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

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 "EmissionsInfluenced 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 (SIT-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 midrange concentrations (30-50 ppb) of the frequency distribution. (C) 2013 Elsevier Ltd. All rights reserved.