| 1. |
- Berg, Olle H., et al.
(author)
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Comparison of observed and modeled hygroscopic behavior of atmospheric particles
- 1998
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In: Contributions to Atmospheric Physics. - 0005-8173. ; 71:1, s. 47-64
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Research review (peer-reviewed)abstract
- The hygroscopic behavior of sub-micrometer atmospheric aerosol particles was studied with a Tandem Differential Mobility Analyzer (TDMA) at a field site in the Po Valley, Italy. The measurements were done in a continental polluted aerosol during the CHEMDROP fog and haze field experiment at San Pietro di Capofiume in November 1994. In this study, hygroscopic diameter growth factors of individual particles were measured when taken from a dry state, to a relative humidity of 90 %. The aerosol consisted of two groups of particles with different hygroscopic properties, as also seen in an earlier field experiment at the same location in 1989 and at other continental sites. The present work is a closure study on the hygroscopic behavior of sub-micrometer aerosol particles and their mass. Ammonium sulfate was used to model the hygroscopic growth with a model based on thermodynamic data for non-ideal aqueous solutions at water vapor subsaturation. The study was made in two steps: The first step is a comparison between hygroscopic active aerosol volume fractions derived from TDMA measurements on individual particles integrated over the particle size distributions, and collected volume fractions of major ions sampled by size resolved cascade impactors. The model of hygroscopic growth was also used, in the second step, to calculate ambient sizes of individual aerosol particles. These sizes were then compared to the actual ambient sizes as measured by the Droplet Aerosol Analyzer. The result shows agreement, within the estimated errors, between the integrated hygroscopic active volume fractions and the collected volume fractions of inorganic salts, for five events out of six studied. A mass balance could also be obtained between the masses collected with the impactors and the integrated volume distributions, by attributing reasonable densities to the hygroscopically active and inactive fractions. The differences between the calculated and measured ambient sizes were within measurement errors, when Raoult's law was used to model the occasions with relative humidities larger than 95 %.
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| 2. |
- Frank, Göran, et al.
(author)
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Droplet Formation and Growth in Polluted Fogs
- 1998
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In: Contributions to Atmospheric Physics. - 0005-8173. ; 71:1, s. 65-85
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Journal article (peer-reviewed)abstract
- Fog droplet formation and growth related to fog droplet activation were studied in a polluted region. The joint field experiments were carried out at San Pietro Capofiume in northern Italy during November 1994. It was found that the fog droplet number distribution was continuous in the size region 1-47 µm and that for most of the time the fog consisted of unactivated droplets, i.e. the droplets were smaller than the critical diameter for activation according to the Köhler equation. During a few time periods some of the droplets were possibly slightly larger than the critical diameter for activation. The solute concentration in the fog droplets was found to be strongly dependent and decreasing with increasing droplet size. The experimental results were compared with results obtaied using a fog model. Good overall agreement was found between the model and the experimental results, with respect to fog droplet size related to dry residue size, and to fog droplet number distribution. The fog model was also used to study the influence on fog droplet growth of the rate of temperature decrease, the aerosol particle mass load and fog liquid water content. In addition the effect of aging of the fog was also considered.
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| 3. |
- Fuzzi, Sandro, et al.
(author)
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Overview of the Po valley fog experiment 1994 (CHEMDROP)
- 1998
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In: Contributions to Atmospheric Physics. - 0005-8173. ; 71:1, s. 3-19
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Research review (peer-reviewed)abstract
- The paper presents an outline of the CHEMDROP field experiment, carried out in November 1994 at the field station of S. Pietro Capofiume in the Po Valley, Italy. The main objective of the project was to address the issue of the size-dependent chemical composition of fog droplets, by experimentally investigating the following processes, which are expected to affect (or be affected by) the chemical composition of fog droplets as a function of size: a) the connection of the size-dependent chemical composition of CCN to the size-dependent composition of fog droplets; b) the gas/liquid partitioning of the gaseous species NH3, SO2, HCHO, HNO3 in fog; c) the Fe(II)/Fe(III) redox cycle in fog water. Some general results and overall conclusions of the experiment are reported in this paper, while more specific scientific questions are discussed in other companion papers in this issue. CHEMDROP results show that several processes concur in determining the size-dependence of fog droplets chemical composition: nucleation scavenging of pre-existing CCN, fog dynamical evolution and gas/liquid exchange between interstitial air and fog droplets. Chemical transformations in the liquid phase can cause further changes in the chemical composition of the droplets. Only by taking into account the combination of all these processes, is it possible to explain the inhomogeneities in fog droplet chemical composition.
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| 4. |
- Heintzenberg, Jost, et al.
(author)
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Characteristics of haze, mist and fog
- 1998
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In: Contributions to Atmospheric Physics. - 0005-8173. ; 71:1, s. 21-31
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Journal article (peer-reviewed)abstract
- An aerosol and fog data set from a field experiment in November 1994 at San Pietro Capofiume, Northern Italy was analysed. With objective criteria developed from visibility measurements average aerosol characteristics in the dry and wet state were calculated for haze, mist and fog conditions which can be seen as representative for continental air masses in an industrialised region. Dry particle size distributions between three and 800 nm and ambient size distributions between and 50 μm were measured with a system consisting of the three size-segregating particle sensors differential mobility particle sizer, droplet aerosol analyser and FSSP. Systematic changes in particle properties were found for the transition from haze to fog that can be used to derive internally consistent optical aerosol properties. The analysis of the data set demonstrates that atmospheric sensors which have been developed for cloud and fog studies yield useful data for haze conditions.
