Interstitial Aerosol Research Articles

A Collector for Fog Water and Interstitial Aerosol

Abstract An active heatable cloud water collector for ground sampling is presented. The collector can be operated unattended for approximately one week, even in harsh winter conditions. The collection strands are Teflon tubes. A preset cycle of 15-min sampling followed by 250 s of mild heating using wires inserted into the tubes is used. The lower cutoff diameter for fog droplets is 7.3 μm, and its overall collection efficiency is 79% for the liquid water content of fogs at the experimental site in central Europe. It performed reliably during a 2-yr experiment. The collected fog water interacts exclusively with inert materials such as Teflon and Perspex so the collector is well suited for trace analyses of fog water. The collector can be upgraded with an interstitial aerosol collection unit, at the expense of unattended operation. The lower cutoff diameter of the fog water collection strands is 8.1 μm when the interstitial aerosol module is installed. The module efficiently collects particles with diameters <3.5 μm. For these particles, size-segregated samples in four size classes at diameters down to 0.06 μm are collected with a Berner-type impactor. The collector was successfully employed in a mountainous region of central Europe. Over 400 samples were collected within 2 yr. With the collection unit for interstitial aerosol added, 31 samples were collected in a 2-month period.

Microphysical and chemical characteristics of cloud droplet residuals and interstitial particles in continental stratocumulus clouds

During June and July 2003 the Sources and Origins of Atmospheric Cloud Droplets experiment (SOACED) was carried out on a mountain-top site in central Sweden. The main objective of the experiment was to characterise the microphysical and chemical properties of cloud droplet residuals and interstitial aerosol particles in continental clouds and to understand the processes controlling cloud properties at this location. Interstitial and residual aerosol size distributions, cloud liquid water content and species- and size-resolved aerosol mass concentrations are the main variables employed to address questions pertaining to the cloud droplet number concentration and scavenging efficiency during a stratocumulus cloud event observed on July 28, 2003. In this cloud event, about 56% of the aerosol mass was associated with organic species, whilst SO 4 accounted for 23% and NH 4 for 14%. NO 3 and Cl made up about 7% of the total mass. The partitioning of the aerosol particles between cloud droplets and interstitial air has been studied in terms of their microphysical properties. The scavenging efficiency, defined as the fraction of particles activated into cloud elements compared to the total amount of particles, was investigated as a function of size. The scavenging efficiency curves displayed different shapes during the cloud event, from an S-shaped curve, with low scavenging efficiency in the Aitken mode and larger scavenging efficiency in the accumulation mode, to more unusual shapes where Aitken-mode particles were either solely activated or activated in addition to accumulation-mode particles. This study suggests that alterations of the aerosol chemical composition occurred during the measurement period, changing the hygroscopic nature of the CCN and decreasing their activation diameter. It is also hypothesized that entrainment of drier air aloft may have introduced inhomogeneities in the supersaturation field and modified the S-shaped scavenging curves.

Scavenging of black carbon in mixed phase clouds at the high alpine site Jungfraujoch

Abstract. The scavenging of black carbon (BC) in liquid and mixed phase clouds was investigated during intensive experiments in winter 2004, summer 2004 and winter 2005 at the high alpine research station Jungfraujoch (3580 m a.s.l., Switzerland). Aerosol residuals were sampled behind two well characterized inlets; a total inlet which collected cloud particles (droplets and ice particles) as well as interstitial (unactivated) aerosol particles; an interstitial inlet which collected only interstitial aerosol particles. BC concentrations were measured behind each of these inlets along with the submicrometer aerosol number size distribution, from which a volume concentration was derived. These measurements were complemented by in-situ measurements of cloud microphysical parameters. BC was found to be scavenged into the condensed phase to the same extent as the bulk aerosol, which suggests that BC was covered with soluble material through aging processes, rendering it more hygroscopic. The scavenged fraction of BC (FScav,BC), defined as the fraction of BC that is incorporated into cloud droplets and ice crystals, decreases with increasing cloud ice mass fraction (IMF) from FScav,BC=60% in liquid phase clouds to FScav,BC~5–10% in mixed-phase clouds with IMF>0.2. This can be explained by the evaporation of liquid droplets in the presence of ice crystals (Wegener-Bergeron-Findeisen process), releasing BC containing cloud condensation nuclei back into the interstitial phase. In liquid clouds, the scavenged BC fraction is found to decrease with decreasing cloud liquid water content. The scavenged BC fraction is also found to decrease with increasing BC mass concentration since there is an increased competition for the available water vapour.

