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

Abstract

This paper examines the operational performance of the Community MultiscaleAir Quality (CMAQ) model simulations for 2002-2006 using both 36-km and12-km horizontal grid spacing, with a primary focus on the performance ofthe CMAQ model in predicting wet deposition of sulfate (SO4=), ammonium (NH4+) and nitrate (NO3-). Performance of thewet deposition estimates from the model is determined by comparing CMAQpredicted concentrations to concentrations measured by the National AcidDeposition Program (NADP), specifically the National Trends Network (NTN).For SO4= wet deposition, the CMAQ model estimates were generallycomparable between the 36-km and 12-km simulations for the eastern US, with the 12-km simulation giving slightly higher estimates of SO 4= wet deposition than the 36-km simulation on average. The result is aslightly larger normalized mean bias (NMB) for the 12-km simulation; howeverboth simulations had annual biases that were less than ±15 % foreach of the five years. The model estimated SO4= wet depositionvalues improved when they were adjusted to account for biases in the modelestimated precipitation. The CMAQ model underestimates NH4+ wetdeposition over the eastern US, with a slightly larger underestimation inthe 36-km simulation. The largest underestimations occur in the winter andspring periods, while the summer and fall have slightly smallerunderestimations of NH4+ wet deposition. The underestimation in NH4+ wet deposition is likely due in part to the poor temporal andspatial representation of ammonia (NH3) emissions, particularly thoseemissions associated with fertilizer applications and NH3 bi-directional exchange. The model performance for estimates of NO3- wet deposition are mixed throughout the year, with the modellargely underestimating NO3- wet deposition in the spring andsummer in the eastern US, while the model has a relatively small bias inthe fall and winter. Model estimates of NO3- wet deposition tendto be slightly lower for the 36-km simulation as compared to the 12-km simulation, particularly in the spring. The underestimation of NO 3- wet deposition in the spring and summer is due in part to alack of lightning generated NO emissions in the upper troposphere, which canbe a large source of NO in the spring and summer when lightning activity isthe high. CMAQ model simulations that include production of NO fromlightning show a significant improvement in the NO3- wetdeposition estimates in the eastern US in the summer. Overall, performancefor the 36-km and 12-km CMAQ model simulations is similar for the easternUS, while for the western US the performance of the 36-km simulation isgenerally not as good as either eastern US simulation, which is not entireunexpected given the complex topography in the western US. © 2011 Author(s).