A numerical model of the partitioning of trace chemical solutes during drop freezing     

A. L. Stuart  M. Z. Jacobson 《Journal of Atmospheric Chemistry》2006,53(1):13-42

Partitioning of volatile chemicals among the gas, liquid, and solid phases during freezing of liquid water in clouds can impact trace chemical distributions in the troposphere and in precipitation. We describe here a numerical model of this partitioning during the freezing of a supercooled liquid drop. Our model includes the time-dependent calculation of the coupled processes of crystallization kinetics, heat transport, and solute mass transport, for a freezing hydrometeor particle. We demonstrate the model for tracer partitioning during the freezing of a 1000 μm radius drop on a 100 μm ice substrate, under a few ambient condition scenarios. The model effectively simulates particle freezing and solute transport, yielding results that are qualitatively and quantitatively consistent with previous experimental and theoretical work. Results suggest that the ice shell formation time is governed by heat loss to air and not by dendrite propagation, and that the location of ice nucleation is not important to freezing times or the effective partitioning of chemical solutes. Even for the case of nucleation at the center of the drop, we found that dendrites propagated rapidly to form surface ice. Freezing then proceeded from the outside in. Results also indicate that the solid-liquid interfacial surface area is not important to freezing times or the effective partitioning of chemical solutes, and that the rate aspects of trapping are more important than equilibrium solid-liquid partitioning to the effective partitioning resulting from freezing.  相似文献   

NUMERICAL MODELING OF CUMULUS CLOUD CHEMISTRY—PART Ⅰ.MODEL DEVELOPMENT     

赵春生  秦瑜 《Acta Meteorologica Sinica》1996,(3)

A one-dimensional cumulus cloud chemistry model(1CCCM)is developed to simulate cloudphysical processes and chemical processes during the evolution of a convective cloud.The cloudphysical submodel includes a detailed microphysical parameterized scheme of 20 processes.Thechemistry submodel is composed of three parts:gas phase chemistry,aqueous phase chemistry andscavenging of soluble gases.The gas phase reaction mechanism contains 85 reactions among 45species including 13 organics.The aqueous phase reaction mechanism contains 54 reactions among40 species and 12 ion equilibria.Mass of 19 gases is transported between the gas phase and theaqueous phase.With this model,studies may be made to analyze the interactions among processesduring lifetime of a cumulus cloud.  相似文献   

On the scavenging of SO2 by cloud and raindrops: II. An experimental study of SO2 absorption and desorption for water drops in air     

C. J. Walcek  H. R. Pruppacher  J. H. Topalian  S. K. Mitra 《Journal of Atmospheric Chemistry》1983,1(3):291-306

For the purpose of testing our previously described theory of SO₂ scavenging a laboratory investigation was carried out in the UCLA 33 m long rainshaft. Drops with radii between 250 and 2500 m were allowed to come to terminal velocity, after which they passed through a chamber of variable length filled with various SO₂ concentrations in air. After falling through a gas separating chamber consisting of a fluorocarbon gas the drops were collected and analyzed for their total S content in order to determine the rate of SO ₂ absorption.The SO₂ concentration in air studied ranged between 1 and 60% (v). Such relatively large concentrations were necessary due to the short times the drops were exposed to SO₂ in the present setup. The present experimental results were therefore not used to simulate atmospheric conditions but rather to test our previously derived theory which is applicable to any laboratory or atmospheric condition. Comparison of our studies with the results from our theory applied to our laboratory conditions led to predicted values for the S concentration in the drops which agreed well with those observed if the drops had radii smaller than 500 m. In order to obtain agreement between predicted and observed S concentrations in larger drops, an empirically derived eddy diffusivity for SO₂ in water had to be included in the theory to take into account the effect of turbulent mixing inside such large drops.In a subsequent set of experiments, drops initially saturated with S (IV) were allowed to fall through S-free air to determine the rate of SO ₂ desorption. The results of these studies also agreed well with the results of our theoretical model, thus justifying the reversibility assumption made in our theoretical models.In a final set of experiments, the effects of oxidation on SO₂ absorption was studied by means of drops containing various amounts of H₂O₂. For comparable exposure times to SO₂, the S concentration in drops with H₂O₂ was found to be up to 10 times higher than the concentration in drops in which no oxidation occurred.  相似文献   

