The evolution and structure of the planetary boundary layer (PBL) are critical to understanding the formation and fates of ozone (O3), particulate matter (PM), regional haze, and respective precursors. As part of the NE-OPS 1999 field campaign, vertical profiles of temperature, humidity, and trace chemical species were made from an instrumented light aircraft at several locations in the US Mid-Atlantic region during July and August. Temperature and humidity profiles were also obtained from a tethered balloon and radiosondes launched at the core surface site, the Baxter Water Treatment Plant. The meteorological observations and PBL height estimates were then compared with the results from two model simulations using the Fifth-Generation Penn State University/NCAR Mesoscale Model (MM5 Version 3.3). The MM5 domain covered much of the eastern US with a horizontal grid resolution of 12 km, for the period July 1 – August 3. In one MM5 simulation, we used the Blackadar PBL scheme, while in the other we used the Gayno-Seaman PBL scheme. Both PBL schemes are commonly used in conjunction with photochemical models to simulate concentrations of O3 and PM. We focus on comparing the PBL heights and vertical profiles of temperature and specific humidity, to investigate the differences in the PBL evolution between these two schemes. Substantial differences between the two model simulations occurred within the lowest 2-3 km of the atmosphere, which are expected to impact such photochemical predictions. While the model tends to predict afternoon mixing heights fairly well, it also tends to fail at night and during the morning evolution. Hence, while the model may be useful in hindcasting weather associated with O3/PM/haze, improvements are needed during the nighttime (nocturnal inversions, low-level jet development) and morning hours (PBL growth and entrainment dynamics).