Probing into Regional O3 and PM Pollution: A 1-year CMAQ Simulation and Process Analysis over the United States

Regional air pollution may be caused by many factors including emissions, in-situ photochemistry, local meteorological processes, and interstate/intercontinental transport. Process analysis (PA), a probing tool imbedded in the U.S. EPA Models-3/Community Multiscale Air Quality (CMAQ) modeling system, enables an in-depth understanding of the major contributors to the formation and fate of air pollutants. The PA in CMAQ calculates the Integrated Process Rates (IPRs) and the Integrated Reaction Rates (IRRs). The IPRs provide the change in species concentrations due to different physical and chemical processes (e.g., transport, emission, chemistry, aerosol and cloud processes, and deposition). The IRRs provide individual gas-phase reaction rates, permitting a detailed study of the chemical transformation of the species of interest such as O3 and its precursors. An annual simulation with CMAQ is being conducted for year 2001 at a 36-km grid resolution over the contiguous U.S., southern Canada, and northern Mexico. Model predictions of gas and aerosol concentrations and aerosol radiative properties such as aerosol optical depth are evaluated against measurements from satellites and ground-based monitoring networks. In addition, PA is performed with hourly IPRs for 30 species and IRRs for 96 reactions from the surface to an altitude of 3.9 km at sixteen locations that represent air masses of different origins (e.g., urban vs. rural, inland vs. coast, NOx-limited vs. VOC-limited regions). Controlling processes are identified and quantified for the formation and fate of total odd oxygen (Ox) and aerosols. Their total masses exported from the urban/regional scale to the global atmosphere are estimated. Our preliminary results have shown that CMAQ's performance for O3 and PM is generally satisfactory. CMAQ generally reproduces the magnitudes and spatial variations of the monthly-mean AOD obtained from MODIS. Vertical transport, chemistry, and dry deposition are the major processes contributing to the formation and fate of O3 in winter/spring, and those for aerosols include emissions, vertical transport, PM processes, and dry deposition. The net export from the planetary boundary layer (defined as 0-2.9 km in this study) to the free troposphere during January-March 2001 is 0.75 Gigamoles/day for Ox and 1.54 Gigagrams/day for aerosols over the entire modeling domain. Monthly and seasonal-mean results are being analyzed and contrasted to provide a complete assessment of major factors for regional O3 and PM pollution and aerosol radiative/visibility impact in the U.S. throughout the year.