Determination of the Tropospheric Hydroxyl Radical by Liquidphase Scrubbing and HPLC: Preliminary Results     

Xiaohui Chen  Kenneth Mopper 《Journal of Atmospheric Chemistry》2000,36(1):81-105

A preliminary study was carried out toexamine the feasibility of measuring tropospherichydroxyl radicals (OH) by liquidphase scrubbing andhigh performance liquid chromatography (HPLC). Thepotential advantages of this approach are itssimplicity, portability, and low expense. Thesampling system employs glass bubblers to trapatmospheric OH into a buffered solution of salicylicacid (o-hydroxybenzoic acid, OHBA). Rapidreaction of OH with OHBA produces a stable fluorescentproduct, 2,5-dihydroxybenzoic acid (2,5-DHBA), whichis determined by reverse-phase HPLC and fluorescencedetection. Our preliminary field results indicatethat this method is most suitable for OH measurementsin clean tropospheric air, where interferences fromother atmospheric species appear to be negligible orminor relative to polluted air. In clean air, thesampling period is about 45–90 minutes, which yieldsa detection limit of approximately 3–6 ×10⁵ radicalscm^-3. During an OHintercomparison experiment at the Caribou samplingsite in Colorado, our liquidphase scrubber method wascompared with the ion-assisted mass spectrometry (MS)method. Our results were within the same range asthose of the ion-assisted MS method (1–5 ×10⁶ radicals cm^-3) within our precision atthat time (about ±30–50%). Preliminary testsin Pullman, WA indicated that the method might alsofunction in moderately polluted air by acidifying thescrubbing solution or by adding a scavenger tosuppress interferences. In Pullman, mid-day OHconcentrations were usually in the range of 2–20 ×10⁶ radicals cm^-3. Nighttime OHconcentrations were always low, either at or slightlyabove the detection limit.  相似文献   

Study on the Free Radical Scavenging Activity of Sea Cucumber(Paracaudina chinens var.) Gelatin Hydrolysate     

ZENG Mingyong  XIAO Feng  ZHAO Yuanhui  LIU Zunying  LI Bafang  and DONG Shiyuan College of Food Science  Engineering  Ocean University of China  Qingdao  P. R. China 《中国海洋大学学报(英文版)》2007,6(3)

Gelatin from the sea cucumber(Paracaudina chinens var.) was hydrolyzed by bromelain and the hydrolysate was found to have a high free radical scavenging activity. The hydrolysate was fractionated through an ultrafiltration membrane with 5 kDa molecular weight cutoff(MWCO). The portion(less than 5 kDa) was further separated by Sephadex G-25. The active peak was col-lected and assayed for free radical scavenging activity. The scavenging rates for superoxide anion radicals(O2·-) and hydroxyl radi-cals(·OH) of the fraction with the highest activity were 29.02% and 75.41%,respectively. A rabbit liver mitochondrial free radical damage model was adopted to study the free radical scavenging activity of the fraction. The results showed that the sea cucumber gelatin hydrolysate can prevent the damage of rabbit liver and mitochondria.  相似文献   

On the scavenging of gaseous nitrogen compounds by large and small rain drops I. A wind tunnel and theoretical study of the uptake and desorption of NH3 in the presence of CO2     

A. U. Hannemann  S. K. Mitra  H. R. Pruppacher 《Journal of Atmospheric Chemistry》1995,21(3):293-307

An experimental study has been carried out in the Mainz vertical wind tunnel to determine the rate at which NH₃ in the presence of CO₂ is absorbed by freely suspended water drops. The experimental uptake rates were found to be in good agreement with the rates predicted by the Kronig-Brink convective diffusion model and, for gas concentrations in the ppbv range also by the model in which it is assumed that the absorbed gas is well mixed inside the drop (henceforth called well mixed model). The same conclusion was shown to apply also to the desorption of NH₃ from a drop previously exposed to NH₃. The latter result is in contrast to the desorption of SO₂ which must be described by a model which accounts for the diffusion of the species inside the drop. Comparison of our experimental results with theory show further that the uptake of NH₃ in presence of CO₂ is significantly overestimated if the slow reaction CO_2(aq)+H₂OHCO ₃ ^– +H⁺ is neglected in the theoretical computation.  相似文献   

