Development of a statistical model to identify spatial and meteorological drivers of elevated O₃ in Nevada and its application to other rural mountainous regions

Measurements of O₃ at relatively remote monitoring sites are useful for quantifying baseline O₃, and subsequently the magnitude of O₃ not controllable by local regulations. As the National Ambient Air Quality Standard (NAAQS) for O₃ becomes more stringent, there is an increased need to quantify baseline O₃ particularly in the Western US, where regional and global sources can significantly enhance O₃ measured at surface sites, yielding baseline mixing ratios approaching or exceeding the NAAQS threshold. Past work has indicated that meteorological conditions as well as site specific spatial characteristics (e.g. elevation, basin size, gradient) are significantly correlated with O₃ intercepted at rural monitoring sites. Here, we use 3years of measurements from sites throughout rural Nevada to develop a categorical tree model to identify spatial and meteorological characteristics that are associated with elevated baseline O₃. Data from other sites in the Intermountain Western US are used to test the applicability of the model for sites throughout the region. Our analyses indicate that increased elevation and basin size were associated with increased frequency of elevated O₃. On a daily time scale, relative humidity had the strongest association with observed MDA8 O₃. Seventy-four percent of MDA8 O₃ observations>60ppbv occurred when daily minimum relative humidity was <15%. Further, we found that including ancillary pollutant data did not improve the predictive accuracy for measurements >60ppbv whereas including upper air meteorological measurements improved the accuracy of predicting periods when O₃ was >60ppbv. These findings indicate that transport, rather than local production, influences O₃ measurements in Nevada, and that high elevation sites in rural Nevada, are representative of baseline conditions in the Intermountain Western US.