Simulations of fine particulate matter (PM2.5) in Houston, Texas

Abstract

Simulations of fine particulate matter (PM2.5) during an eight‐day episode (24 to 31 August 2000) is conducted in association with the 2000 Texas Air Quality Study (TexAQS 2000) and the Houston Supersite Project using the EPA's Models‐3 Community Multiscale Air Quality model (CMAQ). The mass concentrations of PM2.5 and major chemical constituents during the episode are calculated and compared with available field measurements. The predicted daily PM2.5 mass concentrations are about 8.5–13.0 μg/m3, consistent with the observed values. The diurnal patterns of PM2.5 mass concentrations are similar throughout the region, with a strong morning peak and a weak peak in the late afternoon to the early evening. High primary emissions, high formation rates of the secondary fine particulate matter, and low planetary boundary layer (PBL) heights contribute to the morning peak. The major components of the fine particulate matter in this region are sulfate, organic carbon, elemental carbon and ammonium. The model predicts about 30% sulfate, 32% organics (including elemental carbon (EC)), and 10% ammonium of the total PM2.5 mass. The balance of the primary cations and anions indicates that fine particulate matter in this region is acidic. Comparison with field observation reveals that CMAQ produces good simulations of averaged daily mass concentrations of major components such as sulfate, organic carbon, elemental carbon and ammonium with normalized mean biases (NMB) of less than ±25%. Uncertainties in the aerosol precursor emissions, the aerosol chemistry especially about secondary organic aerosol (SOA) formation and aqueous reactions, and the calculated PBL heights are likely responsible for the differences.