Photochemical model calculations have been performed for air masses encountered by the National Research Council's Twin Otter aircraft during the 1993 summer North Atlantic Regional Experiment (NARE) intensive. These calculations use observed values of O3, NOy, CO, and hydrocarbons as constraints. NO is determined using the ratio NO/NOy measured from the National Center for Atmospheric Research King Air under comparable circumstances. Measurements over coastal locations indicate photochemically aged air masses with relatively low concentrations of NO and an OH reactivity that is dominated by CO and CH4. Samples over land have higher NO and an OH reactivity that is dominated by isoprene. Ozone production rates and H2O2 concentrations are analyzed using radical budget arguments that are applicable to low NOx conditions. The ozone production rate, P(O3), is predicted to be proportional to Q1/2, where Q is the production rate for free radicals. This relation explains 99% of the variance in P(O3). Over 90% of the variance is explained by alone. P(O3) in the coastal samples is about a factor of 4 lower than previous estimates for the eastern United States. This is a consequence of low in the air masses that are advected to Nova Scotia. ¿ 1998 American Geophysical Union