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- ItemCloud phase identification of Arctic boundary-layer clouds from airborne spectral reflection measurements: Test of three approaches(München : European Geopyhsical Union, 2008) Ehrlich, A.; Bierwirth, E.; Wendisch, M.; Gayet, J.-F.; Mioche, G.; Lampert, A.; Heintzenberg, J.Arctic boundary-layer clouds were investigated with remote sensing and in situ instruments during the Arctic Study of Tropospheric Aerosol, Clouds and Radiation (ASTAR) campaign in March and April 2007. The clouds formed in a cold air outbreak over the open Greenland Sea. Beside the predominant mixed-phase clouds pure liquid water and ice clouds were observed. Utilizing measurements of solar radiation reflected by the clouds three methods to retrieve the thermodynamic phase of the cloud are introduced and compared. Two ice indices IS and IP were obtained by analyzing the spectral pattern of the cloud top reflectance in the near infrared (1500–1800 nm wavelength) spectral range which is characterized by ice and water absorption. While IS analyzes the spectral slope of the reflectance in this wavelength range, IS utilizes a principle component analysis (PCA) of the spectral reflectance. A third ice index IA is based on the different side scattering of spherical liquid water particles and nonspherical ice crystals which was recorded in simultaneous measurements of spectral cloud albedo and reflectance. Radiative transfer simulations show that IS, IP and IA range between 5 to 80, 0 to 8 and 1 to 1.25 respectively with lowest values indicating pure liquid water clouds and highest values pure ice clouds. The spectral slope ice index IS and the PCA ice index IP are found to be strongly sensitive to the effective diameter of the ice crystals present in the cloud. Therefore, the identification of mixed-phase clouds requires a priori knowledge of the ice crystal dimension. The reflectance-albedo ice index IA is mainly dominated by the uppermost cloud layer (τ<1.5). Therefore, typical boundary-layer mixed-phase clouds with a liquid cloud top layer will be identified as pure liquid water clouds. All three methods were applied to measurements above a cloud field observed during ASTAR 2007. The comparison with independent in situ microphysical measurements shows the ability of the three approaches to identify the ice phase in Arctic boundary-layer clouds.
- ItemTraffic restrictions in Beijing during the Sino-African Summit 2006: Aerosol size distribution and visibility compared to long-term in situ observations(München : European Geopyhsical Union, 2008) Cheng, Y.F.; Heintzenberg, J.; Wehner, B.; Wu, Z.J.; Su, H.; Mao, J.T.Based on the long-term in-situ observations of aerosol particle number size distributions and meteorological parameters, the measures of traffic restriction during the Sino-African Summit (4–6 November 2006) in Beijing, China have been found to be efficient in reducing the number concentration of aerosol particles, in particular Aitken and accumulation mode particles, and in improving the visibility when local emissions dominated. The influence of traffic restrictions on the particle concentrations differed for different particle sizes. More significant effects on fine particles with diameters ranging from 40 to 500 nm have been found. Based on statistical analysis of long-term observations, under comparable weather conditions, the number concentrations of the particles in Aitken and accumulation modes seemingly were reduced by 20–60% when the traffic restrictions were in place. This change may be mainly due to the reduction of secondary particle contributions. However, it is worth to notice that the reduction of 60% might overestimate the effect of the measures of traffic control, due to the inherent data shortage with very high wind speeds in the comparison data population. Our size-dependent aerosol data also indicate that measures led to reductions in particulate air pollution in the optically most important diameter range, whereas further vehicle restriction measures might lead to an increase in ultrafine particle formation if the condensational sink further decreased. Assuming that there were no traffic restrictions and with normal levels of the vehicle emissions, the visibilities during the Summit would have been lower by about 20–45%. The fact that over 95% cases with visibility range lower than 5 km during 2004 to 2007 occurred when the local wind speed was lower than 3 m s−1 may suggest that the importance of the emission restrictions is highest when the wind speed is lower than 3 m s−1, concerning the improvement of serious low visibility situations in Beijing.
- ItemSO2 oxidation products other than H2SO4 as a trigger of new particle formation. Part 1: Laboratory investigations(München : European Geopyhsical Union, 2008) Berndt, T.; Stratmann, F.; Bräsel, S.; Heintzenberg, J.; Laaksonen, A.; Kulmala, M.Mechanistic investigations of atmospheric H2SO4 particle formation have been performed in a laboratory study taking either H2SO4 from a liquid reservoir or using the gas-phase reaction of OH radicals with SO2. Applying both approaches for H2SO4 generation simultaneously it was found that H2SO4 evaporated from the liquid reservoir acts considerably less effective for the process of particle formation and growth than the products originating from the reaction of OH radicals with SO2. Furthermore, for NOx concentrations >5×1011 molecule cm−3 the formation of new particles from the reaction of OH radicals with SO2 is inhibited. This suggests that substances other than H2SO4 (potentially products from sulphur-containing peroxy radicals) trigger lower tropospheric new particle formation and growth. The currently accepted mechanism for SO2 gas-phase oxidation does not consider the formation of such substances. The analysis of new particle formation for different reaction conditions in our experiment suggests that a contribution of impurities to the nucleation process is unlikely.
- ItemLACIS-measurements and parameterization of sea-salt particle hygroscopic growth and activation(München : European Geopyhsical Union, 2008) Niedermeier, D.; Wex, H.; Voigtländer, J.; Stratmann, F.; Brüggemann, E.; Kiselev, A.; Henk, H.; Heintzenberg, J.The Leipzig Aerosol Cloud Interaction Simulator (LACIS) was used to investigate the hygroscopic growth and activation of sea-salt particles which were generated from three different sea-water samples. The measurements showed that the sea-salt particles exhibit a slightly reduced hygroscopic growth compared to pure NaCl particles. Köhler theory was utilized to model the hygroscopic growth of these particles. Some parameters used in this model are unknown for sea-salt. These parameters are combined in an "ionic density" ρion. For each sea-salt sample an average ρion was determined by fitting the Köhler equation to the data from the hygroscopic growth measurements. LACIS was also used to measure the activation of the sea-salt particles at three different supersaturations: 0.11%, 0.17% and 0.32%. A CCN-closure was tested by calculating the critical diameters Dcrit for the sea-salt particles at these supersaturations, using the Köhler model and the corresponding ρion as derived from the hygroscopic growth data. These calculated critical diameters were compared to the measured ones. Measured and calculated values of Dcrit agree within the level of uncertainty. Based on this successful closure, a new parameterization to describe sea-salt-particle hygroscopic growth (at RH>95%) and activation has been developed.