bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling halogen chemistry in the marine boundary layer 2. Interactions with sulfur and the cloud‐covered MBL
Authors: von Glasow Roland , Sander Rolf , Bott Andreas , Crutzen Paul J.
Published: October 2002

A companion paper presented a numerical one‐dimensional model of the marine boundary layer (MBL) including chemical reactions in the gas and aqueous phase, focusing on the reaction cycles of halogen compounds. In this paper we study interactions between halogen and sulfur chemistry. HOCl and HOBr were found to be generally more important than H2O2 or O3 in the oxidation of S(IV) in sea salt aerosols in the cloud‐free MBL. The inclusion of halogen chemistry lead to an increase in the oxidation of DMS of roughly 63%. This additional oxidation is caused by BrO. The model was also expanded for the study of the cloudy MBL. We found that the effects of stratiform clouds on the evolution and diurnal cycle of halogen species are widespread; they are not restricted to cloud layers. The diurnal variation of gas and aqueous phase bromine was the opposite of that in cloud‐free runs. Oxidation of S(IV) by HOBr and HOCl was important for cloud droplets, too. However, the relative importance of these oxidants changed compared to the cloud‐free runs.
Winter to summer monsoon variation of aerosol optical depth over the tropical Indian Ocean
Authors: Li F. , Ramanathan V.
Published: October 2002

The northern Indian Ocean undergoes a major transition from anthropogenic aerosols during the northeasterly winter monsoon season to mineral dust and sea salt during the southwest summer monsoon. The former is dominated by low‐level transport from south and southeast Asia, while the latter results from low to mid tropospheric transport from the African continent and the Arabian Peninsula. During the winter monsoon, low‐level transport from the Indian subcontinent and neighboring nations is particularly important. This paper uses 5 years of satellite‐derived aerosol optical depths (AODs) to document the large seasonal variations in AODs modulated by the monsoons. In situ data are used to develop and validate the procedure we employ to derive visible AODs from the radiances observed by the advanced very high resolution radiometer (AVHRR) onboard polar‐orbiting satellites. Monthly mean AOD over the Arabian Sea displays a clear annual cycle with a maximum (0.60 ± 0.10) in July and a minimum (0.20 ± 0.05) in January. The summer monsoon maximum is due both to the southwest winds bringing dust from the Horn of Africa over the Arabian Sea and mid tropospheric transport of dust from the Arabian Peninsula. Such long‐range transports also lead to aerosol loading south of the equator. We also show that the Indonesia forest fires during the 1997 El Niño led to a large increase in AOD over most of the equatorial Indian Ocean.
Two years of continuous aerosol measurements in northern Finland
Authors: Ricard V. , Jaffrezo J.‐L. , Kerminen V.‐M. , Hillamo R. E. , Sillanpaa M. , Ruellan S. , Liousse C. , Cachier H.
Published: October 2002

An investigation of aerosol chemistry was carried out at Sevettijarvi in Finnish Lapland between September 1997 and June 1999. Aerosol particles were collected on a 2‐day basis using two‐stage virtual impactors and were analyzed with ion chromatography for major inorganic cations and anions and for a suite of organic acids. Aerosols were also sampled in parallel on a 4‐day basis for the analysis of organic carbon (OC) and black carbon (BC). The average total mass is about 3 μg m−3 and does not significantly vary according to the season or the type of air mass. The major chemical components are sulfate, sea salts, and organic carbon, which account together for more than 80% of the total aerosol mass. BC, ammonium, nitrate, methanesulfonic acid, and the estimated crustal fraction each accounts for a few percent at most in any situation. Non‐sea‐salt (nss) sulfate concentrations are maximum during late winter and spring, related to the Arctic haze, associated with increased concentrations in BC, ammonium, and nss K+. The organic fraction is at its lowest in winter, as are the concentrations of most organic acids. OC and short‐chain organic acid concentrations increase during springtime, which may be due to enhanced photochemistry at polar sunrise. The chemical profile is rather different during summer, with a strong decrease of the anthropogenic fraction and a larger occurrence of episodic marine events. However, the main characteristic is the very large increase in OC concentrations, which is the main component of the aerosol at that time and may be linked with local and regional enhanced biogenic activity. The aerosol at Sevettijarvi presents some specificity compared with other Arctic sites, with a much smaller impact of Arctic haze and marine events in winter and a much larger impact of biogenic sources in summer. The low contribution of the crustal fraction indicates low occurrences of transport of desert dust from Eurasia. The time series of concentrations indicate a large variability in the chemical profiles on short timescales, linked with changes in the origin of the air masses. It shows that even purely marine aerosol still comprises about 10% of nss sulfate associated with BC and OC. The profile in the continental case is largely dominated by nss sulfate, with strong increases in the ammonium and BC fractions.
Nighttime isoprene trends at an urban forested site during the 1999 Southern Oxidant Study
Authors: Stroud C. A. , Roberts J. M. , Williams E. J. , Hereid D. , Angevine W. M. , Fehsenfeld F. C. , Wisthaler A. , Hansel A. , Martinez‐Harder M. , Harder H. , Brune W. H. , Hoenninger G. , Stutz J. , White A. B.
Published: October 2002

