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1.
The interpretation of the physico-chemical processes in clouds is facilitated by segregating in situ cloud elements from their carrier gas and small particles (interstitial aerosol). Thus, the present study focuses on the quantitative phase segregation of interstitial air from cloud phase by two complementary samplers with microphysical on-line analysis of the separated phases. An improved counterflow virtual impactor (CVI) was developed for the collection and subsequent evaporation of the condensed phase, releasing dissolved gaseous material and residual particles. This sampler operates in the size range of few micrometers up to 50 μm in cloud element diameter and is matched by an interstitial Round Jet Impactor sampling the gas phase with interstitial particles. Calibrations of both samplers verified the calculated cut sizes D50 of 4, 5, and 6 μm and quantified the slope of the collection efficiency curves. Until this study no direct CVI measurements of the residual particle sizes far below the diameter of 0.1 μm were available. For the first time a CVI was connected to a Differential Mobility Particle Sizer (DMPS) scanning between 25 nm and 850 nm, thus, including the entire Aitken mode in the residual size analysis. Cloud studies on the Puy de Dôme, France, revealed residual particle sizes including Aitken mode (diameter D<100 nm) and accumulation mode (D>100 nm). A major feature of the CVI data is expressed by the fact that despite incomplete incorporation of accumulation mode particles in cloud elements there are contributions of particles with diameters smaller than 0.1 μm to the number of residual particles. Cloud entrainment from height levels above the maximum supersaturation as wells as the size-dependent chemical composition of the aerosol population most likely produced the S-shaped size-dependent partitioning of residual particles. Compared to earlier studies the 50% partitioning diameters dropped significantly below 100 nm to roughly 70 nm.  相似文献   

2.
Research flights in November 1990 over the central parts of the United States, Wyoming and Colorado, were aimed to the investigation of the properties and microstructure of cirrus clouds (mainly cirrocumulus lenticularis). Among the other parameters measured on board the NCAR Saberliner were the concentration and size distribution of submicron particles and, in some cases, the particle deliquescence. For coarse insoluble particles found inside and outside of cloud elements, size distributions and morphology information were obtained by evaluating inertial impactor samples with an optical microscope and scanning electron microscope. In addition, the coarse particle composition was determined by x-ray energy spectrum analysis. The following conclusions from these measurements are:The large and coarse particle size distribution can be roughly simulated by a log-normal function with the modus around r=0.5 μm. Particle concentrations are very variable between several tenths and several particles per cm3. Particle volume distribution features a distinct maximum around 0.75 μm without a broad plateau which was observed in the case of sampling at lower altitude. Aerosol composition heterogeneity at cirrus cloud level is well documented by the evaluation of the fine particle sampling taken with the UMR sampling system. This heterogeneity can be partly explained by the interaction between aerosol and cloud elements, which is documented by the measured particle size distribution curves inside and outside of cloud elements. Assuming that particle deliquescence is caused by H2SO4 and/or by (NH4)2SO4, particle soluble mass fractions were found to be around 30% in the first case and about 40% in the second. The most frequently occurring elements in large and coarse particles at cirrus cloud level were Si, Cl, Ba, S, Ca and C.  相似文献   

