Interstitial Aerosol Research Articles

Discrimination of micrometre-sized ice and super-cooled droplets in mixed-phase cloud

Preliminary experimental results are presented from an aircraft-mounted probe designed to provide in situ data on cloud particle shape, size, and number concentration. In particular, the probe has been designed to facilitate discrimination between super-cooled water droplets and ice crystals of 1–25 μm size within mixed-phase clouds and to provide information on cloud interstitial aerosols. The probe acquires spatial light scattering data from individual particles at throughput rates of several thousand particles per second. These data are logged at 100 ms intervals to allow the distribution and number concentration of each particle type to be determined with 10 m spatial resolution at a typical airspeed of 100 m s −1. Preliminary results from flight data recorded in altocumulus castellanus, showing liquid water phase, mixed phase, and ice phase are presented to illustrate the probe's particle discrimination capabilities.

Black carbon and other species at a high‐elevation European site (Mount Sonnblick, 3106 m, Austria): Concentrations and scavenging efficiencies

During a recent measurement project, several intensive campaigns were performed on Mount Sonnblick (3106 m above sea level) in the Austrian central range of the Alps. Cloud water and interstitial aerosol samples were obtained from supercooled clouds by using a specially designed cloud water sampler [Kruisz et al., 1993]. The samples were analyzed for black carbon (BC) by an optical technique (integrating sphere [Hitzenberger et al. 1996]) using liquid samples, for major inorganic ions by ion chromatography and for total carbon (TC) by a combusion method. During the fall campaign of 1996, cloud water BC concentrations ranged from 0.45 to 3.64 μg/mL (average concentration 0.85 μg/mL). During the spring 1997 campaign, cloud water BC concentrations ranged from 0.55 to 2.95 μg/mL (average concentration 1.07 μg/mL). The dominant ion in cloud water was SO42− with concentrations from 0.36 to 86.5 μg/mL (average 6.83 μg/mL) in fall 1996 and 0.31–15.4 μg/mL (average 3.06 μg/mL) in spring 1997. In the individual samples, the BC/SO42− ratio ranged from 0.036 to 1.2 (average 0.316) in fall 1996 and 0.036 to 2.04 (average 0.79) in spring 1997. The extreme values were usually confined to short periods within one cloud event. Scavenging efficiencies ε were calculated by using cloud water and interstitial aerosol concentrations from samples obtained simultaneously with the cloud water sampler for the 1997 campaign. For BC, εBC = 0.74 (±0.19) was found, while the values for SO42− and TC were εSO4 = 0.91 (±0.08) and εTC = 0.57 (±0.21), respectively. The findings of an earlier study [Kasper‐Giebl et al., 2000], where εSO4 depended on the liquid water content, were confirmed here for all the three substances.

Combustion Organic Aerosol as Cloud Condensation Nuclei in Ship Tracks

Polycyclic aromatic hydrocarbons (PAHs) have been sampled in marine stratiform clouds to identify the contribution of anthropogenic combustion emissions in activation of aerosol to cloud droplets. The Monterey Area Ship Track experiment provided an opportunity to acquire data on the role of organic compounds in ambient clouds and in ship tracks identified in satellite images. Identification of PAHs in cloud droplet residual samples indicates that several PAHs are present in cloud condensation nuclei in anthropogenically influenced air and do result in activated droplets in cloud. These results establish the presence of combustion products, such as PAHs, in submicrometer aerosols in anthropogenically influenced marine air, with enhanced concentrations in air polluted by ship effluent. The presence of PAHs in droplet residuals in anthropogenically influenced air masses indicates that some fraction of those combustion products is present in the cloud condensation nuclei that activate in cloud. Although a sufficient mass of droplet residuals was not collected to establish a similar role for organics from measurements in satellite-identified ship tracks, the available evidence from the fraction of organics present in the interstitial aerosol is consistent with part of the organic fraction partitioning to the droplet population. In addition, the probability that a compound will be found in cloud droplets rather than in the unactivated aerosol and the compound’s water solubility are compared. The PAHs studied are only weakly soluble in water, but most of the sparse data collected support more soluble compounds having a higher probability of activation.

