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1.
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   

2.
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   

3.
An aerosol dynamics model, AEROFOR2, is developed in the context of the BIOFOR project focussing on boreal forest aerosol. It is the second version of a Lagrangian type box model AEROFOR for investigating the formation and growth of particles under clear sky atmospheric conditions. Particles can consist of soluble and insoluble material and the particle population can be externally or internally mixed. AEROFOR2 includes gas phase chemistry and aerosol dynamics, and calculates the number and composition distributions of particles as functions of time. Observed growth rates of the nucleation mode particles after a typical nucleation event are 2–3 nm/h. The model simulations predict that 3·107 molecules cm−3 of insoluble organic vapour and less than 6·106 molecules cm−3 of soluble vapour condensing onto particles are enough to make them grow in good agreement with the observed growth rates. Then the source rate of the organic vapour must be an order of 105 molecules cm−3 s−1, and its saturation vapour density should be below 106 molecules cm−3. If the aerosol was initially an internal mixture of soluble (70%) and insoluble (30%) constituents it transformed to an externally mixed aerosol during the simulation. By applying the externally‐mixed aerosol based on measured soluble volume fractions, it was concluded that the modelled soluble fraction of the nucleation mode was too low in comparison with the measurements, and thus, a part of the condensable organic vapour must be water soluble.  相似文献   

4.
Physical characterization of aerosol particles during nucleation events   总被引：4，自引：3，他引：4  
Particle concentrations and size distributions have been measured from different heights inside and above a boreal forest during three BIOFOR campaigns (14 April–22 May 1998, 27 July–21 August 1998 and 20 March–24 April 1999) in Hyytiälä, Finland. Typically, the shape of the background distribution inside the forest exhibited 2 dominant modes: a fine or Aitken mode with a geometric number mean diameter of 44 nm and a mean concentration of 1160 cm−3 and an accumulation mode with mean diameter of 154 nm and a mean concentration of 830 cm−3. A coarse mode was also present, extending up to sizes of 20 μm having a number concentration of 1.2 cm−3, volume mean diameter of 2.0 μm and a geometric standard deviation of 1.9. Aerosol humidity was lower than 50% during the measurements. Particle production was observed on many days, typically occurring in the late morning. Under these periods of new particle production, a nucleation mode was observed to form at diameter of the order of 3 nm and, on most occasions, this mode was observed to grow into Aitken mode sizes over the course of a day. Total concentrations ranged from 410–45 000 cm−3, the highest concentrations occurring on particle production days. A clear gradient was observed between particle concentrations encountered below the forest canopy and those above, with significantly lower concentrations occurring within the canopy. Above the canopy, a slight gradient was observed between 18 m and 67 m, with at maximum 5% higher concentration observed at 67 m during the strongest concentration increases.  相似文献   

5.
基于云微物理过程完善的TAU2D分档云模式, 模拟研究了用热泡扰动生成的暖云在不同气溶胶数浓度(Na)背景条件下各演变阶段云微物理量的变化特征。结果表明: 在暖云的发展过程中Na越高, 云滴尺度变小, 云滴间的碰并增长发动较晚, 云滴谱因碰并增长而实现的滴谱拓宽变弱, 云滴谱相对较窄, 云滴谱标准差较小, 因此云体发展越缓慢, 云体生命周期越长, 形成降水就越晚; 反之, 云滴尺度越大, 碰并增长发动越早, 云滴谱拓宽更明显, 云滴谱标准差越大, 云体生命周期相对更短, 降水开始时刻越早。高Na背景下, 碰并阶段云滴谱较凝结阶段更宽, 沉降阶段因云体下沉蒸发导致小尺度云滴减少, 使其滴谱较碰并阶段略有拓宽。在凝结阶段, 低气溶胶背景下云滴数浓度(N)和离散度(ε)间呈现正相关关系, 而高气溶胶背景下两者为负相关关系。在碰并阶段, N与ε的相关性关系为负相关, 且随着气溶胶数浓度的增加, 负相关程度降低。在沉降阶段, N和ε间为负相关关系。  相似文献   

