Resolving and Quantifying Ozone Contributions from Boundary Conditions Within Regional Models

In regional model simulations, ozone introduced by boundary conditions (BCs) is important to model performance, model sensitivity and for policy reasons such as transport assessments. Quantitatively tracking the contribution of ozone BCs throughout a regional model is highly desirable. A simple and efficient solution is to add a tracer (O3BC) to the regional model simulation with a source term equal to the ozone BC. However, it is also necessary to model the removal of O3BC by deposition and chemistry. Modeling tracer deposition is relatively straightforward but modeling the photochemical removal of O3BC is complex. We developed a chemical mechanism for the photochemical removal of O3BC that is compact, represents all major removal pathways (i.e., photolysis, reactions with HOx radicals), accounts for important dependencies on NO, and can be implemented as an extension of the main gas-phase chemical mechanism. The method provides great flexibility by permitting O3BC to be factored into components that are then tracked independently. Example components of potential interest are different lateral boundaries segments (north vs. south boundary, etc.), altitude segments (boundary layer vs. free troposphere vs. stratosphere) and geographic origin (e.g., Asia vs. North America vs. Europe). Information about the geographic origin of O3BC can be derived from global model sensitivity simulations. The method is implemented in the CAMx regional model by using an existing model option to add reactive tracers (RTRAC) with complex chemistry. The method is applied and evaluated in simulations for continental North America.