Annual PM2.5 State Implementation Plan Modeling for Allegheny County, Pennsylvania, Using a Hybrid Plume/Grid Approach

In 2012, USEPA revised the National Ambient Air Quality Standard (NAAQS) for annual PM_2.5­, lowering it from 15 to 12 µg/m³. Allegheny County, Pennsylvania was designated nonattainment of that standard, based on a measured 2012-2014 Design Value of 13.0 µg/m³ at the South Allegheny High School (Liberty) monitoring site. The Liberty site is in southern Allegheny County, within an area of complex river-valley terrain. The full nonattainment area encompasses the entire county, and contains a mixture of urban and rural areas, including the cities of Pittsburgh, McKeesport, and Bethel Park, with a couty-wide population of 1.2 million. PM_2.5 exceedances in the nonattainment area and specifically at the Liberty monitoring site are due to the combination of local sources and long-range transport, and include both primary and secondarily-formed PM_2.5. The CAMx photochemical grid model (PGM) was used to simulate the year 2011 using four nested domains: a continental 36 km, a 12 km northeast, a 4 km Pennsylvania, and 1.33 km Allegheny County domain. WRF meteorological modeling was conducted for the 36/12/4/1.33 km domains. The 2011 National Emissions Inventory (NEI) was processed to generate the gridded hourly speciated emissions needed for the CAMx PGM model. Local source emissions were updated by Allegheny County, and treated using the CAMx Plume-in-Grid puff model. A GEOS-Chem global model run was used to supply boundary conditions (BCs) for the 36km CAMx domain. A two-way nested 36/12 km CAMx simulation was performed, and used to supply BCs to a 4/1.33 km CAMx simulation. A base year model performance evaluation (MPE) was conducted for the CAMx 1.33 km domain modeling results, with particular focus on PM_2.5 performance at the Liberty monitoring site. Emissions were projected to the year 2021, and 36/12 km and 4/1.33 km CAMx simulations were performed. For an attainment demonstration, annual and 24-hour PM_2.5 Design Values were extracted from the 2021 simulation and compared against the NAAQS.