bluuebunny/crossref_metadata_2025 huggingface dataset:
Total VOC reactivity in the planetary boundary layer: 1. Estimation by a pump and probe OH experiment
Authors: Jeanneret Francois , Kirchner Frank , Clappier Alain , van den Bergh Hubert , Calpini Bertrand
Published: February 2004

The reactivity of hydroxyl radical (OH) with total volatile organic compounds (VOC), ΣkVOC[VOC], is directly obtained in the planetary boundary layer (PBL). The method is based on a perturbation technique where high initial OH concentrations are created by flash photolysis of ozone and subsequent reaction of O(1D) with H2O. Laser‐induced fluorescence is used to measure the residual OH concentration at different time delays after the perturbation (<100 ms) and obtain a direct estimate of the OH lifetime τOH for different atmospheric conditions. For specific experimental conditions the transport by the wind may be neglected, and the chemical processes governing the OH decay may be expressed by a detailed box model. With a simple chemical equation derived in this paper using τOH and complementary measurements of CO, O3, and NOx, an in situ estimate of ΣkVOC[VOC] in the PBL is retrieved with an uncertainty of less than 20% in comparison to the detailed box model calculations. This analysis is applied to laboratory measurements with three synthetic NOx/VOC/O3 gas mixtures, and the retrieved OH lifetimes and total VOC reactivity are discussed against model predictions.
Identification of an El Niño‐Southern Oscillation signal in a multiyear global simulation of tropospheric ozone
Authors: Peters Wouter , Krol Maarten , Dentener Frank , Lelieveld Jos
Published: February 2004

We present the first study of the El Niño‐Southern Oscillation (ENSO) interannual variability in tropical tropospheric ozone in a multiyear simulation with a global three‐dimensional chemistry‐transport model. A 15‐year period (1979–1993) was simulated using European Centre for Medium‐Range Weather Forecasts meteorological reanalysis data and a time‐varying emission data set. A comparison of model calculations with observations shows good agreement for surface ozone, seasonal cycles, and ozone concentrations at remote stations, but reveals an underestimate of ozone in the free troposphere, which is most pronounced during biomass burning seasons. The ENSO signal is the most important component of interannual variability in tropical tropospheric ozone columns (TTOC), being responsible for nearly 25% of the total interannual variability of ozone in the tropics. The amplitude of the modeled ENSO signal in TTOC is 3 Dobson units, in close agreement with satellite observations. This signal is evenly distributed with height, indicating that rapid vertical transport plays an important role. The ENSO signal is also detectable at the surface, for example at the Pacific island Samoa, for which model‐calculated and measured ozone agree well.
Climatology of dust aerosol size distribution and optical properties derived from remotely sensed data in the solar spectrum
Authors: Tanré D. , Kaufman Y. J. , Holben B. N. , Chatenet B. , Karnieli A. , Lavenu F. , Blarel L. , Dubovik O. , Remer L. A. , Smirnov A.
Published: February 2004

Simultaneous spectral remote observations of dust properties from space and from the ground create a powerful tool for the determination of ambient dust properties integrated on the entire atmospheric column. The two measurement methods have a complementary sensitivity to variety of dust properties. The methodology is demonstrated using spectral measurements (0.47–2.21 μm) from Landsat TM over the bright Senegalian coast and dark ocean, and Aerosol Robotic Network (AERONET) radiances measured in several locations. We derive (1) the dust size distribution, showing a dominant coarse mode at 1–5 μm and a secondary mode around 0.5 μm effective radius; (2) dust absorption, which is found to be substantially smaller than reported from previous measurements; (3) the real part of the refractive index which varies within the range 1.53–1.46; and we show that (4) the effect of the dust nonspherical shape on its optical properties is not significant for scattering angles <120°.
Spatial and temporal characteristics of dust storms in China and its surrounding regions, 1960–1999: Relations to source area and climate
Authors: Sun Jimin , Zhang Mingying , Liu Tungsheng
Published: February 2004

Compilation and analysis of the past 40 years of dust storm reports from China allow estimation of the meteorological conditions of dust storms, dust transport routes, and eolian source regions. Our results indicate that dust storms in China are highly associated with the frontal systems and the Mongolian cyclonic depression. The spatial distribution of dust storms indicates that there are two dominant source regions of eolian dust raised from China and its surrounding regions. The major source is the gobi deserts in Mongolia and northern China. Another source region is the Taklimakan Desert in western China. However, dust entrained from the two sources makes different contributions to downwind deposition regions. In most cases, dust materials entrained from the gobi deserts of Mongolia and China can only be entrained to an elevation of <3000 m. They are the dominant source materials of the eolian sediments in the Loess Plateau, southeastern China, offshore regions, and the near North Pacific Ocean. Dust materials from the Taklimakan Desert can be entrained to an elevation of >5000 m and then transported over long distances (∼5000 km) by the westerlies. These materials are not the main sources of the dust deposited in the proximal region, such as the Chinese Loess Plateau, but they are important sources of the eolian fraction of pelagic sediment in the remote North Pacific Ocean.
Laboratory investigation of possible ice nucleation by ionizing radiation in pure water at tropospheric temperatures
Authors: Seeley L. H. , Seidler G. T. , Dash J. G.
Published: February 2004

