| 1. |
- Kommula, S. M., et al.
(författare)
-
Effect of Long-Range Transported Fire Aerosols on Cloud Condensation Nuclei Concentrations and Cloud Properties at High Latitudes
- 2024
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 51:6
-
Tidskriftsartikel (refereegranskat)abstract
- Active vegetation fires in south-eastern (SE) Europe resulted in a notable increase in the number concentration of aerosols and cloud condensation nuclei (CCN) particles at two high latitude locations—the SMEAR IV station in Kuopio, Finland, and the Zeppelin Observatory in Svalbard, high Arctic. During the fire episode aerosol hygroscopicity κ slightly increased at SMEAR IV and at the Zeppelin Observatory κ decreased. Despite increased κ in high CCN conditions at SMEAR IV, the aerosol activation diameter increased due to the decreased supersaturation with an increase in aerosol loading. In addition, at SMEAR IV during the fire episode, in situ measured cloud droplet number concentration (CDNC) increased by a factor of ∼7 as compared to non-fire periods which was in good agreement with the satellite observations (MODIS, Terra). Results from this study show the importance of SE European fires for cloud properties and radiative forcing in high latitudes.
|
|
| 2. |
- Dalirian, Maryam, et al.
(författare)
-
CCN activation of fumed silica aerosols mixed with soluble pollutants
- 2015
-
Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 15:7, s. 3815-3829
-
Tidskriftsartikel (refereegranskat)abstract
- Particle-water interactions of completely soluble or insoluble particles are fairly well understood but less is known of aerosols consisting of mixtures of soluble and insoluble components. In this study, laboratory measurements were performed to investigate cloud condensation nuclei (CCN) activity of silica particles mixed with ammonium sulfate (a salt), sucrose (a sugar) and bovine serum albumin known as BSA (a protein). The agglomerated structure of the silica particles was investigated using measurements with a differential mobility analyser (DMA) and an aerosol particle mass analyser (APM). Based on these data, the particles were assumed to be compact agglomerates when studying their CCN activation capabilities. Furthermore, the critical super-saturations of particles consisting of pure and mixed soluble and insoluble compounds were explored using existing theoretical frameworks. These results showed that the CCN activation of single-component particles was in good agreement with Kohler- and adsorption theory based models when the agglomerated structure was accounted for. For mixed particles the CCN activation was governed by the soluble components, and the soluble fraction varied considerably with particle size for our wet-generated aerosols. Our results confirm the hypothesis that knowing the soluble fraction is the key parameter needed for describing the CCN activation of mixed aerosols, and highlight the importance of controlled coating techniques for acquiring a detailed understanding of the CCN activation of atmospheric insoluble particles mixed with soluble pollutants.
|
|
| 3. |
- Leung, Wing -Y. H., et al.
(författare)
-
Sensitivity of a continental night-time stratocumulus-topped boundary layer to varying environmental conditions
- 2016
-
Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 142:700, s. 2911-2924
-
Tidskriftsartikel (refereegranskat)abstract
- Large-eddy simulation of a nocturnal stratocumulus-topped boundary layer in a continental midlatitude environment has been performed to examine the sensitivity of the cloud to a number of different environmental parameters. The simulations showed that the stratocumulus cloud was strongly affected by the presence of an overlying free tropospheric cirrus cloud (FTC), in agreement with previous studies of marine nighttime stratocumulus. When introducing an FTC with an optical thickness of 2, stratocumulus liquid water path decreased by 30%. Enhancing the optical thickness of the FTC to 8 further decreased the liquid water path by almost 10%. The presence of an FTC decreased the cloud-top radiative cooling which decreased the turbulent mixing in the boundary layer, so that the liquid water content and cloud depth were reduced. The sensitivity of the stratocumulus cloud to an overlying FTC was found to be affected by the moisture content in the free troposphere. When a clear positive or negative moisture gradient above the inversion was imposed, and an overlying FTC with an optical thickness of 8 was introduced, the stratocumulus cloud LWP decreased by more than 40%. Furthermore, the effect of changes in free tropospheric moisture content and an overlying FTC on the stratocumulus cloud properties was found to be nonlinear; the combined response was in general weaker than the two responses added together. The modeled response to changes in cloud condensation nuclei (CCN) concentrations was found to be non-significant, unless the CCN concentrations were so low that drizzle was induced (similar to 50 cm(-3))
|
|
| 4. |
- Stevens, Robin G., et al.
(författare)
-
A model intercomparison of CCN-limited tenuous clouds in the high Arctic
- 2018
-
Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 18:15, s. 11041-11071
-
Tidskriftsartikel (refereegranskat)abstract
- We perform a model intercomparison of summertime high Arctic ( > 80 degrees N) clouds observed during the 2008 Arctic Summer Cloud Ocean Study (ASCOS) campaign, when observed cloud condensation nuclei (CCN) concentrations fell below 1 cm(-3). Previous analyses have suggested that at these low CCN concentrations the liquid water content (LWC) and radiative properties of the clouds are determined primarily by the CCN concentrations, conditions that have previously been referred to as the tenuous cloud regime. The intercomparison includes results from three large eddy simulation models (UCLALES-SALSA, COSMO-LES, and MIMICA) and three numerical weather prediction models (COSMO-NWP, WRF, and UM-CASIM). We test the sensitivities of the model results to different treatments of cloud droplet activation, including prescribed cloud droplet number concentrations (CDNCs) and diagnostic CCN activation based on either fixed aerosol concentrations or prognostic aerosol with in-cloud processing. There remains considerable diversity even in experiments with prescribed CDNCs and prescribed ice crystal number concentrations (ICNC). The sensitivity of mixed-phase Arctic cloud properties to changes in CDNC depends on the representation of the cloud droplet size distribution within each model, which impacts autoconversion rates. Our results therefore suggest that properly estimating aerosol-cloud interactions requires an appropriate treatment of the cloud droplet size distribution within models, as well as in situ observations of hydrometeor size distributions to constrain them. The results strongly support the hypothesis that the liquid water content of these clouds is CCN limited. For the observed meteorological conditions, the cloud generally did not collapse when the CCN concentration was held constant at the relatively high CCN concentrations measured during the cloudy period, but the cloud thins or collapses as the CCN concentration is reduced. The CCN concentration at which collapse occurs varies substantially between models. Only one model predicts complete dissipation of the cloud due to glaciation, and this occurs only for the largest prescribed ICNC tested in this study. Global and regional models with either prescribed CDNCs or prescribed aerosol concentrations would not reproduce these dissipation events. Additionally, future increases in Arctic aerosol concentrations would be expected to decrease the frequency of occurrence of such cloud dissipation events, with implications for the radiative balance at the surface. Our results also show that cooling of the sea-ice surface following cloud dissipation increases atmospheric stability near the surface, further suppressing cloud formation. Therefore, this suggests that linkages between aerosol and clouds, as well as linkages between clouds, surface temperatures, and atmospheric stability need to be considered for weather and climate predictions in this region.
|
|