Aerosol size spectra and CCN activity spectra: Reconciling the lognormal, algebraic, and power laws

Empirical power law expressions of the form NCCN = Csk have been used in cloud physics for over 4 decades to relate the number of cloud condensation nuclei (CCN) and droplets formed on them to cloud supersaturation, s. The deficiencies of this parameterization are that the parameters C and k are usually constants taken from the empirical data and not directly related to the CCN microphysical properties and this parameterization predicts unbounded droplet concentrations at high s. The activation power law was derived in several works from the power law Junge‐type aerosol size spectra and parameters C and k were related to the indices of the power law, but this still does not allow to describe observed decrease of the k‐indices with s and limited NCCN. Recently, new parameterizations for cloud drop activation have been developed based upon the lognormal aerosol size spectra that yield finite NCCN at large s, but does not explain the activation power law, which is still traditionally used in the interpretation of CCN observations, in many cloud models and some large‐scale models. Thus the relation between the lognormal and power law parameterizations is unclear, and it is desirable to establish a bridge between them. In this paper, algebraic and power law equivalents are found for the lognormal size spectra of partially soluble dry and wet interstitial aerosol, and for the differential and integral CCN activity spectra. This allows derivation of the power law expression for cloud drop activation from basic thermodynamic principles (Köhler theory). In the new power law formulation, the index k and coefficient C are obtained as continuous analytical functions of s and expressed directly via parameters of aerosol lognormal size spectra (modal radii, dispersions) and physicochemical properties. This approach allows reconciliation of this modified power law and the lognormal parameterizations and their equivalence is shown by the quantitative comparison of these models as applied to several examples. The advantages of this new power law relationship include bounded Nd at high s, quantitative explanation of the experimental data on the k‐index and possibility to express k(s) and C(s) directly via the aerosol microphysics. The modified power law provides a framework for using the wealth of data on the C, k parameters accumulated over the past decades, both in the framework of the power law and the lognormal parameterizations.

Effects of Externally-Through-Internally-Mixed Soot Inclusions within Clouds and Precipitation on Global Climate

This paper examines the incremental global climate response of black carbon (BC), the main component of soot, due to absorption and scattering by BC inclusions within cloud and precipitation particles. Modeled soot is emitted as an externally mixed aerosol particle. It evolves to an internal mixture through condensation, hydration, dissolution, dissociation, crystallization, aqueous chemistry, coagulation, and cloud processing. Size-resolved cloud liquid and ice particles grow by condensation onto size-resolved soot and other particles. Cloud particles grow to precipitation by coagulation and the Bergeron process. Cloud and precipitation particles also undergo freezing, melting, evaporation, sublimation, and coagulation with interstitial aerosol particles. Soot, which is tracked in cloud and precipitation particles of all sizes, is removed by rainout, washout, sedimentation, and dry deposition. Two methods of treating the optics of BC in size-resolved cloud liquid, ice and graupel are compared: the core-shell approximation (CSA) and the iterative dynamic effective medium approximation (DEMA). The 10-year global near-surface incremental temperature response due to fossil fuel (ff), biofuel (bf), and biomass burning (bb) BC within clouds with the DEMA was slightly stronger than that with the CSA, but both enhancements were <+0.05 K. The ff+bf portion may be approximately 60% of the total, suggesting that BC inclusions within clouds may enhance the near-surface temperature response of ff+bf soot due to all processes (estimated as approximately 0.27 K), by <10%, strengthening the possible climate impact of BC. BC cloud absorption was also found to increase water vapor, decrease precipitation, and decrease cloud fraction. The increase in water vapor at the expense of precipitation contributed to warming in addition to that of the cloud BC absorption itself. Aerosol-hydrometeor coagulation followed by hydrometeor evaporation may have caused almost twice the BC internal mixing as aerosol-aerosol coagulation.