A variety of mechanisms have been proposed to explain observed correlations between tropospheric climate variables and cosmic ray flux levels. The most direct hypothesis involves the formation of ice nuclei along ion tracks formed when cosmic rays pass through cloud water droplets. To test this hypothesis, we performed nucleation studies of a single water droplet undergoing repeated linear cooling cycles to −40°C with and without a radioactive source in the vicinity of the drop. A comparison of the distributions of nucleation temperatures obtained with and without a source of 5.3 MeV α‐particles present supports the null hypothesis that ionization tracks do not act as nucleation sites. In both cases we observe mean nucleation temperatures of −34.5°C consistent with previous studies of homogeneous nucleation.
Relative contribution of biogenic and anthropogenic sources to formic and acetic acids in the atmospheric boundary layer
Authors: Glasius M. , Boel C. , Bruun N. , Easa L. M. , Hornung P. , Klausen H. S. , Klitgaard K. C. , Lindeskov C. , Møller C. K. , Nissen H. , Petersen A. P. F. , Kleefeld S. , Boaretto E. , Hansen T. S. , Heinemeier J. , Lohse C.
Published: February 2004

Formic and acetic acids are ubiquitous trace gases in ambient air, but their sources remain to be fully understood. They originate from photochemical oxidation of volatile organic compounds (VOC). Direct emissions of these carboxylic acids are supposedly negligible at North European conditions. Radiocarbon analysis of formic and acetic acids provides information on the origin of the precursor VOC, since biogenic VOC have the same 14C/12C ratio as atmospheric carbon dioxide, while VOC emitted from fossil fuels are devoid of 14C. The origin of atmospheric formic and acetic acids was studied by accelerator mass spectrometry of air and rainwater samples. Sampling sites were selected at different distances from centers of anthropogenic activities in Europe, ranging from urban via rural to remote sites. The procedure for preparation of samples was checked by several quality assurance samples, and no significant contamination from atmospheric carbon dioxide was observed. The results show very high (55–100%) biogenic contributions to both carboxylic acids in all samples. Only in semiurban and urban areas were the biogenic fractions less than 80%. The results indicate that biogenic VOC can have substantial influence on formation of photochemical oxidation products and thus photochemical oxidants.
Unique markers of chlorine atom chemistry in coastal urban areas: The reaction with 1,3‐butadiene in air at room temperature
Authors: Wang Weihong , Finlayson‐Pitts Barbara J.
Published: February 2004

Chlorine atoms are generated in the marine boundary layer, including coastal urban areas, through heterogeneous reactions of sea‐salt particles. However, there are no measurements of tropospheric chlorine atoms, and few measurements of their precursors due to the difficulty in specifically identifying and measuring parts per trillion levels of these highly reactive species. We report here studies which suggest an alternate approach for elucidating the role of chlorine atoms in the chemistry of coastal urban areas: the identification and measurement of unique chlorine‐containing products from the reactions of Cl with organics, in this case 1,3‐butadiene, in air. The reaction of chlorine atoms with 1,3‐butadiene (19–61 ppm), which is emitted by motor vehicles, was studied at 1 atm pressure in air at room temperature in both the absence and presence of NO (0–20 ppm). Cl2 (4–16 ppm) was photolyzed to generate chlorine atoms, and the products were identified and measured using gas chromatography‐mass spectrometry (GC‐MS) and long path (40 m) Fourier transform infrared spectroscopy (FTIR). We identified 4‐chlorocrotonaldehyde (CCA) as a unique chlorine‐containing product from the Cl atom reaction with 1,3‐butadiene in both the presence and absence of NO. The yield of CCA was (29±7)% (2σ) in the absence of NO and (65±20)% (2σ) in its presence. The yield of chloromethyl vinyl ketone (CMVK), another chlorine‐containing compound from this reaction, was measured to be (19±4)% (2σ) in the absence of NO; it was not observed in its presence. The reaction system was modeled using ACUCHEM to clarify the reaction paths and to allow extrapolation to ambient air conditions. These studies predict that CCA will be formed in coastal urban areas in the early morning hours at concentrations of ∼100 ppt, which should be readily detectable by GC‐MS and GC with electron capture detection (ECD). Some studies were also carried out on the isoprene reaction, and some potential unique products of its reaction with chlorine atoms are suggested as candidates for field investigations.
Momentum transfer over the coastal zone
Authors: Sun Jielun , Vandemark Douglas , Mahrt Larry , Vickers Dean , Crawford Timothy , Vogel Christoph
Published: February 2004

