Abstract

Aerosol optical depth can be retrieved from measurements performed by Moderate Resolution Imaging Spectroradiometer (MODIS) satellite instrument. The MODIS satellite system includes two polar satellites, Terra and Aqua. Each of them flies over the Pierre Auger Observatory once a day, providing two measurements of aerosols per day and covering the whole area of the Observatory. MODIS aerosol data products have been generated by three dedicated algorithms over bright and dark land and over ocean surface. We choose the Deep Blue algorithm data to investigate the distribution of aerosols over the Observatory, as this algorithm is the most appropriate one for semi-arid land of the Pierre Auger Observatory. This data algorithm allows us to obtain aerosol optical depth values for the investigated region, and to build cloud-free aerosol maps with a horizontal resolution 0.1°×0.1°. Since a suffcient number of measurements was obtained only for Loma Amarilla and Coihueco fluorescence detector (FD) sites of the Pierre Auger Observatory, a more detailed analysis of aerosol distributions is provided for these sites. Aerosols over these FD sites are generally distributed in a similar way each year, but some anomalies are also observed. These anomalies in aerosol distributions appear mainly due to some transient events, such as volcanic ash clouds, fires etc. We conclude that the Deep Blue MODIS algorithm provides more realistic aerosol optical depth values than other available algorithms.

Highlights

  • Terra and Aqua satellites with Moderate Resolution Imaging Spectroradiometer (MODIS) instruments attached fly on the sun-synchronous orbits and pass over the same spot of the Earth at about the same local time every day

  • Since the Pierre Auger Observatory is located in a semi-arid area, which is a bright surface type, the focus of this research is on the Deep Blue (DB) over-land data

  • During that period of time, a widespread ash cloud was formed at the altitude about 1.5 km. This cloud dispersed rapidly over large part of Argentina, including Córdoba, Mendoza and San Luis provinces making the sky almost invisible. Taking into account this information, it can be concluded that the increase of daily , which started in Figure 5(a1) after 170th day of a year and continued up to the end of the year, can be caused by appearing of clouds with volcanic ash in the atmosphere above the Pierre Auger Observatory, taken there by the strong winds

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Summary

MODIS instruments and algorithms

Terra and Aqua satellites with MODIS instruments attached fly on the sun-synchronous orbits and pass over the same spot of the Earth at about the same local time every day. Due to the large swath of data collected by MODIS (over 2300 km wide) it is possible to observe almost the entire Earth surface every day. MODIS level 2 aerosol data products are used in this work. The level 1B MODIS data include measurements of so-called “sensor pixels” [4]. These are single measured pixels with the size 250 m, 500 m, or 1 km at nadir, depending on the measured wavelength band. The level 2 aerosol products include only retrieval pixels. Since the Pierre Auger Observatory is located in a semi-arid area, which is a bright surface type, the focus of this research is on the DB over-land data

Analysis procedure
Interpretation of maps and daily plots
Summary

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