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

Abstract

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 “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 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 ppb) of the frequency distribution.