Measurements of isoprene and its oxidation products, methacrolein, methyl vinyl ketone and peroxymethacrylic nitric anhydride, were conducted between 13 June and 14 July 1999, at the Cornelia Fort Airpark during the Nashville intensive of the Southern Oxidant Study. Trends in isoprene and its oxidation products showed marked variability from night‐to‐night. The reaction between isoprene and the nitrate radical was shown to be important to the chemical budget of isoprene and often caused rapid decay of isoprene mixing ratios in the evening. Trends in methacrolein, methyl vinyl ketone, and peroxymethacrylic nitric anhydride were steady during the evening isoprene decay period, consistent with their slow reaction rate with the nitrate radical. For cases when isoprene sustained and even increased in mixing ratio throughout the night, the observed isoprene oxidation rates via the hydroxyl radical, ozone, and the nitrate radical were all small. Sustained isoprene mixing ratios within the nocturnal boundary layer give a unique opportunity to capture hydroxyl radical photochemistry at sunrise as isoprene was observed to rapidly convert to its first stage oxidation products before vertical mixing significantly redistributed chemical species. The observed nighttime isoprene variability at urban, forested sites is related to a complex coupling between nighttime boundary layer dynamics and chemistry.
Chemical evolution of the Sacramento urban plume: Transport and oxidation
Authors: Dillon M. B. , Lamanna M. S. , Schade G. W. , Goldstein A. H. , Cohen R. C.
Published: October 2002

Measurements of anthropogenic hydrocarbons, ozone, and meteorological variables were obtained during the summer of 1997 near the University of California Blodgett Forest Research Station on the western slope of the Sierra Nevada Mountains. These measurements and related observations obtained upwind in Sacramento and Folsom, California, by the California Air Resources Board and the National Weather Service demonstrate that the transport of the Sacramento plume is controlled by extremely consistent, terrain‐driven winds that draw polluted air into the Sierra Nevada by day and flush the mountains at night with clean, continental background air. In effect the plume serves as a mesoscale (100 km) daytime flow reactor where the daily evolution of the Sacramento urban plume can be characterized as a Lagrangian air parcel transported from the urban core into the sparsely populated Sierra Nevada mountains. Using observations of slowly reacting anthropogenic hydrocarbons, we demonstrate that at the peak impact of the Sacramento plume the air at Blodgett Forest can be characterized as a mixture of 32% air from the urban core and 68% from the relatively clean background. From measurements of more reactive hydrocarbons we infer an average daytime OH concentration of 1.1 × 107 molecules cm−3 during the transit of the urban plume.
Marine biogenic and anthropogenic contributions to non‐sea‐salt sulfate in the marine boundary layer over the North Atlantic Ocean
Authors: Savoie Dennis L. , Arimoto Richard , Keene William C. , Prospero Joseph M. , Duce Robert A. , Galloway James N.
Published: October 2002

As a part of the Atmosphere/Ocean Chemistry Experiment (AEROCE), daily aerosol samples were collected in the marine boundary layer at Barbados, West Indies (13.17°N, 59.43°W), Bermuda (32.27°N, 64.87°W), and Mace Head, Ireland (53.32°N, 9.85°W), and in the free troposphere at Izaña, Tenerife, Canary Islands (28.30°N, 16.48°W; 2360 m asl). In this report, we use multiple variable regression analyses with methanesulfonate (MSA) and Sb and/or NO3− as the independent variables to assess the relative contributions of the marine biogenic and anthropogenic sources to the total non‐sea‐salt (nss) SO42− concentrations at the AEROCE sites. On the basis of 2 years of data at Bermuda and Barbados, the marine nss SO42−/MSA mass ratios (19.6 ± 2.1 and 18.8 ± 2.2) were consistent throughout the year and comparable to those at American Samoa in the tropical South Pacific (18.1 ± 0.9). At Mace Head (based on 1 year of data), this ratio was about 3.01 (±0.53). An analysis of the residuals and an assessment of the root mean square deviations indicate that the ratio at Mace Head can also be reasonably applied throughout the year. However, there is enough uncertainty during the winter that we cannot rule out a significant increase (to about 20) during periods with low concentrations of both MSA and NO3−. The results from 4 years indicate that the marine contribution is too low to permit a reasonable assessment of the biogenic nss SO42−/MSA ratio at Izaña. The continental nss SO42−/Sb mass ratio varies significantly from one location to another. At Bermuda, where North American sources are expected to dominate, the ratio is about 29,000, about a factor of 2 higher than the average of 13,500 at Mace Head where European sources dominate. Intermediate values occurred at Barbados (18,000) and Izaña (24,000) where both European and North African sources are significant. Estimates based on these ratios indicate that, on an annual basis, the contributions from anthropogenic sources account for about 50% of the total nss SO42− in aerosols at Barbados, 70% at Bermuda, 85–90% at Mace Head, and about 90% at Izaña. If the same biogenic nss SO42−/MSA ratios are applicable to rainwater, then the relative contributions in precipitation at Barbados and Bermuda are comparable to those in aerosols.
On some characteristic variations in warm season precipitation over the central United States (1910–2000)
Authors: Englehart Phil J. , Douglas Arthur V.
Published: October 2002

