The diurnal variation of the aerosol optical depth at the ARM SGP site

Abstract

13 This study examines the diurnal variation of the aerosol optical depth (AOD) at 355 nm 14 observed by Raman lidar (RL) at the Atmospheric Radiation Measurement Program (ARM) 15 Southern Great Plains (SGP) site under both clear and cloudy-sky conditions. Here only 16 cloudy-skies when the lidar signal is not fully attenuated are considered. The daytime 17 AOD and its variation from the RL showed an excellent agreement with the Aerosol Robotic 18 Network, demonstrating that the RL-retrieved AOD is not affected by solar background 19 contamination. The climatological annual-mean daytime-mean AOD is only slightly larger 20 than the nighttime-mean AOD (by 1-3%). However, day-to-day variations are observed 21 such that the daytimeand nighttime-mean AOD difference for a given day can be large 22 (about 95% of days have differences within 0.2). The seasonal AOD diurnal range (i.e., 23 the difference between the maximum and minimum values) relative to the mean was ∼1024 15% except in the winter when it was ∼44%. The seasonal-mean cloudy-sky AOD di25 urnal variation is similar to that for clear-sky, except that the AODs are larger (the annual26 mean cloudy-sky AOD is larger than the clear-sky by ∼24%). The aerosol lidar ratio di27 urnal variations are also examined, which are ∼10-20% for all seasons with a minimum 28 near 9 am to 15 pm for all seasons except winter. Also presented is the annual-mean AOD 29 from the CloudAerosol Lidar and Infrared Pathfinder Satellite at SGP site: its daytime 30 AOD is about 0.1 smaller than nighttime AOD because of daytime solar background con31 tamination. 32