Abstract
Abstract. Multi-scale tracer and full-chemistry simulations with the STEM atmospheric chemistry model are used to analyze the effects of transported background ozone (O3) from the eastern Pacific on California air quality during the ARCTAS-CARB experiment conducted in June, 2008. Previous work has focused on the importance of long-range transport of O3 to North America air quality in springtime. However during this summer experiment the long-range transport of O3 is also shown to be important. Simulated and observed O3 transport patterns from the coast to inland northern California are shown to vary based on meteorological conditions and the O3 profiles over the oceans, which are strongly episodically affected by Asian inflows. Analysis of the correlations of O3 at various altitudes above the coastal site at Trinidad Head and at a downwind surface site in northern California, show that under long-range transport events, high O3 air-masses (O3>60 ppb) at altitudes between about 2 and 4 km can be transported inland and can significantly influence surface O3 20–30 h later. These results show the importance of characterizing the vertical structure of the lateral boundary conditions (LBC) needed in air quality simulations. The importance of the LBC on O3 prediction during this period is further studied through a series of sensitivity studies using different forms of LBC. It is shown that the use of the LBC downscaled from RAQMS global model that assimilated MLS and OMI data improves the model performance. We also show that the predictions can be further improved through the use of LBC based on NASA DC-8 airborne observations during the ARCTAS-CARB experiment. These results indicate the need to develop observational strategies to provide information on the three-dimensional nature of pollutant distributions, in order to improve our capability to predict pollution levels and to better quantify the influence of these Asian inflows on the US west coast air quality.
Highlights
Tropospheric ozone (O3) is an atmospheric pollutant harmful to human health and agriculture, and is one of the most important green house gases
While estimates vary due to differences in season, location, elevation, experimental and modeling methods, there is considerable agreement that less than 40 ppb of Policy-Relevant Background O3 (PRB) is due to natural sources in North America, peaking in spring, and 5–15 ppb is due to trans-continental transport of O3 (Fiore et al, 2002; Vingarzan et al, 2004; McKendry et al, 2006)
In a previous study we have shown that the use of temporal and spatial variations in top and lateral boundary conditions downscaled from global models can enhance model performance, especially for longlive species and over areas less impacted by local pollution (Tang et al, 2007)
Summary
Tropospheric ozone (O3) is an atmospheric pollutant harmful to human health and agriculture, and is one of the most important green house gases. The PRB concentration defines the level below which national O3 regulatory standards cannot be set This includes O3 formed through photochemical reactions involving precursors originating exclusively from continental biogenic sources, wildfires, and lightning, as well as O3 transported from outside of North America and from the stratosphere. Previous work has reported a wide range of background O3 in the northern hemisphere based on modeling studies and observational analysis, with estimates varying from 15 ppb to 60 ppb (Fiore et al, 2003; Lefohn et al, 2001; Jaffe et al, 2003) Many of these studies have analyzed observations at remote sites to determine the contributions of long-range O3 transport. While estimates vary due to differences in season, location, elevation, experimental and modeling methods, there is considerable agreement that less than 40 ppb of PRB is due to natural sources in North America, peaking in spring, and 5–15 ppb is due to trans-continental transport of O3 (Fiore et al, 2002; Vingarzan et al, 2004; McKendry et al, 2006)
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