EVALUATION OF A COMPREHENSIVE EULERIAN AIR-QUALITY MODEL WITH MULTIPLE CHEMICAL-SPECIES MEASUREMENTS USING PRINCIPAL COMPONENT ANALYSIS

Using a principal component analysis technique and data on atmospheric gases and aerosols at a rural site in Ontario, Canada from the Euleri an model evaluation field study (EMEFS), the Eulerian acid deposition and oxidant model (ADOM) is evaluated. Seventy-nine and 76% of the var iances in the data and model output, respectively, are explained by th ree principal components. They are a chemically aged/transported compo nent, a diurnal cycle component, and an area emission component, all c haracterized by their ratios of gases and temporal variation patterns. The ADOM component contributions to sulphur species are in general ag reement with the EMEFS components, but with notable differences for ke y photochemical species including O-3. The temporal variations of the ADOM components are close to those of the EMEFS components. The EMEFS chemically aged/transported component shows a high degree of photochem ical processing, with the ratios [NOx]/[TNOy]=0.3 and [O-3]/([TNOy]-[N Ox])=9+/-1. The corresponding ADOM component predicts lower [NOx]/[TNO y] and [O-3]/([TNOy]-[NOx]) ratios, probably caused by a chemical mech anism in the model that is too fast, and lower contributions to O-3, N O2, TNO3, PAN, TNOy, and HCHO, probably caused by model grid dilution or lower model emissions. The EMEFS diurnal component owes its varianc e to the daily photochemistry and nighttime dry deposition of the chem ical species. In comparison, the matching ADOM component underpredicts the ratio [O-3]/([TNOy]-[NOx]) and the NO2 consumption and O-3 produc tion but overpredicts the contributions to the other species. The EMEF S emission component represents emissions from local/regional area sou rces. The corresponding ADOM component underpredicts TNOy by 44% and t he fraction of TNOy as NOx compared to the EMEFS component, suggesting that the model has lower emissions of NOx and a photochemical mechani sm that converts NOx faster than indicated by the EMEFS results.