Spatial variations of surface stress over the coastal shoaling zone are studied offshore of Duck, North Carolina, by the LongEZ research aircraft, equipped to measure both atmospheric turbulence and oceanic waves. We find that the spatial variation of the friction velocity with offshore distance is much larger with offshore flow than with onshore flow. In general, the mean square slope of the short waves (wavelength shorter than 2 m) decreases with offshore distance, while the mean square slope of the long waves (wavelength longer than 2 m) increases with offshore distance. With onshore flow the friction velocity is strongly correlated with surface waves. In addition, the variation of the neutral drag coefficient is well correlated with the atmospheric bulk Richardson number. With offshore flow the observed momentum flux significantly decreases with offshore distance. Within the first few kilometer offshore, the relationship between the friction velocity and the mean square slope of the short waves and the relationship between the neutral drag coefficient and the atmospheric bulk Richardson number are obscured by the direct influence of the upstream land surface on the measured turbulence. These relationships for offshore flow agree well with those for onshore flow if the fetch is beyond the immediate influence of the land surface. The results in this study suggests that the effects of the strong turbulence advected from over the nearby land surface in offshore flow may lead to ambiguous physical interpretation of the correlation between the momentum flux and the wave state.
Collision‐induced absorption of solar radiation in the atmosphere by molecular oxygen at 1.27 μm: Field observations and model calculations
Authors: Smith K. M. , Newnham D. A. , Williams R. G.
Published: February 2004

Significant uncertainties in the spectroscopy of collision‐induced absorption by oxygen have a limited knowledge of the contribution made by molecular oxygen to the short‐wave radiation budget of the Earth's atmosphere. As part of a programme of high‐resolution (up to 0.02 cm−1) studies of atmospheric short‐wave transmittance under clear‐sky conditions, field measurements in the region of the 1.27 μm (7874 cm−1) oxygen absorption are reported here. Measurements of direct solar radiation incident at the ground were made using a ground‐based Fourier transform spectrometer at the Rutherford Appleton Laboratory (51.6°N, 1.3°W, altitude 135 m). Model calculations of the atmospheric transmittance in the region of the broad continuum feature underlying the a1Δg ← X3∑g¯ (0,0) band of oxygen were made using absorption cross sections derived from recent laboratory measurements. The agreement between the field observations and the model calculations validates a simple model developed to describe the atmospheric oxygen continuum absorption centered at 1.27 μm for atmospheric short‐wave radiative transfer and remote sensing applications.
SSM/T‐2 measurements of regional changes in three‐dimensional water vapor fields during ENSO events
Authors: Blankenship Clay B. , Wilheit Thomas T.
Published: February 2004

Observations of water vapor fields are presented from the SSM/T‐2 (Special Sensor Microwave Temperature‐2) radiometer during both El Niño (November 1997) and La Niña (November 1995). Increased SST associated with the El Niño Southern Oscillation is found to cause enhanced circulation with drying in the upper troposphere of subtropical subsidence zones. Although regional rather than global in nature, this is similar to the effect proposed by Lindzen [1997] that could lead to a negative water vapor feedback. These observations provide a useful test of climate models to see how well they model currently observed interannual changes in the water vapor field. Water vapor profiles were retrieved from SSM/T‐2 observations by a physical algorithm for all ocean areas between 40°N and 40°S. These data reveal moistening (drying) in regions of equatorial warming (cooling). They also reveal drying (moistening) of the upper troposphere in subsidence zones to the north and south. Patterns of computed and observed clear‐sky OLR changes match changes in upper tropospheric humidity closely, with increased OLR in regions of drying. Partitioning of the water vapor field into changes explained by local SST or rainfall and dynamically induced changes indicates that the changes in subtropical upper tropospheric humidity are caused, in part, by large‐scale dynamic effects. Numerical values of area averages and plots of zonally averaged quantities are also presented, although analysis is complicated by compensating processes.
Evidence for nonlinearity in observed stratospheric circulation changes
Authors: Gillett Nathan P. , Baldwin Mark P. , Allen Myles R.
Published: February 2004

The leading mode of variability of the lower atmosphere circulation in the Northern Hemisphere is a largely zonally symmetric mode known as the Arctic Oscillation. We calculate Arctic Oscillation (AO) indices on a range of levels from 1000 to 10 hPa by means of a principal component analysis of National Centers for Environmental Prediction daily geopotential height anomalies. We find the apparent downward propagation of anomalies noted by other authors to be statistically significant compared to a red noise model. By examining histograms of these indices for each month, we note that the distribution of the index is generally close to Gaussian in the troposphere. In the stratosphere, however, the index is negatively skewed in the winter and positively skewed in the spring. We conclude that the positive skewness in April results from the coexistence of distinct summer and winter circulation states, and by examining polar stratospheric temperatures, we conclude that the negative skewness in January may be due to the radiatively determined limit on the vortex strength. This radiative limit responds relatively slowly to anthropogenic forcing, whereas changes in planetary wave forcing could have a much faster impact on the number of warm events. This suggests a hypothesis that the vortex strength may respond nonlinearly to anthropogenic forcing, which is supported by an observed change in the shape of the histograms of 20–200 hPa AO indices in January over the past 40 years.
Experimental conditions for ball lightning creation by using air gap discharge embedded in a microwave field
Authors: Ofuruton Hideho , Kondo Norihisa , Kamogawa Masashi , Aoki Masao , Ohtsuki Yoshi‐Hiko
Published: February 2004

An experiment was carried out to reproduce ball lightning by using microwave radiation and electric discharge without a metal cavity. When the energy of the discharge was high, a plasma fireball was produced. When high‐power microwave existed, a plasma fireball was produced even if the power of the discharge was low. We must pay more attention to places where high‐power electromagnetic waves exist to see ball lightning.
Sensitivity of tracers and a stratospheric aircraft perturbation to two‐dimensional model transport variations
Authors: Fleming Eric L. , Jackman Charles H. , Considine David B. , Stolarski Richard S.
Published: February 2004

