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1.
During the 1st Lagrangian experiment of the North Atlantic Regional Aerosol Characterisation Experiment (ACE‐2), a parcel of air was tagged by releasing a smart, constant level balloon into it from the Research Vessel Vodyanitskiy . The Meteorological Research Flight's C‐130 aircraft then followed this parcel over a period of 30 h characterising the marine boundary layer (MBL), the cloud and the physical and chemical aerosol evolution. The air mass had originated over the northern North Atlantic and thus was clean and had low aerosol concentrations. At the beginning of the experiment the MBL was over 1500 m deep and made up of a surface mixed layer (SML) underlying a layer containing cloud beneath a subsidence inversion. Subsidence in the free troposphere caused the depth of the MBL to almost halve during the experiment and, after 26 h, the MBL became well mixed throughout its whole depth. Salt particle mass in the MBL increased as the surface wind speed increased from 8 m s−1 to 16 m s−1 and the accumulation mode (0.1μm to 3.0 μm) aerosol concentrations quadrupled from 50 cm−3 to 200 cm−3. However, at the same time the total condensation nuclei (>3 nm) decreased from over 1000 cm−3 to 750 cm−3. The changes in the accumulation mode aerosol concentrations had a significant effect on the observed cloud microphysics. Observational evidence suggests that the important processes in controlling the Aitken mode concentration which, dominated the total CN concentration, included, scavenging of interstitial aerosol by cloud droplets, enhanced coagulation of Aitken mode aerosol and accumulation mode aerosol due to the increased sea salt aerosol surface area, and dilution of the MBL by free tropospheric air.  相似文献   

2.
Aircraft measurements are presented of the Lagrangian evolution of a marine boundary layer over a 30‐h period during the ACE‐2 field campaign. At the start of the observational period, a 500‐m deep polluted marine internal boundary layer (MIBL) was overlain by the remnants of a polluted continental boundary layer extending to around 2 km below a clean, dry free troposphere. The MIBL grew rapidly to a thickness of 900–1000 m in response to increasing sea surface temperatures. No significant aerosol spectral evolution was observed in the boundary layer. Low concentrations of SO2 were observed in the MIBL suggesting that the air mass contained relatively aged aerosol. Aerosol spectra show a broad mode with a modal diameter of around 0.1μm. The polluted layer between the MIBL and the unpolluted free troposphere was only weakly and intermittently turbulent which prevented significant entrainment of clean air into the polluted layer from aloft. The polluted layer depth was thus controlled mainly by subsidence which as a result becomes shallower, decreasing from over 2000 m to around 1200 m during the observational period. The aerosol characteristics of the polluted layer were similar to those in the MIBL and so although the MIBL entrained considerable amounts of air from above the MIBL the aerosol characteristics underwent no significant change. This has important implications for the rate at which a polluted continental air mass is converted to a clean marine one. The dataset should prove useful in the validation of the modelling of continental pollution outbreaks.  相似文献   

3.
During the ACE‐2 field campaign in the summer of 1997 an intensive, ground‐based physical and chemical characterisation of the clean marine and continentally polluted aerosol was performed at Sagres, Portugal. Number size distributions of the dry aerosol in the size range 3–10 000 nm were continuously measured using DMPS and APS systems. Impactor samples were regularly taken at 60% relative humidity (RH) to obtain mass size distributions by weighing the impactor foils, and to derive a chemical mass balance by ion and carbon analysis. Hygroscopic growth factors of the metastable aerosol at 60% RH were determined to estimate the number size distribution at a relative humidity of 60%. A size segregated 3‐way mass closure study was performed in this investigation for the first time. Mass size distributions at 60% RH derived from number size distribution measurements and impactors samples (weighing and chemical analysis) are compared. A good agreement was found for the comparison of total gravimetrically‐determined mass with both number distribution‐derived (slope=1.23/1.09; R2>0.97; depending on the parameters humidity growth and density) and chemical mass concentration (slope=1.02; R2= 0.79) for particles smaller than 3 μm in diameter. Except for the smallest impactor size range relatively good correlations (slope=0.86–1.42) with small deviations (R2=0.76–0.98) for the different size fractions were found. Since uncertainties in each of the 3 methods are about 20% the observed differences in the size‐segregated mass fractions can be explained by the measurement uncertainties. However, the number distribution‐derived mass is mostly higher than the chemically and gravimetrically determined mass, which can be explained by sampling losses of the impactor, but as well with measurement uncertainties as, e.g., the sizing of the DMPS/APS.  相似文献   

