One-hour and eight-hour average ozone in the California South Coast air quality management district: trends in peak values and sensitivity to precursors

Exceedences of the 1 and 8 h O3 air quality standard were identified for 24 routine air quality monitor stations in the California South Coast Air Quality Management District (AQMD) from 1980 to 2001. Yearly trends, temporal and spatial distributions of O3 exceedences and “weekend effect” on O3 exceedences are presented. There were considerably more exceedences of the 8 h O3 standard than exceedences of the 1 h standard in the period of 1997–2001. Prescursor control strategies in the AQMD were very successful in controlling O3 with dramatic reductions in the frequency of days on which both the 1 and 8 h O3 standards were exceeded in 1990s. However, there was less reduction in frequency of moderate exceedences of the 8 h standard, where “moderate” was defined as the day on which 8 h average O3 exceeded 80 ppb but the peak hour O3 was <120 ppb. Observation-Based Methods (OBMs) were used to determine a possible shift in O3 control strategies for attaining the 8 h standard. The photochemical indicator, extent of reaction, was calculated for the days with exceedence of the 1 or 8 h O3 standards. Analysis of the results indicates that the promulgation of an 8 h standard would significantly increase the number of non-attainment in station days in the AQMD. The O3 control in the AQMD was general VOCs control benefit. The new 8 h O3 standard would lend the O3 control strategy little more VOCs control benefit.