On the basis of an observational data set that spans most of the twentieth century, we show that warm season precipitation over the central United States aligns into six large‐scale regions. Using two different measures of seasonal precipitation, frequency and intensity, we explore the basic question: To what extent are the observed interannual variations in regional precipitation associated with fluctuations in both large‐scale climate modes, the El Niño/Southern Oscillation (ENSO) phenomena and the Pacific Decadal Oscillation (PDO), and more local‐scale antecedent moisture conditions? The principal study findings are that the teleconnection between ENSO and regional rainfall is best developed during the early part of the warm season in association with El Niño events, but that modest spatial shifts in the teleconnection occur depending upon phase of the PDO. The investigation reveals little evidence for a teleconnection between ENSO and regional rainfall during La Niña events. Moreover, late in the season, ENSO teleconnectivity is confined largely to the northern part of the study area. While the influence of large‐scale climate modes apparently diminishes late in the warm season, this study provides additional empirical evidence to connect early season moisture demand with subsequent rainfall regimes.
A study of internal and external mixing scenarios and its effect on aerosol optical properties and direct radiative forcing
Authors: Lesins Glen , Chylek Petr , Lohmann Ulrike
Published: October 2002

The optical properties and hence the radiative forcing of atmospheric aerosols are determined, in part, by the way in which the various constituents are externally or internally mixed. The mixing state must be known to compute the effective refractive index, water activity, and size distribution of the aerosols. In this study we found that the percentage difference in the optical properties, including extinction, single scattering albedo, and asymmetry parameter, between an internal mixture and external mixture of black carbon and ammonium sulfate can be over 25% for the dry case and over 50% for the wet case for typical mass mixing ratios. The differences are a result of a complicated combination of nonlinear Mie theory on the refractive index, assumptions about the coagulated particle sizes for internal mixtures, and the role of water uptake and deliquescence as a function of relative humidity. The computed optical properties are used to estimate the globally average clear‐sky direct radiative forcing for different mixing assumptions. The results are displayed as a function of relative humidity to conveniently see the mixing effects for dry aerosols at less than the crystallization point, for dry internal and wet external mixtures between the crystallization and deliquescence points, and for fully wet mixtures above the deliquescence point. For a 9:1 ammonium sulfate to black carbon mass ratio, nearly all the cooling effect predicted for an external mixture is lost for the internally mixed assumption, especially for relative humidities less than the deliquescence point.
Impact of surface inhomogeneity on solar radiative transfer under overcast conditions
Authors: Li Zhanqing , Cribb Maureen C. , Trishchenko Alexander P.
Published: October 2002

The goal of this study was to assess the ability of the Moderate‐Resolution Transmittance 4 (MODTRAN‐4) code to simulate high‐resolution shortwave (SW) fluxes given detailed and complete input information under overcast conditions. The study underlines the impact of surface inhomogeneity on the closure of SW radiative transfer. It also leads to a method of estimating surface spectral areal‐mean albedo from downwelling solar transmittance measurements. The investigation made use of ample Atmospheric Radiation Measurement (ARM) field data collected by a suite of instruments, including broadband and narrowband radiometers and spectrometers, cloud radar and lidar, microwave radiometer, atmospheric sounding instruments, and satellite data. Furnishing the MODTRAN‐4 code with observed atmospheric, cloud, and surface parameters generates spectral solar transmittance at the surface and reflectance at the top of the atmosphere (TOA). The transmittances were compared with the Rotating Shadowband Spectroradiometer measurements and showed significant discrepancies in the near‐infrared (NIR) region, the bulk of which was attributed to the use of unrepresentative surface spectral albedos. A field campaign was undertaken to collect surface albedo data for a wide variety of land cover types near the ARM Central Facility. The sampled data were combined with thematic mapper/Landsat‐based land cover classification data to map surface spectral albedo. Substitution of the derived areal‐mean spectral albedo into the MODTRAN‐4 model eliminates major discrepancies in the NIR, and also leads to good agreements with surface solar broadband fluxes and TOA satellite spectral reflectance. On the basis of these findings, one may use downwelling spectral transmittance data, together with detailed cloud and atmospheric information, to estimate surface effective areal‐mean albedo. The estimated values agree well with those derived from the ground survey data. Following the method, a data set of effective areal‐mean spectral albedo throughout a year was obtained.
Effect of regional‐scale transport on oxidants in the vicinity of Philadelphia during the 1999 NE‐OPS field campaign
Authors: Fast Jerome D. , Zaveri Rahul A. , Bian Xindi , Chapman Elaine G. , Easter Richard C.
Published: October 2002

A new meteorological‐chemical model is used to determine the relative contribution of regional‐scale transport and local photochemical production on air quality over Philadelphia. The model performance is evaluated using surface and airborne meteorological and chemical measurements made during a 30‐day period in July and August of 1999 as part of the Northeast Oxidant and Particulate Study (NE‐OPS). Good agreement between the simulations and observations was obtained. The bias in the vicinity of Philadelphia over the simulation period was −5.8 ppb for the peak ozone mixing ratio during the day and 2.0 ppb for the minimum ozone mixing ratio at night. Layers of ozone above the convective boundary layer were measured by both research aircraft and ozonesondes during the morning between 0900 and 1100 LT. The model demonstrates that upwind vertical mixing processes the previous afternoon, subsequent horizontal transport aloft, and depletion of ozone by NO titration within the stable boundary layer at night lead to the development of these layers. Ozone aloft was then entrained into the growing convective boundary, contributing to surface ozone concentrations. Through a series of sensitivity studies, we find that most of the ozone is the result of emissions in the vicinity of Philadelphia and Chesapeake Bay area, but up to 30–40% of the ozone during high ozone episodes was due to transport from upwind sources. Local emissions and meteorological conditions were largely responsible for one high ozone episode because of light winds.
Background ozone over the United States in summer: Origin, trend, and contribution to pollution episodes
Authors: Fiore Arlene M. , Jacob Daniel J. , Bey Isabelle , Yantosca Robert M. , Field Brendan D. , Fusco Andrew C. , Wilkinson James G.
Published: October 2002

