The Diurnal Variation of the Aerosol Optical Depth at the ARM SGP Site

This study examines the diurnal variation of the aerosol optical depth (AOD) at 355 nm observed by Raman lidar (RL) at the Atmospheric Radiation Measurement Program Southern Great Plains (SGP) site under both clear and cloudy-sky conditions. Here only cloudy-skies when the lidar signal is not fully attenuated are considered. The daytime AOD and its variation from the RL showed an excellent agreement with the Aerosol Robotic Network, demonstrating that the RL-retrieved AOD is not affected by solar background contamination. The climatological annual-mean daytime-mean AOD is only slightly larger than the nighttime-mean AOD (by 1%-3%). However, day-to-day variations are observed such that the daytime- and nighttime-mean AOD difference for a given day can be large (about 95% of days have differences within 0.2). The seasonal AOD diurnal range (i.e., the difference between the maximum and minimum values) relative to the mean was similar to 10%-15% except in the winter when it was similar to 44%. The seasonal-mean cloudy-sky AOD diurnal variation is similar to that for clear-sky, except that the AODs are larger (the annual-mean cloudy-sky AOD is larger than the clear-sky by similar to 24%). The aerosol lidar ratio diurnal variations are also examined, which are similar to 10%-20% for all seasons with a minimum near 9 a.m. to 15 p.m. for all seasons except winter. Also presented is the annual-mean AOD from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite at SGP site: its daytime AOD is about 0.1 smaller than nighttime AOD because of daytime solar background contamination.