Investigation of Aerosol and Cloud Properties Using Multiwavelength Raman Lidar Measurements
Document ID: 14
Doctoral Dissertation
The Pennsylvania State University
The Graduate School
Department of Electrical Engineering
Abstract
Lidar measurements obtained during several field campaigns have provided an extensive dataset for investigating aerosol characteristics and cloud properties. In this thesis we use measurements of multi-wavelength optical extinction measured with a Raman lidar to infer aerosol and cloud particle size variations. Aerosol extinction depends on both size and number density of the scatterers. The optical extinction at different wavelengths depends on the sixth power of the size parameter for aerosols much smaller than the scattering wavelength, and on the second power of the size parameter for aerosols much larger than the wavelength. Changes in the density of a particular size aerosol lead to a proportional response. The extinction profiles at several wavelengths are simultaneously examined to study changes in the aerosol size distribution over an interesting range of sizes corresponding to accumulation-mode particles. Model calculations based on Mie scattering theory are compared with extinction profiles at different wavelengths, water vapor profiles, and other simultaneous measurements, to investigate the formation and dissipation of cloud structures. The optical scattering measurements from aerosols and cloud particles demonstrate that various characteristics of aerosols and visibility can be determined.
We demonstrate the capability of the new technique using the multi-wavelength extinction ratios to profile information about changes in CCN particle size in the range of 50 nm to 0.5 μm. Examples taken from three different field campaigns demonstrate that changes in the size of the cloud particles during the different stages of growth and dissipation are observed in the multi-wavelength aerosol extinction using this technique. We also show the relationship that exists between particle size increase or decrease in cloud regions, based on the extinction coefficients and changes in relative humidity.
The deliquescence relative humidity (DRH) is found to exert a strong control on the optical extinction and visibility. Our results show that relative humidity values above 85% accompany drastic drops in visibility in the U.S. north-east regions. Increase in the relative humidity values beyond the DRH results in rapid growth of particle size, which in turn causes a simultaneous increase in the optical extinction and a drop in visibility.
Comparison of data from the eastern and western regions of the United States show that different sources control the changes in optical extinction values in the lower boundary layer. During the Southern California Ozone Study (SCOS) campaign an increase of optical extinction was observed after sunset in the nocturnal boundary layer due to the growth of particles caused by the increasing relative humidity. On the other hand, the optical extinction during the North-East Particle and Oxidant Study (NEOPS) campaigns was controlled more by pollutant concentrations and showed an increase in values after sunrise and decreased values after sunset; opposite from that observed at Hesperia, Ca.
We used theoretical simulations along with field measurements of multi-wavelength extinction coefficients to investigate the differences that particle growth and pollutant concentration have on the extinction coefficient as well as on the extinction coefficient ratios (visible/ultraviolet). Our results show that the increase in the extinction coefficient in a region of pollutants, typically composed of smaller size particles, depends on the number density of the scatterers, which has the same effect at all wavelengths.
We additionally demonstrate the capability of the Raman lidar to measure atmospheric visibility conditions and transmission properties using the optical extinction measurements.
Keywords: aerosols, cloud properties, multiwavelength Raman lidar
| Citation: | S. J. Verghese, "Investigation of Aerosol and Cloud Properties Using Multiwavelength Raman Lidar Measurements", The Pennsylvania State University, Doctoral Dissertation, May 2008, 165 pages |