6.
Long-term measurements of fine particle number-size distributions were carried out over 9.5 yr (May 1997–December 2006), in the urban background atmosphere of Helsinki. The total number of days was 3528 with about 91.9% valid data. A new particle formation event (NPF) is defined if a distinct nucleation mode of aerosol particles is observed below 25 nm for several hours, and it shows a growth pattern. We observed 185 NPF events, 111 d were clear non-events and most of the days (around 83.5%) were undefined. The observed events were regional because they were observed at Hyytiälä (250 km north of Helsinki). The events occurred most frequently during spring and autumn. The observed formation rate was maximum during the spring and summer (monthly median 2.87 cm−3 s−1) and the modal growth rate was maximum during late summer and Autumn (monthly median 6.55 nm h−1). The events were observed around noon, and the growth pattern often continued on the following day. The observation of weak NPF events was hindered due to pre-existing particles from both local sources. It is clear that regional NPF events have a clear influence on the dynamic behaviour of aerosol particles in the urban atmosphere.  相似文献   

7.
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   

8.
A two-moment bulk stratiform microphysics scheme, including recently developed physically-based droplet activation/ice nucleation parameterizations has been implemented into the Grid-point Atmospheric Model of IAP LASG (GAMIL) as an effort to enhance the model’s capability to simulate aerosol indirect effects. Unlike the previous one-moment cloud microphysics scheme, the new scheme produces a reasonable representation of cloud particle size and number concentration. This scheme captures the observed spatial variations in cloud droplet number concentrations. Simulated ice crystal number concentrations in cirrus clouds qualitatively agree with in situ observations. The longwave and shortwave cloud forcings are in better agreement with observations. Sensitivity tests show that the column cloud droplet number concentrations calculated from two different droplet activation parameterizations are similar. However, ice crystal number concentration in mixed-phased clouds is sensitive to different heterogeneous ice nucleation formulations. The simulation with high ice crystal number concentration in mixed-phase clouds has less liquid water path and weaker cloud forcing. Furthermore, ice crystal number concentration in cirrus clouds is sensitive to different ice nucleation parameterizations. Sensitivity tests also suggest that the impact of pre-existing ice crystals on homogeneous freezing in old clouds should be taken into account.  相似文献   

9.
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   

10.
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   

11.
The second Aerosol Characterisation Experiment (ACE‐2) was aimed at investigating the physical, chemical and radiative properties of aerosol and their evolution in the North Atlantic region. In the 2nd "Lagrangian" experiment, an air mass was tracked over a 30‐h period during conditions of extensive stratocumulus cover. Boundary‐layer measurements of the aerosol size distribution obtained with a passive cavity aerosol spectrometer probe (PCASP) during the experiment show a gradual growth in size of particles in the 0.1–0.2 μm diameter mode. Simultaneously, SO2 concentrations were found to decrease sharply from 800 to 20 ppt. The fraction of sulphate in aerosol ionic mass increased from 0.68±0.07 to 0.82±0.09 for small particles (diameter below 1.7 μm) and from 0.21±0.04 to 0.34±0.03 for large particles (diameter above 1.7 μm). The measurements were compared with a multicyclic parcel model of gas phase diffusion into cloud droplets and aqueous phase chemical reactions. The model was able to broadly reproduce the observed transformation in the aerosol spectra and the timescale for the transformation of SO2 to sulphate aerosol. The modelled SO2 concentration in the boundary layer fell to below half its initial value over a 6.5‐h time period due to a combination of the entrainment of cleaner tropospheric air and cloud chemical reactions. NH3 and HCl gas were also found to play an important rôle in cloud processing in the model.  相似文献   

12.
The major finding of this study is that aerosols over the tropical Indian Ocean enhance clear sky atmospheric solar heating significantly and decrease the surface solar heating by even a larger amount. The results presented here are based on aerosol chemical, microphysical, and optical and radiometric data collected at the island of Kaashidhoo (4.97°N, 73.47°E) during February and March of 1998, as part of the first field phase of the Indian Ocean experiment (INDOEX). The aerosol optical properties were integrated with a multiple scattering Monte Carlo radiative transfer model which was validated at the surface with broadband flux measurements and at the top of the atmosphere (TOA) with the clouds and earth's radiant energy system (CERES) radiation budget measurements. We consider both externally and internally mixed aerosol models with very little difference between the two models in the estimated forcing. For the February–March period, the aerosols increase the monthly mean clear sky atmospheric solar heating by about 12 W/m2(about 15% of the total atmospheric solar heating) and decrease the sea surface clear sky solar heating by about 16 W/m2 with a daily range from 5 to 23 W/m2. The net aerosol forcing at the top of the atmosphere is about −4 W/m2 with a daily range from −2 to −6 W/m2. Although the soot contributes only about 10% to the aerosol optical thickness, it contributes more than 50% to the aerosol induced atmospheric solar heating. The fundamental conclusion of this study is that anthropogenic aerosols over the tropical Indian Ocean are altering the clear sky radiation budget of the atmosphere and surface in a major manner.  相似文献   

