| 1. |
- Miao, J., et al.
(author)
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The potential of polarization measurements from space at mm and sub-mm wavelengths for determining cirrus cloud parameters
- 2003
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In: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 3:1, s. 39-48
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Journal article (peer-reviewed)abstract
- The millimeter and sub-millimeter waves have been attracting a lot of attention recently in the cloud remote sensing community. This is largely because of their potential use in measuring cirrus cloud parameters with airborne or space-borne radiometers. In this study, we examine the possibility of using polarization measurements in this frequency range to get information on the microphysical properties of cirrus clouds. By using a simple radiative transfer model, we calculated the brightness temperature differences at the vertical and horizontal polarization channels for the following seven frequencies: 90, 157, 220, 340, 463, 683, and 874 GHz. The ice crystals in cirrus clouds are modeled with nearly spherical particles, circular cylinder, and circular plate, as well as with mixtures of these types. We found that the polarization difference signal shows a unique "resonance'' feature with the change of ice particle characteristic size: it has a strong response only in a certain range of ice particle size, beyond that range it approaches zero. The size range where this resonance happens depends to a large extent on particle shape and aspect ratio, but to a much less extent on particle orientation. This resonance feature appears even when ice clouds are composed of a mixture of ice crystals in different shapes, although the magnitude and the position of the resonance peak may change, depending on how the mixture is made. Oriented particles generally show larger polarization difference than randomly oriented ones, and plates have larger polarization difference than cylinders. However, the state of particle orientation has a significantly stronger effect on the polarization difference than the particle shape (cylinder or plate). This makes it difficult to distinguish particle shapes using millimeter and sub-millimeter radiometric measurements, if there is no information available on particle orientations. However, if the state of particle shape mixture can be predetermined by other approaches, polarization measurements can help to determine ice particle characteristic size and orientation. This information, in turn, will benefit our retrieval of the ice water path of cirrus clouds.
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| 2. |
- Ketzel, M, et al.
(author)
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Particle size distribution and particle mass measurements at urban, near-city and rural level in the Copenhagen area and Southern Sweden
- 2004
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In: Atmospheric Chemistry and Physics. - 1680-7324. ; 4, s. 281-292
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Journal article (peer-reviewed)abstract
- Particle size distribution (size-range 3-900 nm) and PM10 was measured simultaneously at an urban background station in Copenhagen, a near-city background and a rural location during a period in September-November 2002. The study investigates the contribution from urban versus regional sources of particle number and mass concentration. The total particle number (ToN) and NOx are well correlated at the urban and near-city level and show a distinct diurnal variation, indicating the common traffic source. The average ToN at the three stations differs by a factor of 3. The observed concentrations are 2500# cm(-3), 4500# cm(-3) and 7700# cm(-3) at rural, near-city and urban level, respectively. PM10 and total particle volume (ToV) are well correlated between the three different stations and show similar concentration levels, in average within 30% relative difference, indicating a common source from long-range transport that dominates the concentrations at all locations. Measures to reduce the local urban emissions of NOx and ToN are likely to affect both the street level and urban background concentrations, while for PM10 and ToV only measurable effects at the street level are probable. Taking into account the supposed stronger health effects of ultrafine particles reduction measures should address particle number emissions. The traffic source contributes strongest in the 10-200 nm particle size range. The maximum of the size distribution shifts from about 20-30 nm at kerbside to 50-60 nm at rural level. Particle formation events were observed in the 3-20 nm size range at rural location in the afternoon hours, mainly under conditions with low concentrations of preexisting aerosol particles. The maximum in the size distribution of the "traffic contribution" seems to be shifted to about 28 nm in the urban location compared to 22 nm at kerbside. Assuming NOx as an inert tracer on urban scale allows to estimate that ToN at urban level is reduced by 15-30% compared to kerbside. Particle removal processes, e. g. deposition and coagulation, which are most efficient for smallest particle sizes (<20 nm) and condensational growth are likely mechanisms for the loss of particle number and the shift in particle size.
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| 3. |
- Rissler, Jenny, et al.