Modeling studies of the sulfur cycle in low-level, warm stratiform clouds

A one-dimensional cloud model with size-resolved microphysics has been fully coupled with size-resolved aqueous-phase chemistry. The model, driven by prescribed dynamics, has been used to study the sulfur cycle in low-level, warm stratiform clouds. Model evaluation and sensitivity tests show that the model can reasonably simulate the time-evolution of cloud properties, interstitial aerosols, gas to aqueous conversions and size-resolved cloud water acidity. Model results show that gaseous NH 3 and H 2O 2 can be removed quickly by the aqueous-phase process within the cloud layer due to the very small size of cloud droplets. 10% to 50% of SO 2 within the cloud layer can be scavenged in a 1-h period depending on the concentrations of other gaseous species, especially those of NH 3 and H 2O 2. Aqueous-phase chemistry can effectively transfer mass from gas to aqueous particles in low-level, warm stratiform clouds. The rate of transfer from gaseous SO 2 to aqueous sulfate depends on the cloud properties, which are controlled by the initial cloud condensation nuclei (CCN) from which the cloud forms, and on gaseous concentrations of other species such as NH 3 and H 2O 2. Aqueous-phase chemistry might broaden the cloud droplet spectra by increasing the number concentrations of both smallest and largest droplets, and thus enhance precipitation formation.

MIRAGE: Model description and evaluation of aerosols and trace gases

The Model for Integrated Research on Atmospheric Global Exchanges (MIRAGE) modeling system, designed to study the impacts of anthropogenic aerosols on the global environment, is described. MIRAGE consists of a chemical transport model coupled online with a global climate model. The chemical transport model simulates trace gases, aerosol number, and aerosol chemical component mass (sulfate, methane sulfonic acid (MSA), organic matter, black carbon (BC), sea salt, and mineral dust) for four aerosol modes (Aitken, accumulation, coarse sea salt, and coarse mineral dust) using the modal aerosol dynamics approach. Cloud‐phase and interstitial aerosol are predicted separately. The climate model, based on Community Climate Model, Version 2 (CCM2), has physically based treatments of aerosol direct and indirect forcing. Stratiform cloud water and droplet number are simulated using a bulk microphysics parameterization that includes aerosol activation. Aerosol and trace gas species simulated by MIRAGE are presented and evaluated using surface and aircraft measurements. Surface‐level SO2 in North American and European source regions is higher than observed. SO2 above the boundary layer is in better agreement with observations, and surface‐level SO2 at marine locations is somewhat lower than observed. Comparison with other models suggests insufficient SO2 dry deposition; increasing the deposition velocity improves simulated SO2. Surface‐level sulfate in North American and European source regions is in good agreement with observations, although the seasonal cycle in Europe is stronger than observed. Surface‐level sulfate at high‐latitude and marine locations, and sulfate above the boundary layer, are higher than observed. This is attributed primarily to insufficient wet removal; increasing the wet removal improves simulated sulfate at remote locations and aloft. Because of the high sulfate bias, radiative forcing estimates for anthropogenic sulfur given in 2001 by S. J. Ghan and colleagues are probably too high. Surface‐level dimethyl sulfide (DMS) is ∼40% higher than observed, and the seasonal cycle shows too much DMS in local winter, partially caused by neglect of oxidation by NO3. Surface‐level MSA at marine locations is ∼80% higher than observed, also attributed to insufficient wet removal. Surface‐level BC is ∼50% lower than observed in the United States and ∼40% lower than observed globally. Treating BC as initially hydrophobic would lessen this bias. Surface‐level organic matter is lower than observed in the United States, similar to BC, but shows no bias in the global comparison. Surface‐level sea salt concentrations are ∼30% lower than observed, partly caused by low temporal variance of the model's 10 m wind speeds. Submicrometer sea salt is strongly underestimated by the emissions parameterization. Dust concentrations are within a factor of 3 at most sites but tend to be lower than observed, primarily because of neglect of very large particles and underestimation of emissions and vertical transport under high‐wind conditions. Accumulation and Aitken mode number concentrations and mean sizes at the surface over ocean, and condensation nuclei concentrations aloft over the Pacific, are in fair agreement with observations. Concentrations over land are generally higher than observations, with mean sizes correspondingly lower than observations, especially at some European locations. Increasing the assumed size of emitted particles produces better agreement at the surface over land, and reducing the particle nucleation rate improves the agreement aloft over land.