We examine the sensitivity of two‐dimensional model simulations of stratospheric tracers to uncertainties in the model transport and explore how such uncertainties impact the simulation of a lower stratospheric perturbation due to high‐speed civil transport (HSCT) aircraft emissions. To define the transport uncertainty, we vary the model transport fields so that the resulting tracer simulations roughly bracket the observations. This provides an estimate of the upper and lower limits on realistic transport rates in our two‐dimensional (2‐D) model. Increasing the advective residual circulation strength or the lower stratospheric vertical diffusion (Kzz) decreases the mean age and residence time of the HSCT emissions and diminishes the negative response in total column ozone globally. Increasing the stratospheric horizontal diffusion (Kyy) either globally or in the tropics only has the opposite effect of increasing the age and emission residence time and enhancing the negative total ozone response. Uncertainties in the mechanical eddy forcing derivation affect both Kyy and the residual circulation simultaneously, resulting in some cancellation of effects. This produces a smaller range of uncertainty in the tracer and perturbation simulations than given by uncertainties in the circulation or Kyy components separately. The model simulations in the lower and middle stratosphere are relatively insensitive to the strength of the mesospheric gravity wave effects and the magnitude of the horizontal diffusive transport across the tropopause. The base model transport compares most favorably with tracer data and gives a global and annual mean steady state HSCT perturbation response in total ozone of −0.62%, assuming a NOx emission index of 5 g/kg, 500 airplanes, and a 10% gas‐to‐particle conversion of the SO2 emission. For the range of transport uncertainty examined in this study, the global total ozone perturbation response ranges from −0.34% to −0.74%, with a mainly strong correlation between the total ozone response and mean age.
Aerosol properties derived from aircraft multiangle imaging over Monterey Bay
Authors: Kahn Ralph , Banerjee Pranab , McDonald Duncan , Martonchik John
Published: February 2004

The first generic and climatological aerosol retrievals using AirMISR data are presented. Multiangle observations at 672 and 867 nm, in a cloud‐free region over dark water in Monterey Bay on June 29, 1999, yield complementary generic and climatological results. The generic retrieval produces cross‐section‐weighted, column‐mean aerosol properties: midvisible aerosol optical depth between 0.05 and 0.10, with a preference for values on the low side of the range, particle number‐mean characteristic radius between 0.25 and 0.45 μm, and imaginary index of refraction <0.004, with 0.0 as the most likely value. These properties correspond to a “medium‐to‐large, spherical” column‐average particle. The climatological retrieval identifies a maritime air mass, having a total aerosol optical depth about 0.1, and mixing ratio for sea‐salt particles (large, spherical) of 50%, based on optical depth in MISR Band 2, and 40% for the sulfate plus carbonaceous (medium, spherical) components, to an accuracy of about ±15%. These results are in good agreement with the limited nearby surface‐based and aircraft observations available. The analysis also shows that over dark water, pixel‐to‐pixel scene variability can contribute more to the retrieval uncertainty than camera calibration and that high spatial variance of the reflectance, in addition to geometric considerations, is a better indicator of Sun glint contamination than geometry alone. This work represents an early step toward the goal of using MISR multiangle data to add spatial detail and information about temporal variability to the global aerosol climatology.
Global three‐dimensional modeling of the water vapor concentration of the mesosphere‐mesopause region and implications with respect to the noctilucent cloud region
Authors: Körner U. , Sonnemann G. R.
Published: February 2004

A global three‐dimensional model of the dynamics (0–150 km) and chemistry (30–150 km) of the middle atmosphere has been developed and applied to the problem of the water vapor distribution of the mesosphere‐mesopause region. The mesopause region is one of the most intricate domains of the atmosphere and requires an extraordinarily careful modeling. In order to interpret the specific feature of the water vapor distribution, particular attention was paid to the problem of the effective characteristic chemical time and the comparison of this time with the characteristic transport time. The results confirm measurements which show highest concentrations during the summer months and lowest concentrations in the winter in the middle and high latitudes of the mesosphere because of the seasonal variation of the vertical wind system and change in the altitudes of levels of constant pressure as a result of a mean cooling or warming below the respective heights. The equator region is marked by a semiannual variation with maxima around the equinoxes. By way of contrast, the Halogen Occultation Experiment (HALOE) measurements show maxima around solstices, however, with some exceptions for which a maximum also occurs during the equinox. This points to a dynamically sensitive equatorial region. At high latitudes the extremely low temperatures within the mesopause region in summer and the relatively high water vapor concentrations cause a supersaturation of water vapor which is the condition for the formation of noctilucent clouds (NLCs). The calculated seasonal and latitudinal border of the domain of supersaturation corresponds quite well with the mean areas of the occurrence of NLCs; however, it is impossible to model specific events on the basis of such a coarse model as the occurrence of NLCs at middle latitudes. There is no direct hemispheric transport from the summer to the winter hemisphere within the mesosphere‐lower thermosphere, but the meridional transport occurs in a more complicated manner.
Trans‐Pacific yellow sand transport observed in April 1998: A numerical simulation
Authors: Uno Itsushi , Amano Hiroyasu , Emori Seita , Kinoshita Kisei , Matsui Ichiro , Sugimoto Nobuo
Published: February 2004

