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1.
Weekly bulk aerosol samples collected at Funafuti, Tuvalu (8°30S, 179°12E), American Samoa (14°15S, 170°35W), and Rarotonga (21°15S, 159°45W), from 1983 through most of 1987 have been analyzed for nitrate and other constituents. The mean nitrate concentration is about 0.11 g m–3 at each of these stations: 0.107±0.011 g m–3 at Funafuti; 0.116±0.008 at American Samoa; and 0.117±0.010 at Rarotonga. Previous measurements of mineral aerosol and trace metal concentrations at American Samoa are among the lowest ever recorded for the near-surface troposphere and indicate that this region is minimally affected by transport of soil material and pollutants from the continents. Consequently, the nitrate concentration of 0.11 g m–3 can be regarded as the natural level for the remote marine boundary layer of the tropical South Pacific Ocean. In contrast, over the tropical North Pacific which is significantly impacted by the transport of material from Asia and North America, the mean nitrate concentrations are about three times higher, 0.29 and 0.36 g m–3 at Midway and Oahu, respectively. The major sources of the nitrate over the tropical South Pacific are still very uncertain. A very significant correlation between the nitrate concentrations at American Samoa and the concentrations of 210Pb suggests that transport from continental sources might be important. This continental source could be lightning, which occurs most frequently over the tropical continents. A near-zero correlation with 7Be indicates that the stratosphere and upper troposphere are probably not the major sources. A significant biogenic source would be consistent with the higher mean nitrate concentrations, 0.16 to 0.17 g m–3, found over the equatorial Pacific at Fanning Island (3°55N, 159°20W) and Nauru (0°32S, 166°57E). The lack of correlation between nitrate and nss sulfate at American Samoa does not necessarily preclude an important role for marine biogenic sources.  相似文献   

2.
Radiative Heat Transfer and Hydrostatic Stability in Nocturnal Fog   总被引：1，自引：0，他引：1  
We have performed a one-dimensional and transient radiative heat transfer analysis in order to investigate interaction between atmospheric radiation and convective instability within a nocturnal fog. The radiation element method using the Ray Emission Model (REM2), which is a generalized numerical method, in conjunction with a line-by-line (LBL) method, is employed to attain high spectral resolution calculations for anisotropically scattering fog. The results show that the convective instability has a strong dependence on radiative properties of the fog. For the condition of a 20-m droplet diameter and liquid water content of 0.1 × 10–3 kg m–3;, the temperature profile within the fog becomes S shaped, and a convective instability layer forms in the middle or lower level of the fog. However, for the same water content and a 40-m diameter droplet, no strong convective instability layer forms, whereas for a 10-m diameter droplet a strong convective instability is observed.  相似文献   

3.
Daily measurements of atmospheric concentrations of dimethylsulfide (DMS) were carried out for two years in a marine site at remote area: the Amsterdam Island (37°50S–77°31E) located in the southern Indian Ocean. DMS concentrations were also measured in seawater. A seasonal variation is observed for both DMS in the atmosphere and in the sea-surface. The monthly averages of DMS concentrations in the surface coastal seawater and in the atmosphere ranged, respectively, from 0.3 to 2.0 nmol l-1 and from 1.4 to 11.3 nmol m-3 (34 to 274 pptv), with the highest values in summer. The monthly variation of sea-to-air flux of DMS from the southern Indian Ocean ranges from 0.7 to 4.4 mol m-2 d-1. A factor of 2.3 is observed between summer and winter with mean DMS fluxes of 3.0 and 1.3 mol m-2 d-1, respectively.  相似文献   