Thermal stability analysis of particles incorporated in cirrus crystals and of non-activated particles in between the cirrus crystals: comparing clean and polluted air masses

Abstract. A thermal volatility technique is used to provide indirect information about the chemical composition of the aerosol involved in cirrus cloud formation. The fraction of particles that disappears after being heated to 125°C is termed volatile and the fraction that disappears between 125 and 250°C is termed semi-volatile. Particles that still remain after being heated to 250°C make up the non-volatile fraction. The thermal composition of residual particles remaining from evaporated cirrus crystals is presented and compared to interstitial aerosol particles (non-activated particles in between the cirrus crystals) for two temperature regimes (cold: T&lt; -38°C, warm: -38≤T&lt; -23°C), based on in-situ observations. The observations were conducted in cirrus clouds in the Southern Hemisphere (SH) and Northern Hemisphere (NH) midlatitudes during the INCA project. In the cold temperature regime, the non-volatile fraction of the residual particles was typically in the range 10 to 30% in the NH and 30 to 40% in the SH. In the warm temperature regime, the non-volatile residual fraction was typically 10 to 30% (NH) and 20 to 40% (SH). At high crystal number densities the non-volatile fraction in both temperature regimes was even higher: in the range of 30 to 40% (NH) and 40 to 50% (SH). The semi-volatile fraction was typically less than 10% in both hemispheres, causing the volatile fraction to essentially be a complement to the non-volatile fraction. In terms of the fractioning into the three types of particles, the SH cold case is clearly different compared to the other three cases (the two warm cases and the cold NH case), which share many features. In the NH data the distribution of different particle types does not seem to be temperature dependent. In all the cases, the non-volatile fraction is enriched in the residual particles compared to the fractions observed for the interstitial particles. This enrichment corresponds to about 15 (NH) and 30 (SH) percent units in the two cold cases and to 15-25 (NH) and 25-35 (SH) percent units in the two warm cases. In the NH cold case, there is a clear relation between the fractions observed in the interstitial particles and what is observed in the residual particles. The observed large fractions of non-volatile particles show that particles forming ice crystals are not entirely made up of water-soluble sulfate particles.

Single particle measurements of the chemical composition of cirrus ice residue during CRYSTAL‐FACE

The first real‐time, in situ, investigation of the chemical composition of the residue of cirrus ice crystals was performed during July 2002. This study was undertaken on a NASA WB‐57F high‐altitude research aircraft as part of CRYSTAL‐FACE, a field campaign which sought to further our understanding of the relation of clouds, water vapor, and climate by characterizing, among other parameters, anvil cirrus formed about the Florida peninsula. A counter flow virtual impactor (CVI) was used to separate cirrus ice from the unactivated interstitial aerosol particles and evaporate condensed‐phase water. Residual material, on a crystal‐by‐crystal basis, was subsequently analyzed using the NOAA Aeronomy Laboratory's Particle Analysis by Laser Mass Spectrometry (PALMS) instrument. Sampling was performed from 5 to 15 km altitude and from 12° to 28° north latitude within cirrus originating over land and ocean. Chemical composition measurements provided several important results. Sea salt was often incorporated into cirrus, consistent with homogeneous ice formation by aerosol particles from the marine boundary layer. Size measurements showed that large particles preferentially froze over smaller ones. Meteoritic material was found within ice crystals, indicative of a relation between stratospheric aerosol particles and tropospheric clouds. Mineral dust was the dominant residue observed in clouds formed during a dust transport event from the Sahara, consistent with a heterogeneous freezing mechanism. These results show that chemical composition and size are important determinants of which aerosol particles form cirrus ice crystals.

Aerosol-cirrus interactions: a number based phenomenon at all?

Abstract. In situ measurements of the partitioning of aerosol particles within cirrus clouds were used to investigate aerosol-cloud interactions in ice clouds. The number density of interstitial aerosol particles (non-activated particles in between the cirrus crystals) was compared to the number density of cirrus crystal residuals. The data was obtained during the two INCA (Interhemispheric Differences in Cirrus Properties from Anthropogenic Emissions) campaigns, performed in the Southern Hemisphere (SH) and Northern Hemisphere (NH) midlatitudes. Different aerosol-cirrus interactions can be linked to the different stages of the cirrus lifecycle. Cloud formation is linked to positive correlations between the number density of interstitial aerosol (Nint) and crystal residuals (Ncvi), whereas the correlations are smaller or even negative in a dissolving cloud. Unlike warm clouds, where the number density of cloud droplets is positively related to the aerosol number density, we observed a rather complex relationship when expressing Ncvi as a function of Nint for forming clouds. The data sets are similar in that they both show local maxima in the Nint range 100 to 200cm, where the SH- maximum is shifted towards the higher value. For lower number densities Nint and Ncvi are positively related. The slopes emerging from the data suggest that a tenfold increase in the aerosol number density corresponds to a 3 to 4 times increase in the crystal number density. As Nint increases beyond the ca. 100 to 200cm, the mean crystal number density decreases at about the same rate for both data sets. For much higher aerosol number densities, only present in the NH data set, the mean Ncvi remains low. The situation for dissolving clouds allows us to offer two possible, but at this point only speculative, alternative interactions between aerosols and cirrus: evaporating clouds might be associated with a source of aerosol particles, or air pollution (high aerosol number density) might retard ice particle evaporation rates.

