PUN

Abstract

Data from the 1990 San Joaquin Valley Air Quality Study/ Atmospheric Utility Signatures, Predictions, and Experiments (SJVAQS/AUSPEX) field program in California’s San Joaquin Valley (SJV) suggest that both urban and rural areas would have difficulty meeting an 8-hr average O3 standard of 80 ppb. A conceptual model of O3 formation and accumulation in the SJV is formulated based on the chemical, meteorological, and tracer data from SJVAQS/ AUSPEX. Two major phenomena appear to lead to high O3 concentrations in the SJV: (1) transport of O3 and precursors from upwind areas (primarily the San Francisco Bay Area, but also the Sacramento Valley) into the SJV, affecting the northern part of the valley, and (2) emissions of precursors, mixing, transport (including longrange transport), and atmospheric reactions within the SJV responsible for regional and urban-scale (e.g., downwind of Fresno and Bakersfield) distributions of O3. Using IMPLICATIONS The development of State Implementation Plans to address the O3 air quality standard will require meteorological and air quality models that provide accurate representations of the relevant atmospheric processes over regional scales. The California SJV is an appropriate test bed for evaluating state-of-the-science models, because of its variety of chemical and meteorological regimes and the availability of data to execute and test models. A conceptual model of O3 formation in the SJV was developed and used to critically assess the suitability of meteorological and air quality models. Specific recommendations are provided to improve existing models, thereby increasing the level of confidence in the use of model predictions of O3 and precursors to develop cost-effective emission control strategies. this conceptual model, we then conduct a critical evaluation of the meteorological model and air quality model. Areas of model improvements and data needed to understand and properly simulate O3 formation in the SJV are highlighted. INTRODUCTION Summertime O3 air pollution is a serious problem in California’s San Joaquin Valley (SJV). The 1990 San Joaquin Valley Air Quality Study (SJVAQS) and the Atmospheric Utility Signatures, Predictions, and Experiments (AUSPEX) were two components of a comprehensive study to address this issue.1 The comprehensive study involved (1) a large-scale air quality measurement program, (2) analyses of the causes of poor O3 air quality, and (3) the development and application of a regional-scale air quality modeling system (SJVAQS/AUSPEX Regional Model Adaptation Project, or SARMAP). The field program was conducted during an eight-week period from July 9, 1990, to August 24, 1990. Fourteen intensive days were selected, which corresponded to high O3 episodes, for detailed measurements. Figure 1 depicts the SJVAQS/AUSPEX domain.1 Time series of the daily maximum 1-hr and 8-hr average O3 concentrations are shown in Figures 2 and 3, respectively.2 At urban sites, the 8-hr average concentrations can be 20 to 30 ppb lower than the 1-hr maximum when O3 concentrations are above 100 ppb (e.g., in Fresno). On the other hand, at rural sites such as Sequoia National Park, the daily maximum 8-hr concentrations of O3 tend to be similar (< 10 ppb difference) to the daily 1-hr maximum concentrations. As a result, while urban areas exceed the 1-hr average National Ambient Air Quality Standard (NAAQS) of 120 ppb, both urban and rural areas may exceed an 8-hr average NAAQS of 80 ppb.