4.
Eighteen soil samples from central Sudan were fractionated by dry sieving ina size fraction from <45 m to >300 m while aerosols generatedfrom these soils were fractionated in the particle size range from 0.25 mto >16 m. The elemental concentrations of soil samples were determinedby energy-dispersive X-ray fluorescence, while the elemental concentrationsof generated aerosols were analysed by particle-induced X-ray emission. Theelements Al, K and Rb show a slight positive fractionation with decreasingparticle size throughout the particle size range studied. The concentrationsof Ca, Mn, Fe, Sr and Y are maximum in the small soil size fraction (<45m) and decrease for the coarse soil size fractions, while in the mineralaerosol particle sizes (0.25– > 16 m) the concentrations remainmore or less constant. The size distributions for Cr, Ti and Zr show a maximumin the particle size range 45–100 m and the concentrations of theseelements decrease sharply in the aerosol fraction down to 16 m to remainconstant in the smaller aerosol fractions.Enrichment factors for the elements were calculated relative to five referencematerials: average crustal rock, average soil, the investigated Sahara bulksoil, the finest fraction of this soil and the aerosol generated from thissoil, and using four reference elements: Al, Si, Ti and Fe. The enrichmentfactors were found to vary significantly depending on the choice of thereference material or the reference element. The enrichment factors for theSudan mineral aerosol were almost identical to those for Khartoum atmosphericaerosol but different from those for Namib mineral aerosol and Israelatmospheric aerosol following dust storms. Multivariate display methods(cluster analysis, principal component analysis and linear discriminantanalysis) were applied to the element ratios in the mineral aerosol from theSahara and Namib and this showed that these mineral aerosol can bedifferentiated into different groups. An attempt was also made to relate themineral aerosol to its parent soil through the use of these multivariatetechniques and the elemental ratios in both the mineral aerosols and the bulksoils (Namib and Sahara). It was also possible using the elemental ratios andthe multivariate display methods to associate the crustal component to themineral aerosol generated from the Sahara.  相似文献   

5.
Airborne measurements made during August 1985 over Greenland and its environs show that both accumulation-mode (0.1 m D2.0 m) and giant (D2 m) particles were present in relatively high concentrations in arctic haze layers and that the accumulation-mode particles dominated light scattering. Particles with diameters (D) between 1 and 4 m consisted predominately of mixed materials, small and dense inclusions, and probably organic compounds containing sulfur. Many of the particles from 0.1 to 1 m in diameter were also of mixed composition, with sulfuric acid, ammonium sulfate and organics probably the dominant constituents.  相似文献   

6.
Although extensive areas of forests and grasslands are burned in the tropics, relatively little scientific attention has been focused on this phenomenon. In order to determine the land area burned and estimate the charcoal (elemental or graphitic carbon) produced, I monitored agricultural burning in a 1145 km2 area in central Panama during the 1981 dry season. Over 10% of the land surface was burned in that year. Charcoal concentrations in the aerosol were also measured and reached values of 3.1 gC/m3 during the peak in burning. Off-peak values of aerosol charcoal are less than 1 gC/m3. The high charcoal concentration reflects the massive amounts of vegetational burning occurring in the area.The charcoal advected by the air mass flowing over the area has been estimated using a box model. Assuming an average aerosol concentration of charcoal of 1 gC/m3 for a three-month burning period, a 2 km atmospheric mixed layer, a 14 km/h wind velocity to the south, and a 150 km wide zone across the western Gulf of Panama watershed, I estimate that, during the dry season, 9×109 g charcoal are mobilized by the troposphere. If 4.1×1012 g phytomass are annually burned in this region, then the charcoal emission factor to the troposphere is 2.2×10–3.  相似文献   

7.
We present the first application of a multi-stage impactor to study volcanic particle emissions to the troposphere from Masaya volcano, Nicaragua. Concentrations of soluble SO4 2–,Cl–, F–, NO3 –, K+, Na+,NH4 +, Ca2+ and Mg2+ were determined in 11 size bins from 0.07 m to >25.5 m. The near-source size distributions showed major modes at 0.5m (SO4 2–, H+,NH4 +); 0.2 m and 5.0 m (Cl–) and 2.0–5.0 m(F–). K+ and Na+ mirrored the SO4 2– size-resolvedconcentrations closely, suggesting that these were transported primarily asK2SO4 and Na2SO4 in acidic solution, while Mg2+ andCa2+ presented modes in both <1 m and >1 m particles. Changes in relative humidity were studied by comparing daytime (transparent plume) and night-time (condensed plume) results. Enhanced particle growth rates were observed in the night-time plume as well as preferential scavenging of soluble gases, such as HCl, by condensed water. Neutralisation of the acidic aerosol by background ammonia was observed at the crater rim and to a greater extent approximately 15 km downwind of the active crater. We report measurements of re-suspended near-source volcanic dust, which may form a component of the plume downwind. Elevated levels ofSO4 2–, Cl–, F–,H+, Na+, K+ and Mg2+ were observed around the 10 m particle diameter in this dust. The volcanic SO4 2– flux leaving the craterwas 0.07 kg s–1.  相似文献   

