Spatiotemporal variability of ground-level ozone and influence of smoke in Treasure Valley, Idaho

The hourly concentrations of atmospheric ozone (O3) were measured at seven locations throughout the Treasure Valley during the summer of 2007, indicating a concentration gradient up to 30 percent, with the lowest levels measured on the northeast and the highest on the southwest parts of the Treasure Valley. A weak “weekend” effect was observed on Saturday despite morning NOx levels that were lower than those measured during the weekdays. The lowest ozone levels were measured on Sundays because of the absence of NOx emissions. The contribution of wildfire smoke was qualitatively assessed through the geospatial analysis of air mass backward trajectories along with the locations and durations of wildfires. For at least three sites, there was evidence of the direct contribution of wildfire smoke in four of the nine multi-day O3 events in which the measured ozone levels were higher than the 80th percentile value. For the rest five high O3 events, four of them had weaker indications of the influence of smoke, while one event (on July 4, 2007) was associated with the extended use of fireworks. During the high O3 events influenced by wildfires, max hourly and mean daily O3 concentrations were up to 15 ppbv and 12 ppbv, respectively, higher than those regularly measured in the Treasure Valley.