A Study of the Effect of Molecular and Aerosol Conditions in the
  Atmosphere on Air Fluorescence Measurements at the Pierre Auger Observatory
  The air fluorescence detector of the Pierre Auger Observatory is designed to
perform calorimetric measurements of extensive air showers created by cosmic
rays of above 10^18 eV. To correct these measurements for the effects
introduced by atmospheric fluctuations, the Observatory contains a group of
monitoring instruments to record atmospheric conditions across the detector
site, an area exceeding 3,000 km^2. The atmospheric data are used extensively
in the reconstruction of air showers, and are particularly important for the
correct determination of shower energies and the depths of shower maxima. This
paper contains a summary of the molecular and aerosol conditions measured at
the Pierre Auger Observatory since the start of regular operations in 2004, and
includes a discussion of the impact of these measurements on air shower
reconstructions. Between 10^18 and 10^20 eV, the systematic uncertainties due
to all atmospheric effects increase from 4% to 8% in measurements of shower
energy, and 4 g/cm^2 to 8 g/cm^2 in measurements of the shower maximum.