3.
A closure study of sub-micrometer aerosol particle hygroscopic behaviour   总被引：2，自引：0，他引：2  
The hygroscopic properties of sub-micrometer aerosol particles were studied in connection with a ground-based cloud experiment at Great Dun Fell, in northern England in 1995. Hygroscopic diameter growth factors were measured with a Tandem Differential Mobility Analyser (TDMA) for dry particle diameters between 35 and 265 nm at one of the sites upwind of the orographic cloud. An external mixture consisting of three groups of particles, each with different hygroscopic properties, was observed. These particle groups were denoted less-hygroscopic, more-hygroscopic and sea spray particles and had average diameter growth factors of 1.11–1.15, 1.38–1.69 and 2.08–2.21 respectively when taken from a dry state to a relative humidity of 90%. Average growth factors increased with dry particle size. A bimodal hygroscopic behaviour was observed for 74–87% of the cases depending on particle size. Parallel measurements of dry sub-micrometer particle number size distributions were performed with a Differential Mobility Particle Sizer (DMPS). The inorganic ion aerosol composition was determined by means of ion chromatography analysis of samples collected with Berner-type low pressure cascade impactors at ambient conditions. The number of ions collected on each impactor stage was predicted from the size distribution and hygroscopic growth data by means of a model of hygroscopic behaviour assuming that only the inorganic substances interacted with the ambient water vapour. The predicted ion number concentration was compared with the actual number of all positive and negative ions collected on the various impactor stages. For the impactor stage which collected particles with aerodynamic diameters between 0.17–0.53 μm at ambient relative humidity, and for which all pertinent data was available for the hygroscopic closure study, the predicted ion concentrations agreed with the measured values within the combined measurement and model uncertainties for all cases but one. For this impactor sampling occasion, the predicted ion concentration was significantly higher than the measured. The air mass in which this sample was taken had undergone extensive photochemical activity which had probably produced hygroscopically active material other than inorganic ions, such as organic oxygenated substances.  相似文献   

4.
Measurements of the small-, intermediate-, and large-ion concentrations and the air–earth current density along with simultaneous measurements of the concentration and size distribution of aerosol particles in the size ranges 4.4–163 nm and 0.5–20 μm diameter are reported for a drifting snow period after the occurrence of a blizzard at a coastal station, Maitri, Antarctica. Ion concentrations of all categories and the air–earth current simultaneously decrease by approximately an order of magnitude as the wind speed increases from 5 to 10 ms− 1. The rate of decrease is the highest for large ions, lowest for small ions and in-between the two for intermediate ions. Total aerosol number concentration decreases in the 4.4–163 nm size range but increases in the 0.5–20 μm size range with wind speed. The size distribution of the nanometer particles shows a dominant maximum at ~ 30 nm diameter throughout the period of observations and the height of the maximum decreases with wind speed. However, larger particles show a maximum at ~ 0.7 μm diameter but the height of the maximum increases with increasing wind speed. The results are explained in terms of scavenging of atmospheric ions and aerosols by the drifting snow particles.  相似文献   

5.
Spatial/temporal variabilities of rainwater constituents are examined based on soluble/insoluble trace elements, pH and electrical conductivity measurements in rainfall sampled during December 2003–May 2005 at two urban and two suburban sites in Mersin, an industrialized city of 850,000 inhabitants on the southern coast of Turkey. In the analyses, backward air mass trajectories for rainy days were used in addition to factor analyses, enrichment factors, phase distributions and correlations between trace elements. The pH varied from 4.8 to 8.5 with an average value of 6.2, reflecting a mainly alkaline regime. Mean concentrations of trace elements collected from urban and suburban sites are spatially variable. Based on the overall data, total concentrations of trace elements were ordered as Ca > Na > Fe > Al > Mg > K > Zn > Mn > Sr > Pb > Ni > Cr > Ba > Cu > Co > Cd. Mainly terrigeneous (Ca, Fe, Al) and, to a lesser extent, sea salt particles (Na, Mg) were shown to be the major source of trace elements. Excluding major cations, the solubilities of trace elements were found to be ordered as Sr > Zn > Ba > Mn > Cu > Ni > Cr > Fe > Al, confirming the lower solubility of crustal elements. Cd, Co and Pb were excluded from the above evaluation because of the low numbers of soluble samples allowing quantitative measurements. The solubilities of Al, Fe, Mn and particularly of Ni were found to be considerably lower than those reported for various sites around the world, most likely due to the effect of pH. During the entire sampling period, a total of 28 dust transport episodes associated with 31 red rain events were identified. Extremely high mean concentration ratios of Al (8.2), Fe (14.4) and Mn (13.1) were observed in red rain, compared to normal rain. The degree of this enhancement displayed a decrease from crustal to anthropogenic origin elements and the lowest enhancements were found for anthropogenic origin elements of Zn and Cd (both having a ratio of 1.1). Aerosol dust was found to be the main source of almost all analyzed elements in Mersin precipitation, regardless that they are crustal or anthropic derived elements. The magnitude of crustal source contribution to trace element budget of precipitation was at its highest levels for crustal originated elements, most probably due to much higher scavenging ratios of crustal elements compared to anthropogenic ones.  相似文献   