4.
The COSAM intercomparison exercise (comparison of large‐scale sulfur models) was organized to compare and evaluate the performance of global sulfur cycle models. Eleven models participated, and from these models the simulated surface concentrations, vertical profiles and budget terms were submitted. This study focuses on simulated budget terms for the sources and sinks of SO2 and sulfate in three polluted regions in the Northern Hemisphere, i.e., eastern North America, Europe, and Southeast Asia. Qualitatively, features of the sulfur cycle are modeled quite consistently between models, such as the relative importance of dry deposition as a removal mechanism for SO2, the importance of aqueous phase oxidation over gas phase oxidation for SO2, and the importance of wet over dry deposition for removal of sulfate aerosol. Quantitatively, however, models may show large differences, especially for cloud‐related processes, i.e., aqueous phase oxidation of SO2 and sulfate wet deposition. In some cases a specific behavior can be related to the treatment of oxidants for aqueous phase SO2 oxidation, or the vertical resolution applied in models. Generally, however, the differences between models appear to be related to simulated cloud (micro‐)physics and distributions, whereas differences in vertical transport efficiencies related to convection play an additional rôle. The estimated sulfur column burdens, lifetimes and export budgets vary between models by about a factor of 2 or 3. It can be expected that uncertainties in related effects which are derived from global sulfur model calculations, such as direct and indirect climate forcing estimates by sulfate aerosol, are at least of similar magnitude.  相似文献   

5.
Microphysical measurements performed during 8 flights of the CLOUDYCOLUMN component of ACE‐2, with the Meteo‐France Merlin‐IV, are analyzed in terms of droplet number concentration and size. The droplet concentration is dependent upon the aerosol properties within the boundary layer. Its mean value over a flight varies from 55 cm−3, for the cleanest conditions, to 244 cm−3, for the most polluted one. For each flight, the variability of the concentration, in selected cloud regions that are not affected by mixing with dry air or drizzle scavenging, ranges from 0.5 to 1.5 of the mean value. The mean volume diameter increases with altitude above cloud base according to the adiabatic cloud model. The frequency distribution of mean droplet volume normalized by the adiabatic value, for the selected regions, shows the same dispersion as the distribution of normalized concentration. The values of droplet concentration versus mean volume diameter are then examined in sub‐adiabatic samples to characterize the effects of mixing and drizzle scavenging. Finally, the ratio of mean volume diameter to effective diameter is analyzed and a simple relationship between these 2 crucial parameters is proposed.  相似文献   

6.
As part of the 2nd A erosol C haracterisation E xperiment (ACE‐2), conducted during summer 1997 in the North Atlantic region between the Canary Islands and Portugal, we measured aerosol optical depths (AOD) at a mid‐tropospheric site, near the top of the volcanic mountain "El Teide"(28°16'N, 16°36' W, 3570 m asl). Our instrument was located at the highest altitude in a network of sunphotometers that extended down to sea level. Clear conditions dominated the ACE‐2 period, and, although suggested by back‐trajectories at 300 hPa, no evidence of anthropogenic pollution was found in our data. Three distinct dust episodes were observed. Vertical soundings and back trajectories suggested mineral dust from the Sahel region as a source. During these episodes, AOD increased an order of magnitude with respect to background conditions (from 0.017 up to 0.19 at λ=500 nm). A shift towards neutrality of the extinction spectral dependence (Ångstrom exponent α down to 0.13), indicated that the coarse mode (particle diameter >2 μm) dominated the aerosol size distribution. For 6 days during the episodes of mineral dust, a monomodal size distribution between 2 and 20 μm diameter was obtained from Mie based size distribution calculations. Estimates, at 500 nm, of the single scattering albedo ω0(0.87–0.96), and the aerosol asymmetry parameter g (0.72–0.73) suggest that the dust layer causes a net cooling forcing at the top of the atmosphere.  相似文献   