Cirrus cloud occurrence as function of ambient relative humidity: a comparison of observations obtained during the INCA experiment

Abstract. Based on in-situ observations performed during the Interhemispheric differences in cirrus properties from anthropogenic emissions (INCA) experiment, we introduce and discuss the cloud presence fraction (CPF) defined as the ratio between the number of data points determined to represent cloud at a given ambient relative humidity over ice (RHI) divided by the total number of data points at that value of RHI. The CPFs are measured with four different cloud probes. Within similar ranges of detected particle sizes and concentrations, it is shown that different cloud probes yield results that are in good agreement with each other. The CPFs taken at Southern Hemisphere (SH) and Northern Hemisphere (NH) midlatitudes differ from each other. Above ice saturation, clouds occurred more frequently during the NH campaign. Local minima in the CPF as a function of RHI are interpreted as a systematic underestimation of cloud presence when cloud particles become invisible to cloud probes. Based on this interpretation, we find that clouds during the SH campaign formed preferentially at RHIs between 140 and 155%, whereas clouds in the NH campaign formed at RHIs somewhat below 130%. The data show that interstitial aerosol and ice particles coexist down to RHIs of 70-90%, demonstrating that the ability to distinguish between different particle types in cirrus conditions depends on the sensors used to probe the aerosol/cirrus system. Observed distributions of cloud water content differ only slightly between the NH and SH campaigns and seem to be only weakly, if at all, affected by the freezing aerosols.

In-situ observations of aerosol particles remaining from evaporated cirrus crystals: Comparing clean and polluted air masses

Abstract. In-situ observations of aerosol particles contained in cirrus crystals are presented and compared to interstitial aerosol size distributions (non-activated particles in between the cirrus crystals). The observations were conducted in cirrus clouds in the Southern and Northern Hemisphere mid-latitudes during the INCA project. The first campaign in March and April 2000 was performed from Punta Arenas, Chile (54°S) in pristine air. The second campaign in September and October 2000 was performed from Prestwick, Scotland (53°N) in the vicinity of the North Atlantic flight corridor. Size distribution measurements of crystal residuals (particles remaining after evaporation of the crystals) show that small aerosol particles (Dp&lt; 0.1 mm) dominate the number density of residuals. The crystal residual size distributions were significantly different in the two campaigns. On average the residual size distributions were shifted towards larger sizes in the Southern Hemisphere. For a given integral residual number density, the calculated particle volume was on average three times larger in the Southern Hemisphere. This may be of significance to the vertical redistribution of aerosol mass by clouds in the tropopause region. In both campaigns the mean residual size increased slightly with increasing crystal number density. The form of the residual size distribution did not depend on temperature as one might have expected considering different modes of nucleation. The observations of ambient aerosol particles were consistent with the expected higher pollution level in the Northern Hemisphere. The fraction of residual particles only contributes to approximately a percent or less of the total number of particles, which is the sum of the residual and interstitial particles. Excellent agreement between the CVI and FSSP-300 probes was found supporting the assumption that each crystal is associated with only one residual particle.