Observations indicate that ozone (O3) concentrations in surface air over the United States in summer contain a 20–45 ppbv background contribution, presumably reflecting transport from outside the North American boundary layer. We use a three‐dimensional global model of tropospheric chemistry driven by assimilated meteorological observations to investigate the origin of this background and to quantify its contribution to total surface O3on both average and highly polluted summer days. The model simulation is evaluated with a suite of surface and aircraft observations over the United States from the summer of 1995. The model reproduces the principal features in the observed distributions of O3and its precursors, including frequency distributions of O3concentrations and the development of regional high‐O3episodes in the eastern United States. Comparison of simulations with 1995 versus 1980 global fossil fuel emissions indicates that the model captures the previously observed decrease in the high end of the O3probability distribution in surface air over the United States (reflecting reduction of domestic hydrocarbon emissions) and the increase in the low end (reflecting, at least in the model, rising Asian emissions). In the model, background O3produced outside of the North American boundary layer contributes an average 25–35 ppbv to afternoon O3concentrations in surface air in the western United States. and 15–30 ppbv in the eastern United States during the summer of 1995. This background generally decays to below 15 ppbv during the stagnation conditions conducive to exceedances of the 8‐hour 0.08 ppmv (80 ppbv) National Ambient Air Quality Standard (NAAQS) for O3. A high background contribution of 25–40 ppbv is found during 9% of these exceedances, reflecting convective mixing of free tropospheric O3from aloft, followed by rapid production within the U.S. boundary layer. Anthropogenic emissions in Asia and Europe are found to increase afternoon O3concentrations in surface air over the United States by typically 4–7 ppbv, under both average and highly polluted conditions. This enhancement is particularly large (up to 14 ppbv) for O3concentrations in the 50–70 ppbv range, and would represent a major concern if the NAAQS were to be tightened.
Climatology of ozone transport from the free troposphere into the boundary layer south of the Alps during North Foehn
Authors: Weber Rudolf O. , Prévôt André S. H.
Published: October 2002

In the Swiss Alpine foothills south of the Alps, on the northern edge of the Po Basin of Italy, 10 years of ozone data were analyzed to deduce the climatological influence of North Foehn on the boundary layer concentrations of ozone. It was found that northerly winds are connected with low relative humidity and small differences in potential temperature between the Jungfraujoch (3600 m above sea level (asl)) and Mendrisio (350 m asl) and therefore are primarily caused by North Foehn. This confirms that North Foehn transports air masses from high altitudes above the Alpine crest down into the lower boundary layer. On days with North Foehn the yearly cycles of ozone are very similar at Jungfraujoch and Mendrisio, around 40 ppb in winter and 60 ppb in summer. Compared to the whole data set in Mendrisio, these values are, at comparable temperatures, high in winter and very low in summer, when ozone concentrations often exceed 120 ppb during daytime. In contrast, during nighttime the North Foehn causes an enhancement of the ozone concentration near the ground during the whole year. The frequencies of the North Foehn events are 4–6% from May to October and 6–10% from November to April. Thus North Foehn is an important process for the ozone concentrations in the Po Basin.
Vertical variations in O3 concentrations before and after a gust front passage
Authors: Darby Lisa S. , Banta Robert M. , Brewer W. Alan , Neff William D. , Marchbanks Richard D. , McCarty Brandi J. , Senff Christoph J. , White Allen B. , Angevine Wayne M. , Williams Eric J.
Published: October 2002

Two consecutive increases in surface ozone, occurring after the cessation of photochemical ozone production, are investigated. A unique suite of instruments, including an ozone profiling lidar, a Doppler lidar, surface chemistry sensors, surface meteorological sensors, and a radar wind profiler, was deployed during the Southern Oxidants Study of 1999 in Nashville, Tennesee. Time series of ozone at 10, 405, and 1035 m above ground level (AGL) on 22 June 1999 showed variations in the vertical gradient of ozone over the course of the afternoon and evening. Analysis of time series of vertical velocity at 8 m AGL and its standard deviation, the variance in Doppler lidar calculated horizontal wind speed from near the surface to 500 m AGL, wind profiles, and meteorological surface station data, revealed mechanisms responsible for these changes in ozone. An early evening rise in surface ozone occurred with the passage of a thunderstorm gust front. Analysis showed that downward mixing of ozone‐rich air from a residual layer (RL) of ozone above 400 m caused a sharp rise in ozone at the surface and a decrease of ozone in the RL. Doppler lidar measurements showed details of the postgust front wind flow, such as the depth of the air mass behind the front, the turbulent wake region, and inferred vertical velocities throughout a layer several hundred meters deep. The second rise in surface ozone was caused by turbulent mixing due to elevated directional wind shear, which mixed ozone‐rich air down to the surface.
Moisture budget of the bimodal pattern of the summer circulation over South America
Authors: Herdies Dirceu L. , da Silva Arlindo , Silva Dias Maria A. F. , Nieto Ferreira Rosana
Published: October 2002