7.
The equivalent wavelength ( λ E), at which the aerosol optical depth (AOD) is equal to broadband AOD (BAOD), can change in a wide range from 0.619 μm to 1.575 μm in the usual aerosol conditions. By using the least squares technique and some empirical corrections, a parameterized relationship of λ E with BAOD, Ångström wavelength exponent ( α ), solar zenith angle ( θ 0) and H2O amount is developed. Using this relationship, and based on the strong sensitivity of BAOD on θ 0 when θ 0>70°, the broadband extinction method to derive the spectral AOD and α is further proposed. As shown in comparative simulations to retrieve AOD by the present, Molineaux et al. and Gueymard methods, the present method has the best accuracy in most simulations using Junge, MODTRAN, log‐normal and Deirmendjian aerosol models. A key question of the pyrheliometer method to determine wavelength-dependent AODs is the effect of uncertainty in the aerosol size istribution. It is found that the AOD solution around λ E is less sensitive to the uncertainty. The wavelength exponent α is derived using an assumption of the stable atmospheric turbidity. If the pyrheliometer data from θ 0=85° to 70° are used and the change of the turbidity is ±10%, the error of solution α is usually within ±0.32. If the variation of the turbidity is random, the mean value of a lot of the measurements of α would be very reasonable.  相似文献   

8.
In this study, we present a relationship between total accumulation mode aerosol mass concentrations and cloud droplet number concentrations ( N d). The fundamental aim with the present method is to arrive at a physically‐based conversion algorithm in which each step in the conversion is based on real physical processes that occur and can be observed in the atmosphere, and in which all of the fields involved can be observed or modeled. In the last conversion (the critical part in the algorithm), we use measurements of the size distributions of cloud droplet residual particles for different pollution conditions. This conversion assumes that the size of the residual particles can be described with a lognormal distribution function and uses the Hatch–Choate relationship to convert between residual volume and number. The relatively sparse data set with which we have developed the present algorithm results in a course classification of the aerosol mass field. Consequently, uncertainties need to be recognized when using the algorithm in its present form in model calculations. The algorithm has been used on data from 15 days and the agreement between calculated and observed N d values is, with one exception, within a factor of 2 and for many of these cases also much better than a factor of 2. In addition to the results of the algorithm itself, we also present a least‐squares fit to the predicted N d values. To improve the algorithm in the longer‐term requires more data of scavenging fractions, particle chemical composition and density, and residual particle size distributions as a function of aerosol mass loading and cloud type.  相似文献   

9.
The amounts of microbial and root‐respired CO2 in a maize/winter wheat agricultural system in south western Germany were investigated by measurements of the CO2 mixing ratio and the 13C/12C ratio in soil air. CO2 fluxes at the soil surface for the period of investigation (1993–1995) were also determined. Root respired CO2 shows a strong correlation with the plant mass above ground surface of the respective vegetation (R2≥0.88); the maximum CO2 release from roots was in August for the maize (2.0±0.5 mmol m−2 h−1) and in June for winter wheat (1.5±0.5 mmol m−2 h−1). Maximum CO2 production by roots correlate well with the maximum amount of plant root matter. Integrating the CO2 production over the whole growing season and normalizing to the dry root matter yields, the CO2 production per gram dry organic root matter (DORM) of maize was found to be 0.14±0.03 gC (g DORM)−1. At the sites investigated, root‐produced CO2 contributed (16±4)% for maize, and (24±4)% for winter wheat, respectively, to the total annual CO2 production in the soil (450±50 gC m−2 for maize, 210±30 gC m−2 for winter wheat).  相似文献   

10.
The Saharan Mineral Dust Experiment (SAMUM) was conducted in May/June 2006 in southern Morocco. As part of SAMUM, airborne in situ measurements of the particle size distribution in the diameter range 4 nm < D p < 100 μm were conducted. The aerosol mixing state was determined below D p < 2.5 μm. Furthermore, the vertical structure of the dust layers was investigated with a nadir-looking high spectral resolution lidar (HSRL). The desert dust aerosol exhibited two size regimes of different mixing states: below 0.5 μm, the particles had a non-volatile core and a volatile coating; larger particles above 0.5 μm consisted of non-volatile components and contained light absorbing material. In all cases, particles larger than 10 μm were present, and in 80% of the measurements no particles larger than 40 μm were present. The abundance of large particles showed almost no height dependence. The effective diameter D eff in the dust plumes investigated showed two main ranges: the first range of D eff peaked around 5 μm and the second range of D eff around 8 μm. The two ranges of D eff suggest that it may be inadequate to use one average effective diameter or one parametrization for a typical dust size distribution.  相似文献   