Dependence of in‐cloud scavenging of polar organic aerosol compounds on the water solubility

In spring 1997 at the Sonnblick Observatory, located at 3106m elevation in the Austrian Alps, interstitial aerosol and cloud water samples were simultaneously collected in supercooled convective clouds. These samples were analyzed for their polar organic composition using a newly developed analytical method that allows the simultaneous determination of dicarboxylic acids, monocarboxylic acids, and other polar organic constituents. Using the obtained data set, in‐cloud scavenging efficiencies (ε) for individual polar organic compounds were calculated. For the different organic substances, scavenging efficiencies ranged from 0.16 to 0.98, compared with sulfate, which exhibited an average scavenging efficiency of 0.94. For dicarboxylic acids, scavenging efficiencies (average of about 0.8) were of the same order as for sulfate. Distinctly lower values (average of about 0.6) were achieved for polar aromatic compounds like phthalic acid or diisobutylphenol. The lowest scavenging efficiencies (average about 0.4) were found for alcohols and monocarboxylic acids. Thus we found in the Sonnblick cloud experiment that more polar organic aerosol constituents are more efficiently scavenged into cloud droplets than less polar compounds. In addition, the scavenging efficiencies exhibited a dependence on the solubilities of the examined compounds. For highly water soluble compounds (1–1000 g L−1) a decrease of the water solubility for an individual compound leads to a decrease in the scavenging efficiency for this compound. For “poorly soluble” substances with water solubilities below l g L−1, a near‐constant value for the scavenging efficiency was found, indicating that their scavenging behavior is then dominated by the scavenging of the bulk noncarbonate carbon independent of the physical and chemical properties of the individual substances.

In situ studies on volatile jet exhaust particle emissions: Impact of fuel sulfur content and environmental conditions on nuclei mode aerosols

In situ measurements of ultrafine aerosol particle emissions were performed at cruise altitudes behind the Deutsches Zentrum für Luft‐und Raumfahrt ATTAS research jet (Rolls‐Royce/Snecma M45H M501 engines) and a B737‐300 aircraft (CFM International 56‐3B1 engines). Measurements were made 0.15–20 s after emission as the source aircraft burned fuel with sulfur contents (FSC) of 2.6, 56, or 118mg kg−1. Particle size distributions of from 3‐ to 60‐nm diameter were determined by using condensationnuclei‐counters with varying lower size detection limits. Volatile particle concentrations in the aircraft plumes strongly increased as diameter decreased toward the sizes of large molecular clusters, illustrating that apparent particle emissions are extremely sensitive to the smallest particle size detectable by the instrument used. Environmental conditions and plume age alone could influence the number of detected ultrafine (volatile) aerosols within an order of magnitude, as well. The observed volatile particle emissions decreased nonlinearly as FSC decreased to 60mg kg−1, reaching minimum values of about 2×1017kg−1 and 2×1016kg−1 for particles >3nm and >5nm, respectively. Volatile particle emissions did not change significantly as FSCs were further reduced below 60mg kg−1. Volatile particle emissions did not differ significantly between the two studied engine types. In contrast, soot particle emissions from the modern CFM56‐3B1 engines were 4–5 times less (4×1014kg−1) than from the older RR M45H M501 engines (1.8×1015kg−1). Contrail processing has been identified as an efficient sink/quenching parameter for ultrafine particles and reduces the remaining interstitial aerosol by factors of 2–10 depending on particle size. These and previously published data are consistent with volatile particle emissions of 2.4×1017kg−1 independent of environmental conditions, engine type and FSCs ranging between 2.6 and 2700mg kg−1. There are clear experimental indications that nonsulfate compounds (probably nonmethane hydrocarbons) begin to dominate the volatile particle composition as FSC decreases below ∼l00mg kg−1.

Study of humic-like substances in fog and interstitial aerosol by size-exclusion chromatography and capillary electrophoresis

Recently, there have been implications that the bulk of the organic carbon in the atmospheric aerosol and fog is contained in an “air polymer” whose chemical nature is poorly understood. Since several properties (e.g. acidity, UV–VIS absorbance, fluorescence) of this polymer were found to be very similar to those of humic substances the term humic-like substances (HULIS) was proposed. In this work size-exclusion chromatography and capillary electrophoresis are used to obtain new information about some properties of the HULIS found in fog water and aqueous extract of interstitial aerosol. Retention and migration behaviour as well as mass spectra are investigated and compared to those of reference humic substances. The capillary electrophoresis is applied for the determination of the range of protonation constants of the HULIS, as well. Scavenging ratio for the HULIS is calculated and found to be about the same as those of ammonium and sulphate. The results imply that such compounds may play a role in cloud condensation.