Submonthly variations in warm‐season (January–February) precipitation over South America, in particular over the Amazon basin, central southwest Brazil, north Argentina, and Paraguay are studied. Two distinct regimes of lower tropospheric winds (westerlies and easterlies) were observed in Rondonia during the Wet Season Atmospheric Mesoscale Campaign (WETAMC) component of the Large‐Scale Atmosphere‐Biosphere Experiment in Amazonia (LBA) and the Tropical Rainfall Measuring Mission (TRMM) field campaign. The westerly (easterly) winds were associated with strong (weak) convective activity over the South Atlantic Convergence Zone (SACZ). The period of this study (January and February of 1999) was divided into SACZ and no SACZ (NSACZ) regimes. The vertically integrated moisture fluxes over South America obtained from the National Aeronautics and Space Administration/Goddard Data Assimilation Office (NASA/DAO) assimilation system show that during the SACZ (NSACZ) period, strong (weak) convergence occurred over the Amazon basin with divergence (convergence) over southwestern Brazil, northern Argentina, and Paraguay. These moisture budgets also indicated that moisture transport from the tropics to the extratropics in the South American sector occurs more efficiently during the SACZ regime than during the NSACZ regime.
Ozone loss from quasi‐conservative coordinate mapping during the 1999–2000 SOLVE/THESEO 2000 campaigns
Authors: Lait L. R. , Schoeberl M. R. , Newman P. A. , McGee T. , Burris J. , Browell E. V. , Richard E. , Braathen G. O. , Bojkov B. R. , Goutail F. , von der Gathen P. , Kyrö E. , Vaughan G. , Kelder H. , Kirkwood S. , Woods P. , Dorokhov V. , Zaitcev I. , Litynska Z. , Kois B. , Benesova A. , Skrivankova P. , De Backer H. , Davies J. , Jorgensen T. , Mikkelsen I. S.
Published: October 2002

Ozone observations made by the Airborne Raman Ozone, Temperature, and Aerosol Lidar (AROTEL) and Differential Absorption Lidar (DIAL) on board the NASA DC‐8 aircraft, the NOAA in situ instrument on board the NASA ER‐2 aircraft, and Third European Stratospheric Experiment on Ozone 2000 (THESEO 2000) ozonesondes are analyzed by applying a quasi‐conservative coordinate mapping technique. Measurements from the late winter/early spring SAGE III Ozone Loss and Validation Experiment (SOLVE) period (January through March 2000) are incorporated into a time‐varying composite field in a potential vorticity‐potential temperature coordinate space; ozone loss rates are calculated both with and without diabatic effects. The average loss rate from mid‐January to mid‐March near the 450 K isentropic surface in the polar vortex is found to be approximately 0.03 ppmv/d.
A simple model of multiple climate regimes
Authors: Emanuel Kerry
Published: October 2002

Among the most intriguing enigmas of the climate system is that on the one hand, the Earth's climate appears to be exquisitely sensitive to relatively minor variations in the distribution of insolation owing to orbital variations, but on the other hand, it is in a grosser sense stable, in that it has varied only moderately in response to a roughly 30% increase in solar insolation over the life of the planet. To this enigma may be added the evidence that climate may undergo extraordinarily abrupt transitions. An attractive idea to help explain these characteristics is the notion that the Earth possesses a limited number of stable climate regimes that may overlap to produce multiple equilibrium states for the same solar forcing. Here we present a simple model that produces such overlapping stable equilibria, based on a few key feedback processes. These include control of atmospheric clouds and water vapor by the large‐scale circulation of the atmosphere, control of the depth and intensity of the ocean's thermohaline circulation by tropical cyclones, and the dependence of atmospheric CO2 content on ocean temperature and the strength of the thermohaline circulation. We will show that these key feedback processes produce a climate with two or three stable, overlapping climate regimes. Subjecting this system to variations in climate forcing can account for several observed features of the climate system, including abrupt transitions, sensitivity to orbital variations, arctic warmth and high bottom water temperature during the Eocene and late Cretaceous, tempestites, and possible episodes of deep ocean anoxia during the Cretaceous.
Analysis and presentation of in situ atmospheric methane measurements from Cape Ochi‐ishi and Hateruma Island
Authors: Tohjima Y. , Machida T. , Utiyama M. , Katsumoto M. , Fujinuma Y. , Maksyutov S.
Published: October 2002

In situ measurements of atmospheric methane (CH4) from the monitoring stations at Cape Ochi‐ishi (latitude 43°10′N, longitude 145°30′E) during the period from July 1995 to July 2000 and at Hateruma Island (latitude 24°03′N, longitude 123°48′E) during the period from January 1996 to January 2000 are presented. At each station a fully automated gas chromatograph equipped with a flame ionization detector measures the CH4 mixing ratios at a frequency of more than 82 air samples per day. Over the above observation periods, average growth rates of CH4 mixing ratios were 4.5 parts per billion (ppb) yr−1 for Ochi‐ishi and 4.7 ppb yr−1 for Hateruma, but there were considerable fluctuations in the instantaneous growth rate. The peak‐to‐peak amplitudes in the seasonal cycles determined from the smooth curve fits were 71 ppb for Ochi‐ishi and 94 ppb for Hateruma. These seasonal amplitudes are larger than those at the sampling network sites of the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory. Analysis of the back trajectories indicated that the air masses at both sites are transported from continental Asia in winter and from the Pacific Ocean in summer. Because there are strong CH4 sources in continental Asia but no strong sources in the Pacific Ocean, the seasonality of airflow patterns can explain the enhancement of the seasonal cycles. The significantly large seasonality, especially at Hateruma, is a result of air masses originating from a wide range of latitudes, the clear seasonality of airflows, and the steep latitudinal gradient of the CH4 mixing ratio.
A Bayesian algorithm for the retrieval of liquid water cloud properties from microwave radiometer and millimeter radar data
Authors: McFarlane Sally A. , Evans K. Franklin , Ackerman Andrew S.
Published: October 2002