6.
The change of the chemical composition of the near-ground level atmospheric aerosol was studied during two summer episodes by a Lagrangian type of experimental approach. Bulk and single-particle chemical analyses of ions and elements in the particulate phase were deployed. N(-III) and N(V) components were also measured in the gas-phase. The measurements were completed by particle size distributions.Secondary inorganic aerosols (SIA) and fine particles of ≈0.2–0.4 μm size were still elevated 50 km downwind of the city. The direct comparison of transport over the city in contrast to transport over the surrounding areas showed that SIA was formed from emission from the city within less than 3 h. Relative increases, i.e., enrichment during transport were observed for primary and secondary aerosol components. The degree of mixing on the individual particle level increased significantly during transport in the area. In particular, newly emitted carbonaceous particles became internally mixed within hours with pre-existing sulphate particles. Mostly due to secondary aerosol formation the average particle size (mass median diameter) of major constituents of the aerosol was significantly decreased while being transported over 13 h. Given recent insights which link fine particles number and mass concentrations with health risks, the results suggest that rural populations in areas which frequently are located within an urban plume might run an elevated health risk relative to populations in areas not affected by urban plumes.  相似文献   

7.
Electrical charges on aerosol particles and droplets modify the droplet–particle collision efficiencies involved in scavenging, and the droplet–droplet and particle–particle collision efficiencies involved in coalescence of droplets and particles, even in only weakly electrified clouds and aerosol layers. This work places electrically enhanced scavenging, and the electrical inhibition of scavenging in the context of the microphysics of weakly electrified clouds.Collision efficiencies are calculated by numerical integration to obtain particle trajectories, that are determined by the complex interplay of electrical, gravitational and phoretic forces together with inertia. These modify the trajectory of a particle as it is carried by flow around the falling droplet. Conversely, the flow around the particle also modifies the trajectory of the droplet. The flows are specified analytically, using a hybrid of the Proudman–Pearson stream function for that region close to the droplet or particle, where it is accurate, merging into the exact Oseen stream function for larger distances, where that becomes accurate. The effect of the flow around the particle on the motion of the droplet was simulated using Langmuir's superposition technique on the hybrid stream functions. The treatment of inertia in the present calculations allows an extension of the scope of our previous work by a factor of 10 larger in particle size (103 in mass). The coverage is extended to a wide range of atmospheric conditions and particle densities.The pressures and temperatures used in the models ranged from a representation of the lower troposphere at  1 km altitude (900 hPa, 10 °C) to that of the middle stratosphere at  30 km altitude (12 hPa, − 47 °C). The particles considered range from 0.1 μm to 10 μm radius; the droplet radii range from 4 μm to 50 μm; particle densities range from 300 kg m− 3 to 2500 kg m− 3; particle charges range from 2e to 100e with droplet charges of like sign of 100e; and relative humidities range from 10% to 100%.For the larger particles (radii greater than about 3 μm) interacting with the larger droplets (radii greater than about 15 μm) the effects of inertia increase with particle density and dominate at the larger densities. For particles with radii in the range 1–3 μm the ‘Greenfield Gap’ of very low collision efficiencies was found, and was determined to be due to the effects of the gravitational force causing a reduction of collisions of particles with the front of the droplet, and the effect of inertia overcoming the tendency for the weight to produce a collision in the slow velocity region in the rear. When the electrical or phoretic forces are sufficiently large the Greenfield Gap is closed.When the particles have radii < 3 μm inertial effects no longer dominate the collisions, although inertia modifies the weight effects for particles with radii down to about 0.5 μm. For charged aerosol particles with radii smaller than about 0.1 μm interacting with droplets or background aerosol particles smaller than a radius of about 15 μm, the long range electrical repulsive force is effective in opposing the phoretic forces and keeping the particle out of range of the short range attractive image force. Thus ‘electroscavenging’ gives way to ‘electroprotection’ against the scavenging or coagulation processes otherwise caused by Browninan diffusion or phoretic forces.In an atmosphere of temperature 10 °C and pressure 900 hPa the net phoretic force reduces to zero and becomes repulsive for particles with radii above about 2 μm (depending on particle conductivity). This enhances the development of the Greenfield Gap. However, the value of this radius (at which the net phoretic force is zero) increases strongly with decreasing temperature and pressure (increasing altitude) as expected from theory, and is about 5 μm in the middle troposphere and more than 10 μm in the stratosphere. Thus a net attractive phoretic force on particles extends into the 1–3 μm radius range in the upper troposphere; however, the weight and inertial effects can ensure the presence of the Greenfield Gap in that range for 2000 kg m− 3 particles up to the middle stratosphere.  相似文献   