8.
The Petryanov air filters combined into half-year sets were analyzed for the presence of 40K, 137Cs and 22Na by means of low-background gamma rays spectrometry. Each sample contains aerosols from more than 1 Mm3 of air. Samples were collected in ground level air at Kraków (Southern Poland) from 1996 to 2002. Activity concentrations of 40K are almost constant with the mean of 14.7± 4.5 Bq m–3. Activity concentrations of 137Cs, which are on the level of single Bq m– 3 show exponential decrease with effective half-life time of 7.07± 0.77 years. The cosmogenic 22Na shows a strong seasonal variation with significant different mean values activity concentration between 0.333± 0.095 Bq m–3 and 0.137± 0.045 Bq m–3, for summer and winter, respectively. Moreover, the activity ratio for two cosmogenic radionuclides: 22Na and measured previously 7Be show also changes with statistically significant seasonal differences. The lower values were found during winters. The mechanisms which might govern this ratio are discussed. The conclusion is that transport of 22Na during summer seems to be so much effective, that results in kind of relative depletion of stratosphere of this nuclide.  相似文献   

9.
Rain and air of Florence have been collected in a continuous way andanalysed by flow analysis spectrofluorimetric methods for formaldehydeand hydrogen peroxide. Diurnal and seasonal variations were observed;the mean/maximum concentrations of all data (as gm–3) are 3.3/23.4 for HCHO and 0.4/4.93 forH2O2. The effect of external sources and ofphotochemical reactions produces periods of positive and negativecorrelations for these compounds. The mean/maximum rain concentration ofall data are 98/443 g l–1 for HCHO and 84/685 g l–1 for H2O2. Concentrationratios rain/air and discrepancies to Henry's Law equilibrium arediscussed.  相似文献   

10.
Emission of nitrous oxide from temperate forest soils into the atmosphere   总被引：5，自引：0，他引：5  
N2O emission rates were measured during a 13-month period from July 1981 till August 1982 with a frequency of once every two weeks at six different forest sites in the vicinity of Mainz, Germany. The sites were selected on the basis of soil types typical for many of the Central European forest ecosystems. The individual N2O emission rates showed a high degree of temporal and spatial variabilities which, however, were not significantly correlated to variabilities in soil moisture content or soil temperatures. However, the N2O emission rates followed a general seasonal trend with relatively high values during spring and fall. These maxima coincided with relatively high soil moisture contents, but may also have been influenced by the leaf fall in autumn. In addition, there was a brief episode of relatively high N2O emission rates immediately after thawing of the winter snow. The individual N2O emission rates measured during the whole season ranged between 1 and 92 g N2O-N m–2 h–1. The average values were in the range of 3–11 g N2O-N m–2 h–1 and those with a 50% probability were in the range of 2–8 g N2O-N m–2 h–1. The total source strength of temperate forest soils for atmospheric N2O may be in the range of 0.7–1.5 Tg N yr–1.  相似文献   

11.
Levels of formate and acetate in dew were measured at Dayalbagh, India, usingsurrogate surfaces. The dew formed per night ranged between 0.06 lm–2 and 1.38 l m–2, with an average of 0.59l m–2. pH ranged between 6.7 and 7.4. Mean concentrations offormate and acetate in dew were 10.2 ± 10.2 eql–1 and 7.5 ± 4.5 eq l–1,respectively. The correlation coefficient between the two ions was 0.80 (p =0.001), which suggested that concentrations of these species in dew are linkedtogether. They have either common or different sources with fairly constantstrengths or products of same reaction. Good correlation of formate andacetate with Ca (r = 0.82 and r = 0.70, respectively) and Mg (r = 0.74 and r= 0.71, respectively) suggested that these ions may be associated with Ca andMg after the neutralization process. Deposition rates for formate and acetatein dew per night were 10.2 ± 7.22 mol m–2 pernight and 4.6 ± 2.2 mol m–2 per night,respectively. The theoretical Henry's law constant (K* H)and the field-observed Henry's law coefficient (K* H) ascalculated from concurrent measurements of gas phase and dew for both acidsshowed large discrepancies of three orders of magnitude.  相似文献   

