Development of a 2007-Based Air Quality Modeling Platform

EPA's Office of Air Quality Planning and Standards is developing a 2007-based air quality modeling platform to support upcoming regulatory and policy assessments. This platform includes meteorology for 2007 obtained from simulations of WRFv3.3 and emissions for the US that are representative of the 2007/2008 time period which can be used with either the CAMx or CMAQ photochemical model . The national-scale modeling domain covers the lower 48 states plus adjacent portions of Canada and Mexico using a horizontal grid resolution of 12 x 12 km. As part of the development of this platform we have performed a series of sensitivity tests to understand the effects on predicted pollutant concentrations of several optional science features that are included in the most recent public release of CMAQ, version 5.0.1. We created a benchmark annual simulation for 2007 using CMAQ version 4.7.1 (released June 2010) coupled with meteorology from a previous version of WRF, version 3.1. We then used these same inputs to run CMAQ version 5.0.1 (released February 2012). Science updates in the default configuration of CMAQv5.0.1 include expansion of the CB05 chemical mechanism to explicitly treat toluene reactions, updates to the turbulent mixing and vertical advection schemes, an updated aerosol module (AERO6) and inline photolysis. The AERO6 module includes updates to the thermodynamic partitioning of inorganic species (ISORROPIAv2.0), speciation and tracking of trace metals from primary PM, revisions to the aqueous sulfate chemistry, and aging of primary organic matter. Inline photolysis incorporates radiative impacts of aerosols and land-surface dependant albedo on photolysis rates. The baseline CMAQv5.0.1 simulation was followed by three incremental sensitivity tests which evaluated the effects of various meteorology-driven emissions processes: (1) bi-directional flux for fertilizer ammonia emissions, (2) lightning NOx emissions, and (3) emissions of wind-blown dust. Finally, we remodeled the 2007 case using meteorology based on simulations from WRF version 3.3 to examine the impacts of changes in meteorology between WRF version 3.1 and 3.3 along with increasing the vertical resolution of the lowest layer from a thickness of 38 meters with version 3.1 to 20 meters with version 3.3. In this paper, we will present an overview of the components of the 2007 platform and describe the impacts on species concentrations and deposition of simulating 2007 with CMAQ version 5.0.1 compared to version 4.7.1 along with the effects of the incremental changes to emissions and meteorology identified above. We will also discuss the effects on model performance of each simulation by comparing model predictions to the corresponding measured concentrations and deposition.