8.
In November 1993 an airborne field study was performed in order to investigate the microphysical and radiative properties of cooling tower water clouds initiated by water vapour emissions and polluted by the exhaust from coal-fired power plants. The number-median diameter of the droplet size distributions of these artificial clouds was in the range of 13 μm. The concentration of smaller droplets (diameters dD < 10 μm) increased with height and horizontal distance from the cooling towers. Close to the cooling towers, bimodal spectra were found with a second mode at 19 μm. The liquid water content (LWC) ranged between 2 and 5 g/m3 and effective droplet radii (Re) between 6 and 9 μm were measured. LWC and Re decreased with altitude, whereas the droplet concentration (ND) remained approximately constant (about 2000 cm−3 ). An enrichment of interstitial aerosol particles with particle diameters (dp) smaller 0.2 μm compared to the power plant plume in the vicinity of the clouds was observed. Particle activation for dm > 0.3 μm. was evident, especially in cooling tower clouds further apart and separated from their sources. Furthermore, radiation measurements were performed, which revealed differences in the vertical profiles of downwelling solar and UV radiation flux densities inside the clouds.The effective droplet radius Re was parameterized in terms of LWC and ND using equations known from literature. The close agreement between measured and parameterized Re indicates a similar coupling of Re, LWC and ND as in natural clouds.By means of Mie calculations, volume scattering coefficients and asymmetry factors are derived for both the cloud droplets and the aerosol particles. For the cloud droplets, the optical parameters were described by parameterizations from the literature. The results show, that the link between radiative and microphysical properties of natural clouds is not changed by the extreme pollution of the artificial clouds.  相似文献   

9.
Direct physical measurements of particle mass and number concentration indicate an increase in overall aerosol mass resulting from cloud processing, most likely through aqueous-phase chemistry (e.g., SO2 oxidation). Measurements conducted in the Pennines of Northern England reveal an average increase of 14 to 20% in dry aerosol mass (0.003<particle diameter<0.9 μm) after aerosol passage through an orographic cloud. The rate of in-cloud mass production is most sensitive to changes in upwind particle size distributions, SO2 concentration, and cloud water acidity. Newly-formed mass appears in size range between 200 and 600 nm and enhances the bimodality of the particle number distribution after cloud processing. Furthermore, the cloud-produced mass is estimated to increase total light scattering, bsp, by 18 to 24%. The scattering efficiency of the dry, cloud-generated aerosol is 5.0±0.3 m2 g−1 and increases to 7.4±0.7 m2 g−1 when adjusted to 90% relative humidity by incorporating particle hygroscopicity data.  相似文献   

10.
《Atmospheric Research》2005,73(1-2):87-100
Coarse (>2.2 μm) and fine (<2.2 μm) atmospheric particulate material samples were collected from an urban area (Al-Hashimya, Jordan), from August 2000 to August 2001 using a “GENT” stack filter unit (SFU). Collected samples were analyzed for 19 elements using inductively coupled plasma mass spectrometry (ICP-MS). The crustal elements exhibit atmospheric concentrations that are comparable to those in urban and industrial areas. The anthropogenic elements, on the other hand, are clearly less abundant in Al-Hashimya than in other industrial regions. Results indicated that, elements of crustal origin are associated with the coarse particles, while elements of anthropogenic origins are more associated with fine particles. Concentrations of crustal-derived elements were higher in summer and those of anthropogenic elements were higher in winter. Crustal enrichment factor calculations showed that concentrations of Pb, Zn, Cd, Sb and Ag are highly enriched and of As, Cu, Co, Ca and Ni are moderately enriched. Factor analysis calculations permitted the identification of four source groups for the fine fraction, namely oil combustion, crustal and urban dust, smelting industries and motor vehicles.  相似文献   