12.
A mass-flux approach is applied to observational data obtained in a convective boundary layer topped with stratocumulus clouds. The observational data were obtained from aircraft measurements during the Atlantic Stratocumulus Transition Experiment (ASTEX). A conditional sampling method is used to calculate average updraft and downdraft values. The vertical fluxes calculated with the mass-flux approach are found to be proportional to the real (measured) fluxes, with a proportionality factor being about 0.6. This value is predicted by theory for two variables having a joint Gaussian distribution function; proportionality factor = 2-1 0.637. The horizontal fractional entrainment and detrainment rates calculated from the data ( 1–2 × 10-2 m-1) are an order of magnitude higher than the rates obtained by large eddy simulations for cumulus convection ( 2–3 × 10-3 m-1) and two orders of magnitude higher than those used in modelling cumulus convection with a mass-flux scheme in an operational weather forecast model ( 3 × 10-4 m-1). A numerical mass-flux model for the thermodynamics was developed and showed that results are in good agreement when compared with measured profiles of the liquid water content.  相似文献   

13.
Characterisation of the carbonate content of atmospheric aerosols   总被引：4，自引：0，他引：4  
A programme of aerosol sampling by dichotomous sampler has been undertaken with analysis of soluble ions plus carbonate. The technique for carbonate involved release of CO2 by HCl vapour and quantitative measurement of the CO2 by FTIR spectroscopy. The method is suitable for amounts down to below 10g CO3 2- per filter. The prevailing atmospheric levels in an urban area were found to be below 1g m-3. Information on the particle size from the dichotomous sampler was supplemented by use of a cascade impactor. Although CaCO3 is clearly the dominant species initial analyses demonstrated higher carbonate levels than could be accounted for on the basis of the concentrations of calcium and magnesium ions which are the most likely cations in mineral carbonates. Analysis with and without heating to 100°C in a vacuum oven demonstrates the presence of carbonate in volatile form. This could be due to carbon dioxide adsorbed onto particulate matter such as soot.  相似文献   

14.
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   

15.
Aerosol chemical composition and trace gas measurements were made at twolocations on the northeastern peninsula of Tenerife during the ACE-2HILLCLOUD experiment, between 28 June and 23 July 1997. Measurementswere made of coarse (#gt;2.5 m aerodynamic diameter) and fine (#lt; 2.5m) aerosol Cl–, NO3 –,SO4 2–, non-sea saltSO4 2– (NSSS),CH3SO3 – (MSA) andNH4 +, and gas phase dimethylsulphide (DMS), HCl,HNO3, SO2, CH3COOH, HCOOH andNH3. Size distributions were measured using a cascadeimpactor. Results show that in marine air masses NSSS and MSA wereformed via DMS oxidation, with additional NSSS present in air massescontaining a continental component. Using a Eulerian box model approachfor aerosols transported between upwind and downwind sites, a mean NSSSproduction rate of 4.36 × 10–4 gm–3 s–1 was calculated for daytimeclear sky periods (highest insolation), with values for cloudy periodsduring daytime and nighttime of 3.55 × 10–4 and2.40 × 10–4 g m–3s–1, respectively. The corresponding rates for MSA were6.23 × 10–6, 8.49 × 10–6and 6.95 × 10–6 g m–3s–1, respectively. Molar concentration ratios forMSA/NSSS were 8.7% (1.8–18.2%) and 1.9%(1.3–3.5%) in clean and polluted air masses, respectively.Reactions occurring within clouds appeared to have a greater influenceon rates of MSA production, than of NSSS, while conversely daytime gasphase reactions were more important for NSSS. For MSA, nighttimein-cloud oxidation rates exceeded rates of daytime gas phase productionvia OH oxidation of DMS. NSSS, MSA and ammonium had trimodal sizedistributions, with modes at 0.3, 4.0 and >10.0 m (NSSS andNH4 +), and 0.3, 1.5 and 4.0 m (MSA). Nosignificant production of other aerosol species was observed, with theexception of ammonium, which was formed at variable rates dependent onneutralisation of the aerosol with ammonia released from spatiallynon-uniform surface sources. Seasalt components were mainly present incoarse particles, although sub-micrometre chloride was also measured.Losses by deposition exceeded calculated expectations for all species,and were highest for the seasalt fraction and nitrate.  相似文献   