A yellow sand transport episode from the Asian continent to Japan and North America which occurred in April 1998 is simulated. A new on‐line dust tracer model coupled with a regional‐scale meteorological model is developed and applied to this dust storm episode. The results for two large dust events that started during April 14–15 and 19–20, 1998, have been analyzed and discussed. The first dust storm was trapped in a cutoff vortex developed over the China plain. A modeled 3‐D structure of dust associated with this cutoff vortex agreed with an observed time‐height cross section of dust concentration. Results show that the strong subsidence at the backside of the vortex restricted the dust layer below 3 km level. Model analysis revealed that the second dust event that started during April 19–20 over inland China was the origin of a dust episode reported over North America. The trans‐Pacific dust transport simulation successfully showed the dust onset near the West Coast of North America. Elevation of the dust layer during the long‐range transport was below 3 km. The model is extended to include the transport of an Asian origin anthropogenic tracer over the North Pacific Rim. Both the natural‐origin mineral dust and the Asian‐origin anthropogenic tracer are simultaneously transported even if their emission regions are different.
Arctic weather during the FIRE/ACE flights in 1998
Authors: Wylie Donald P.
Published: February 2004

The weather systems, cyclones and anticyclones, along with air trajectories and cloud forms, are compared to past studies of the Arctic to assess compatibility of the 4‐month study of FIRE/ACE with past climatologies. The frequency and movement of cyclones (lows) and anticyclones (highs) followed the general eastward and northeastward directions indicated by past studies. Most cyclones (lows) came from eastern Siberia and the Bering Sea to the south and moved north across the Bering Straight or Alaska into the Arctic Ocean. They generally weakened in central pressure as they moved poleward. Anticyclones (highs) were most common in the eastern Beaufort Sea near Canada in June and July, as predicted from previous studies. However, many cyclones and anticyclones moved in westward directions, which is rare in other latitudes. Erratic changes in shape and intensity on a daily basis also were observed. The NOAA National Center for Environmental Prediction (NCEP) analysis generally reflected the SHEBA Ship WMO observations which it used. However, NCEP temperatures were biased warm by 1.0° to 1.5°C in April and early May. In July, when the surface temperatures were at the freezing/thawing point, the NCEP analysis changed to a cold bias of −1.0°C. Dew points had smaller biases except for July where they were biased cold by −1.4°C. Wind speeds had a −2 m/s low bias for the six windiest days. Surface barometric pressures had consistently low biases from −1.2 to −2.8 hPa in all four months. Air parcel historical trajectories were mainly from the south or from local anticyclonic gyres in the Beaufort Sea. Most air came to the SHEBA ship from the North Pacific Ocean or from Alaska and Canada and occasionally from eastern Siberia. Very few trajectories traced back across the pole to Europe and central Asia. Cloud cover was high, as expected, from 69 to 86% of the time. Satellite data also indicate frequent stratus, altostratus, and cirrus clouds (occurring 61% of the time) above the expected boundary layer fog and Arctic stratus clouds.
Chemistry‐transport model comparison with ozone observations in the midlatitude lowermost stratosphere
Authors: Bregman A. , Krol M. C. , Teyssèdre H. , Norton W. A. , Iwi A. , Chipperfield M. , Pitari G. , Sundet J. K. , Lelieveld J.
Published: February 2004

About 600 ozone sonde profiles and in situ ozone observations on 2300 aircraft flights were compared with several three‐dimensional global chemistry‐transport models, in the midlatitude lowermost stratosphere during 1996. The models use the same top boundary conditions and parameterized ozone chemistry. The comparisons show that the models using general circulation model (GCM) winds do not capture the seasonal ozone accumulation in the lowermost stratosphere. The models using winds from the European Centre for Medium‐Range Weather Forecasts (ECWMF) overestimate ozone above this maximum, especially during spring. This overestimation increases with increasing latitude. Close to the tropopause, the best agreement between models and observations is found during winter and the worst occurs during summer, where most models underestimate the mean ozone concentration. This underestimation is partly caused by inaccurate description of the relatively small‐scale transport processes, mainly associated with convective activity, and partly by incomplete ozone chemistry in the parameterized scheme. The models that use assimilated winds show significant differences in calculated ozone, despite their common source (ECWMF) to calculate the transport. We illustrate that the model performance significantly depends on the ECWMF processing method and that interpolation of wind data should be avoided. In addition, the results seem relatively insensitive to vertical resolution. To improve the model performance further in this region, the horizontal resolution should be higher than 2.5°.
Causes of enhanced fluoride levels in Alpine ice cores over the last 75 years: Implications for the atmospheric fluoride budget
Authors: Preunkert Susanne , Legrand Michel , Wagenbach Dietmar
Published: February 2004