We present a new algorithm for retrieving optical depth and liquid water content and effective radius profiles of nonprecipitating liquid water clouds using millimeter wavelength radar reflectivity and dual‐channel microwave brightness temperatures. The algorithm is based on Bayes' theorem of conditional probability and combines prior information on cloud microphysics with the remote sensing observations. Prior probability distribution functions for liquid water clouds were derived from the second, third, and sixth moments of droplet size distributions measured by in situ aircraft probes in shallow tropical cumuli. The algorithm also calculates error bars for each retrieved parameter. To assess the algorithm, we perform retrieval simulations using radar reflectivity and brightness temperatures simulated from tropical cumulus fields calculated by a large eddy simulation model with explicit microphysics. These retrieval simulations show that the Bayesian algorithm has similar magnitude errors to current retrieval methods for liquid water content and liquid water path retrievals but has much smaller errors for effective radius and optical depth. We also perform retrievals on three months of data from the Atmospheric Radiation Measurement (ARM) Program's site on Nauru in the tropical west Pacific. For nonprecipitating liquid water clouds over Nauru during June–August 1999 we retrieve a mean optical depth of 9.2, mean liquid water content of 0.112 g/m3, and mean effective radius of 7.8 μm. The Bayesian method is a flexible approach to cloud profile remote sensing and could be expanded to other sites or cloud types.
Denitrification inside the stratospheric vortex in the winter of 1999–2000 by sedimentation of large nitric acid trihydrate particles
Authors: Flentje Harald , Dörnbrack Andreas , Fix Andreas , Meister Alexander , Schmid Heidi , Füglistaler Stefan , Luo Beiping , Peter Thomas
Published: October 2002

Synoptic‐scale polar stratospheric clouds (PSCs) have been measured with an aircraft‐borne lidar south of Spitsbergen in early February 2000. The PSCs of moderately depolarizing stratified layers extended from near the tropopause (10–11 km) up to 20 km altitude. Their internal structure in backscatter ratio reflects the variability of the particles' concentration rather than manifold sizes and phases of different optical efficiency. Actually, their size distribution was nearly monodisperse. According to T‐matrix calculations, low backscatter ratios (γ < 3), color ratios of 1–1.5, and moderate depolarization ratios (δa ≈ 0.15–0.2) indicate that the observed layers were composed of aspherical nitric acid trihydrate (NAT) particles larger than 2 μm. During three turning point missions into the cold vortex core a significant settling of the particle layers was observed between aligned outgoing and incoming legs. Low stratospheric winds and quasi‐Lagrangian flight heading minimized horizontal drifts between the probed volumes. A NAT particle sedimentation velocity of 69 ± 14 m/h is derived by cross‐covariance analysis of corresponding pairs of two‐dimensional backscatter sections of the layers. This implies a particle size of rp ≈ 7–8 μm and a mixing ratio of condensed HNO3 near 2 ppbv. The downward NOy flux below 19 km altitude amounts to j ≈ 3 ppbv km/d. Based on particle back trajectory analysis, this flux, derived from the observations in different regions of the Northern European Ice Sea, has a significant denitrification potential.
Measurement of the specific surface area of 176 snow samples using methane adsorption at 77 K
Authors: Legagneux Loïc , Cabanes Axel , Dominé Florent
Published: October 2002

To help quantify exchanges between the atmosphere and the snow cover, we have measured the specific surface area (SSA) of 176 snow samples taken from the seasonal snowpack in the Alps, Svalbard, and the Canadian high Arctic around Alert. A volumetric method was used, and the adsorption isotherm of CH4 on snow at 77 K was recorded. The data were analyzed by the Brunauer‐Emmett‐Teller method to yield SSA and ΔQCH4, the mean heat of adsorption of the first CH4 monolayer. SSA values obtained were between 100 and 1580 cm2/g. The reproducibility of the method is estimated at 6%, and the accuracy is estimated at 12%. We propose that ΔQCH4 = 2240 ± 200 J/mol should be used as a criterion of reliability of the measurement. The method is described in detail to promote its use. Aged snow samples have lower SSA than fresh ones. The lowest values were found for faceted crystals and depth hoar, and the highest values were found for fresh rimed dendritic snow. A method that field investigators can use to estimate SSA from a visual examination of the snow and from a density measurement is suggested. Snow samples are classified into 14 types based on snow age and crystal shapes. Within each type, a density versus SSA correlation is determined. Our data indicate that, depending on snow type, SSA can then be estimated within 25 to 40% at the 1σ confidence level with the method proposed. Preliminary data suggest that SSA spatial variability of a given snow layer is low (<5%), but metamorphism can increase it.
Using gas‐phase nitric acid as an indicator of PSC composition
Authors: von König Miriam , Bremer Holger , Kleinböhl Armin , Küllmann Harry , Künzi Klaus F. , Goede Albert P. H. , Browell Edward V. , Grant William B. , Burris John F. , McGee Thomas J. , Twigg Laurence
Published: October 2002