Size‐dependent scavenging efficiencies of multicomponent atmospheric aerosols in clouds

The size‐dependent scavenging efficiencies of inorganic and carbonaceous fractions of aerosol have been investigated in clouds present at the Puy de Dôme (1465 m above sea level, France) in winter. In‐cloud interstitial aerosols and residues of evaporated cloud droplets are separated by a round jet impactor (RJI) collecting the interstitial phase and a counterflow virtual impactor (CVI) collecting cloud droplets hereinafter evaporated. In addition, interstitial and droplet phases are collected together in a whole air inlet. Particles (interstitial aerosol and evaporated cloud droplets) are subsequently sampled using low‐pressure cascade impactors, weighted, and analyzed for major ions, organic (OC) and elemental (EC) carbon. RJI and CVI data provide mass‐scavenging efficiencies of the different aerosol components. The scavenged fraction of inorganic species ions (εinorg) is 0.76 ± 0.07 on a first accumulation mode (0.2 ± 0.1 μm geometric diameter (GMD) (Acc1) and 0.93 ± 0.03 on a second (0.5 ± 0.2 μm GDM) accumulation mode. Instead, the scavenged fraction of organic carbon (εOC) is much lower with averaged values of 0.14 ± 0.07 on both modes. These differences are clearly indicative of a high degree of external mixing between inorganic and organic aerosols. Gravimetric data confirm these values of εOC and εinorg with total mass scavenging efficiency of 0.41 ± 0.07 and 0.54 ± 0.07 for Acc1 and Acc2 modes, respectively. Higher mass scavenging efficiencies are obtained for EC than OC (εEC(Acc1) = 0.33 ± 0.09 and εEC (Acc2) = 0.74 ± 0.09). This suggests either internal mixing between black carbon and more soluble species such as sulfate or nitrate or presence on the surface of EC particle of species containing more hygroscopic properties.

Mass balance of free tropospheric aerosol at the Puy de Dôme (France) in winter

The size‐segregated chemical composition of aerosols was investigated during winters 2000 and 2001 at Puy de Dôme (1465 m above sea level, France), a site most of the time located in the free troposphere. Aerosols have been sampled using low‐pressure cascade impactors (Electrical Low Pressure Impactor (ELPI) and Small Deposition Impactor (SDI) 13 and 12 stages) and analyzed for inorganic (Na+, NH4+, K+, Mg2+, Ca2+, Cl−, NO3−, and SO42−) and organic (HCOO−, CH3COO−, and C2O42−) ions, organic and elemental carbon (OC and EC), insoluble dust, and total mass. Under cloudy conditions, the sampling includes interstitial aerosol as well as the residue of evaporated cloud droplets. Aerosols (and residues of cloud droplets) were sampled in different air masses, which can be classified into three different categories according to their aerosol load and composition: background (BG), anthropogenic (ANT), and specific events (EV) that include advection of Saharan dust and upward transport from the polluted boundary layer to the site. On the basis of the presence or absence of coarse sea‐salt particles, a further classification permits us to distinguish air masses that have or have not been exposed to the ocean. A closed mass balance is achieved on submicron ranges (mean departure of 18.5%) for the three main air mass categories, providing a reliable description of main aerosol types in the west European free troposphere. The total aerosol mass at 50% relative humidity is close to 2.7 ± 0.6 μg m−3 in BG, 5.3 ± 1.0 μg m−3 in ANT, and 15 to 22 μg m−3 in EV air masses. The aerosol mass distribution generally exhibits two submicron modes (Acc1 at 0.2 ± 0.1 μm and Acc2 at 0.5 ± 0.2 μm geometric mean diameter (calculated for every impactor stage) and a supermicron mode (2 ± 1 μm). Aerosols exhibit a high degree of external mixing with carbonaceous (EC and OC) and ionic species associated with Acc1 and Acc2. Concentrations of light carboxylates and mineral dust never exceed 4% of the total content of analyzed compounds, except for a Saharan dust event during which the contribution of insoluble dust reaches 26% of the total aerosol mass. Depending on the sampled air mass, bulk water‐soluble inorganic species and carbonaceous material account for 25–70% and 15–60% of the total mass, respectively. The OC fraction is higher in air masses with low aerosol load (53%, 32%, and 22% for BG, ANT, and EV, respectively). Conversely, the EC fraction is enhanced from 4% in BG to 10% in ANT and 14% in EV. The inorganic fraction is more abundant in EV (55%) and ANT (60%) than in BG (40%) air masses as a result of enhanced nit .

