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

摘要

We analyze background surface ozone (O_3) 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 O_3 entering North America, stratospheric O_3 over North America, and natural O_3 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 "Emissions-Influenced Background" (EIB), which includes chemical interactions with anthropogenic emissions and thus reflects "current" background levels at the sites analyzed. We further define global background O_3 (GBO_3) as the sum of the global tropospheric and stratospheric components and find that higher modeled GBO_3 occurs during the spring at sites across the US. At many of the sites during the spring, fall, and winter months higher GBO_3 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 O_3 are followed by lower values during the summer, due to heightened chemical interaction with anthropogenic sources, which are then followed by rising EIB O_3 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 O_3 estimated throughout the year. EIB O_3 at all high-elevation sites contributes a significant proportion to total O_3 throughout the year and throughout the observed total O_3 frequency distribution, while EIB O_3 at most urban sites contributes a major portion to total O_3 during non-summer months and to the mid-range concentrations (30-50 ppb) of the frequency distribution.