The composition of polar stratospheric cloud particles is investigated using data from several remote sensing instruments: gas‐phase HNO3 measured by the Airborne Submillimeter Radiometer (ASUR), temperature measured by the Airborne Raman Ozone, Temperature and Aerosol Lidar (AROTEL), and aerosol backscatter wavelength dependence and depolarization measured by the UV Differential Absorption Lidar (DIAL). All three instruments have been operated onboard the NASA DC‐8 during the SOLVE winter 1999/2000. A simple thermodynamical model is used to calculate the gas‐phase amount of HNO3 in the presence of polar stratospheric clouds (PSCs) along the flight track of the DC‐8 for one flight into the polar vortex on 7 December 1999. Three types of PSCs are considered in the model: nitric acid trihydrate (NAT), nitric acid dihydrate (NAD), and supercooled ternary solutions (STS). The comparison of the modeled and measured gas‐phase HNO3 in the presence of PSCs shows a very good agreement if a NAT composition of the particles is assumed. However, sensitivity studies show that while the PSCs observed on this flight are not in agreement with a STS composition, the model is very sensitive to temperature, and a NAD composition of the PSC cannot be ruled out.
Thresholds in atmosphere–soil moisture interactions: Results from climate model studies
Authors: Oglesby Robert J. , Marshall Susan , Erickson David J. , Roads John O. , Robertson Franklin R.
Published: October 2002

The potential predictability of the effects of warm season soil moisture anomalies over the central United States has been investigated in a series of general circulation model (GCM) experiments with the National Center for Atmospheric Research (NCAR) Community Climate Model version 3 Land Surface Model (CCM3/LSM). Three different types of experiments have been developed, all starting in either March (representing precursor conditions) or June (conditions at the onset of the warm season): (1) “anomaly” runs with large, exaggerated initial soil moisture reductions, aimed at evaluating the physical mechanisms by which soil moisture can affect the atmosphere; (2) “predictability” runs aimed at evaluating whether typical soil moisture initial anomalies (indicative of year‐to‐year variability) can have a significant effect, and if so, for how long; and (3) “threshold” runs aimed at evaluating if a soil moisture anomaly must be of a specific size (i.e., a threshold crossed) before a significant impact on the atmosphere is seen. The anomaly runs show a large, long‐lasting response in soil moisture as well as surface temperature, sea level pressure, and precipitation; effects persist for at least a year. The predictability runs, on the other hand, show very little impact of the initial soil moisture anomalies on the subsequent evolution of soil moisture and other atmospheric parameters; internal variability is most important, with the initial state of the atmosphere (representing remote effects such as SST anomalies) playing a more secondary role on seasonal and shorter timescales. The threshold runs, devised to help resolve the dichotomy in anomaly and predictability results, suggest that, at least in CCM3/LSM, the vertical profile of soil moisture is the most important factor and that deep soil zone anomalies exert a more powerful, long‐lasting effect than do anomalies in the near‐surface soil zone. We therefore suggest that soil moisture feedbacks may be more important in explaining interannual to decadal droughts evident in the historic and recent prehistoric records, but less important on monthly to seasonal timescales.
Condensation and coagulation sinks and formation of nucleation mode particles in coastal and boreal forest boundary layers
Authors: Dal Maso M. , Kulmala M. , Lehtinen K. E. J. , Mäkelä J. M. , Aalto P. , O'Dowd C. D.
Published: October 2002

The formation and growth of new particles has been evaluated using a revised version of a simple, but novel, theoretical tool. The concentration of condensable vapors and their source rates has been estimated using the aerosol condensation sink together with the measured particle growth rate. Also, by adding the coagulation sink and the measured formation rate of 3 nm particles, the formation rate of 1 nm particles and their concentration can be estimated. Condensation and coagulation sinks can be obtained from ambient aerosol size distribution data. The method has been applied to analyze the particle formation and growth rates observed during coastal and boreal forest nucleation events. The condensation sinks are typically 4–7 × 10−3 s−1 in the forest and 2 × 10−3 s−1 under coastal conditions, while the coagulation sinks for 1, 2, and 3 nm particles are typically smaller by factors 1.5–2, 5–7, and 11–15, respectively. The measured growth rates are 2–10 nm/h for the boreal forest and range from 15 to 180 nm/h at the coast, corresponding to a vapor concentration of 2–13 × 107 cm−3 and 108 cm−3 to 109 cm−3, respectively. The vapor source rate was 1–2 × 105 cm−3s−1 in the boreal forest and 2–5 × 106 cm−3s−1 in the coastal environment. The estimated formation rate of 1 nm particles in the forest environment was 8–20 cm−3s−1 and 300–10,000 cm−3s−1 at the coast. The concentration of 1 nm particles was estimated to be 2000–5000 and 4 × 104–7 × 106 particles cm−3 in forest and at coast, respectively.
Size effect of hematite and corundum inclusions on the efflorescence relative humidities of aqueous ammonium nitrate particles
Authors: Han Jeong‐Ho , Hung Hui‐Ming , Martin Scot T.
Published: October 2002

