  The Pierre Auger Observatory is making significant contributions towards
understanding the nature and origin of ultra-high energy cosmic rays. One of
its main challenges is the monitoring of the atmosphere, both in terms of its
state variables and its optical properties. The aim of this work is to analyze
aerosol optical depth $\tau_{\rm a}(z)$ values measured from 2004 to 2012 at
the observatory, which is located in a remote and relatively unstudied area of
the Pampa Amarilla, Argentina. The aerosol optical depth is in average quite
low - annual mean $\tau_{\rm a}(3.5~{\rm km})\sim 0.04$ - and shows a seasonal
trend with a winter minimum - $\tau_{\rm a}(3.5~{\rm km})\sim 0.03$ -, and a
summer maximum - $\tau_{\rm a}(3.5~{\rm km})\sim 0.06$ -, and an unexpected
increase from August to September - $\tau_{\rm a}(3.5~{\rm km})\sim 0.055$). We
computed backward trajectories for the years 2005 to 2012 to interpret the air
mass origin. Winter nights with low aerosol concentrations show air masses
originating from the Pacific Ocean. Average concentrations are affected by
continental sources (wind-blown dust and urban pollution), while the peak
observed in September and October could be linked to biomass burning in the
northern part of Argentina or air pollution coming from surrounding urban
areas.