ACE-2 HILLCLOUD. An overview of the ACE-2 ground-based cloud experiment

The ACE‐2 HILLCLOUD experiment was carried out on the island of Tenerife in June–July 1997 to investigate the interaction of the boundary layer aerosol with a hill cap cloud forming over a ridge to the north‐east of the island. The cloud was used as a natural flow through reactor to investigate the dependence of the cloud microphysics and chemistry on the characteristics of the aerosols and trace gases entering cloud, and to simultaneously study the influence of the physical and chemical processes occurring within the cloud on the size distribution, chemical and hygroscopic properties of the aerosol exiting cloud. 5 major ground base sites were used, measuring trace gases and aerosols upwind and downwind of the cloud, and cloud microphysics and chemistry and interstitial aerosol and gases within the cloud on the hill. 8 intensive measurement periods or runs were undertaken during cloud events, (nocturnally for seven of the eight runs) and were carried out in a wide range of airmass conditions from clean maritime to polluted continental. Polluted air was characterised by higher than average concentrations of ozone (>50 ppbv), fine and accumulation mode aerosols (>3000 and >1500 cm−3, respectively) and higher aerosol mass loadings. Cloud droplet number concentrations N, increased from 50 cm−3 in background maritime air to >2500 cm−3 in aged polluted continental air, a concentration much higher than had previously been detected. Surprisingly, N was seen to vary almost linearly with aerosol number across this range. The droplet aerosol analyser (DAA) measured higher droplet numbers than the corrected forward scattering spectrometer probe (FSSP) in the most polluted air, but at other times there was good agreement (FSSP=0.95 DAA with an r2=0.89 for N<1200 cm−3). Background ammonia gas concentrations were around 0.3 ppbv even in air originating over the ocean, another unexpected but important result for the region. NO2 was present in background concentrations of typically 15 pptv to 100 pptv and NO˙3 (the nitrate radical) was observed at night throughout. Calculations suggest NO˙3 losses were mainly by reaction with DMS to produce nitric acid. Low concentrations of SO2(∼30 pptv), HNO3 and HCl were always present. HNO3 concentrations were higher in polluted episodes and calculations implied that these exceeded those which could be accounted for by NO2 oxidation. It is presumed that nitric and hydrochloric acids were present as a result of outgassing from aerosol, the HNO3 from nitrate rich aerosol transported into the region from upwind of Tenerife, and HCl from sea salt aerosol newly formed at the sea surface. The oxidants hydrogen peroxide and ozone were abundant (i.e., were well in excess over SO2 throughout the experiment). Occasions of significant aerosol growth following cloud processing were observed, particularly in cleaner cases. Observations and modelling suggested this was due mainly to the take up of nitric acid, hydrochloric acid and ammonia by the smallest activated aerosol particles. On a few occasions a small contribution was made by the in‐cloud oxidation of S(IV). The implications of these results from HILLCLOUD for the climatologically more important stratocumulus Marine Boundary Layer (MBL) clouds are considered.