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| 5. |
- Martinsson, Bengt G., et al.
(author)
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Droplet nucleation and growth in orographic clouds in relation to the aerosol population
- 1999
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In: Atmospheric Research. - 0169-8095. ; 50:3-4, s. 289-315
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Journal article (peer-reviewed)abstract
- 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.
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| 6. |
- Yuskiewicz, Brett A., et al.
(author)
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Changes in submicrometer particle distributions and light scattering during haze and fog events in a highly polluted environment
- 1998
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In: Contributions to Atmospheric Physics. - 0005-8173. ; 71:1, s. 33-45
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Journal article (peer-reviewed)abstract
- The changes in submicrometer atmospheric particle size distributions measured with a Differential Mobility Particle Sizer (DMPS) system during a polluted fog experiment during November, 1994 are presented in this study. Results reveal three modes commonly evident in the size distribution (3 < DN < 843 nm) measurements; the ultrafine, Aitken and accumulation with respective geometric diameters, (Dgn), of 17, 110 and 400 nm. An additional mode, appears between the ultrafine and Aitken modes (Dgn = 52 nm) in approximately one quarter of the measurements and is linked to several industrial cities upwind of the measurement site. A stabile ultrafine mode appears consistently (84% of measurements) at 16-17 nm throughout the campaign, suggestive of a source, such as a highway in the near vicinity. During fog and haze periods number concentrations for particles less than 25 nm and greater than 400 nm decrease by 78 and 95%, respectively. These changes do not affect the aerosol scattering efficiency significantly. The overall aerosol mass scattering efficiency determined for the Po Valley region is 4.3 ± 0.6 m2 g-1. Closure is achieved for light extinction predicted from droplet distributions and measured with a transmissiometer in 37 of 39 cases during fog periods. Measured and calculated light extinction, bext, covary strongly with an R2 of 0.92.
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| 7. |
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| 8. |
- Laj, Paolo, et al.
(author)
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The size dependent composition of fog droplets
- 1998
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In: Contributions to Atmospheric Physics. - 0005-8173. ; 71:1, s. 115-130
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Journal article (peer-reviewed)abstract
- The size dependency of fog droplet concentration and composition was studied using newly developed droplet impactors during the CHEMDROP campaign in the Po Valley (Italy). A strong size dependency of solute concentrations was measured during several fog episodes. The ionic strength of the droplet solutions varies as a function of droplet diameter, showing maximum values in the 9-19 μm diameter range. The solute concentration varies up to a factor of 10 among droplets of different diameter. Similarly, differences of up to 2 pH units are found among droplets of different diameter. The solute dependency of aerosol and droplets species from 0.1 μm to 50 μm is investigated. The monomodal behaviour of the solute concentration in droplets can be explained by both diffusional condensation of the aerosols serving as cloud condensation nuclei (CCN) and the air/liquid transfer of volatile species, in particular for HNO3 and NH3. The distribution of sulphur species is also size-dependent and is directly linked to the pH variations across the droplet spectrum, resulting in HMSA formation in small droplets and S(IV) oxidation large droplets.
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| 9. |
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| 10. |
- Swietlicki, Erik, et al.
(author)
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A closure study of sub-micrometer aerosol particle hygroscopic behaviour
- 1999
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In: Atmospheric Research. - 0169-8095. ; 50:3-4, s. 205-240
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Journal article (peer-reviewed)abstract
- The hygroscopic properties of sub-micrometer aerosol particles were studied in connection with a ground-based cloud experiment at Great Dun Fell, in northern England in 1995. Hygroscopic diameter growth factors were measured with a Tandem Differential Mobility Analyser (TDMA) for dry particle diameters between 35 and 265 nm at one of the sites upwind of the orographic cloud. An external mixture consisting of three groups of particles, each with different hygroscopic properties, was observed. These particle groups were denoted less-hygroscopic, more-hygroscopic and sea spray particles and had average diameter growth factors of 1.11-1.15, 1.38-1.69 and 2.08-2.21 respectively when taken from a dry state to a relative humidity of 90%. Average growth factors increased with dry particle size. A bimodal hygroscopic behaviour was observed for 74-87% of the cases depending on particle size. Parallel measurements of dry sub-micrometer particle number size distributions were performed with a Differential Mobility Particle Sizer (DMPS). The inorganic ion aerosol composition was determined by means of ion chromatography analysis of samples collected with Berner-type low pressure cascade impactors at ambient conditions. The number of ions collected on each impactor stage was predicted from the size distribution and hygroscopic growth data by means of a model of hygroscopic behaviour assuming that only the inorganic substances interacted with the ambient water vapour. The predicted ion number concentration was compared with the actual number of all positive and negative ions collected on the various impactor stages. For the impactor stage which collected particles with aerodynamic diameters between 0.17-0.53 μm at ambient relative humidity, and for which all pertinent data was available for the hygroscopic closure study, the predicted ion concentrations agreed with the measured values within the combined measurement and model uncertainties for all cases but one. For this impactor sampling occasion, the predicted ion concentration was significantly higher than the measured. The air mass in which this sample was taken had undergone extensive photochemical activity which had probably produced hygroscopically active material other than inorganic ions, such as organic oxygenated substances.
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