A continuous high‐resolution record from a Col du Dôme (Mont Blanc massif, 4250 m above sea level (asl), French Alps) ice core in addition to discontinuous samples from a Colle Gnifetti (Monte Rosa massif, 4450 m asl, Swiss Alps) ice core were used to reconstruct the history of the atmospheric fluoride pollution at the scale of Europe. Such studies are mandatory by large uncertainties in our understanding of the natural fluoride cycle which have confounded assessment of the environmental impact of anthropogenic emissions. For fluoride, advantages of Alpine ice core records with respect to the Greenland ones include less efficient post‐depositional effects in relation with higher snow accumulation rates, and less contamination by quasi‐permanent passive volcanic HF emissions at midlatitudes compared to the situation at high northern latitudes. Hence Alpine ice records permit detailed examination of natural sources of fluoride for the free troposphere over Europe and the impact of anthropogenic sources such as aluminium smelters, coal burning, and contribution of the stratospheric reservoir built up from the chlorofluorocarbon (CFC) degradation since the beginning of the twentieth century. At Col du Dôme (CDD), fluoride concentrations in summer snow layers were close to 0.30 ng g−1 in 1930, started to increase in the late 1930s, reaching 1.4 ng g−1 in 1940 and 2.4 ng g−1 in the late 1960s. From 1970 to 1980 they were strongly decreased, exhibiting a plateau value close to 1.3 ng g−1 between 1980 and 1995. It is shown that at the scale of Europe in summer, soil dust emissions dominated the atmospheric fluoride budget prior to 1880. In the late 1960s the soil contribution decreased to 6±1% due to enhanced release of fluoride by aluminium smelters and coal burning which accounted for 86±3% and 8±2% of the total fluoride content, respectively. From 1970 to 1980, effective precautions have been taken to minimize the release of fluoride from aluminium smelters to the atmosphere. Thus, over the 2 last decades, 26±8% of the fluoride summer level of Alpine snow was due to coal combustion. The remaining part was related to the release from the less pollutant aluminium smelters as well as other anthropogenic processes (cement and phosphate industrial processings) (56±11%) and to soil dust emissions (18±2%). Winter levels close to 0.10 ng g−1 or less in 1930 were gradually increased after the late 1930s, reaching a maximum of 0.4 ng g−1 in 1970. Similarly to the summer level, the winter one has then strongly decreased (∼0.12 ng g−1 in 1980). A major difference between summer and winter trends is the reincrease of winter level up to 0.4 ng g−1 (i.e., similar to the 1970 maximum) in 1990. Such a very recent change of fluoride background levels may be partly related to the impact via stratosphere/troposphere exchanges of the growing HF stratospheric load related to the CFC degradation.
Model and measurement analysis of springtime transport and chemistry of the Pacific basin
Authors: Hess P. G.
Published: February 2004

Extensive chemical measurements from the Mauna Loa Observatory (MLO) Photochemistry Experiment 2c (MLOPEX 2c) (April 15 to May 15, 1992) field campaign are analyzed in relation to the photochemistry, transport, and physical losses in the Pacific basin utilizing an episodic chemical transport model christened HANK. The budgets of NOy and O3 are examined in particular, as are model‐measurement discrepancies in the NOy budget. For the long‐lived species the bias of the simulated means (measured minus simulated divided by measured) of CO, CH4, C2H6, C3H8, O3, and NOx is less than 20%. The bias of ln ([C3H8]/[C2H6]) is less than 4%, and the modeled and measured means of C3H8 are not significantly different. H2O2, CH3OOH, and radon agree within a standard deviation. The simulated concentrations of NOy, HNO3, and peroxyacetyl nitrate (PAN) are high, while the short‐lived hydrocarbons (C2H6 and C2H4) are low. Frequency distributions and species' cross correlations are also used to diagnose and evaluate the simulation. Sensitivities in the model formulation are discussed. The results indicate that the subtropics demarcate a transition between a regime characterized by Hadley cell transport to the south and one characterized by rapid isentropic mixing to the north. Vigorous photochemical processing occurs in the remote subtropical Pacific as PAN is converted to NOx, which in turn is converted to HNO3. This rapid photochemistry is related to the slope of the isentropic surfaces in the subtropics. It maintains elevated levels of NOx in the remote Pacific, reduced levels of HNO3/NOx, and increases the net ozone production. The model and measurements show that as air masses are processed photochemically (as measured by AGE) the relative concentrations of PAN and NOx decrease, as does PAN/NOy; corresponding increases occur in HNO3, HNO3/NOx, and HNO3/NOy. In both the model and measurements, H2O2, CH3OOH, and HNO3/NOx are tracers of photochemically processed air, while high concentrations of C3H8, C2H6, NOx, and PAN are associated with younger air.
Air mass factor formulation for spectroscopic measurements from satellites: Application to formaldehyde retrievals from the Global Ozone Monitoring Experiment
Authors: Palmer Paul I. , Jacob Daniel J. , Chance Kelly , Martin Randall V. , Spurr Robert J. D. , Kurosu Thomas P. , Bey Isabelle , Yantosca Robert , Fiore Arlene , Li Qinbin
Published: February 2004

