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 – O3/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.