The influence of relative humidity (RH) and mineral dust inclusions on the physical state of NH4NO3/H2O particles is investigated through infrared aerosol flow tube experiments. Neat submicron aqueous particles do not effloresce, while those bearing mineral inclusions ranging from 150 to 400 nm in diameter effloresce between 3 and 10% RH at 298 K. NH4NO3/H2O coatings are achieved on hematite (α‐Fe2O3), corundum (α‐Al2O3), mullite (Al6Si2O13), and amorphous silica (am‐SiO2) through a HNO3 vaporization‐condensation technique followed by neutralization with NH3. The extent of internal mixing in the aerosol ranges from 7 to 27% depending on the mineral dust diameter. The size‐dependent efflorescence results are successfully rationalized within the framework of an active‐site model, and the physical parameters obtained from the model optimization are consistent with previous work on the efflorescence of (NH4)2SO4/H2O by these mineral dusts. Evidence is presented for a mechanism of heterogeneous nucleation that proceeds by chemisorption of nitrate at the surface of the oxide minerals followed by epitaxial germ formation of crystalline NH4NO3, all of which is driven by the saturation of the aqueous solution with respect to the solid.
Kelvin waves in stratospheric temperature observed by the Microwave Limb Sounder
Authors: Mote Philip W. , Dunkerton Timothy J. , Wu Dong
Published: October 2002

The three‐dimensional (3‐D) structure of stratospheric Kelvin waves is revealed using data from the Microwave Limb Sounder for the period July 1992 to April 1993. Four Kelvin wave modes are identified, two for zonal wave number 1 (k = 1) and two for k = 2, using extended empirical orthogonal functions. The 3‐D structures of the modes agree with linear theory and satisfy the dispersion relation for Kelvin waves, and with periods ranging from 4.5 to 10 days the modes mostly fall near the boundary between “fast” and “slow” Kelvin waves [Canziani et al., 1998]. The analysis also identifies a feature that may correspond with the “pancake structures” associated with inertial instability.
Chemical ionization mass spectrometry technique for detection of dimethylsulfoxide and ammonia
Authors: Nowak J. B. , Huey L. G. , Eisele F. L. , Tanner D. J. , Mauldin R. L. , Cantrell C. , Kosciuch E. , Davis D. D.
Published: October 2002

A chemical ionization mass spectrometer (CIMS) was used to study reactions of protonated ethanol clusters (C2H5OH)nH+ with dimethylsulfoxide (DMSO), dimethylsulfone (DMSO2), ammonia (NH3), and a series of nonmethane hydrocarbons (NMHCs) and volatile organic compounds (VOCs). The reactivity of the (C2H5OH)nH+ cluster ions is a function of cluster size with reactivity decreasing as cluster size increases. Ethanol cluster ion distributions that formed at atmospheric pressure from 24 ppbv, 900 ppmv, and 1% ethanol/N2 gas mixtures were studied. Small (C2H5OH)nH+ clusters, those formed using the 24 ppbv ethanol/N2 mixture, react at or near the collisional rate with DMSO, NH3, acetone, and methyl vinyl ketone (MVK). The effective ion molecule rate coefficients are 1.8 × 10−9, 1.5 × 10−9, 1.0 × 10−9, and 1.6 × 10−9 cm3 molecule−1 s−1, respectively. Only DMSO and NH3 react efficiently with the two larger (C2H5OH)nH+ cluster ion distributions studied. The effective rate coefficients for DMSO and NH3 with the 900 ppmv ethanol cluster ion distribution are 1.5 × 10−9 and 0.7 × 10−9 cm3 molecule−1 s−1, respectively. The effective rate coefficient for DMSO with the 1% ethanol/N2 mixture is 0.35 × 10−9 cm3 molecule−1 s−1, while NH3 reaches equilibrium with this cluster ion distribution. Experiments show that large (C2H5OH)nH+ ion clusters must be used at relative humidities greater than 50% at 20°C to prevent formation of and subsequent interferences from H3O+ ions. These results demonstrate that the (C2H5OH)nH+ ion chemistry can selectively detect DMSO and NH3 under most ambient atmospheric conditions with high sensitivity.
A mechanism of the onset of the Australian summer monsoon
Authors: Kawamura Ryuichi , Fukuta Yuichiro , Ueda Hiroaki , Matsuura Tomonori , Iizuka Satoshi
Published: October 2002

An onset mechanism of the Australian summer monsoon that incorporates possible air‐sea feedback processes is investigated using the National Centers for Environmental Prediction /National Center for Atmospheric Research daily reanalysis data aided by an ocean general circulation model. Rapid intensification of land‐ocean thermal contrast during the premonsoon period results in a well‐organized continental‐scale shallow vertical circulation over the Australian continent. The shallow vertical circulation is dynamically coupled both with a thermally induced low at the lower level below 850 hPa and a thermal high at 600–700 hPa level. Intensified low‐level westerly anomalies and increased solar radiation in less cloudy air induced by the subsidence in the periphery of the Australian thermal low results in increasing sea surface temperature (SST) along the northern coast of Australia. The thermal high concurrent with the shallow vertical circulation leads to dry intrusion into the layer at ∼700 hPa over the Arafura Sea and Coral Sea through the horizontal and vertical advective processes. A combination of the SST increase and the dry intrusion creates a more convectively unstable condition. When convective instability is intensified while subsidence suppresses convection, the arrival of large‐scale disturbances with ascending motion (such as the Madden‐Julian oscillation) at the domain where the instability is enhanced triggers deep cumulus convection, implying the onset of the monsoon. The onset mechanism proposed in this study may apply not only to the Australian monsoon but also to other monsoon systems that have continental masses in the subtropics.