(author)
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Physical properties of the sub-micrometer aerosol over the Amazon rain forest during the wet-to-dry season transition - comparison of modeled and measured CCN concentrations
- 2004
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In: Atmospheric Chemistry and Physics. - 1680-7324. ; 4:8, s. 2119-2143
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Journal article (peer-reviewed)abstract
- Sub-micrometer atmospheric aerosol particles were studied in the Amazon region, 125 km northeast of Manaus, Brazil (-1degrees55.2'S, 59degrees28.1'W). The measurements were performed during the wet-to-dry transition period, 4-28 July 2001 as part of the LBA (Large-Scale Biosphere Atmosphere Experiment in Amazonia) CLAIRE-2001 (Cooperative LBA Airborne Regional Experiment) experiment. The number size distribution was measured with two parallel differential mobility analyzers, the hygroscopic growth at 90% RH with a Hygroscopic Tandem Mobility Analyzer (H-TDMA) and the concentrations of cloud condensation nuclei (CCN) with a cloud condensation nuclei counter. A model was developed that uses the H-TDMA data to predict the number of soluble molecules or ions in the individual particles and the corresponding minimum particle diameter for activation into a cloud droplet at a certain supersaturation. Integrating the number size distribution above this diameter, CCN concentrations were predicted with a time resolution of 10 min and compared to the measured concentrations. During the study period, three different air masses were identified and compared: clean background, air influenced by aged biomass burning, and moderately polluted air from recent local biomass burning. For the clean period 2001, similar number size distributions and hygroscopic behavior were observed as during the wet season at the same site in 1998, with mostly internally mixed particles of low diameter growth factor (similar to1.3 taken from dry to 90% RH). During the periods influenced by biomass burning the hygroscopic growth changed slightly, but the largest difference was seen in the number size distribution. The CCN model was found to be successful in predicting the measured CCN concentrations, typically within 25%. A sensitivity study showed relatively small dependence on the assumption of which model salt that was used to predict CCN concentrations from H-TDMA data. One strength of using H-TDMA data to predict CCN concentrations is that the model can also take into account soluble organic compounds, insofar as they go into solution at 90% RH. Another advantage is the higher time resolution compared to using size-resolved chemical composition data.
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| 4. |
- Tunved, P, et al.
(author)
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One year boundary layer aerosol size distribution data from five nordic background stations
- 2003
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In: Atmospheric Chemistry and Physics. - 1680-7324. ; 3, s. 2183-2205
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Journal article (peer-reviewed)abstract
- Size distribution measurements performed at five different stations have been investigated during a one-year period between 01 June 2000 and 31 May 2001 with focus on diurnal, seasonal and geographical differences of size distribution properties. The stations involved cover a large geographical area ranging from the Finnish Lapland (67degrees N) down to southern Sweden (56degrees N) in the order Varrio, Pallas, Hyytiala, Aspvreten and Vavihill. The shape of the size distribution is typically bimodal during winter with a larger fraction of accumulation mode particles compared to the other seasons. Highest Aitken mode concentration is found during summer and spring during the year of study. The maximum of nucleation events occur during the spring months at all stations. Nucleation events occur during other months as well, although not as frequently. Large differences were found between different categories of stations. Northerly located stations such as Pallas and Varrio presented well-separated Aitken and accumulation modes, while the two modes often overlap significantly at the two southernmost stations Vavihill and Aspvreten. A method to cluster trajectories was used to analyse the impact of long-range transport on the observed aerosol properties. Clusters of trajectories arriving from the continent were clearly associated with size distributions shifted towards the accumulation mode. This feature was more pronounced the further south the station was located. Marine- or Arctic-type clusters were associated with large variability in the nuclei size ranges. A quasi-lagrangian approach was used to investigate transport related changes in the aerosol properties. Typically, an increase in especially Aitken mode concentrations was observed when advection from the north occurs, i.e. allowing more continental influence on the aerosol when comparing the different measurement sites. When trajectory clusters arrive to the stations from SW, a gradual decrease in number concentration is experienced in all modes as latitude of measurement site increases.
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| 5. |
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| 6. |
- von Hessberg, P, et al.
(author)
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Ultra-violet absorption cross sections of isotopically substituted nitrous oxide species: (NNO)-N-14-N-14, (NNO)-N-15-N-14, (NNO)-N-14-N-15 and (NNO)-N-15-N-15
- 2004
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In: Atmospheric Chemistry and Physics. - 1680-7324. ; 4:5, s. 1237-1253
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Journal article (peer-reviewed)abstract
- The isotopically substituted nitrous oxide species (NNO)-N-14-N-14, (NNO)-N-15-N-14, (NNO)-N-14-N-15 and (NNO)-N-15-N-15 were investigated by ultra-violet (UV) absorption spectroscopy. High precision cross sections were obtained for the wavelength range 181 to 218 nm at temperatures of 233 and 283 K. These data are used to calculate photolytic isotopic fractionation constants as a function of wavelength. The fractionation constants were used in a three-dimensional chemical transport model in order to simulate the actual fractionation of N2O in the stratosphere, and the results were found to be in good agreement with field studies.
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