We present a new formulation for the air mass factor (AMF) to convert slant column measurements of optically thin atmospheric species from space into total vertical columns. Because of atmospheric scattering, the AMF depends on the vertical distribution of the species. We formulate the AMF as the integral of the relative vertical distribution (shape factor) of the species over the depth of the atmosphere, weighted by altitude‐dependent coefficients (scattering weights) computed independently from a radiative transfer model. The scattering weights are readily tabulated, and one can then obtain the AMF for any observation scene by using shape factors from a three dimensional (3‐D) atmospheric chemistry model for the period of observation. This approach subsequently allows objective evaluation of the 3‐D model with the observed vertical columns, since the shape factor and the vertical column in the model represent two independent pieces of information. We demonstrate the AMF method by using slant column measurements of formaldehyde at 346 nm from the Global Ozone Monitoring Experiment satellite instrument over North America during July 1996. Shape factors are computed with the Global Earth Observing System CHEMistry (GEOS‐CHEM) global 3‐D model and are checked for consistency with the few available aircraft measurements. Scattering weights increase by an order of magnitude from the surface to the upper troposphere. The AMFs are typically 20–40% less over continents than over the oceans and are approximately half the values calculated in the absence of scattering. Model‐induced errors in the AMF are estimated to be ∼10%. The GEOS‐CHEM model captures 50% and 60% of the variances in the observed slant and vertical columns, respectively. Comparison of the simulated and observed vertical columns allows assessment of model bias.
Microphysics of INDOEX clean and polluted trade cumulus clouds
Authors: Heymsfield Andrew J. , McFarquhar Greg M.
Published: February 2004

In this study, we examine how emissions from the Indian subcontinent modify the microphysical properties of trade wind cumulus clouds over the Indian Ocean. In situ microphysical data from 18 National Center for Atmospheric Research C130 research flights during the Indian Ocean Experiment (INDOEX) in February‐March 1999, in polluted to pristine regions, as delineated from the concentrations of condensation nuclei (CN), were analyzed. Cloud properties were found to vary in a systematic way during the five gradient flights: i.e., those flights in which the C130 flew from about 4.5° N latitude in the high‐CN regions, south across the intertropical convergence zone (ITCZ) into the clean regime, then farther south to about 8° S latitude. The high‐CN regime contained relatively large concentrations of small droplets as compared to the low‐CN regime, where low concentrations of large droplets, and more frequent drizzle, were measured. An analysis of the data from penetrations into thousands of clouds during all 18 flights supports these qualitative observations: In the polluted regions, the droplet concentrations were a factor of 3 higher, the droplet effective diameters were 35% smaller, and drizzle was observed 25% as often than in the pristine regions; in both polluted and pristine regions, the bulk cloud properties (liquid water content (LWC), vertical velocity, cloud horizontal dimensions) were approximately the same. Even larger differences in the microphysical properties between the high‐CN, intermediate‐CN, and low‐CN regimes were noted when the data set was partitioned by the LWC. A high ratio offorward scattering spectrometer probe (FSSP) to CN concentrations was noted in the low‐CN regime, whereas in the high‐CN regime this ratio was small. Droplet growth calculations in an adiabatic, ID parcel model over a 300 m cloud depth support the droplet observations and indicate that the cloud optical depths in the high CN regime could have been as large as twice those in the low CN regime as a result of differences in the cloud condensation nucleus (CCN) population. The corresponding albedos of about 0.54 in the high CN regime and 0.47 in the low CN regime signify potentially large differences in albedo for the typical vertical velocities observed in INDOEX clouds, but it is unclear how entrainment would affect this difference. The calculations indicate that stronger updrafts associated with deep convection could lead to a larger difference between the microphysics of high‐CN and low‐CN regions.
Closure between aerosol particles and cloud condensation nuclei at Kaashidhoo Climate Observatory
Authors: Cantrell Will , Shaw Glenn , Cass Glen R. , Chowdhury Zohir , Hughes Lara S. , Prather Kimberly A. , Guazzotti Sergio A. , Coffee Keith R.
Published: February 2004

Predicting the cloud condensation nuclei (CCN) supersaturation spectrum from aerosol properties is a fairly straightforward matter, as long as those properties are simple. During the Indian Ocean Experiment we measured CCN spectra, size‐resolved aerosol chemical composition, and aerosol number distributions and attempted to reconcile them using a modified form of Köhler theory. We obtained general agreement between our measured and modeled CCN spectra. However, the agreement was not as good during a time period when organic carbon comprised a quarter of the total mass of the aerosol in the submicron size range. The modeled concentrations overpredict those actually measured during that time period. This suggests that some component, presumably organic material, can inhibit the uptake of water by the electrolytic fraction of the mass.
Vertical profiling of optical and physical particle properties over the tropical Indian Ocean with six‐wavelength lidar: 2. Case studies
Authors: Müller Detlef , Franke Kathleen , Wagner Frank , Althausen Dietrich , Ansmann Albert , Heintzenberg Jost , Verver Ge
Published: February 2004