Deposition of trace substances via cloud droplets in the Atlantic Rain Forest of the Serra Do Mar, São Paulo State, SE Brazil

The Atlantic Rain Forest of the Serra do Mar close to Cubatão, São Paulo State, SE Brazil, is severely affected by the emissions of a big industrial complex. Measurements of trace substance concentrations in air as well as in rain were carried out over a period of 6 years to investigate the interrelation of pollution and vegetation damage. Due to the local atmospheric circulation, orographic clouds occur very frequently at the top of the Serra do Mar, where vegetation damage also is very high. Therefore, additional information was required about trace substance deposition via cloud water. During three field experiments, various samples of cloud water and of the interstitial aerosol were taken by help of a sampling device especially modified for this purpose, and were analysed for the major anions and cations (hydrogen ions, ammonium, chloride, sodium, nitrate and sulphate). The trace substance concentrations found in cloud water were in the same range as for polluted sites in Europe. Over all samples, about 90–100% of the trace substances—both gaseous and particulate matter—found in the atmosphere before a cloud event were found in the cloud water. A resistance model using meteorological input data (wind speed, atmospheric liquid water content) was adapted to the characteristics of the Atlantic Rain Forest to estimate the cloud water deposition to vegetation. The results from cloud water analyses and from modelled cloud water deposition were combined to investigate the ion deposition to the vegetation. A rough estimate of the annual deposition showed, that the deposition via rain is in the order of one magnitude (factor 6–40) higher than that via cloud. The high amount of water deposition via precipitation overcompensates the higher trace substance concentrations in cloud water. Furthermore the trace substance deposition to vegetation via cloud water in the Atlantic Rain Forest is in the order of one magnitude lower than for typical German spruce forests due to the higher capture efficiency of the needles compared to that one of the leaves in the Serra do Mar.

Aerosol distributions and cloud interactions at a mountaintop laboratory

The first measurements of ultrafine (3–10 nm diameter) condensation nuclei concentrations and particle size distributions at Storm Peak Laboratory (3210 MSL) are presented. In‐cloud average hourly average ultrafine concentrations peaked at 1608 cm−3 in the 1300 hour, MST. Under clear conditions in April and May 2001, average hourly average ultrafine concentrations reached 8947 cm−3 during the 1400 hour, MST. The highest observed 5‐minute average ultrafine concentration was 32000 cm−3. A comparison of clear‐air and interstitial aerosol size distributions indicates that particles larger than 0.5 μm were efficiently removed by snowing, mixed‐phase clouds. Particles with diameters between 0.03–0.04 and 0.2 μm appeared to serve as cloud condensation nuclei. The observed loss of particles smaller than 16 nm in‐cloud may be explained by diffusion to cloud drops.

Size-dependent aerosol activation at the high-alpine site Jungfraujoch (3580 m asl)

Microphysical and chemical aerosol properties and their influence on cloud formation were studied in a field campaign at the high-alpine site Jungfraujoch (JFJ, 3580 m asl). Due to its altitude, this site is suitable for ground-based in-cloud measurements, with a high cloud frequency of 40%. Dry total and interstitial aerosol size distributions [18 nm <particle diameter (Dp)<800 nm] were determined with a time resolution of 6 min. A forward scattering spectrometer probe (FSSP-100) measured the cloud droplet size distribution, and a particle volume monitor (PVM-100) was used to measure liquid water content (LWC). In addition, the aerosol chemical composition (major soluble ions) was determined in two size classes (total and sub-micron particles). Agreement within the range of measurement uncertainties was observed between the droplet number concentrations derived from the aerosol size distribution measurements (total minus interstitial) and those measured by the FSSP. The observed particle diameter at 50% activation (D50) was typically around 100 nm for LWC > 0.15 g m−3. Below this value, D50 increased with decreasing LWC. A dependence of D50 on the accumulation mode (Dp>100 nm) number concentration (Ntot,Dp>100) was only found for concentrations less than 100 cm−3. For higher values of Ntot,Dp>100 the D50 remained constant. Furthermore, a decrease of the effective radius of cloud droplets (Reff) with increasing Ntot,Dp>100 was observed, providing experimental evidence for the microphysical relation predicted by the Twomey effect. A modified Köhler model was used to quantify the critical supersaturation for the aerosol observed at the JFJ. Ambient supersaturations were determined from the derived supersaturation curve and the calculated D50. As an example, a critical supersaturation of 0.2% was found for 100 nm particles.

