  The ATLAS liquid argon calorimeter has been operating continuously since
August 2006. At this time, only part of the calorimeter was readout, but since
the beginning of 2008, all calorimeter cells have been connected to the ATLAS
readout system in preparation for LHC collisions. This paper gives an overview
of the liquid argon calorimeter performance measured in situ with random
triggers, calibration data, cosmic muons, and LHC beam splash events. Results
on the detector operation, timing performance, electronics noise, and gain
stability are presented. High energy deposits from radiative cosmic muons and
beam splash events allow to check the intrinsic constant term of the energy
resolution. The uniformity of the electromagnetic barrel calorimeter response
along eta (averaged over phi) is measured at the percent level using minimum
ionizing cosmic muons. Finally, studies of electromagnetic showers from
radiative muons have been used to cross-check the Monte Carlo simulation. The
performance results obtained using the ATLAS readout, data acquisition, and
reconstruction software indicate that the liquid argon calorimeter is
well-prepared for collisions at the dawn of the LHC era.