11.
Ice sphere backscatter has been calculated using both Mie theory and the discrete dipole approximation (DDA) at a wavelength of 3.2 mm (94 GHz). The electric dipole, magnetic dipole and electric quadrupole contributions to spherical particle backscatter have been analyzed. The results show that there is a resonance area around particle size parameter of 1.5, where the calculated backscatter errors are very large due to the neglect of the magnetic dipole, and this is confirmed by applying Mie theory to 8.66 mm (35 GHz) and 3.21 cm (X-band) wavelengths. Based on the backscatter calculation using a cube and a hexagon column randomly oriented in space, it was found that the backscatter error from the inaccurate representation of the particle surface shape is much smaller than that from the neglect of the magnetic dipole, and the resonance occurs at different particle sizes depending upon the exact particle shapes. At a wavelength of 3.2 mm, the particle shape has little effect on backscatter when volume-equivalent spherical particle radius rv < 500 μm, and Rayleigh backscatter can be used as a reasonable approximation for rv < 300 μm.  相似文献   

12.
A field study was conducted at a mountain-top site in northwestern Colorado. Supercooled cloud water, collected as a function of droplet size, was analyzed for anions, cations and trace elements. Enrichment factors (EF) of SO 4 2– , K+, Na+ and Cl– relative to crustal and marine reference elements (Al and Na) were calculated to determine whether chemical fractionation of the aerosol occurs during cloud droplet formation. The largest EF's for all ions were found for droplets less than 10–15 µm diameter. Ratios of the small to large droplet mean EF's ranged from 1 to 2, for SO 4 2– relative to both Al and Na+, to 10 to 12 for Na+, Cl– and K+, relative to Al. EF's of K+ and Cl– in the bulk cloud water were in crustal and marine proportions, respectively. It was concluded that although bulk could chemistry may indicate a lack of enrichment of a species, this may not be true throughout the droplet size distribution. The higher enrichments in small droplets is likely a result of their formation on small aerosol particles whereas the large droplets form on the largest aerosol particles. This may suppress EF's in precipitation relative to the total aerosol.  相似文献   

13.
A high-volume cascade impactor, equipped with a PM10 inlet, was used to collect size-segregated aerosol samples during the summer of 2004 at two Portuguese locations: a coastal-rural area (Moitinhos) and an urban area (Oporto). Concentrations of airborne particulate matter (PM), total carbon (TC), organic carbon (OC), elemental carbon (EC), and water-soluble organic carbon (WSOC) were determined for the following particle size ranges: < 0.49, 0.49–0.95, 0.95–3.0, and 3.0–10 µm. The total PM mass concentrations at the urban and coastal-rural sites ranged from 22.8 to 79.6 μg m− 3 and 19.9 to 28.2 μg m− 3, respectively, and more than 56% of the total aerosol mass was found in the fractions below 3.0 μm. At both locations the highest concentrations of OC and EC were found in the submicrometer size range. The regional variability for the OC and EC concentrations, with the highest concentrations being found in the urban area, was related to the contribution of local primary sources (mostly traffic emissions). It was also verified an enrichment of the small size particles in WSOC, representing on average 37.3(± 12.4)% and 59.7(± 18.0)% of OC in the very fine aerosol at the coastal-rural and urban areas, respectively. The amount of secondary OC calculated by the minimum OC/EC ratio method indicates that secondary organic aerosol formation was important throughout the study at both sites. The obtained results suggest that long-range transport and favourable summer conditions for photochemical oxidation are key factors determining secondary OC formation in the coastal-rural and urban areas. The ultraviolet absorption properties of the chromophoric constituents of the WSOC fractions were also different among the different particle size ranges and also between the two sampling locations, thus suggesting the strong impact of the diverse emission sources into the composition of the size-segregated organic aerosol.  相似文献   