Size-dependent aerosol activation at the high-alpine site Jungfraujoch (3580 m asl)

Microphysical and chemical aerosol properties and their influence on cloud formation were studied in a field campaign at the high-alpine site Jungfraujoch (JFJ, 3580 m asl). Due to its altitude, this site is suitable for ground-based in-cloud measurements, with a high cloud frequency of 40%. Dry total and interstitial aerosol size distributions [18 nm &lt;particle diameter (Dp)&lt;800 nm] were determined with a time resolution of 6 min. A forward scattering spectrometer probe (FSSP-100) measured the cloud droplet size distribution, and a particle volume monitor (PVM-100) was used to measure liquid water content (LWC). In addition, the aerosol chemical composition (major soluble ions) was determined in two size classes (total and sub-micron particles). Agreement within the range of measurement uncertainties was observed between the droplet number concentrations derived from the aerosol size distribution measurements (total minus interstitial) and those measured by the FSSP. The observed particle diameter at 50% activation (D₅₀) was typically around 100 nm for LWC &gt; 0.15 g m⁻³. Below this value, D₅₀ increased with decreasing LWC. A dependence of D₅₀ on the accumulation mode (Dp&gt;100 nm) number concentration (N_{tot,Dp&gt;100}) was only found for concentrations less than 100 cm⁻³. For higher values of N_{tot,Dp&gt;100} the D50 remained constant. Furthermore, a decrease of the effective radius of cloud droplets (R_eff) with increasing Ntot,Dp&gt;100 was observed, providing experimental evidence for the microphysical relation predicted by the Twomey effect. A modified Köhler model was used to quantify the critical supersaturation for the aerosol observed at the JFJ. Ambient supersaturations were determined from the derived supersaturation curve and the calculated D50. As an example, a critical supersaturation of 0.2% was found for 100 nm particles.

The Cloud Ice Mountain Experiment (CIME) 1998: experiment overview and modelling of the microphysical processes during the seeding by isentropic gas expansion

The second field campaign of the Cloud Ice Mountain Experiment (CIME) project took place in February 1998 on the mountain Puy de Dôme in the centre of France. The content of residual aerosol particles, of H 2O 2 and NH 3 in cloud droplets was evaluated by evaporating the drops larger than 5 μm in a Counterflow Virtual Impactor (CVI) and by measuring the residual particle concentration and the released gas content. The same trace species were studied behind a round jet impactor for the complementary interstitial aerosol particles smaller than 5 μm diameter. In a second step of experiments, the ambient supercooled cloud was converted to a mixed phase cloud by seeding the cloud with ice particles by the gas release from pressurised gas bottles. A comparison between the physical and chemical characteristics of liquid drops and ice particles allows a study of the fate of the trace constituents during the presence of ice crystals in the cloud. In the present paper, an overview is given of the CIME 98 experiment and the instrumentation deployed. The meteorological situation during the experiment was analysed with the help of a cloud scale model. The microphysics processes and the behaviour of the scavenged aerosol particles before and during seeding are analysed with the detailed microphysical model ExMix. The simulation results agreed well with the observations and confirmed the assumption that the Bergeron–Findeisen process was dominating during seeding and was influencing the partitioning of aerosol particles between drops and ice crystals. The results of the CIME 98 experiment give an insight on microphysical changes, redistribution of aerosol particles and cloud chemistry during the Bergeron–Findeisen process when acting also in natural clouds.

Sulfur dioxide oxidation in clouds at Whiteface Mountain as a function of drop size

In situ oxidation of SO2 has been determined in clouds as a function of droplet size using a trace element technique during July 1998 at Whiteface Mountain, New York. The pH of the cloud water ranged from 2.8 to 4.7 with a mean of 3.4, and therefore SO2 oxidation was dominated by hydrogen peroxide. Size‐fractioned cloud samples were collected from six different events at the mountain's summit (1.5 km above mean sea level) using a size‐fractionating California Institute of Technology Active Strand Cloudwater Collector. Bulk samples were collected using both passive and active collectors. During each event, below‐cloud and interstitial aerosols were collected every 2 hours. Cloud water and aerosol samples were analyzed for major ions and selected trace elements. Continuous measurements of gas phase species SO2, H2O2, and O3 were carried out at the summit and below‐cloud sites. Concentrations of cloud water SO42−, NO3−, H2O2, and trace elements, as well as pH, were largely independent of droplet size. The component of cloud water SO42− produced from in situ oxidation (SO2−4in) was also largely independent of droplet size. The results are in agreement with calculated relative production rates in the small and large drop sizes based on known laboratory reaction rates.