Modelling cloud processing of aerosol during the ACE-2 HILLCLOUD experiment

A numerical model has been used to simulate the conditions observed during the ACE-2 Hillcloud experiment and to study the processes which may be taking place. The model incorporates gas phase chemistry of sulphur and nitrogen compounds upstream of the cloud, and the interaction of aerosol, precursor trace gases and oxidants within the cloud. Gas phase and aerosol inputs to the model have been provided from measurements made in the field. Dynamics of the air flow over the hill consisted of simple prescribed dynamics based on wind speed measurements, and also for some cases modelled dynamics. In this modelling study, it was found that during clean case studies particles down to 40-55 nm diameter were activated to form cloud droplets, the total number of droplets formed ranging from 200 to 400 drops/cm(3) Significant modification of the aerosol spectra due to cloud processing was observed. In polluted cases particles down to 65-80 nm diameter were activated to form cloud droplets, the total number of droplets ranging from 800 to 2800 drops/cm(3). Modification of the aerosol spectra due to cloud processing was slight. In all cases, changes in the aerosol spectra were due to both the uptake of HNO3, HCl, NH3 and SO2 from the gas phase, (the SO2 being oxidised to sulphate) and the repartitioning of species such as HNO3, HCl, and NH3 from larger particles onto smaller ones. Modelling results have been compared with observations made. Modelled droplet numbers are typically within 20% of the best measured values. The mode of the droplet distribution typically around 10 20 mu m for clean cases and 4-8 mu m for polluted cases was found to be in good agreement with the measured values of 10-25 mu m for clean cases, but not in such good agreement for polluted cases. Measurements of upwind and interstitial aerosol distributions showed that the smallest particles activated were 30 and 50 nm for clean and polluted cases respectively, slightly smaller than the model values quoted above. Measured upwind and downwind aerosol spectra showed similar modification to that predicted by the model in eight out of the eleven model runs carried out. Chemistry measurements also give general evidence for both the uptake of species from the gas phase, and repartitioning of species from large particles onto smaller ones, though comparisons For individual cases are more difficult. From this modelling study, it can be concluded that in general, in the remote environment the exchange of hydrochloric acid, nitric acid and ammonia between aerosol particles and take up from the gas phase in the vicinity of cloud may be a very important mechanism in regulating the evolution of the aerosol spectrum. Further, the much more linear relationship between cloud droplet and accumulation mode aerosol number, which was observed in the measurements made during the ACE-2 HILLCLOUD project is supported by these modelling results. The implications of this for the indirect effect will be explored in future work.

Validation of very high cloud droplet number concentrations in air masses transported thousands of kilometres over the ocean

The microstructure of orographic clouds related to the aerosol present was studied during the second Aerosol Characterisation Experiment (ACE-2). Very high cloud droplet number concentrations (almost 3000 cm(-3)) were observed. These high concentrations occurred when clouds formed on a hill slope at Tenerife in polluted air masses originating in Europe that had transported the order of 1000 km over the Atlantic Ocean. The validity of the measured droplet number concentrations was investigated by comparing with measurements of the aerosol upstream of the cloud and cloud interstitial aerosol. Guided by distributions of the ratios between the measurements, three criteria of typically 30% in maximum deviation were applied to the measurements to test their validity. Agreement was found for 88% of the cases. The validated data set spans droplet number concentrations of 150 3000 cm(-3). The updraught velocity during the cloud formation was estimated to 2.2 m s(-1) by model calculations, which is typical of cumuliform clouds. The results of the present study are discussed in relation to cloud droplet number concentrations previously reported in the literature. The importance of promoting the mechanistic understanding of the aerosol/cloud interaction and the use of validation procedures of cloud microphysical parameters is stressed in relation to the assessment of the indirect climatic effect of aerosols.

Observations of the evolution of the aerosol, cloud and boundary-layer characteristics during the 1st ACE-2 Lagrangian experiment

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. DOI: 10.1034/j.1600-0889.2000.00087.x

CUT SIZE MINIMIZATION AND CLOUD ELEMENT BREAK-UP IN A GROUND-BASED CVI

A ground-based Counterflow Virtual Impactor (CVI) was optimized to achieve nearly complete in situ segregation of cloud droplets and ice crystals (with subsequent evaporation, releasing dissolved gaseous and non-volatile material) from their surrounding carrier gas and interstitial aerosol particles. With a one-dimensional numerical model, the CVI cut size D 50 was reduced to 4 μm from 7 μm in an earlier design (Anderson et al., 1993). This could be achieved by a velocity increase to 225 m s −1 inside the wind tunnel forming part of the ground-based CVI, and by minimizing all dimensions contributing to the stagnation length L stag (distance from the wind intersection plane tunnel/CVI to the stagnation plane inside the CVI that cloud elements have to reach to be sampled). CVI and high-speed wind tunnel were designed and constructed according to the modeling results. Subsequent calibrations verified the calculated lower cut sizes D 50 and quantified the slope of the collection efficiency curve in terms of cut sharpness S cut. With the new CVI lower cut sizes between 4 and 6 μm can be achieved. A cloud chamber experiment was performed with CVI measurements supplemented by a Forward Scattering Spectrometer Probe (FSSP). It could be demonstrated that significant drop break-up is caused by wind tunnel velocities well beyond 150 m s −1. For a reduced wind tunnel velocity of 150 m s −1 a reasonable cut size of at least 5 μm could be maintained, while avoiding break-up. The demonstration of break-up should have consequences for any cloud sampling technique featuring high relative velocities of cloudy air past the inlet. In particular, in-cloud retrieval of cloud nuclei concentrations on high-speed airborne platforms could be affected to a significant extent.