11.
A micro‐pulse lidar system (MPL) was used to measure the vertical and horizontal distribution of aerosols during the Aerosol Characterization Experiment 2 (ACE‐2) in June and July of 1997. The MPL measurements were made at the Izaña observatory (IZO), a weather station located on a mountain ridge (28°18' N, 16°30' W, 2367 m asl) near the center of the island of Tenerife, Canary Islands. The MPL was used to acquire aerosol backscatter, extinction, and optical depth profiles for normal background periods and periods influenced by Saharan dust from North Africa. System tests and calibration procedures are discussed, and an analysis of aerosol optical profiles acquired during ACE‐2 is presented. MPL data taken during normal IZO conditions (no dust) showed that upslope aerosols appeared during the day and dissipated at night and that the layers were mostly confined to altitudes a few hundred meters above IZO. MPL data taken during a Saharan dust episode on 17 July showed that peak aerosol extinction values were an order of magnitude greater than molecular scattering over IZO, and that the dust layers extended to 5 km asl. The value of the dust backscatter–extinction ratio was determined to be 0.027±0.007 sr−1. Comparisons of the MPL data with data from other co‐located instruments showed good agreement during the dust episode.  相似文献   

12.
A set of 152 vertical profiles of aerosol number concentration and size distribution with diameter ranging from 0.12 to 3.0 μm observed by the airborne optical spectrometer probe in Beijing, China, between February 2005 and September 2006 is analysed and discussed. The statistic of aerosol number concentration ( N a) reveals a high aerosol number density in this region with average surface level number concentration ( N 0) of about 6600 cm−3 (0.12–3.0 μm). The average vertical profile of N a approximately satisfies an exponential decline function with a scale height of 1419 m. The N a profiles are influenced by the structures of planetary boundary layer (PBL) significantly and two typical types of N a profile under different conditions of PBL are presented and parametrized in this study. The observations of aerosol size distribution show that, in most cases the aerosol size distributions are not very sensitive to altitude, with effective radii ranging from 0.16 to 0.28 μm. Comparison between aircraft-derived aerosol optical depth (AOD) and Moderate Resolution Imaging Spectroradiometer-derived AOD shows good agreement. The Mie model calculations suggest that the surface level number concentration, the PBL height and the structure of PBL can influence the AOD significantly.  相似文献   

13.
A comparison of large‐scale models simulating atmospheric sulfate aerosols (COSAM) was conducted to increase our understanding of global distributions of sulfate aerosols and precursors. Earlier model comparisons focused on wet deposition measurements and sulfate aerosol concentrations in source regions at the surface. They found that different models simulated the observed sulfate surface concentrations mostly within a factor of two, but that the simulated column burdens and vertical profiles were very different amongst different models. In the COSAM exercise, one aspect is the comparison of sulfate aerosol and precursor gases above the surface. Vertical profiles of SO2, SO2−4, oxidants and cloud properties were measured by aircraft during the North Atlantic Regional Experiment (NARE) experiment in August/September 1993 off the coast of Nova Scotia and during the Second Eulerian Model Evaluation Field Study (EMEFSII), in central Ontario in March/April 1990. While no single model stands out as being best or worst, the general tendency is that those models simulating the full oxidant chemistry tend to agree best with observations although differences in transport and treatment of clouds are important as well.  相似文献   