14.
Cloud microphysical property retrievals from the active microwave instrument on a satellite require the cloud droplet size distribution obtained from aircraft observations as a priori data in the iteration procedure.The cloud lognormal size distributions derived from 12 flights over Beijing,China,in 2008-09 were characterized to evaluate and improve regional CloudSat cloud water content retrievals.We present the distribution parameters of stratiform cloud droplet （diameter ＜500 tm and ＜1500 μm） and discuss the effect of large particles on distribution parameter fitting.Based on three retrieval schemes with different lognormal size distribution parameters,the vertical distribution of cloud liquid and ice water content were derived and then compared with the aircraft observations.The results showed that the liquid water content （LWC） retrievals from large particle size distributions were more consistent with the vertical distribution of cloud water content profiles derived from in situ data on 25 September 2006.We then applied two schemes with different a priori data derived from flight data to CloudSat overpasses in northern China during April-October in 2008 and 2009.The CloudSat cloud water path （CWP） retrievals were compared with Moderate Resolution Imaging Spectroradiometer （MODIS） liquid water path （LWP） data.The results indicated that considering a priori data including large particle size information can significantly improve the consistency between the CloudSat CWP and MODIS CWP.These results strongly suggest that it is necessary to consider particles with diameters greater than 50 tm in CloudSat LWC retrievals.  相似文献   

15.
Radiative transfer calculations have been performed to demonstrate the usefulness of the Meteosat observations in the relative narrow-band of the water vapour absorption (WV, 5.7–7.1 μm) in addition to the observations in the atmospheric infrared window (IR, 10.5–12.5 μm) to deduce the integrated thermal outgoing longwave radiation (OLR). A statistical analysis of colocated and nearly simultaneous Meteosat and the Earth Radiation Budget Experiment (ERBE) data has yielded regression coefficients for estimating the OLR with Meteosat data during the months of April and July 1985. These results have been used to study the mean diurnal variation of the outgoing longwave radiation. The results show that in some cases, because of inadequate time sampling, the form (and especially the phase) of the longwave (LW) diurnal cycle is incorrectly determined by ERBE, but that Meteosat data can improve the determination. In nearly all cases, such errors have little or no influence on the determination of monthly mean LW flux fields.  相似文献   

16.
Aerosol mass size distribution has been measured by using an optical particle counter. The measurements were done in an urban background location in the western Mediterranean during winter 2006. The study has been focused in determining the mass size distribution under special meteorological conditions like moderate rain, considerable winds and high atmospheric stability. The results obtained showed a mass predominance of accumulation mode during rain and high stability periods although for different reasons. In the case of rain, it is due to greater atmospheric cleansing effectiveness that rain has upon coarse mode particles. However, during stagnant periods, the meteorological situation favored coagulation processes among nucleation mode particles giving like result a mass increase in the accumulation mode. Finally, strong winds favor the resuspension of the largest particles and the dispersion of particles with sizes inferior to 7.5 μm. Similar results have been reproduced using principal component analysis (PCA). In this way, three components were identified. The first (PC1) represents particles in the accumulation mode. The second component (PC2) is constituted by coarse particles to 7.5 μm, and the third (PC3) corresponds to coarser particles. The contribution of each group to the overall average concentration was determined: 27.2% corresponds to particles with sizes belonging within the first component, 35.4% to PC2 and 37.3% to PC3. Important percentage variability for each component under meteorological episodes has been obtained. Results obtained showed an important increase of PC1 during Rainy Days (53.8%) and High Pollution Days (40.2%). Contrary to this on Windy Days this component decreases to 7.4%. However, during this kind of day PC3 increases to 64.6%.  相似文献   

