Measurements of the concentrations of O3, CO, NO, NOy, condensation nuclei, and aerosols in 12 size bins ranging from 0.1 to 3.1 μm as well as a variety of meteorological parameters were collected aboard the NCAR King Air in August 1993 during 16 flights over or near the Gulf of Maine. The purpose of this measurement campaign was to describe the sources and variation of ozone and its precursors in the western NARE region during the summer season. Correlations between O3 and CO, O3 and NOy, and other chemical and aerosol measurements are examined. Several features in the altitude profiles are investigated using principal component analysis. The results of the trace gas analyses indicate that anthropogenically influenced ozone production dominated ozone throughout the experimental region at altitudes below 1500 m. Other factors identified include a distinct signature from fresh emissions below 1500 m. Above 1500 m the sources of ozone were determined by transport from a variety of sources. ¿ American Geophysical Union 1996