16.
Zusammenfassung Auf dem Feldberg (1493 m NN) im südlichen Schwarzwald wurden von Januar 1953 bis Mai 1953 und von September 1954 bis Februar 1955 Messungen von Tropfengröße, Wassergehalt und Sichtweite in Wolken durchgeführt. Die Tropfen der einzelnen Spektren haben einen mittleren Durchmesser zwischen 6 und 30 ; die Durchmesser der größten gemessenen Tropfen liegen zwischen 50 und 60 . Der Wassergehalt hat Werte zwischen 0,003 und 1,54 g/m3, wobei über 90% aller Messungen einen Wassergehalt unter 0,50 g/m3 zeigen. Ein einfacher Zusammenhang zwischen Sichtweite, Tropfengröße und Wassergehalt nach derTrabertschen Formel besteht nicht.Eine enge beziehung wurde zwischen der Form der Spektren und dem Niederschlag gefunden. In Zeiten ohne Niederschlag und bei geringfügigem Niederschlag überwiegen schmale ( < 12,0 ), in Zeiten mit länger andauerndem Niederschlag breite Spektren ( > 12,0 ). Eine Ausnahme machen alle Messungen in Hochnebel, die breite Spektren mit zwischen 13 und 17 zeigen, ohne daß Niederschlag fällt.Das gleichzeitige Vorhandensein der drei Phasen Wasserdampf, flüssiges Wasser und Eisteilchen über längere Zeiträume hinweg wurde festgestellt; somit kann die Eiskristall-Niederschlagstheorie vonBergeron und vonFindeisen für den Mechanismus der Niederschlagsbildung nicht im allgemein angenommenen Ausmaß zutreffen. Es erscheint notwendig, die Frage der Wolkenphysik nicht nur thermodynamisch, sondern auch in vermehrtem maß kolloidphysikalisch im Sinne vonSchmauss zu untersuchen.
Summary Measurements of drop size, water content and visibility in clouds have been carried out from January till May 1953 and from September 1954 to February 1955 on the Feldberg (1493 m. a. s. 1.) in the southern Black Forest. The drops of the different spectra have a mean diameter between 6 and 30 ; the diameters of the largest measured drops lie between 50 and 60 . The water content shows values between 0.003 and 1.54g/m3 whereby more than 90 per cent of all measurements lead to a water content below 0.50 g/m3. A simple relation between visibility, drop size and water content according to the formula ofTrabert does not exist.A closed connection between the form of the spectra and the precipitations has been found. In periods without or with only slight precipitation narrow spectra ( < 12.0 ) prevail, in periods with persistent precipitation broad ones ( > 12.0 ). All measurements in elevanted fog make an exception, showing broad spectra with between 13 and 17 without any precipitation.The simultaneous existance of the 3 phases: water vapour, liquid water and ice particles over alonger periods has been stated; the ice crystall precipitation theory ofBergeron andFindeisen for the mechanism of precipitation can therefore not hold in the generally accepted extent. The examination of the problems of cloud physics appears to be necessary, not only in thermodynamical respect, but also in the sense of coloid physics according toSchmauss.
Résumé On a effectué de janvier à mai 1953 et de septembre 1954 à février 1955 au Feldberg (1493 m.) en Forêt-Noire des mesures de grosseur de gouttelettes, de teneur en eau et de visibilité dans les nuages. Les gouttelettes ont und diamètre moyen compris entre 6 et 30 ; les plus grosses mesurent 50 à 60 . La teneur en eau varie de 0,003 à 1,54 g/m3; dans plus de 90% des mesures, elle reste inférieure a 0,5g/m3. Il n'y a pas de relation simple, qui suivrait la formula deTrabert, entre visibilité, grosseur des gouttelettes et teneur en eau.Il existe par contre une relation étroite entre la forme du spectre de fréquence et les précipitations. Lorsque celles-ci sont nulles ou très faibles ce sont les spectres étroit ( < 12,0 ) qui dominent; par précipitations durables, ce sont les spectres larges ( > ). Dans le stratus, sans pluie, les spectres larges ( entre 13 et 17 ) prédominent.On a constaté l'existence simultanée des trois phases solide, liquide et gazeuse pendant d'assez longs intervalles de temps, de sorte que le théorie deBergeron et deFindeisen concernant la genène des précipitations n'a pas la généralité qu'on lui prête. Il apparaît nécessaire d'aborder les problèmes de la physique des nuages non seulement du point de vue thermodynamique, mais aussi de celui de l'état colloïdal au sens deSchmauss.

Mit 9 Textabbildungen.