17.
The diffusion aerosol spectrometer for the measurements of particle size spectra and concentration levels is described. It includes three principal parts: (i) a block of diffusion batteries for measuring the particles, whose size does not exceed 0.15 μm, (ii) the particle amplifier for growing the particles passing through the diffusion batteries up to optically distinguishable sizes and (iii) the laser aerosol spectrometer, which counts the amplified particles and may also serve for independent measurements of particle size spectra within submicron size range. The tandem including: diffusion batteries+laser aerosol spectrometer allows for detecting particles of radius >3 nm at maximal concentration up to 2×104 particles/cm3. The tandem is managed either by PC or manually. The instrument is designed for studying aerosols in the atmosphere and for ecological measurements.  相似文献   

18.
Balloon-borne aerosol measurements were performed with an optical particle counter between 1994 and 2000 at Ny-Ålesund (79°N), Svarbard. Throughout the observation period, continuous decay was found in the concentrations of particles with 0.4–0.6 μm in radius in the Arctic stratosphere, suggesting that Pinatubo aerosols remained even at the end of the 1990s. The decay rate was clearly higher for larger particle sizes, and higher at higher altitude (e-folding time of 970–526 days), suggesting a gravitational sedimentation effect. For smaller particles with R<0.4 μm, slight increases in concentration with time were found, which agreed with the measurements at mid-latitude. The sulfate mass mixing ratio in the Arctic stratosphere before 1998 showed values higher than those at middle latitude, while values were almost the same in both regions after 1998. A possible explanation of the latitudinal difference is a time lag (of 0.5–1 year) in the arrival of Pinatubo aerosols in the Arctic.  相似文献   

19.
Aqueous concentrations of ionic species observed in cloud water studies often have been in conflict with expectations from model predictions. These inconsistencies result from the size-dependent chemical composition of cloud drops during different stages in the lifetime of a cloud. To study this phenomenon, droplets of clouds need to be collected in different size ranges with high resolution in space and time. The only possibility for this kind of study is the use of an aircraft. Therefore, during the last several years, an attempt was made to develop a mobile cascade impactor, which can be installed outside an aircraft. The cloud water sampled in different size fractions can be transferred into the interior of the aircraft during the measuring flight. The collector is able to sample two size fractions. For continental clouds, the cutoffs are chosen to be >5 and >13.5 μm in diameter. For maritime clouds, the cutoff for the first stage could be shifted to 18.6 μm by lowering the nozzle speed. Prior to field application, the collector was characterized with the aid of “calibration fogs” produced in the laboratory with different drop sizes and different chemical compositions. The characterization included the examination of the cutoffs and the reliability of the sampling procedure with regard to the subsequent chemical analysis. With a collection period of 2 min, collection rates in the order of 0.1–1 cm3 min−1 can be obtained. The collector characterized in this manner was successfully used during measuring flights in clouds over northern Germany. Preliminary concentrations of NH4+, SO42− and Cl− found in the two size fractions of the cloud drops are presented.  相似文献   

20.
Aerosol size spectra (d=10 nm–10 μm) were measured with an electrical aerosol spectrometer (EAS) at Mace Head on the west coast of Ireland. Several small aerosol particle (diameter 10–32 nm) concentration bursts were observed during the measurement period. Relationships between the events, air mass trajectories, tide height, and meteorological parameters are examined. Series of bursts were observed when a spectral transformation due to subsequent particle growth from 10 to 56–100 nm can be identified in an Eulerian experiment. Particle growth rates of between 1 and 3 nm/h were determined. These bursts appear in cold and comparatively clean arctic or polar air masses with temperature and relative humidity fluctuations, and do not correlate with low tide in some cases. These episodes, similar to those frequently found in the continental boundary layer, are thought to occur over a wide area and, for clear detection, require stable airflow for a few days. Elevated small-particle concentration events are more common during low tide or shortly after, and are typically associated with low wind speeds. Here, the increased shore exposure during low tide is thought to influence the nucleation and the subsequent growth of these aerosol particles. The occurrences of the bursts are found to depend on local wind direction. The highest d=10–32 nm particle concentrations appeared for wind sectors furthest from the tidal regions when the wind direction was 150–160°(south-easterly). Most of the events occurred during daytime when solar irradiation is most intense.  相似文献   

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