Incorporation of aerosol particles between 25 and 850 nm into cloud elements: measurements with a new complementary sampling system

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 D 50 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.

Scavenging Efficiency of ‘Aerosol Carbon’ and Sulfate in Supercooled Clouds at Mt. Sonnblick (3106 m a.s.l., Austria)

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 (eSO) ranged from 0.52 to 0.99 with an average of 0.80. ‘Aerosol carbon’ was scavenged less efficiently with an average value (eAC) of 0.45 and minimum and maximum values of 0.14 and 0.81, respectively. Both eSO and eAC showed a marked, but slightly different, dependence on the liquid water content (LWC) of the cloud. At low LWC, eSO 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 eAC up to an LWC of ≈ 0.5 g/m3. At higher LWCs, e_ remained relatively constant at 0.60. As the differences between eSO and eA 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.

Observations of the evolution of the aerosol, cloud and boundary-layer characteristics during the 1st ACE-2 Lagrangian experiment

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.

Partitioning of the organic aerosol component between fog droplets and interstitial air

Limited information is available on the nature of organic compounds in the tropospheric aerosol and their effect on aerosol hygroscopic properties and cloud condensation nuclei (CCN) ability. Here we analyze samples of liquid droplets and interstitial aerosol, concurrently collected during fog episodes, to determine how the organic compounds are partitioned between the two reservoirs. By comparing the nature and concentration of different organic carbon classes found in the two reservoirs, we find that fog acts as an efficient separator for carbon (C) species on the basis of their chemical properties, with polar water soluble species representing the greater part of total C within fog droplets, and water insoluble C species preferentially found in the interstitial reservoir. Water‐soluble organic species are scavenged by fog droplets to a comparable extent to major inorganic ions and are therefore expected to play an important role in the droplet nucleation process. The main classes of water soluble organic carbon (WSOC) identified in fog water and interstitial aerosol by the techniques traditionally used in aerosol analysis are aliphatic dicarboxylic acids, sugars, aliphatic alcohols, and aliphatic carboxylic acids. However, such species, ∼120 individual compounds, only account for a few percent (&lt;5% on average) of total WSOC. A new class of water soluble macromolecular compounds (MMC), detected in aerosol samples from different areas of the globe, are found to constitute a large fraction (∼40% on average) of WSOC in the fog system (fog droplets plus interstitial aerosol) and represent the main class of water soluble species identified. More than 50% of WSOC still remains undetermined.

A numerical model of the cloud-topped planetary boundary-layer: cloud processing of aerosol particles in marine stratus

In a numerical study with the one-dimensional chemical microphysical stratus model CHEMISTRA the effect of stratiform clouds on aerosol particles is investigated. The microphysical part of CHEMISTRA consists of a detailed description of cloud microphysical processes by means of a joint two-dimensional particle distribution for aerosols and cloud droplets. In the chemical part of the model the particle spectrum is subdivided into three categories referring to inactivated aerosols, small and large cloud droplets. Aqueous phase chemical reactions are separately treated in the two droplet size classes. Numerical results are presented demonstrating that the uptake of trace gases by cloud drops and the sulfate production yield a distinct modification of the physico-chemical microstructure of the activated aerosols. After cloud evaporation a local minimum evolves in the aerosol spectra separating cloud processed from interstitial aerosols. The residual aerosols have an increased mass forming a second maximum in the accumulation mode of the size distributions.

