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21.
R. N. Colvile R. Sander T. W. Choularton K. N. Bower D. W. F. Inglis W. Wobrock D. Schell I. B. Svenningsson A. Wiedensohler H. -C. Hansson A. Hallberg J. A. Ogren K. J. Noone M. C. Facchini S. Fuzzi G. Orsi B. G. Arends W. Winiwarter T. Schneider A. Berner 《Journal of Atmospheric Chemistry》1994,19(1-2):189-229
The airflow, cloud microphysics and gas- and aqueous-phase chemistry on Kleiner Feldberg have been modelled for the case study of the evening of 1 November 1990, in order to calculate parameters that are not easily measured in the cloud and thus to aid the interpretation of the GCE experimental data-set. An airflow model has been used to produce the updraught over complex terrain for the cloud model, with some care required to ensure realistic modelling of the strong stable stratification of the atmosphere. An extensive set of measurements has been made self-consistent and used to calculate gas and aerosol input parameters for the model. A typical run of the cloud model has calculated a peak supersaturation of 0.55% which occurs about 20 s after entering cloud where the updraught is 0.6 m s–1. This figure has been used to calculate the efficiency with which aerosol particles were scavenged; it is higher than that calculated by other methods, and produces a cloud with slightly too many droplets. A broad cloud droplet size spectrum has been produced by varying the model inputs to simulate turbulent mixing and fluctuations in cloud parameters in space and time, and the ability of mixing processes near cloud-base to produce a lower peak supersaturation is discussed. The scavenging of soluble gases by cloud droplets has been observed and departures from Henry's Law in bulk cloud-water samples seen to be caused by variation of pH across the droplet spectrum and the inability of diffusion to adjust initial distributions of highly soluble substances across the spectrum in the time available. Aqueous-phase chemistry has been found to play a minor role in the cloud as modelled, but circumstances in which these processes would be more important are identified. 相似文献
22.
Kong Fanyou 《大气科学进展》1994,11(1):1-12
A two-dimensional, non-reactive convective cloud transport model is used to simulate in detail the vertical transport and wet scavenging of soluble pollutant gases by a deep thunderstorm system, Simulations show that for gases with not very high solubility, a deep and intense thunderstorm can still rapidly and efficiently transport them from boundary layer (PBL) up to mid and upper troposphere, resulting in a local significant increase of concentration in the upper layer and a reduction in PBL. Dissolution effects decrease both the incloud gas concentration and the upward net fluxes. The higher the solubility is, the more remarkable the decrease is. However, for very low soluble gases (H < 102 M atm-1), the influences are very slight. In addition, the effects of irreversible dissolution and aqueous reactions in drops on the vertical transport of gaseous pollutants are estimated in extreme. 相似文献
23.
24.
Vertical and temporal variations in the activities of234Th,210Po and210Pb have been measured, in both dissolved and paniculate phases, at several stations in the eastern Arabian Sea and north-central
Bay of Bengal. A comparative study allows us to make inferences about the particle associated scavenging processes in these
two seas having distinct biogeochemical properties.
A common feature of the234Th profiles, in the Arabian Sea and Bay of Bengal, is that the dissolved as well as total (dissolved + particulate) activity
of234Th is deficient in the surface 200 m with respect to its parent,238U. This gross deficiency is attributed to the preferential removal of234Th by adsorption onto settling particles which account for its net loss from the surface waters. The scavenging rates of dissolved234Th are comparable in these two basins. The temporal variations in the234Th-238U disequilibrium are significantly pronounced both in the Arabian Sea and Bay of Bengal indicating that the scavenging rates
are more influenced by the increased abundance of particles rather than their chemical make-up. In the mixed layer (0–50 m),
the scavenging residence time of234Th ranges from 30 to 100 days.
The surface and deep waters of both the seas show an enhanced deficiency of dissolved210Po relative to210Pb and that of210Pb relative to226Ra. The deficiencies of both210Po and210Pb in the dissolved phases are not balanced by their abundance in the particulate form indicating a net loss of both these
nuclides from the water column. The scavenging rates of210Po and210Pb are significantly enhanced in the Bay of Bengal compared to those in the Arabian Sea. The mean dissolved210Po/210Pb and210Pb/226Ra activity ratios in deep waters of the Bay of Bengal are ∼ 0.7 and 0.1, respectively, representing some of the most pronounced
disequilibria observed to date in the deep sea. The Bay of Bengal and the Arabian Sea appear to be the regions of most intense
particle moderated scavenging processes in the world oceans. This is evidenced by the gross disequilibria exhibited by the
three isotope pairs used in this study. 相似文献
25.
A one-dimensional cloud model with size-resolved microphysics and size-resolved aqueous-phase chemistry, driven by prescribed
dynamics, has been used to study gas scavenging by weak precipitation developed from low-level, warm stratiform clouds. The
dependence of the gas removal rate on the physical and chemical properties of precipitation has been explored under controlled
initial conditions. It is found that the removal of four gaseous species (SO2, NH3, H2O2 and HNO3) strongly depends on the total droplet surface area, regardless the mean size of droplets. The removal rates also correlate
positively with the precipitation rate, especially for precipitation having a mean radius larger than 20 μm. The dependence
of the scavenging coefficients on the total droplet surface area is stronger than on the precipitation rate.
The removal rates of SO2, NH3 and H2O2 by precipitation strongly depend on the others' initial concentrations. When NH3 (or H2O2) concentration is much lower than that of SO2, the removal rate of SO2 is then controlled by the concentration of H2O2 (or NH3). The removal of NH3 (or H2O2) also directly depends on the concentration of SO2. NH3 and H2O2 can also indirectly affect each other's removal rate through interaction with SO2. The scavenging coefficient of SO2 increases with the concentration ratio of NH3 to SO2 if the ratio is larger than 0.5, while the scavenging coefficient of NH3 increases with the concentration ratio of SO2 to NH3 when the ratio is smaller than 1. The scavenging coefficient of H2O2 generally increases with the concentration ratio of SO2 to H2O2. Although the Henry's law equilibrium approach seems to be able to simulate gas scavenging by cloud droplets, it causes large
errors when used for simulating the scavenging of soluble gas species by droplets of precipitating sizes. 相似文献
26.
