We have investigated the phase behavior of DPPC (dipalmitoylphosphatidylcholine) monolayers at the water-air interface using molecular dynamics simulations, where the phospholipids and the water molecules are modeled atomistically. We report pressure-area isotherms in the interval of 273-310 K. Our results show evidence for a liquid condensed (LC) to liquid expanded (LE) phase transition and indicate that ordered condensed phases can nucleate from a starting disordered phase on a time scale of approximately 50 ns. The existence of the phase transition is confirmed with structural analyses of the phospholipid pair correlation functions and of the monolayer thickness. We find that the change in the monolayer thickness associated with the LC-LE transition is largely due to a shortening of the hydrocarbon chains, with little modification in the average tilt angle of the choline head group. This result is compatible with recent sum frequency spectroscopy experiments, which concluded that the transition occurs without major changes in the orientation of the head group with respect to the monolayer plane. The dependence of the simulated pressure-area isotherms on temperature, in particular, the reduction in width of the coexistence plateau with increasing temperature, is consistent with published experimental pressure-area isotherms.