We present the seasonal cycle of optical and physical particle properties over the Indian Ocean based on case studies of six‐wavelength aerosol lidar observations performed in the framework of the Indian Ocean Experiment. From February 1999 to March 2000 the lidar system made routine measurements at the International Airport of the Maldives on Hulule island (4.1° N, 73.3° E). The measurement from February 18, 1999, during the northeast monsoon showed high optical depths of 0.27 at 532 nm. Lidar ratios of 40–45 sr at 532 nm indicated the long‐range transport of well‐aged polluted air from Southeast Asia. Effective radii of <0.14 μm, complex refractive indices >1.6–0.01i, and single‐scattering albedos of 0.86–0.93 at 532 nm were retrieved for the pollution layer above 1000 m height. Similar physical particle properties followed from the measurement on March 22, 2000. Optical depths were 0.4; lidar ratios of 60 sr at 1000 m height indicated anthropogenic pollution from India. Values of 45 sr at 3000 m height indicated a considerable influence by clean‐continental particles from Africa and Arabia. Accordingly, the single‐scattering albedo ranged from 0.85–0.9. During the southwest monsoon in July, air was advected from the western Indian Ocean, eastern Africa, and Arabia. Particles were transported into maximum heights of 5 km from these regions on July 12, 1999. Small effective radii of 0.18–0.3 μm, considerably lower mean complex refractive indices of <1.6–0.02i, and single‐scattering albedos >0.88 indicated the prevailing clean‐marine and clean‐continental conditions. The intermonsoon season was characterized by strong washout processes during the crossover of the Intertropical Convergence Zone. On October 16, 1999, low optical depths of 0.13 and lidar ratios from 20–35 sr indicated clean‐marine conditions. Accordingly, large effective radii of 0.2–0.35 μm, low complex refractive indices ∼1.45–0.005i, and single‐scattering albedos >0.95 were found.
Asian dust events of April 1998
Authors: Husar R. B. , Tratt D. M. , Schichtel B. A. , Falke S. R. , Li F. , Jaffe D. , Gassó S. , Gill T. , Laulainen N. S. , Lu F. , Reheis M. C. , Chun Y. , Westphal D. , Holben B. N. , Gueymard C. , McKendry I. , Kuring N. , Feldman G. C. , McClain C. , Frouin R. J. , Merrill J. , DuBois D. , Vignola F. , Murayama T. , Nickovic S. , Wilson W. E. , Sassen K. , Sugimoto N. , Malm W. C.
Published: February 2004

On April 15 and 19, 1998, two intense dust storms were generated over the Gobi desert by springtime low‐pressure systems descending from the northwest. The windblown dust was detected and its evolution followed by its yellow color on SeaWiFS satellite images, routine surface‐based monitoring, and through serendipitous observations. The April 15 dust cloud was recirculating, and it was removed by a precipitating weather system over east Asia. The April 19 dust cloud crossed the Pacific Ocean in 5 days, subsided to the surface along the mountain ranges between British Columbia and California, and impacted severely the optical and the concentration environments of the region. In east Asia the dust clouds increased the albedo over the cloudless ocean and land by up to 10–20%, but it reduced the near‐UV cloud reflectance, causing a yellow coloration of all surfaces. The yellow colored backscattering by the dust eludes a plausible explanation using simple Mie theory with constant refractive index. Over the West Coast the dust layer has increased the spectrally uniform optical depth to about 0.4, reduced the direct solar radiation by 30–40%, doubled the diffuse radiation, and caused a whitish discoloration of the blue sky. On April 29 the average excess surface‐level dust aerosol concentration over the valleys of the West Coast was about 20–50 μg/m3 with local peaks >100 μg/m3. The dust mass mean diameter was 2–3 μm, and the dust chemical fingerprints were evident throughout the West Coast and extended to Minnesota. The April 1998 dust event has impacted the surface aerosol concentration 2–4 times more than any other dust event since 1988. The dust events were observed and interpreted by an ad hoc international web‐based virtual community. It would be useful to set up a community‐supported web‐based infrastructure to monitor the global aerosol pattern for such extreme aerosol events, to alert and to inform the interested communities, and to facilitate collaborative analysis for improved air quality and disaster management.
Present‐day variability of background ozone in the lower troposphere
Authors: Lefohn Allen S. , Oltmans Samuel J. , Dann Tom , Singh Hanwant B.
Published: February 2004

There is a substantial background of ozone (O3) present in the lower troposphere in the Northern Hemisphere that has both a stratospheric and photochemical tropospheric origin. Levels of hourly averaged O3 concentrations in the range 0.04–0.08 ppm are often measured as part of the “background O3” burden. Stratospheric processes play a significant role in defining these background O3 concentrations. In order to better understand the frequency, spatial, and temporal characteristics of this background O3 burden, we have analyzed hourly average O3 concentrations ≥0.05 and ≥0.06 ppm that were experienced during the photochemically quiescent months in the winter and spring at several rural sites across southern Canada, the northern United States, and northern Europe. Our results were mostly consistent and indicated that hourly average O3 concentrations ≥0.05 and ≥0.06 ppm occur frequently during the winter and spring months. Most occurrences were during April and May but sometimes as late as June. In some, but not all, of the cases that were studied, a plausible explanation for the higher O3 values was the presence of upper tropospheric and stratospheric air that was transported down to the surface. For purposes of making the analysis more straightforward, this study was limited to the more northerly latitudes. It might be expected that a significant contribution from natural sources is also important in producing O3 levels in the 0.05–0.06 ppm or higher range at more southerly locations where stratospheric intrusions also have an important impact on the tropospheric O3 distribution.