A multi-scale observation-modeling study of summertime California air quality.

摘要

Multi-scale tracer and full-chemistry simulations with the STEM atmospheric chemistry model have been used to analyze the effects of both transported and local production of pollutants on California air quality during the ARCTAS--CARB experiment conducted in June 2008.During this summer experiment, simulated and observed Ozone (O3 ) transport patterns from the coast to inland northern California are shown to vary based on the meteorological conditions and the oceanic O 3 profiles, which are strongly episodically affected by Asian inflows. During a specific period (June 21--June 24), high coastal O3 air-masses at altitudes ~2--4 km can be transported inland and can significantly influence the surface O3 20--30 hours later over the northern Sacramento valley and the southern California can be indirectly affected by in-state transport. The model performance was improved by using lateral boundary conditions (LBCs) downscaled from RAQMS global model that assimilated satellite data, as well as the LBC based on NASA DC-8 airborne observations during the experiment. The effects of oxidized sulfur (SOx) in these transported Asian air-masses over California are relatively less strong than O3 and its precursors. Local emissions are the major contributor to the elevated sulfur concentrations below 5 km. Several SOx emission inventories (EI) are compared and the simulated SOx are validated with various observational datasets, with special focus on three regions--South Coast, San Francisco and Central Valley. The resolutions and the spatial and/or temporal variations of SOx emissions in all EIs remain to be further improved. Both terrestrial and maritime emissions are found to be important to coastal SOx distributions. Their percentile contributions to coastal SO x spatial distributions for the experiment week are estimated, and their absolute contributions during flight periods are then quantified with observational-based scaling factors. The California-Mexico pollutant interaction mainly occurred between two sister city pairs--the coastal city pair (San Diego--Tijuana) was heavily affected by both California and Mexico maritime emissions through both directions, due to the northwesterly winds during daytime and the opposite winds through the nights. This effect was extended to the inland city pair (Mexicali--Calexico) by dominant inland westerly winds, where was also shortly impacted by southerly winds from Mexico, during which time limited areas over the southeast of California were also affected.