Estimates of background surface ozone concentrations in the United States based on model-derived source apportionment

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• 作者：Allen S. Lefohn ; Christopher Emery ; Douglas Shadwick ; Heini Wernli ; Jeagun Jung ; Samuel J. Oltmans
• 关键词：Background ozone ; CAMx ; Emissions-Influenced Background ; Photochemical modeling ; Source apportionment ; Stratosphere-to-troposphere transport ; Surface ozone ; Trajectory modeling
• 刊名：Atmospheric Environment
• 出版年：February, 2014
• 年：2014
• 卷：84
• 期：Complete
• 页码：275-288
• 全文大小：2994 K

文摘

We analyze background surface ozone (O₃) concentrations as estimated by coupled GEOS-Chem/CAMx models for 23 monitoring sites across the US at high- and low-elevation, rural and urban locations during 2006. Specifically, we consider hourly contributions from global tropospheric O₃ entering North America, stratospheric O₃ over North America, and natural O₃ formed from continental biogenic, fire, and lightning sources according to CAMx source apportionment calculations. Unlike historical modeled background definitions that reflect the absence of anthropogenic emissions, we define 鈥淓missions-Influenced Background鈥?(EIB), which includes chemical interactions with anthropogenic emissions and thus reflects 鈥渃urrent鈥?background levels at the sites analyzed. We further define global background O₃ (GBO₃) as the sum of the global tropospheric and stratospheric components and find that higher modeled GBO₃ occurs during the spring at sites across the US. At many of the sites during the spring, fall, and winter months higher GBO₃ is associated with more frequent stratosphere-to-troposphere transport to the surface (STT-S) events according to independent three-dimensional trajectories based on global meteorological analyses. Patterns of higher spring EIB O₃ are followed by lower values during the summer, due to heightened chemical interaction with anthropogenic sources, which are then followed by rising EIB O₃ during the fall and winter months. For some high-elevation western US sites, this seasonal pattern is less discernible due to relatively small anthropogenic contributions and the high EIB O₃ estimated throughout the year. EIB O₃ at all high-elevation sites contributes a significant proportion to total O₃ throughout the year and throughout the observed total O₃ frequency distribution, while EIB O₃ at most urban sites contributes a major portion to total O₃ during non-summer months and to the mid-range concentrations (30-50聽ppb) of the frequency distribution.