LIDAR versus satellite-measured optical thickness of a wildfire aerosol

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 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.