LIDAR versus satellite-measured optical thickness of a wildfire aerosol

Abstract

A dual UV, Rayleigh/nitrogen Raman LIDAR system was developed for the purpose of profiling aerosols at vertical ranges between 0.025 and 5 km. The 355 nm LIDAR was operated in El Segundo, California during June and July 2008, during a period of intense wildfire activity in Northern California. From the two independent measurements we calculated the particle backscatter, and using the humidity-corrected LIDAR backscatter-to-extinction ratios given by Ackermann[1] we calculated aerosol optical thickness (AOT) profiles. Preliminary validation studies revealed that under most conditions the calculated LIDAR AOT data agreed with total AOT measured from a collocated sun photometer, except for cases when high-altitude smoke from wildfires was present. To account for high-altitude smoke, a two-layer atmospheric model was assumed, where the lower layer's AOT was calculated using the backscatter-to-extinction method and the high-altitude AOT was found through direct attenuation of the Raman signal. A comparison of AOT measurements from the ground-based LIDAR and the MODIS (Aqua and Terra) overpasses was then performed during the peak period of transport of smoke from Northern California, between 19 June 2008 and 2 July 2008. While the LIDAR and Sun Photometer were found to be in good agreement, it was found that the MODIS overpasses consistently indicated a larger AOT.