Typical atmospheric aerosol behavior at the Cherenkov Telescope Array candidate sites in Argentina

TL;DR

This study characterizes aerosol concentrations and atmospheric transmittance at two Argentine sites, crucial for optimizing Cherenkov Telescope Array observations by understanding atmospheric variability and its impact on UV and visible light detection.

Contribution

It provides detailed aerosol size distribution, concentration data, and satellite-derived AOD values for CTA candidate sites, enhancing atmospheric characterization for gamma-ray astronomy.

Findings

Low aerosol concentrations with a peak in 2.5-5.0 μm range.

Very low mean AOD values indicating clear atmospheric conditions.

Spectral solar irradiance data supports UV transmittance assessment.

Abstract

Aerosols from natural and antropogenic sources are one of the atmospheric components that have the largest spacial-temporal variability, depending on the type (land or ocean) surface, human activity and climatic conditions (mainly temperature and wind). Since Cherenkov photons generated by the incidence of a primary ultraenergetic cosmic gamma photon have a spectral intensity distribution concentrated in the UV and visible ranges [Hillas AM. Space Science Reviews, 75, 17-30, 1996], it is important to know the aerosol concentration and its contribution to atmospheric radiative transfer. We present results of this concentration measured in typical rather calm (not windy) days at San Antonio de los Cobres (SAC) and El Leoncito/CASLEO proposed Argentinean Andes range sites for the placement of the Cherenkov Telescope Array (CTA). In both places, the aerosol concentration has a peak in the 2.5-5.0$\mu$m range of the mean aerosol diameter and a very low mean total concentration of 0.097$\mu$g/m$^3$ (0.365$\mu$g/m$^3$) for SAC(CASLEO). The data were collected the days 15/Dec/2012 (27-28/Dec/2012) for the first (second) place with a GRIMM aerosol spectrometer, who determines the aerosol concentration at a given diameter (0.22-32 $\mu$m range) with a laser technique. We also present AOD values for each CTA proposed place, derived with the improved Deep Blue algorithm, from data measured by SEAWiFS instrument on board of SeaStar/NASA satellite. They have very low mean values, determined for the 1998-2010 period, AOD$_{550nm, SAC}$ = 0.026 $\pm$ 0.011 and AOD$_{550nm, CASLEO}$ = 0.030 $\pm$ 0.014. Also, we introduce the spectral (380 nm) solar irradiance as a test in these sites of the tropospheric UV atmospheric transmittance.