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Author

Klaus Schäfer

Bio: Klaus Schäfer is an academic researcher from Karlsruhe Institute of Technology. The author has contributed to research in topics: Ceilometer & Air quality index. The author has an hindex of 34, co-authored 156 publications receiving 3479 citations. Previous affiliations of Klaus Schäfer include Heinrich Hertz Institute & Chengdu University of Information Technology.
Topics: Ceilometer, Air quality index, SODAR, Aerosol, Venus


Papers
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Journal ArticleDOI
TL;DR: In this paper, the authors observed the atmospheric mixing layer height (MLH) in Beijing from July-2009 to December-2012 using a ceilometer and found that the estimated MLH is low in autumn and winter and high in spring and summer in Beijing.
Abstract: . The mixing layer is an important meteorological factor that affects air pollution. In this study, the atmospheric mixing layer height (MLH) was observed in Beijing from July 2009 to December 2012 using a ceilometer. By comparison with radiosonde data, we found that the ceilometer underestimates the MLH under conditions of neutral stratification caused by strong winds, whereas it overestimates the MLH when sand-dust is crossing. Using meteorological, PM2.5, and PM10 observational data, we screened the observed MLH automatically; the ceilometer observations were fairly consistent with the radiosondes, with a correlation coefficient greater than 0.9. Further analysis indicated that the MLH is low in autumn and winter and high in spring and summer in Beijing. There is a significant correlation between the sensible heat flux and MLH, and the diurnal cycle of the MLH in summer is also affected by the circulation of mountainous plain winds. Using visibility as an index to classify the degree of air pollution, we found that the variation in the sensible heat and buoyancy term in turbulent kinetic energy (TKE) is insignificant when visibility decreases from 10 to 5 km, but the reduction of shear term in TKE is near 70 %. When visibility decreases from 5 to 1 km, the variation of the shear term in TKE is insignificant, but the decrease in the sensible heat and buoyancy term in TKE is approximately 60 %. Although the correlation between the daily variation of the MLH and visibility is very poor, the correlation between them is significantly enhanced when the relative humidity increases beyond 80 %. This indicates that humidity-related physicochemical processes is the primary source of atmospheric particles under heavy pollution and that the dissipation of atmospheric particles mainly depends on the MLH. The presented results of the atmospheric mixing layer provide useful empirical information for improving meteorological and atmospheric chemistry models and the forecasting and warning of air pollution.

336 citations

Journal ArticleDOI
TL;DR: In this article, the authors summarized and compared the available methods for the determination of the mixing-layer height from surface-based acoustic, optical and electro-magnetic remote sensing.
Abstract: Presently available methods for the determination of the mixing-layer height from surface-based acoustic, optical and electro-magnetic remote sensing are summarized and compared. Most complete information on the structure of the ABL can be expected from a combined use of acoustic together with optical or electro-magnetic remote sensing.

198 citations

Journal ArticleDOI
TL;DR: In this paper, different retrieval methods to derive the aerosol backscatter coefficient βp, with special focus on the calibration of the ceilometers, are compared with respect to their accuracy and applicability, and it is shown that advanced lidar systems such as those being operated in the framework of the European Aerosol Research Lidar Network (EARLINET) are excellent tools for the calibration.
Abstract: . With the establishment of ceilometer networks by national weather services, a discussion commenced to which extent these simple backscatter lidars can be used for aerosol research. Though primarily designed for the detection of clouds it was shown that at least observations of the vertical structure of the boundary layer might be possible. However, an assessment of the potential of ceilometers for the quantitative retrieval of aerosol properties is still missing. In this paper we discuss different retrieval methods to derive the aerosol backscatter coefficient βp, with special focus on the calibration of the ceilometers. Different options based on forward and backward integration methods are compared with respect to their accuracy and applicability. It is shown that advanced lidar systems such as those being operated in the framework of the European Aerosol Research Lidar Network (EARLINET) are excellent tools for the calibration, and thus βp retrievals based on forward integration can readily be implemented and used for real-time applications. Furthermore, we discuss uncertainties introduced by incomplete overlap, the unknown lidar ratio, and water vapor absorption. The latter is relevant for the very large number of ceilometers operating in the spectral range around λ = 905–910 nm. The accuracy of the retrieved βp mainly depends on the accuracy of the calibration and the long-term stability of the ceilometer. Under favorable conditions, a relative error of βp on the order of 10% seems feasible. In the case of water vapor absorption, corrections assuming a realistic water vapor distribution and laser spectrum are indispensable; otherwise errors on the order of 20% could occur. From case studies it is shown that ceilometers can be used for the reliable detection of elevated aerosol layers below 5 km, and can contribute to the validation of chemistry transport models, e.g., the height of the boundary layer. However, the exploitation of ceilometer measurements is still in its infancy, so more studies are urgently needed to consolidate the present state of knowledge, which is based on a limited number of case studies.

