AN INTEGRATED AIR-POLLUTION MODELING SYSTEM FOR URBAN AND REGIONAL SCALES .2. SIMULATIONS FOR SCAQS-1987

A new air quality modeling system, the surface meteorology and ozone g eneration (SMOG) model, is used to investigate the evolution and prope rties of air pollution in the Los Angeles basin during the southern Ca lifornia air quality study (SCAQS) intensive field program. The SMOG m odel includes four major components: a meteorological model, a tracer transport code, a chemistry and aerosol microphysics model, and a radi ative transfer code. The fidelity of the coupled modeling system is ev aluated by comparing model predictions against SCAQS data. Predictions of surface winds and temperatures are found to be in excellent agreem ent with measurements during daylight hours, when a strong sea breeze and mountain-upslope flows are predominant but are less reliable at ni ght when winds are typically lighter and more variable. Winds aloft, i ncluding shear and temporal variations, are also simulated quite well, although the forecasts (which are not constrained through continuous data assimilation) tend to drift from actual conditions as time progre sses. Accordingly, the large-scale flow is reinitialized each morning in the simulations. The dispersion patterns of two inert tracers relea sed during the SCAQS period are accurately reproduced by the model. Th e two releases, one in the early morning hours and one around noon, le d to quite different transport rates and distributions owing to the ev olution of the sea breeze over the course of the day. Overall, the thr ee-dimensional development of thermally induced winds and their influe nces on tracer transport in the Los Angeles basin are accurately captu red by the model. The predicted surface concentrations of ozone and ot her key pollutants have been spatially and temporally correlated with measured abundance, and the values agree to within 25-30% for ozone, w ith somewhat larger mean differences for several other species. In the case of the vertical distribution of ozone, the SMOG simulations gene rate dense oxidant (ozone) layers embedded in the temperature inversio n, explaining for the first time similar features seen during SCAQS. S ources of error and uncertainty in the simulations are identified and discussed. The broad agreement between SMOG model predictions and SCAQ S observations suggests that an integrated modeling approach is well s uited for representing the coupled effects of mesoscale meteorology, t racer dispersion, and chemical transformations on urban and regional a ir quality.