Modeling the intercontinental transport of PM2.5 across the North Pacific Ocean

It has been well documented that the increasing outflow of pollutants from Asia can significantly impact the atmospheric chemistry and air quality in North America through transport over the Pacific Ocean. In order to quantify the magnitude and impacts of present-day Asian anthropogenic emissions on US air quality through intercontinental transport, a 3-D regional air quality model, the EPA Models-3 Community Multiscale Air Quality (CMAQ) system, is employed for fine particulate matter (PM_2.5) study. The model is applied to both standard and the sensitivity simulations with anthropogenic Asian emissions turned on and off, respectively, for 2001. The standard simulation was evaluated with nationwide networks of observations in the United States (Interagency Monitoring and Protected Visual Environments (IMPROVE)). Model evaluation and simulation results of PM_2.5 in the surface air over the study region during January, April, July and October were presented. The results show that except in January, over 50% of the simulated PM_2.5 concentrations falling within a factor of two of the observations in other three months. We also find that the impact of Asian anthropogenic emissions on North America appeared to be persistent throughout the entire year, although exhibiting seasonal variations. The model simulations showed that despite the fact that the monthly average maximum of PM_2.5 in Asia occurred in January, the maximum impact on the U.S. appeared in spring (April) with up to 2~2.5 μg/m³ (monthly average) PM_2.5 in the western U.S. and up to 1~1.5 μg/m³ PM_2.5 in the eastern U.S. This is likely due to the optimal combination of PM_2.5 source strength in Asia and atmospheric circulation that created the most efficient intercontinental transport during that period.