13.
Climate sensitivity to cloud optical properties   总被引：1，自引：0，他引：1  
A radiative–convective model was developed to investigate the sensitivity of climate to cloud optical properties and the related feedback processes. This model demonstrates that the Earth's surface temperature increases with cloud optical depth when the clouds are very thin but decreases with cloud optical depth when the cloud shortwave (solar) radiative forcing is larger than the cloud longwave (terrestrial) radiative forcing. When clouds are included in the model, the magnitude of the greenhouse effect due to a doubling of the CO2 concentration varies with the cloudoptical depth: the thicker the clouds, the weaker the greenhouse warming. In addition, a small variation in the cloud droplet size has a larger impact on the equilibrium state temperature in the lower atmosphere than the warming caused by a doubling of the CO2 concentration: a 2% increase in the average cloud droplet size per degree increase in temperature doubles the warming caused by the doubling of the CO2 concentration. These findings suggest that physically reliable correlations between the cloud droplet size and macrophysical meteorological variables such as temperature, wind and water vapor fields are needed on a global climate scale to assess the climate impact of increases in greenhouse gases.  相似文献   

14.
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   

15.
2018年1月6日河北省出现一次低槽冷锋天气系统,利用两次云粒子垂直探测资料分析层状云结构特征和演化规律,探讨冬季低槽冷锋系统层状云催化条件和催化时机.结果表明:天气系统初期西南风风速中心发生在6500 m高空时,风速随高度呈不连续分布,层状云云顶温度为-29.2℃,中低云层稀薄;3000 m以上云层粒子基本被冰化,云...  相似文献   

16.
This study simulates optical depth of marine warm clouds for year 2001 based on interactively predicted aerosol concentrations with a global chemical transport model (CTM) driven by the ERA-40 re-analysis meteorological data. The simulated aerosol and cloud droplet number concentrations (CDNC) largely reproduce the variations between polluted and pristine marine environment as revealed by surface and aircraft measurements. By constraining cloud liquid water path (CLWP) with satellite microwave measurements, the simulated global and southern hemispheric aerosol optical depth (AOD) and cloud optical depth (COD) are well within 10% of the observed values. As a result of larger anthropogenic aerosol loadings over the northern oceans, the simulated CDNC and COD are, respectively, by 51 and 18% higher than those over the southern oceans, while the column-averaged droplet effective radius is 13% smaller. These simulated interhemispheric differences, while qualitatively consistent with satellite observations, are larger than the observations. Inclusion of drizzle effect improved the disparities but not entirely. The constrained CTM generally captures the seasonality in AOD and CLWP observations, and demonstrates that annual cycle of COD is dominated by CLWP. During winter monsoon the simulated and observed COD correlate more strongly with changes in AOD over the N. Indian Ocean.  相似文献   

17.
利用机载粒子测量系统资料、天气雷达和Ka波段云雷达资料,分析了2017年5月22日河北省一次低槽冷锋降水过程积层混合云的微物理结构。结果表明:降水云系出现在低槽槽前西南气流中,积层混合云为大范围的层状云系中镶嵌大量对流云核结构,0℃层高度位于3577—4004m,随降水过程发展0℃层高度降低,嵌入的对流加强将抬升云顶高度。云内粒子浓度随云内对流的发生和加强而提高,云粒子浓度从1.8×10^5L^-1上升至5.0×10^5L^-1;云内过冷水含量大幅提高,从0.05g·m^-3上升至0.60g·m^-3,冷云中上层过冷水含量可长时间维持在0.20g·m^-3,中上层过冷水占比达60%。对流发生和加强可提高冰晶粒子增长速度,弱对流区冷云低层出现冰晶粒子浓度爆发增长区,强对流区冷云中上层成为冰晶粒子浓度快速增长区;最大降水粒子直径从8000μm增长至10000μm以上,直径在10000μm以上降水粒子谱分布区域从云底向中上层拓展。  相似文献   

