| 1. |
- Berg, Olle H., et al.
(författare)
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Comparison of observed and modeled hygroscopic behavior of atmospheric particles
- 1998
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Ingår i: Contributions to Atmospheric Physics. - 0005-8173. ; 71:1, s. 47-64
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Forskningsöversikt (refereegranskat)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.
(författare)
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Droplet Formation and Growth in Polluted Fogs
- 1998
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Ingår i: Contributions to Atmospheric Physics. - 0005-8173. ; 71:1, s. 65-85
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Tidskriftsartikel (refereegranskat)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.
(författare)
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Overview of the Po valley fog experiment 1994 (CHEMDROP)
- 1998
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Ingår i: Contributions to Atmospheric Physics. - 0005-8173. ; 71:1, s. 3-19
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Forskningsöversikt (refereegranskat)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. |
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| 5. |
- Martinsson, Bengt G., et al.
(författare)
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Droplet nucleation and growth in orographic clouds in relation to the aerosol population
- 1999
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Ingår i: Atmospheric Research. - 0169-8095. ; 50:3-4, s. 289-315
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Tidskriftsartikel (refereegranskat)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. |
- Svenningsson, Birgitta, et al.
(författare)
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Cloud droplet nucleation scavenging in relation to the size and hygroscopic behaviour of aerosol particles
- 1997
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Ingår i: Atmospheric Environment. - 1352-2310. ; 31:16, s. 2463-2475
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Tidskriftsartikel (refereegranskat)abstract
- The size distributions and hygroscopic growth spectra of aerosol particles were measured during the GCE cloud experiment at Great Dun Fell in the Pennine Hills in northern England. Hygroscopic growth is defined as the particle diameter at 90% RH divided by the particle diameter at 10% RH. The fraction of the aerosol particles scavenged by cloud droplets as a function of particle size was also measured. The general aerosol type was a mixture of marine and aged anthropogenic aerosols. The Aitken and accumulation mode numbers (average ± 1 S.D.) were 1543 ± 1078 and 1023 ± 682 cm-3, respectively. The mean diameters were in the range 30-100 nm and 100-330 nm. The hygroscopic growth spectra were bimodal about half the time. The less-hygroscopic particles had average growth factors of 1.06, 1.06; 1.03, 1.03, and 1.03 for particle diameters of 50, 75, 110, 165, and 265 nm, respectively. For the more-hygroscopic particles of the same sizes, the average hygroscopic growth was 1.34, 1.37, 1.43, 1.47, and 1.53. The effects of ageing on the aerosol particle size distribution and on hygroscopic behaviour are discussed. The scavenged fraction of aerosol particles was a strong function of particle diameter. The diameter with 50% scavenging was in the range 90-220 nm. No tail of smaller particles activated to cloud drops was observed. A small tail of larger particles that remained in the interstitial aerosol can be explained by there being a small fraction of less-hygroscopic particles. A weak correlation between the integral dry particle diameter and the diameter with 50% scavenging was seen.
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| 7. |
- Swietlicki, Erik, et al.
(författare)
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A closure study of sub-micrometer aerosol particle hygroscopic behaviour
- 1999
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Ingår i: Atmospheric Research. - 0169-8095. ; 50:3-4, s. 205-240
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Tidskriftsartikel (refereegranskat)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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| 8. |
- Swietlicki, Erik, et al.
(författare)
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Source identification during the Great Dun Fell Cloud Experiment 1993
- 1997
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Ingår i: Atmospheric Environment. - 1352-2310. ; 31:16, s. 2441-2451
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Tidskriftsartikel (refereegranskat)abstract
- A characterisation of the sources influencing the site for the final field campaign of the EUROTRAC subproject GCE (Ground-based Cloud Experiment) at Great Dun Fell, Cumbria, Great Britain in April-May 1993 is presented. The sources were characterised mainly by means of aerosol filter and cascade impactor data, single particle analysis, gas data, data on aromatic organic compounds, cloud water ionic composition, measurements of aerosol size distributions and hygroscopic properties and various meteorological information. Receptor models applied on the aerosol filter and impactor data sets separately revealed two major source types being a marine sea spray source and a long-range transported anthropogenic pollution source. The results of the receptor models were largely consistent with the other observations used in the source identification. Periods of considerable anthropogenic pollution as well as almost pure marine air masses were clearly identified during the course of the experiment.
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| 9. |
- Swietlicki, Erik, et al.
(författare)
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Urban air pollution source apportionment using a combination of aerosol and gas monitoring techniques
- 1996
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Ingår i: Atmospheric Environment. - : Elsevier BV. - 1352-2310. ; 30:15, s. 2795-2809
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Tidskriftsartikel (refereegranskat)abstract
- The dominating local and regional sources of gaseous and particulate air pollution in the city of Lund (69,000 inhabitants) in southern Sweden were characterized using a combination of aerosol and gas-phase monitoring techniques. Twelve-hour samples were taken at two stations of both fine and coarse fraction urban aerosols with a stacked filter unit during the four-week measurement campaign in March/April 1993. The aerosol samples were analysed with PIXE for 14 major and trace elements. In addition, gaseous species (SO2, NO2, O-3) were measured simultaneously with the DOAS technique over two stretches, crossing each other only 300 m from the downtown aerosol sampling site. The combined results obtained by the analytical techniques PIXE and DOAS for aerosol and gas measurements, respectively, were used in an absolute receptor model in order to identify the major sources affecting the measurement sites and to apportion the measured air pollutants to the identified sources. The gases (SO2, NO2, O-3) could also be attributed to specific sources by combining the trace elemental concentration data for the fine fraction aerosol with that of the gaseous species in the receptor model. According to the absolute principal component analysis (APCA) model, the dominant sources for the measured species were resuspended road dust, automobile and diesel emissions, combustion of oil and coal, ferrous and non-ferrous smelters and sea spray. Of these, the sources related to traffic are mainly local in character while the others constitute a regional background. For a small city in southern Scandinavia like Lund with only light industry, the long-range transported air pollutants make up a significant part of the fine fraction aerosol. (C) 1996 Elsevier Science Ltd
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