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 O3 and PM2.5, their impacts on global environment, and implications for pollution control policies. Process analyses show that the most influential processes for O3 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 O3 and Ox 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/(O3 + NO2) in winter, and NOy in summer. They indicate a VOC‐limited O3 chemistry in most areas in winter, but a NOx‐limited O3 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 O3 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, O3 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 O3 may increase in summer; surface PM2.5 may increase or decrease. With 0.71°C increase in future surface temperatures in summer, surface O3 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.

A Modeling Study of a Typical Winter PM2.5 Pollution Episode in a City in Eastern China

“…Consequently, there were peak PM 2.5 mass concentration at the nighttime of 13 and 14 December until the diffusion process provided an important pathway for the PM 2.5 removal in the morning of 14 and 15 December under the influence of wind speed increase. By design, the gas-phase chemistry process is accounted as part of aerosol process rather than as a direct contribution to PM 2.5 formation (Zhang et al, 2009b). The pollutants transported from the north of Zhejiang province and Jiangsu province made important contributions to the PM 2.5 production at the industrial station during the episode.…”

“…This technique does not require additional input variables. From CMAQ process analysis result, the relative importance of major atmospheric processes such as emissions of primary species, advection (including horizontal and vertical advection, and advection adjustment), diffusion (including horizontal and vertical diffusion), dry deposition, gas-phase chemistry (including photolysis and kinetic reactions), could process, aerosol process (Zhang et al, 2009b) are examined. Cloud process includes the net effect of cloud attenuation of photolysis rates, convective and nonconvective mixing and scavenging by clouds, aqueousphase chemistry, and wet deposition.…”

Online coupled regional meteorology chemistry models in Europe: current status and prospects

“…Jonson et al, 2006;van Loon et al, 2007;Vautard et al, 2007;Gilliland et al, 2008;Zhang et al, 2009bZhang et al, , 2013Colette et al, 2011;Godowitch et al, 2011;Hogrefe et al, 2011;Zubler et al, 2011). It has been demonstrated that uncertain emission inventories, data assimilation approaches used in meteorology models, and initial/boundary conditions can influence dynamic evaluation of model-predicted changes in ambient air quality (Napolenock et al, 2011;Kang et al, 2012).…”

Urban Emissions and Regional Air Quality in India

“…Tropospheric column observations can be used to evaluate model performance in terms of relevant column output, examining emissions in a region, and drawing inferences for photochemical relationships among precursor species. They have been used to evaluate regional model results [Zhang et al, 2009;Muntaseer Billah Ibn Azkar et al, 2012;Kemball-Cook et al, 2015], in support of O 3 source assessment [McDonald-Buller et al, 2011], and for biogenic contribution assessment [Carlton and Baker, 2011]. The availability and use of satellite retrievals of air pollutant for model evaluation is incredibly useful to assess model performance in between surface monitors, important for ruralregional conclusions.…”

“…Limitations of satellite observations include temporal availability (i.e. once per day), and the fact that cloud or intense smoke coverage can obscure satellite imagery [Zhang et al, 2009;Muntaseer Billah Ibn Azkar et al, 2012]. Despite temporal and retrieval limitations, satellite observations provide a significant amount of data to evaluate a wide range of air quality phenomenon and circumstances.…”