Hinsichtlich der Wiedergabe von Tabellen und Tropfenspektren stark verkürzte Fassung einer bei der Technischen Hochschule in Karlsruhe eingereichten und genehmigten Dissertation; die hier weggelssene Dokumentation kann zur Einsichtnahme bezogen werden bei den Bibliotheken der Technischen Hochschule Karlsruhe und des Deutschen Wetterdienstes, Offenbach (Main).  相似文献   

17.
A liquid jet of 90 m diameter and variable length has been utilized to determine absorption rates and, hence, mass accommodation coefficients , of atmospheric trace gases. The compounds investigated are HCl (0.01), HNO3 (0.01), N2O5 (0.005), peroxyacetyl nitrate (>0.001), and HONO (0.005). It is concluded that the absorption of these trace gases by liquid atmospheric water is not significantly retarded by interfacial mass transport. The strengths and limitations of the liquid jet technique for measuring mass accommodation coefficients are explored.  相似文献   

18.
During April 1986, as part of an international arctic air chemistry study (AGASP-2), ground level observations of aerosol trace elements, oxides of sulphur and nitrogen and particle number size distribution were made at Alert Canada (82.5N, 62.3W). Pollution haze was evident as indicated by daily aerosol number (size > 0.15 m diameter) and SO4 = concentrations in the range 125 – 260 cm–3 and 1.6 – 4.5 g m–3, respectively. Haze and associated acidic gases tended to increase throughout the period. SO2 and peroxyacetylnitrate (PAN) mixing ratios were in the range 140 – 480 and 370 – 590 ppt(v), respectively. About 88% of the total end-product nitrogen was in the form of PAN. In air dried to 2% relative humidity by warming to room temperature, the aerosol mass size distribution had a major mode at 0.3 m diameter and a minor one at 2.5 m. Aerosol mass below 1.5 m was well correlated with SO4 =, K+ and PAN. There was a steady increase in the oxidized fraction of total airborne sulphur and nitrogen oxide throughout April as the sun rose above the horizon and remained above. The mean oxidation rate of SO2 between Eurasia and Alert was estimated as 0.25 – 0.5% h–1. The molar ratio of total nitrogen oxide to total sulphur oxide in the arctic atmosphere (0.67±0.17) was comparable to that in European emissions. A remarkably strong inverse correlation of filterable Br and O3 led to the conclusion that O3 destruction and filterable Br production below the Arctic surface radiation inversion is associated with tropospheric photochemical reactions involving naturally occurring gaseous bromine compounds.  相似文献   

19.
Summary The influence of agricultural management on the CO2 budget of a typical subalpine grassland was investigated at the Swiss CARBOMONT site at Rigi-Seebodenalp (1025m a.s.l.) in Central Switzerland. Eddy covariance flux measurements obtained during the first growing season from the mid of spring until the first snow fall (17 Mai to 25 September 2002) are reported. With respect to the 10-year average 1992–2001, we found that this growing season had started 10 days earlier than normal, but was close to average temperature with above-normal precipitation (100–255% depending on month). Using a footprint model we found that a simple approach using wind direction sectors was adequate to classify our CO2 fluxes as being controlled by either meadow or pasture. Two significantly different light response curves could be determined: one for periods with external interventions (grass cutting, cattle grazing) and the other for periods without external interventions. Other than this, meadow and pasture were similar, with a net carbon gain of –128±17g Cm–2 on the undisturbed meadow, and a net carbon loss of 79±17g Cm–2 on the managed meadow, and 270±24g Cm–2 on the pasture during 131 days of the growing season, respectively. The grass cut in June reduced the gross CO2 uptake of the meadow by 50±2% until regrowth of the vegetation. Cattle grazing reduced gross uptake over the whole vegetation period (37±2%), but left respiration at a similar level as observed in the meadow.  相似文献   

20.
On the determination of the height of the Ekman boundary layer   总被引：1，自引：1，他引：1  
The heighth of the Ekman turbulent boundary layer determined by the momentum flux profile is estimated with the aid of considerations of similarity and an analysis of the dynamic equations. Asymptotic formulae have been obtained showing that, with increasing instability,h increases as ¦¦1/2 (where is the non-dimensional stratification parameter); with increasing stability, on the other hand,h decreases as –1/2. For comparison, a simple estimate of the boundary-layer heighth u determined by the velocity profile is given. As is shown, in unstable stratification,h u behaves asymptotically as ¦¦–1, i.e., in a manner entirely different from that ofh .  相似文献   

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