PHOTOCHEMICAL PRODUCTION AND LOSS RATES OF OZONE AT SABLE-ISLAND, NOVA-SCOTIA DURING THE NORTH-ATLANTIC REGIONAL EXPERIMENT (NARE) 1993 SUMMER INTENSIVE

Three weeks of summertime surface-based chemical and meteorological ob servations at Sable Island, Nova Scotia during the North Atlantic Regi onal Experiment (NARE) 1993 summer intensive are used to study instant aneous photochemical production and loss rates of ozone by means of a numerical photochemical model. Results are most sensitive to the avera ging scheme of data used to constrain the model and the ambient variab ility of the measurements. Model simulations driven by a time series o f 5 min averaged data, most representative of the chemistry at the sit e, yield an average net photochemical ozone production of 3.6 ppbv/d. Estimates of net ozone production designed to filter out local sources , by using 1000-1400 LT median values of observations to drive the mod el and by excluding short-lived hydrocarbons, give values ranging from 1 to 4 ppbv/d. These positive values of net ozone production within t he marine boundary layer over Sable Island demonstrate the impact of p olluted continental plumes on the background photochemistry of the reg ion during the intensive. The dominant ambient variables controlling p hotochemical production and loss rates of ozone at the site during the measurement campaign appear to be levels of nitrogen oxides, ozone, n onmethane hydrocarbons, and solar intensity determined by cloud cover. The model partitioning of nitrogen oxides agrees for the most part wi th measurements, lending credence to calculated photochemical producti on and loss rates of ozone as well as inferred levels of peroxy radica ls not measured at the site. Discrepancies, however, often occur durin g episodes of intermittent cloud cover, fog, and rain, suggesting the influence of cloud processes on air masses reaching the site.