A multi-resolution assessment of the Community Multiscale Air Quality (CMAQ) model v4.7 wet deposition estimates for 2002–2006

This paper examines the operational performance of the Community Multiscale Air Quality (CMAQ) model simulations for 2002–2006 using both 36-km and 12-km horizontal grid spacing, with a primary focus on the performance of the CMAQ model in predicting wet deposition of sulfate (SO<sub>4</sub><sup>=</sup>), ammonium (NH<sub>4</sub><sup>+</sup>) and nitrate (NO<sub>3</sub><sup>−</sup>). Performance of the wet deposition estimates from the model is determined by comparing CMAQ predicted concentrations to concentrations measured by the National Acid Deposition Program (NADP), specifically the National Trends Network (NTN). For SO<sub>4</sub><sup>=</sup> wet deposition, the CMAQ model estimates were generally comparable between the 36-km and 12-km simulations for the eastern US, with the 12-km simulation giving slightly higher estimates of SO<sub>4</sub><sup>=</sup> wet deposition than the 36-km simulation on average. The result is a slightly larger normalized mean bias (NMB) for the 12-km simulation; however both simulations had annual biases that were less than ±15 % for each of the five years. The model estimated SO<sub>4</sub><sup>=</sup> wet deposition values improved when they were adjusted to account for biases in the model estimated precipitation. The CMAQ model underestimates NH<sub>4</sub><sup>+</sup> wet deposition over the eastern US, with a slightly larger underestimation in the 36-km simulation. The largest underestimations occur in the winter and spring periods, while the summer and fall have slightly smaller underestimations of NH<sub>4</sub><sup>+</sup> wet deposition. The underestimation in NH<sub>4</sub><sup>+</sup> wet deposition is likely due in part to the poor temporal and spatial representation of ammonia (NH<sub>3</sub>) emissions, particularly those emissions associated with fertilizer applications and NH<sub>3</sub> bi-directional exchange. The model performance for estimates of NO<sub>3</sub><sup>−</sup> wet deposition are mixed throughout the year, with the model largely underestimating NO<sub>3</sub><sup>−</sup> wet deposition in the spring and summer in the eastern US, while the model has a relatively small bias in the fall and winter. Model estimates of NO<sub>3</sub><sup>−</sup> wet deposition tend to be slightly lower for the 36-km simulation as compared to the 12-km simulation, particularly in the spring. The underestimation of NO<sub>3</sub><sup>−</sup> wet deposition in the spring and summer is due in part to a lack of lightning generated NO emissions in the upper troposphere, which can be a large source of NO in the spring and summer when lightning activity is the high. CMAQ model simulations that include production of NO from lightning show a significant improvement in the NO<sub>3</sub><sup>−</sup> wet deposition estimates in the eastern US in the summer. Overall, performance for the 36-km and 12-km CMAQ model simulations is similar for the eastern US, while for the western US the performance of the 36-km simulation is generally not as good as either eastern US simulation, which is not entire unexpected given the complex topography in the western US