Multistatic lidar profile measurements of lower tropospheric aerosol and particulate matter.

摘要

There is growing interest in the continuous monitoring of the vertical transport and distribution of aerosols and particulates. Laser remote sensing offers a practical tool to accomplish this task. A U.S. Environmental Protection Agency-sponsored field study was conducted in the city of Philadelphia in the summer of 2001 as part of the North-East Oxidant and Particulate Study. This study brought together a consortium of researchers to measure and investigate pollutants and particulate matter in an urban environment. Our own investigation focused on the vertical atmosphere from 10 meters to 100 meters in altitude. Measurements of the parallel and perpendicular polarization components of the laser were taken and then divided to form a polarization ratio. Using the polarization ratio as opposed to analyzing a particular polarization component significantly reduces instrument errors and alleviates the need for range and volume corrections while still retaining the characteristics produced from scattering. In order to obtain aerosol distribution parameters, the polarization ratio measurements were fitted with an atmospheric model that combined Mie theory and a trimodal lognormal aerosol distribution. The aerosol distribution model used particles in the range from 1 nanometer to 35 micrometers. While the monitoring of vertical aerosol profiles was shown to be possible, accurate inversion of the aerosol parameters still poses significant challenges.; A theoretical investigation of the polarization ratio determined that the total observed scattering angle range should be at least 10 degrees for an atmosphere that contains ultrafine, fine and coarse aerosol modes (i.e., trimodal aerosol number density population). However, for distributions that are more monodispersed or for atmospheric distributions that contain a significant number of large particles (such as in the case of radiation fog), fewer observed scattering angles are needed. Additionally, the polarization ratio was found to be insensitive to the imaginary part of the refractive index but very sensitive to changes in the real part of the refractive index. Subsequently, the scattering model could not model the imaginary part of the index of refraction.; Experimental results of the nighttime atmosphere revealed the presence of aerosol variations over altitude. Further, a time sequence of several data sets showed rapid changes in the aerosol profile over time and in space. The overall observed nighttime atmospheric variability is in contrast to the daytime where convective forcing yields a much more uniformly distributed atmosphere. Modeling of the aerosol profile suggested the aerosol variations might depend on several factors, not necessarily only total number density. To specify the exact nature of the altitude dependence, direct measurement of the aerosols at the observed altitudes is needed. However, some data that was taken indicated a uniformly mixed atmosphere and thus allowed the retrieval of aerosol parameters. Fits of the model to this data used real refractive indices of refraction from 1.33 to 1.45 and clearly demonstrated the influence the refractive index has on the accuracy of the retrieved parameters.