Investigation on water soluble organic compounds in fog droplets and interstitial aerosol particles

Investigation on water soluble organic compounds in fog droplets and interstitial aerosol particles

A numerical model of the cloud-topped planetary boundary-layer: chemistry in marine stratus and the effects on aerosol particles

In a numerical study the effect of stratiform clouds on aerosol particles is investigated. This is done with the one-dimensional chemical microphysical stratus model CHEMISTRA. In the microphysical part of the model special emphasis is layed on a detailed description of cloud microphysical processes by means of a joint two-dimensional particle distribution for aerosols and cloud droplets. In the chemical part of the model the particle spectrum is subdivided into three categories referring to unactivated aerosols, small and large cloud droplets. Aqueous-phase chemical reactions are separately treated in the two droplet size classes. Numerical results show that within the boundary-layer, apart from chemical reactions, the time evolution of chemical reactants is strongly controlled by dynamic and microphysical processes. Due to the turbulent mixing the uptake of highly water soluble gas-phase compounds by cloud droplets yields a strong concentration decrease not only in cloudy regions but also below the cloud. The highest liquid-phase concentrations of these species are found in small cloud droplets while reactants with low water solubility yield higher concentrations in big than in small droplets. In the liquid phase the dominant sulfate source is the nucleation scavenging of aerosols. Sulfate production by chemical reactions takes place during the day when in the gas phase SO 2 is photochemically produced. Hereby the oxidation of sulfur by ozone is most important and is mainly observed in small droplets. The uptake of trace gases by cloud droplets and the sulfate production yield a distinct modification of the physico-chemical microstructure of the cloud condensation nuclei. After cloud evaporation a local minimum evolves in the aerosol spectra separating cloud processed from interstitial aerosols. The residual aerosols have an increased mass forming a second maximum in the accumulation mode of the size distributions.

Droplet nucleation and growth in orographic clouds in relation to the aerosol population

The formation and development of orographic clouds was studied in a field experiment comprising several measurement sites at a mountain ridge. The influence of the aerosol population present on the cloud microstructure was studied in relation to the dynamics in the cloud formation. Droplet nucleation scavenging was investigated by the introduction of a non-dimensional particle diameter related to the process, and it was found that the scavenging rose rapidly in a relatively narrow particle size interval. The size dependency of the scavenging could partly be explained by external mixture of the aerosol. The large particles in the cloud interstitial aerosol was found to be of a chemical nature which allows for only a very weak uptake of water, implying that the chemical composition of these particles rather than entrainment of dry air prevented the droplet nucleation. The aerosol particle number concentration was found to strongly influence the cloud microstructure. Droplet number concentrations up to approximately 2000 cm −3 were observed together with a substantially reduced effective droplet diameter. The observed effect of elevated particle number concentrations in orographic clouds was generalised to the climatologically more important stratiform clouds by the use of a cloud model. It was found that the microstructure of stratiform clouds was strongly dependent on the aerosol population present as well on the dynamics in the cloud formation.

Scavenging of atmospheric constituents in mixed phase clouds at the high-alpine site jungfraujoch part III: Quantification of the removal of chemical species by precipitating snow

In order to study scavenging processes of chemical species in mixed phase clouds, in-cloud field experiments were conducted in October and November 1993 at the high-alpine research station Jung-fraujoch, Switzerland (3450 m asl). During eight snowfall events, interstitial aerosol particles and cloud droplets were collected by a filter pack technique. Simultaneously, snow samples were obtained for subsequent chemical analysis of various trace species like, e.g. NO - 3 and SO 2- 4. In addition, the ice water content was measured. From these measurements, air volume based mass balances between the concentration of a chemical species in the aerosol and cloud phase on the one hand, and the ice phase on the other hand, were calculated. This is a new approach to directly link the different phases within the same air parcel. From the mass balances, apparent ice phase scavenging efficiencies for the trace species were determined. They varied from event to event, but were on the average rather low with the highest value of 0.43 for nitrate. Thus, most of the air-borne chemical mass remained in the air parcel after the precipitation event. A simple model was applied to estimate the amount of air processed by a cloud resulting in an imaginary “column” of air of about 6×10 3 m 3 needed as supply for the water and chemical trace species deposited per square metre per hour.