A novel approach for the characterization of transport and optical properties of aerosol particles near sources - Part I: Measurement of particle backscatter coefficient maps with a scanning UV lidar

The physical and chemical properties of aerosols emitted from a livestock farm were determined by a novel approach which combines high-resolution lidar measurements (0.33 s, 30 m) with simulations of a microphysics-chemistry-transport model. This first of two companion papers describes the scanning lidar measurements of optical particle properties. The lidar system employed laser radiation at a wavelength of 355 nm with a power of 9 W and a pulse repetition rate of 30 Hz. The laser beam was expanded before transmission to the atmosphere so that it became eye-safe at distances >270 m to the lidar. The elastic backscatter signal was detected with a resolution of 0.033 s and 3 m. A receiving telescope with a primary-mirror diameter of 40 cm was used. For this system, we developed a novel method for two-dimensional retrievals of the particle backscatter coefficient. With this set up and approach, the lidar was able to identify the aerosol plume up to a range of ~2.5 km from the source, a farm in northern Germany, in daytime. The measurements confirm that the optical particle properties of the emission plume vary largely with distance from the source and that the maximum particle backscatter coefficient is found away from the source. Within a close-to-horizontal scan (elevation angle of 2.3°), we found a mean particle backscatter coefficient of 1.5·10^-5 m^-1 sr^-1 inside the plume between 1.5 and 2.0 km distance from the source. Subtraction of the mean particle backscatter coefficient of the background aerosol present in this case (4.1·10^-6 m^-1 sr^-1) yields a particle backscatter coefficient of the livestock aerosols of 1.1·10^-5 m^-1 sr^-1. The limited extend of the plume is revealed with the scanning lidar: Scans with a slightly higher elevation angle of 4.8° did not pick up the plume.