Identifying sources of ozone to three rural locations in Nevada, USA, using ancillary gas pollutants, aerosol chemistry, and mercury

Ozone (O₃) is a secondary air pollutant of long standing and increasing concern for environmental and human health, and as such, the US Environmental Protection Agency will revise the National Ambient Air Quality Standard of 75ppbv to ≤70ppbv. Long term measurements at the Great Basin National Park (GBNP) indicate that O₃ in remote areas of Nevada will exceed a revised standard. As part of the Nevada Rural Ozone Initiative, measurements of O₃ and other air pollutants were made at 3 remote sites between February 2012 and March 2014, GBNP, Paradise Valley (PAVA), and Echo Peak (ECHO). Exceptionally high concentrations of each air pollutant were defined relative to each site as mixing ratios that exceeded the 90th percentile of all hourly data. Case studies were analyzed for all periods during which mean daily O₃ exceeded the 90th percentile concurrently with a maximum 8-h average (MDA8) O₃ that was "exceptionally high" for the site (65ppbv at PAVA, 70ppbv at ECHO and GBNP), and of potential regulatory significance. An MDA8 ≥ 65ppbv occurred only five times at PAVA, whereas this occurred on 49 and 65days at GBNP and ECHO, respectively. The overall correlation between O₃ and other pollutants was poor, consistent with the large distance from significant primary emission sources. Mean CO at these locations exceeded concentrations reported for background sites in 2000. Trajectory residence time calculations and air pollutant concentrations indicate that exceedances at GBNP and ECHO were promoted by air masses originating from multiple sources, including wildfires, transport of pollution from southern California and the marine boundary layer, and transport of Asian pollution plumes. Results indicate that the State of Nevada will exceed a revised O₃ standard due to sources that are beyond their control.