Acid Rain and Below-Cloud Scavenging in South-Western China 总被引:1,自引:0,他引:1
PETER A. TANNER HENG-CHI LEI MEI-YUAN HUANG ZHI-LAI SHEN 《Journal of Atmospheric Chemistry》1997,27(1):71-78
Major urban areas in south-western China exhibit unique air pollutionproblems due to increasing use of high sulphur-content fuels in an environmentof unfavourable topography and climate. Ambient levels of sulphur dioxideexceed the air quality objectives, and this gas is the major precursor of acidrain. Cloudwater chemistry studies are reported for urban, suburban andcountryside locations, during the period 1985–1989. Although cloudwateracidity was found to increase towards the cloud base, the acidity was muchgreater for rainwater samples collected simultaneously, and was morepronounced in urban rather than neighbouring suburban or countryside regions.The main contribution to the acidity arises from below-cloud scavenging of gasand aerosol and model calculations are able to simulate this behaviour. 相似文献
27.
Nucleation scavenging and the formation of a cloud interstitial aerosol (CIA) were theoretically studied in terms of the chemical composition of atmospheric aerosol particles. For this study, we used our air-parcel cloud model, which includes the entrainment of air and detailed microphysics, for determining the growth and interaction of aerosol particles and drops. Maritime and remote continental aerosol particle spectrums were used whose size distributions were superpositions of three log-normal distributions, each of a prescribed chemical composition. Our results show (1) that the CIA exhibits a size distribution with a distinctive cut-off at a specific radius of the dry as well as of the wet particle size distribution. All particles above this limiting size become activated to cloud drops and, thus, are not present in the CIA spectrum. This limiting size was found to be independent of the chemical composition of the particles and only dependent on the prevailing supersaturation. Below this specific size, the CIA spectrum becomes depleted of dry aerosol particles in a manner which does depend on their chemical composition and on the supersaturation in the air. (2) The number of aerosol particles nucleated to cloud drops depends critically on the chemical composition of the particles and on the prevailing supersaturation. 相似文献
28.
Partition, not kinetics, ultimately determines the concentration of highly soluble gases in cloud droplets. Partition equations are formulated and applied to idealized air-mass thunderclouds and precipitating stratus. Contribution to aqueous concentrations from sub-cloud scavenging of highly soluble gases is estimated at between 10 and 20% under relatively unpolluted conditions. Data indicate that evaporation can produce enhancements in concentration of as much as a factor of 3. The calculations give large-scale mean coefficients of wet removal of highly soluble gases of about 2.8×10-6 s-1 (4-day residence time) for air-mass thunderclouds and precipitating stratus. Removal is so effective that the mean scale heights of these gases should be decreased to 2 km or less. The criterion of high solubility in this paper is that K
H (Henry's Law coefficient) > 105 mol l-1 atm-1. Gases that are effectively highly soluble include HCl, HNO3, H2SO4, H2O2, NH3 in acid droplets, SO2 in oxidizing droplets (and probably some light amines and sulfonic acids), but not SO2 in the absence of oxidants, nor HCHO. A variation of removal coefficient and scale height with solubility is presented. A comparison of atmospheric NH3 concentrations deduced from rain NH4
+ and measured directly gives reasonable agreement. 相似文献
29.
The characteristics of four wet deposition schemes widely used in dust modeling studies are examined within the framework of a regional scale dust model. Since these schemes are based on different formulations, the scavenging coefficients of them deviate by a factor of 103 depending on precipitation rate and particle size. The four schemes coupled with the dust model are applied to simulate a 2002 Asian dust event. The corresponding wet deposition patterns and scavenging efficiencies are compared. It is found that apart from the scheme derived from scavenging coefficient measurements, the other three schemes give similar wet deposition patterns although their scavenging efficiencies are different depending on the particle-size range. The results suggest that the performances of these schemes are affected by the particle size distribution of the dust emission, together with the model's performance of precipitation prediction. 相似文献
30.
A. Kasper-Giebl A. Koch R. Hitzenberger H. Puxbaum 《Journal of Atmospheric Chemistry》2000,35(1):33-46
Cloud water and interstitial aerosol samples collected at Mt. Sonnblick (SBO) were analyzed for sulfate and aerosol carbon to calculate in-cloud scavenging efficiencies. Scavenging efficiencies for sulfate (SO) ranged from 0.52 to 0.99 with an average of 0.80. Aerosol carbon was scavenged less efficiently with an average value (AC) of 0.45 and minimum and maximum values of 0.14 and 0.81, respectively. Both SO and AC showed a marked, but slightly different, dependence on the liquid water content (LWC) of the cloud. At low LWC, SO increased with rising LWC until it reached a relatively constant value of 0.83 above an LWC of 0.3 g/m3. In the case of aerosol carbon, we obtained a more gradual increase of AC up to an LWC of 0.5 g/m3. At higher LWCs, _ remained relatively constant at 0.60. As the differences between SO and A varied across the LWC range observed at SBO, we assume that part of the aerosol carbon was incorporated into the cloud droplets independently from sulfate. This hypothesis is supported by size classified aerosol measurements. The differences in the size distributions of sulfate and total carbon point to a partially external mixture. Thus, the different chemical nature and the differences in the size and mixing state of the aerosol particles are the most likely candidates for the differences in the scavenging behavior. 相似文献