H2O2 and O3 are known to possess a mass‐independent isotopic signature, we have investigated the possible transfer of this anomaly to sulfate. It appears that both these oxidant species transfer their anomaly. Isotopic analysis shows that two oxygen atoms from H2O2 are found in the product H2SO4. This result is in accord with previous work. For O3 we found that only one of the original ozone oxygen transfers to the product sulfate. These isotopic results contradict the free radical reaction mechanism proposed by Penkett et al. [1979] but agree with the nonfree radical mechanism suggested by Erickson et al. [1977]. Therefore, it appears that only aqueous phase oxidation produces a mass‐independent oxygen isotopic composition in sulfate. This finding is a response of the origin of the mass‐independent oxygen isotopic composition of atmospheric and mineral deposits of sulfate on Earth [Bao et al., 2000; Lee, 1997]. Furthermore, this finding allows us to quantify the relative proportion of sulfate production by OH (gas phase formation) and by H2O2 and O3 (aqueous phase formation). The results can be used to test atmospheric chemical/transport models.
Seasonal CO2 balances of a subarctic mire
Authors: Aurela Mika , Laurila Tuomas , Tuovinen Juha‐Pekka
Published: February 2004

Micrometeorological measurements of CO2 and energy fluxes were carried out in a peatland ecosystem in northern Finland (69°08′N, 27°17′E) during a measurement period from April to the end of October 1997. The summer of 1997 was exceptionally warm and dry as compared to the climatological normal period of 1961–1990, and the effects of the high temperature and lowered water table were clearly seen in the CO2 fluxes. The highest individual downward flux densities of about −0.25 mg(CO2) m−2 s−1 took place at the end of July, while the highest respiration rates of 0.15 mg(CO2) m−2 s−1 were observed later in August. During the first days of measurements in April the median of respiration flux densities through the snow cover was about 0.006 mg(CO2) m−2 s−1. In correspondence to the CO2 fluxes the strongest sink terms in the daily net ecosystem exchange (NEE) balances of about −6 g(CO2) m−2 d−1 were observed in July. The highest positive balances of about 4 g(CO2) m−2 d−1 were observed in early June and in August. The daily balances in April were about 0.6 g(CO2) m−2 d−1. The net balances for the sink period (June 15 to August 26, 1997) and for the 6‐month measurement period were −188 g m−2 and −30 g m−2, respectively. The wintertime CO2 balance was estimated by modeling the NEE using the NEE values from the first measurement week in April. The wintertime balances obtained yielded estimates for annual balances in the range of 62 to 72 g m−2 yr−1.
Modeling methane fluxes in wetlands with gas‐transporting plants: 3. Plot scale
Authors: Segers Reinoud , Leffelaar Peter A.
Published: February 2004

A process model based on kinetic principles was developed for methane fluxes from wetlands with gas‐transporting plants and a fluctuating water table. Water dynamics are modeled with the 1‐D Richards equation. For temperature a standard diffusion equation is used. The depth‐dependent dynamics of methane, oxygen, molecular nitrogen, carbon dioxide, soil carbon, electron acceptors in oxidized and in reduced form are affected by transport processes and kinetic processes. Modeled transport processes are convection and diffusion in the soil matrix, ebullition, and plant‐mediated gas transport. Modeled kinetic processes are carbon mineralization, aerobic respiration, methane production, methane oxidation, electron acceptor reduction, and electron acceptor reoxidation. Concentration gradients around gas‐transporting roots in water‐saturated soil are accounted for by the models from the two previous papers, ensuring an explicit connection between process knowledge at the kinetic level (millimeter scale) and methane fluxes at the plot scale. We applied the model to a fen, and without any fitting, simulated methane fluxes are within 1 order of magnitude of measured methane fluxes. The seasonal variations however, are much weaker in the simulations compared to the measurements. Simulated methane fluxes are sensitive to several uncertain parameters such as the distribution over depth of carbon mineralization, the total pool size of reduced and oxidized electron acceptors, and the root‐shoot ratio. Because of the process‐based character of the model it is probable that these sensitivities are present in reality as well, which explains why the measured variability is usually very high. Interestingly, heterogeneities within a rooted soil layer seem to be less important than heterogeneities between different soil layers. This is due to the strong influence of the interaction between water table and profile scale processes on the oxygen input to the system and hence on net methane production. Other existing process models are discussed and compared with the presented model.
Modeling methane fluxes in wetlands with gas‐transporting plants: 2. Soil layer scale
Authors: Segers Reinoud , Rappoldt Cornelis , Leffelaar Peter A.
Published: February 2004

Methane dynamics in a water‐saturated soil layer with gas‐transporting roots is modeled with a weighed set of single‐root model systems. Each model system consists of a soil cylinder with a gas‐transporting root along its axis or a soil sphere with a gas‐transporting root at its center. The weights associated with a different cylinder or sphere radius were deduced from root architecture. Methane dynamics in each single‐root model system are calculated using a single‐root model from the previous paper. From this full model a simplified model was deduced consisting of an oxygen‐saturated and an oxygen‐unsaturated model system. An even more simplified model was deduced, called the homogeneous model. In this model the concentrations are homogeneous in the whole soil layer. Simulation results of the simplified model are closer to the simulation results of the full model than the simulation results of the homogeneous model. The overall effect of the simplifications on simulated methane emissions are small, though the underlying processes are affected more severely, depending on simulation time and parameters. At high root densities and at large times, under stationary conditions, root density is proportional to simulated methane fluxes, provided that carbon availability is proportional to root density. Sensitivity analysis shows that lack of knowledge on root gas‐transport is an important limitation for the predictability of methane fluxes via the processes at the kinetic level.
Intraseasonal variation of monsoon activities associated with the rainfall over Bangladesh during the 1995 summer monsoon season
Authors: Ohsawa Teruo , Hayashi Taiichi , Mitsuta Yasushi , Matsumoto Jun
Published: February 2004

The rainfall over Bangladesh during the 1995 summer monsoon season has been investigated in terms of the intraseasonal variation of monsoon activities. The rainfall over Bangladesh is basically dominated by the north‐south oscillation of the monsoon trough. The rainfall increases when the monsoon trough is located at the foot of the Himalayas, because synoptic‐scale convective activity is much more vigorous to the south of the monsoon trough axis than to the north of it. In addition, the strong southwesterly wind to the south of the monsoon trough intensifies local convective activity owing to the effects of the orography to the north and east of the country. It is also found that the monsoon rainfall over Bangladesh in 1995 varies with a periodicity of ∼20 days, and this rainfall variation is closely associated with synoptic‐scale monsoon activities spreading over South and Southeast Asia. The active/break cycle of the rainfall variation during the 1995 summer monsoon season can be mostly explained by the northward propagation of what is called the 10–20 day variation of monsoon activities.