14.
Measurements of the polarisation state of the atmosphere were performed at Tenerife in June–July 1997, in the framework of ACE‐2 (second Aerosol Characterization Experiment), by 2 ground‐based instruments: RefPol (a LOA prototype) which took measurements at 445, 665, 870, 1610 nm in the solar principal plane; and an automatic CIMEL (CE 318) sun/sky‐photometer which measured polarised radiation at 870 nm in the same observational geometry. Measurements acquired during the campaign, as well as AERONET (AErosol RObotic NETwork) measurements acquired at the sites of Cape Verde and M'Bour, are processed with an algorithm determining the polarised single‐scattering sky‐radiance due to aerosols, directly proportional to the aerosol polarised phase function (representing the probability to scatter polarised radiation in the direction of the scattering angle). A good correlation between the Ångström exponent α, representing the spectral dependence of the extinction measurements, and the polarised phase function is observed on each set of data. The uncertainty of retrievals at 445 nm makes the determination of the spectral dependence of polarisation inconclusive but does not prevent confirming the dependence of the aerosol polarised phase function on α, at all wavelengths. An Ångström exponent of 1 corresponds to a polarised phase function of around 0.1 (±0.04), at 870 nm and at a scattering angle of 60°. For α between 0 and 0.4, the average value of the polarised phase function is 0.05. The correlation shows that polarisation is more sensitive to small particles than to large particles. The discrepancy between retrievals and Mie calculations from an AERONET size distribution, inverted from Izaña measurements acquired during a dust event, suggests the presence of small particles, not detected by total sky‐radiance measurements.  相似文献   

15.
Concentrations of organic carbon (OC), elemental carbon (EC), selected trace elements and water-soluble (WS) ions were determined for samples collected from August 2004 to February 2005 to assess the aerosol background at two remote sites in China. The OC and EC concentrations in PM10 from near the Tibetan Plateau at Zhuzhang (ZUZ) were comparable with other background sites, averaging 3.1 and 0.34 μg m−3, respectively, with no pronounced seasonality. At Akdala (AKD) on northern margin of the Zhungaer Basin, the average concentrations were similar (mean OC = 2.9 μg m−3 and EC = 0.35 μg m−3), but the concentrations were higher in winter. The aerosol mass at both sites was dominated by OC and SO42−, but a stronger contribution from soil dust was observed at AKD. At ZUZ, NO3− showed a unique weather-related fluctuation in PM10 with a periodicity of ∼1 week. Anthropogenic sources in the Sichuan Basin and southeastern Yunnan Province evidently influence ZUZ in summer and autumn while pollutants from Russia and the China–Mongolia border affect AKD nearly all year. The identification of these upwind sources demonstrates that transboundary transport needs to be taken into account when assessing air quality in remote parts of China.  相似文献   

16.
The atmospheric sulfur cycle of the remote Arctic marine boundary layer is studied using trajectories and measurements of sulfur compounds from the International Arctic Ocean Expedition 1991, along with a pseudo-Lagrangian approach and an analytical model. The dimethyl sulfide [DMS(g)] turnover time was  h. Only  % of DMS(g) followed reaction paths to sulfur dioxide [SO2(g)], sub-micrometre aerosol non-seasalt sulfate (nss-SO42−) or methane sulfonate (MSA). During the first 3 d of transport over the pack ice, fog deposition and drizzle resulted in short turnover times;  h for SO2(g),  h for MSA and  h for nss-SO42−. Therefore, DMS(g) will, owing to its origin along or south of the ice edge and longer turnover time, survive the original sub-micrometre sulfur aerosol mass and gradually replace it with new biogenic sulfur aerosol mass. The advection of DMS(g) along with heat and moisture will influence the clouds and fogs over the Arctic pack ice through the formation of cloud condensation nuclei (CCN). If the pack ice cover were to decrease owing to a climate change, the total Arctic Ocean DMS production would change, and potentially there could be an ice–DMS–cloud–albedo climate feedback effect, but it would be accompanied by changes in the fog aerosol sink.  相似文献   

17.
During the 2nd Aerosol Characterization Experiment (ACE‐2), relationships between stratocumulus cloud properties and aerosols were examined. Here, the relevant measurements including the cloud condensation nuclei (CCN) activation spectrum, updraft velocity, cloud microphysical and aerosol properties are presented. It is shown that calculations of droplet concentration based on updraft velocity and the CCN activation spectrum are consistent with direct observations. Also discussed is an apparent disparity among measurements of the CCN activation spectrum, the accumulation mode size distribution, and the composition of the submicrometric aerosol. The observed consistency between CCN, updraft and cloud droplets is a necessary refinement; however, extended analyses of the ACE‐2 data set are needed to guide improvements in model simulations of the interaction between aerosols and cloud microphysics. In particular, there is need for an examination of aerosol size spectra and chemical composition measurements with a view towards validating droplet activation schemes which relate the aerosol and cloud dynamical properties to cloud albedo.  相似文献   

