Atmospheric transparency at TA site using Central Laser Facility

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The Telescope Array (TA) experiment has three fluorescence detector (FD) stations to measure fluorescence light emitted from extensive air showers, especially produced by Ultra High Energy Cosmic Rays(UHECRs). These fluorescence light is attenuated and decreased by aerosols in the atmosphere during their propagation to the FD. Measuring and calibrating the atmospheric attenuation is necessary to estimate energy of primary cosmic rays. The central Laser Facility (CLF) consists of a laser of wavelength of 355 nm, and emits beam vertically with 10 Hz, every half-hour. The atmospheric transparency in terms of the Vertical Atmospheric Optical Depth (VAOD) is measured with the CLF every 30 minutes during FD observation. The CLF is located at the center of three FD stations and observes the atmospheric transparency of the TA site. We calculated VAOD τAS(5km) at the BR station using 1853 events of the CLF laser from January 2012 to September 2016. We found that the median of VAOD at 5 km height from the ground is . We also found a seasonal variation of VAOD using the same data.