Ozone in remote areas of the Southern Rocky Mountains

Abstract Ozone (O 3 ) data are sparse for remote, non-urban mountain areas of the western U.S. Ozone was monitored 2007–2011 at high elevation sites in national forests in Colorado and northeastern Utah using a portable battery-powered O 3 monitor. The data suggest that many of these remote locations already have O 3 concentrations that would contribute to exceedance of the current National Ambient Air Quality Standard (NAAQS) for O 3 and most could exceed a proposed more stringent secondary standard. There were significant year-to-year differences in O 3 concentration. Ozone was primarily in the mid-concentration range, rarely exceeding 100 ppb or dropping below 30 ppb. The small diel changes in concentration indicate mixing ratios of NO x , VOCs, and O 3 that favor stable O 3 concentrations. The large number of mid-level O 3 concentrations contributed to high W126 O 3 values, the metric proposed as a possible new secondary standard. Higher O 3 concentrations in springtime and at night suggest that stratospheric intrusion may be contributing to ambient O 3 at these sites. Highest nighttime O 3 concentrations occurred at the highest elevations, while daytime O 3 concentrations did not have a relationship with elevation. These factors favor O 3 concentrations at many of our remote locations that may exceed the O 3 NAAQS, and suggest that exceedances are likely to occur at other western rural locations.