18.
During February 1997, one of the 2 observational periods of CIME ( c loud i ce m ountain e xperiment), a joint field experiment funded by the European Commission, took place on the summit of the Puy de Dōme in the centre of France. During this experiment the droplet spectra were measured with an FSSP and the aerosol particles in the drops and in the interstitial particle phase were measured with a counterflow virtual impactor and a round jet impactor inside a windtunnel. Very low aerosol particle and drop concentrations were observed and particles as small as 25 nm in diameter were found to activate. Two datasets obtained on 15 February and 17 February were used to study the activation of the small Aitken‐mode particles and the spectral form of the droplet spectrum and the scavenging fraction. Numerous sensitivity studies were performed investigating the rôle of the number density and chemical composition of the aerosol particles. The rôle of mixing inside the orographic cloud was studied by using a new technique. It considers the fact that the air arriving on the summit of the Puy de Dôme is a mixture of air of different origins. Thus, it weighs the results of a spectral scavenging model (DESCAM or EXMIX) calculated along a number of individual trajectories. The weighing function is derived from tracer and trajectory studies with a 3‐dimensional mesoscale model. The model was able to reproduce the activation of aerosol particles as small as 25 nm. It was caused by the low aerosol particle number concentrations. In general, we can conclude that the variability found in the sensitivity tests of the dynamical and chemical factors allows to reproduce the shape of the observed results. As too many free parameters exit at the moment we cannot quantify the contribution of each factor studied to the observed scavenging fraction, however, it seems that dynamics dominates.  相似文献   

19.
The present work summarises the results of a 2‐year study of wet‐only sequential deposition in 2 rural areas (Alcan¯iz and Morella) located at different distances from a large coal‐fired power station. Precipitation chemistry was characterised by relatively high , Ca2+ and average concentrations. Sequential study of single precipitation events showed that concentrations of most of the ions studied decreased exponentially throughout single precipitation events, with a sharper decrease in concentrations at the beginning of the event. Usually, 40 to 80% of the wet‐only deposition of major ions occurred in the first 2 mm. pH measurements, ranging from 5.6 to 8.1, showed a decrease in the pH values throughout a precipitation event. Deposition levels of Ca2+ and accounted for the neutralisation of major acidic species in the precipitation events except in 5 rain episodes sampled at Morella. The sequential study of the evolution of the ratio during a single precipitation allowed us to identify potential acidic rainfall fractions after an initial volume of precipitation, after most of the Ca2+ in atmospheric particles had been scavenged. A higher neutralisation capacity was deduced for Alcan¯iz owing to the higher atmospheric levels of natural carbonates and .  相似文献   

20.
CLOUDYCOLUMN is one of the 6 ACE‐2 projects which took place in June‐July 1997, between Portugal and the Canary Islands. It was specifically dedicated to the study of changes of cloud radiative properties resulting from changes in the properties of those aerosols which act as cloud condensation nuclei. This process is also refered to as the aerosol indirect effect on climate. CLOUDYCOLUMN is focused on the contribution of stratocumulus clouds to that process. In addition to the basic aerosol measurements performed at the ground stations of the ACE‐2 project, 5 instrumented aircraft carried out in situ characterization of aerosol physical, chemical and nucleation properties and cloud dynamical and microphysical properties. Cloud radiative properties were also measured remotely with radiometers and a lidar. 11 case studies have been documented, from pure marine to significantly polluted air masses. The simultaneity of the measurements with the multi‐aircraft approach provides a unique data set for closure experiments on the aerosol indirect effect. In particular CLOUDYCOLUMN provided the 1st experimental evidence of the existence of the indirect effect in boundary layer clouds forming in polluted continental outbreacks. This paper describes the objectives of the project, the instrumental setup and the sampling strategy. Preliminary results published in additional papers are briefly summarized.  相似文献   

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