18.
This paper presents airborne measurements of ice nuclei (IN) number concentration and elemental composition from the mixed-phase Arctic cloud experiment (M-PACE) in northern Alaska during October 2004. Although the project average IN concentration was low, less than 1 L−1 STP, there was significant spatial and temporal variability, with local maximum concentrations of nearly 60 L−1 STP. Immersion and/or condensation freezing appear to be the dominant freezing mechanisms, whereas mechanisms that occur below water saturation played a smaller role. The dominant particle types identified as IN were metal oxides/dust (39%), carbonaceous particles (35%) and mixtures of metal oxides/dust with either carbonaceous components or salts/sulphates (25%), although there was significant variability in elemental composition. Trajectory analysis suggests both local and remote sources, including biomass burning and volcanic ash. Seasonal variability of IN number concentrations based on this study and data from SHEBA/FIRE-ACE indicates that fall concentrations are depleted relative to spring by about a factor of five. Average IN number concentrations from both studies compare favorably with cloud ice number concentrations of cloud particles larger than 125 μm, for temperatures less than −10 °C. Cloud ice number concentrations also were enhanced in spring, by a factor of ∼2, but only over a limited temperature range.  相似文献   

19.
气溶胶的时空分布及其核化成云的转化过程是云降水物理研究的重点,也是气候变化中气溶胶间接效应关注的热点问题。利用2013～2014年期间在华北中部山西地区开展的9架次夏季晴天和积云天气情况下的气溶胶、云凝结核（CCN）及云滴数浓度观测资料,分析研究了气溶胶的垂直分布、谱分布、来源特征及其与云凝结核、云滴数浓度的转化关系。研究结果表明,大气边界层逆温层结对气溶胶、CCN垂直分布有重要影响,不同天气条件下气溶胶谱型在低层差异较大而高层基本一致;垂直方向上CCN数浓度与气溶胶数浓度有较好的相关性,过饱和度0.3%条件下CCN比率（云凝结核/凝结核）与气溶胶有效直径呈线性关系;积云云下气溶胶与云滴的线性拟合方程为y=1.3x?616.3,拟合相关系数为0.96,气溶胶转化为云滴的比率可达到47%。在过饱和度0.3%条件下,云下CCN与云滴的线性拟合方程为y=1.6x?473.8,拟合相关系数也为0.96,CCN转化为云滴的比率可达到69%。  相似文献   

20.
Measurements of direct solar irradiance were taken employing 4 different sun‐photometers at near infrared wavelengths, suitable for use in atmospheric hygrometry. This technique utilising a set of spectral ratios, in and out of selected water vapour absorption bands, was applied to the measurements to obtain accurate evaluations of precipitable water. For all the hygrometric ratios given by the 4 sun‐photometers used at the 3 stations of Sagres, Monchique and Mt. Foia, during the CLEARCOLUMN experiment, we determined the calibration curves by correcting them for the Rayleigh scattering effects and, then, plotting the natural logarithms of such corrected ratios versus the square root of the water vapour mass present along the atmospheric slant path. The regression lines drawn for the various scatter diagrams were estimated to give evaluations of precipitable water with an uncertainty of less than 5%, 3% and 10% at the 3 stations, respectively. The calibration curves of the sun‐photometer located at the Sagres station were determined using the precipitable water evaluations obtained from the local radiosounding measurements taken on 5 clear‐sky days. Those of the sun‐photometers used at the Monchique and Mt. Foia stations were instead determined through intercomparison between subsets of measurements simultaneously taken with various instruments at Sagres and Mt. Foia. Using these calibration curves, we examined all the field measurements determining the time‐patterns of precipitable water at the 3 stations. During the period from 16 June to 25 July 1997, precipitable water was found to vary between 1.1 and 3.7 g cm−2 at the Sagres station (with an accuracy within ±13%), between 1.0 and 2.8 g cm−2 at Monchique (±11%) and between 0.8 and 3.0 g cm−2 at the top of Mt. Foia (±26%).  相似文献   

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