Probing into regional O3 and particulate matter pollution in the United States: 2. An examination of formation mechanisms through a process analysis technique and sensitivity study

Following a comprehensive model evaluation in part 1, this part 2 paper describes results from 1 year process analysis and a number of sensitivity simulations using the Community Multiscale Air Quality (CMAQ) modeling system aimed to understand the formation mechanisms of O-3 and PM2.5, their impacts on global environment, and implications for pollution control policies. Process analyses show that the most influential processes for O-3 in the planetary boundary layer (PBL) are vertical and horizontal transport, gas-phase chemistry, and dry deposition and those for PM2.5 are primary PM emissions, horizontal transport, PM processes, and cloud processes. Exports of O-3 and O-x from the U.S. PBL to free troposphere occur primarily in summer and at a rate of 0.16 and 0.65 Gmoles day(-1), respectively. In contrast, export of PM2.5 is found to occur during all seasons and at rates of 25.68-34.18 Ggrams day(-1), indicating a need to monitor and control PM2.5 throughout the year. Among nine photochemical indicators examined, the most robust include PH2O2/PHNO3, HCHO/NOy, and HCHO/NOz in winter and summer, H2O2/(O-3 + NO2) in winter, and NOy in summer. They indicate a VOC-limited O-3 chemistry in most areas in winter, but a NOx-limited O-3 chemistry in most areas except for major cities in April-November, providing a rationale for nationwide NOx emission control and integrated control of NOx and VOCs emissions for large cities during high O-3 seasons (May-September). For sensitivity of PM2.5 to its precursors, the adjusted gas ratio provides a more robust indicator than that without adjustment, especially for areas with insufficient sulfate neutralization in winter. NH4NO3 can be formed in most of the domain. Integrated control of emissions of PM precursors such as SO2, NOx, and NH3 are necessary for PM2.5 attainment. Among four types of VOCs examined, O-3 formation is primarily affected by isoprene and low molecular weight anthropogenic VOCs, and PM2.5 formation is affected largely by terpenes and isoprene. Under future emission scenarios, surface O-3 may increase in summer; surface PM2.5 may increase or decrease. With 0.71 degrees C increase in future surface temperatures in summer, surface O-3 may increase in most of the domain and surface PM2.5 may decrease in the eastern U. S. but increase in the western U.S.