Annual model simulations and evaluation of multiple pollutants (particulate matter, ozone, and mercury) over the continental United States

An annual model application and evaluation study was performed for the continental United States using the U.S. EPA's Community Multi-scale Air Quality (CMAQ) modeling system. Annual model simulations for 2001 were conducted over the using a 36-km grid resolution. Annual simulations at 12 km resolution were also performed for the Eastern U.S. The focus of this effort is the evaluation of model predictions of PM2.5 component species and precursor gases (including ozone), and deposition of sulfate, nitrate, and mercury. Based on the 36 km 2001 simulation, sulfate predictions compare very well with measurements over the summer months in the eastern U.S. where sulfate is the dominant PM2.5 species. The model appears to simulate the total nitrate (nitrate PM and nitric acid, HNO3) fairly well, but the partitioning between nitrate PM and HNO3 leads to a modest overestimate of nitrate PM. The results indicate that the predictions of nitrate PM and HNO3 are highly sensitive to (1) the amount of ammonia (NH3) emissions, (2) dry deposition of HNO3 and NH3, (3) night time chemistry of N2O5 and HNO3, and (4) the diffusivity of night time atmospheric. In this paper the model performance results from the 36 km run are compared to those from the 12 km run to determine the importance of using finer resolution modeling for predicting PM2.5 and deposition. A second part of this paper includes a characterization of the seasonal behaviors of PM2.5 components and ozone and their temporal and spatial relationships. A series of emissions sensitivity simulations were conducted and reveal that the response of ozone and PM2.5 to emissions changes is highly correlated over the year. During the summer months, NOx emissions reductions led to lower predictions of sulfate, mainly due to lower ozone and related oxidants, while during the winter months, NOx reduction led to higher predicted sulfate (but lower nitrate PM), mainly due to higher ozone as a result of less NO titration. These relationships are further explored in this paper.