THE USE OF PHOTOCHEMICAL INDICATORS TO EVALUATE OZONE-NOX-HYDROCARBONSENSITIVITY - CASE-STUDIES FROM ATLANTA, NEW-YORK, AND LOS-ANGELES

This study examines the use of ambient measurements of a number of ''p hotochemical indicators'' as a basis for determining ozone-NOx-hydroca rbon sensitivity and for evaluating the performance of ozone models. T he successful photochemical indicators are: O-3/NOy, O-3/NOz (where NO z = NOy-NOx), O-3/HNO3, H2O2/HNO3, and H2O2/NOz. Results of Urban Airs hed Model (UAM-IV) simulations for Atlanta, GA, New York, NY, and Los Angeles, CA, show that high values of these species ratios are correla ted with NOx-sensitive chemistry and low values are associated with re active organic gases (ROG)-sensitive chemistry. Correlations between m easured O-3 and NOy in Atlanta and between O-3 and NOz in Los Angeles are consistent with theory and reflect the difference between likely N Ox-sensitive chemistry in Atlanta and hydrocarbon-sensitive chemistry in Los Angeles. Measured O-3, NOx, and NOy are used to evaluate model performance during two air pollution events in Atlanta and Los Angeles . The performance evaluation includes model scenarios for each city wi th different anthropogenic and biogenic emission rates and different N Ox-ROG sensitivity predictions. Simulations with different NOx-ROG che mistry are found to give similar predictions for peak ozone but differ ent values for photochemical indicators. Comparison with measured valu es of photochemical indicators provides a more stringent test of model performance than evaluation versus observed ozone.