The impact of an 8 h ozone air quality standard on ROG and NOx controls in Southern California

The new National Ambient Air Quality Standard for ozone in the US uses 8 h averaging for the concentration. Based on the 1993 ambient data for Southern California, 8 h averaging has a moderate tendency to move the location of the peak ozone concentration east of the location of the peak 1 h ozone concentration. Reducing the area-wide peak 8 h ozone concentration to 80 ppb would require an effective reduction of the area-wide peak 1 h ozone concentration to around 90 ppb. The Urban Airshed Model with improved numerical solvers, meteorological input based on a mesoscale model and an adjusted emissions inventory was used to study the effect of reactive organic gases (ROG) and NOx controls on daily-maximum and peak 8 h ozone concentrations under the 26-28 August 1987 ozone episodic conditions in Southern California. The NOx disbenefit remains prominent for the case of 8 h ozone concentration but is somewhat less prominent, especially when areal ozone exposure is considered, than the case for 1 h ozone concentration. The role of two indicators - O-3/NOy and H2O2/HNO3 - for NOx- and ROG-sensitivity for 1 and 8 h ozone concentrations were also studied. In general, the indicator trends are consistent with model predictions, but the discriminating power of the indicators is rather limited. (C) 1999 Elsevier Science Ltd. All rights reserved.