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

摘要

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 SO₄=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 inNH₄+ wet deposition is likely due in part to the poor temporal andspatial representation of ammonia (NH₃) emissions, particularly thoseemissions associated with fertilizer applications and NH₃bi-directional exchange. The model performance for estimates ofNO₃− 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-kmsimulation, particularly in the spring. The underestimation ofNO₃− 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.