151 citations

Journal ArticleDOI
TL;DR: In this paper, a measurement campaign at the Zurich airport was performed from 30 June 2004 to 15 July 2004 to study the impact of emissions at an airport on local air quality, while other air samples were taken in the plume of an aircraft during engine ignition.

150 citations

Journal ArticleDOI
TL;DR: In this article, a comparison of the determination of boundary-layer structures by a SODAR, by a RASS, and by a ceilometer is presented, focusing on 3 days with an evolution of a convective boundary layer over a larger city in Germany.

145 citations


Cited by
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01 Jan 1989
TL;DR: In this article, a two-dimensional version of the Pennsylvania State University mesoscale model has been applied to Winter Monsoon Experiment data in order to simulate the diurnally occurring convection observed over the South China Sea.
Abstract: Abstract A two-dimensional version of the Pennsylvania State University mesoscale model has been applied to Winter Monsoon Experiment data in order to simulate the diurnally occurring convection observed over the South China Sea. The domain includes a representation of part of Borneo as well as the sea so that the model can simulate the initiation of convection. Also included in the model are parameterizations of mesoscale ice phase and moisture processes and longwave and shortwave radiation with a diurnal cycle. This allows use of the model to test the relative importance of various heating mechanisms to the stratiform cloud deck, which typically occupies several hundred kilometers of the domain. Frank and Cohen's cumulus parameterization scheme is employed to represent vital unresolved vertical transports in the convective area. The major conclusions are: Ice phase processes are important in determining the level of maximum large-scale heating and vertical motion because there is a strong anvil componen...

3,813 citations

Journal ArticleDOI
TL;DR: It is found only 25 out of 190 cities could meet the National Ambient Air Quality Standards of China, and the population-weighted mean of PM2.5 in Chinese cities are 61 μg/m3, ~3 times as high as global population- Weighted mean, highlighting a high health risk.
Abstract: This study presents one of the first long term datasets including a statistical summary of PM2.5 concentrations obtained from one-year monitoring in 190 cities in China. We found only 25 out of 190 cities could meet the National Ambient Air Quality Standards of China, and the population-weighted mean of PM2.5 in Chinese cities are 61 μg/m3, ~3 times as high as global population-weighted mean, highlighting a high health risk. PM2.5 concentrations are generally higher in north than in south regions due to relative large PM emissions and unfavorable meteorological conditions for pollution dispersion. A remarkable seasonal variability of PM2.5 is observed with the highest during the winter and the lowest during the summer. Due to the enhanced contributions from dust particles and open biomass burning, high PM2.5 abundances are also found in the spring (in Northwest and West Central China) and autumn (in East China), respectively. In addition, we found the lowest and highest PM2.5 often occurs in the afternoon and evening hours, respectively, associated with daily variation of the boundary layer depth and anthropogenic emissions. The diurnal distribution of the PM2.5-to-CO ratio consistently displays a pronounced peak during the afternoon periods, reflecting a significant contribution of secondary PM formation.

750 citations

Journal ArticleDOI
TL;DR: In this article, the authors present a review of some facets of combustion control and focus on the sensors that take or could take part to combustion control solutions, together with the associated control concepts.

640 citations

Journal ArticleDOI
TL;DR: A comprehensive updated assessment of aviation is provided, highlighting the impact of aviation emissions of CO2 on global mean surface temperature last for many hundreds of years (in common with other sources), whilst its non-CO2 effects onTemperature last for decades.

606 citations

Journal ArticleDOI
TL;DR: Although energy consumption in China went up continuously during the past two decades, annual emission of PAHs fluctuated depending on the amount of domestic coal consumption, coke production, and the efficiency of energy utilization.
Abstract: Emission of 16 polycyclic aromatic hydrocarbons (PAHs) listed as US Environmental Protection Agency (US EPA) priority pollutants from major sources in China were compiled Geographical distribution and temporal change of the PAH emission, as well as emission profiles, are discussed It was estimated that the total PAH emission in China was 25 300 tons in 2003 The emission profile featured a relatively higher portion of high molecular weight (HMW) species with carcinogenic potential due to large contributions of domestic coal and coking industry Among various sources, biomass burning, domestic coal combustion, and coking industry contributed 60%, 20%, and 16% of the total emission, respectively Total emission, emission density, emission intensity, and emission per capita showed geographical variations In general, the southeastern provinces were characterized by higher emission density, while those in western and northern China featured higher emission intensity and population-normalized emission A

520 citations