The Elemental Importance of Microscale, Mesoscale, and Synoptic Meteorology in Atmospheric Chemistry Studies: Examples from California's San Joaquin Valley

The fields of atmospheric turbulence, synoptic meteorology, and atmospheric chemistry seem to have originated from three separate branches of more fundamental scientific inquiry (e.g., mechanical engineering, physics, and analytical chemistry, respectively.) Yet despite their reticulate interplay in the atmospheric boundary layer (where the principal sources, and even sinks, of most trace gases and aerosols exist) expertise in these fields remains largely segregated. Even the most rudimentary explanation of air pollution episodes, dating back to the great smog of London in 1952, involves the central importance of synoptic high pressure systems. But the relationship is much more intricate than simply resulting from a lack of ventilation and faster photochemical reaction rates that ensue from the weaker pressure gradients and elevated temperatures accompanying surface anticyclones. Furthermore, the coupling between chemistry and atmospheric dynamics at all scales is even stronger in the presence of complex terrain (e.g. Los Angeles, California and Santiago de Chile.)
We present strategies and summarize results from several airborne studies in California's San Joaquin Valley that are based upon fundamental principles of boundary layer turbulence and designed to illuminate important dynamical and chemical mechanisms that impact air quality in a region notorious for its obstinate air pollution problems. Taking a holistic view, we discuss the effects of entrainment, dry deposition, the low-level nocturnal jet, the valley-mountain circulation, the synoptic setting, interhemispheric transport, and photochemical reaction rates on air quality in the valley. Comparing the airborne data with WRF model runs and regional reanalysis data, we argue that the large-scale meteorology influences the mesoscale recirculation and the turbulent mixing which ultimately impacts the valley's atmospheric chemistry. Judicious, systematic airborne sorties are shown to provide accurate quantification of the individual budget terms of important trace gases (e.g., net photochemical ozone production rates, and regional methane and NO_x emission rates), and reveal vertical and horizontal mixing rates (e.g., eddy diffusivities) in the daytime convective mixed layer as well as in the nocturnal stable boundary layer. We suggest that comparisons with regional air quality models may be performed for the individual budget terms thereby isolating distinct model process weaknesses and guiding improvements in the fidelity of chemical transport models.