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

Abstract

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 PM 2.5 , 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 PM 2.5 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 PM 2.5 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 PM 2.5 throughout the year. Among nine photochemical indicators examined, the most robust include PH 2 O 2 /PHNO 3 , HCHO/NO y , and HCHO/NO z in winter and summer, H 2 O 2 /(O 3 + NO 2 ) in winter, and NO y in summer. They indicate a VOC‐limited O 3 chemistry in most areas in winter, but a NO x ‐limited O 3 chemistry in most areas except for major cities in April–November, providing a rationale for nationwide NO x emission control and integrated control of NO x and VOCs emissions for large cities during high O 3 seasons (May–September). For sensitivity of PM 2.5 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. NH 4 NO 3 can be formed in most of the domain. Integrated control of emissions of PM precursors such as SO 2 , NO x , and NH 3 are necessary for PM 2.5 attainment. Among four types of VOCs examined, O 3 formation is primarily affected by isoprene and low molecular weight anthropogenic VOCs, and PM 2.5 formation is affected largely by terpenes and isoprene. Under future emission scenarios, surface O 3 may increase in summer; surface PM 2.5 may increase or decrease. With 0.71°C increase in future surface temperatures in summer, surface O 3 may increase in most of the domain and surface PM 2.5 may decrease in the eastern U.S. but increase in the western U.S.