Thorium sorption in the marine environment: Equilibrium partitioning at the hematite/water interface,sorption/desorption kinetics and particle tracing     

Matthew S. Quigley  Bruce D. Honeyman  Peter H. Santschi 《Aquatic Geochemistry》1995,1(3):277-301

Thorium(IV) sorption onto hematite (-Fe₂O₃) was examined as a function of pH and ionic strength. Sorption behaved Langmuirian over an eleven order of magnitude range in adsorption densities, : 10^–12 to 10^–1 moles Th sorbed per mole hematite sites, indicating that the overall free energy of Th adsorption is independent of adsorption density. Modeling of Th sorption was conducted with the Triple Layer Model of Davis and Leckie; reactions considered included solution-phase hydroxy and carbonato complexes of thorium, and carbonate/hematite surface complexes. The entire Th sorption isotherm can be modeled with a single surface complex formation reaction

相似文献    18. On the scavenging of gaseous nitrogen compounds by large and small rain drops: II. Wind tunnel and theoretical studies of the simultaneous uptake of NH3, SO2 and CO2 by water drops      A. U. Hannemann  S. K. Mitra  H. R. Pruppacher 《Journal of Atmospheric Chemistry》1996,24(3):271-284 An experimental investigation of the simultaneous absorption of NH3 and SO2 from the ambient atmosphere by freely falling water drops has been carried out in the Mainz vertical wind tunnel. The experimental results were found to be in good agreement with the results derived from computations with the Kronig-Brink convective diffusion model and also with a model which assumes a drop to be well mixed at all times. Encouraged by this agreement, these computation schemes for the uptake of gas by single drops where incorporated in a pollution washout model with realistic SO2, NH3 and CO2 gas profiles. This model allows an entire raindrop size distribution to fall through a gas layer. The results of this plume-model show that the SO2 uptake is strongly dependent on the NH3 concentration in the atmosphere and on the rainrate. We also find that the small drops contribute more towards the washout of these gases. In the case of simultaneous presence of NH3 and SO2, desorption of these gases is negligible.  相似文献    19. Assessment of stone crab (Lithodidae) density on the South Georgia slope using baited video cameras   总被引：2，自引：0，他引：2   Collins  M. A.; Yau  C.; Guilfoyle  F.; Bagley  P.; Everson  I.; Priede  I. G.; Agnew  D. 《ICES Journal of Marine Science》2002,59(2):370-379   相似文献    20. 气溶胶核化清除的化学效应──Ⅱ：动力学参数对云滴化学非均匀性的影响          下载免费PDF全文 刘小红  洪钟祥  王明康 《大气科学》1995,(1) 运用已建立的气溶胶核化清除的物理化学模式，研究了云的动力学因子（如：气块上升速度、夹卷作用）对云滴化学非均匀性的影响。计算结果表明：较强烈的云发展（较大的气块上升速度）可加强由于气溶胶核化和云滴凝结增长造成的云滴化学的非均匀程度。夹卷作用抑制了云的发展，因而减弱了这种非均匀程度。夹卷作用同时也造成总体液态水中Ｓ（ＶＩ）、Ｈ＋等浓度的增加，在Ｓｍａｘ附近可达１个量级。如果考虑气溶胶粒子的夹卷，则可使气块内云滴污染物浓度随云滴大小的变化更加复杂化，如：不仅云滴污染物浓度随云滴大小而变化，即使对于相同大小的云滴之间，其污染物浓度也可相差很大。  相似文献    [首页] « 上一页 [1] 2 [3] [4] [5] [6] [7] [8] [9] [10] [11] 下一页 » 末  页»