Variability and sources of surface ozone at rural sites in Nevada, USA: Results from two years of the Nevada Rural Ozone Initiative.

Abstract Ozone (O 3 ) has been measured at Great Basin National Park (GBNP) since September 1993. GBNP is located in a remote, rural area of eastern Nevada. Data indicate that GBNP will not comply with a more stringent National Ambient Air Quality Standard (NAAQS) for O 3 , which is based upon the 3-year average of the annual 4th highest Maximum Daily 8-h Average (MDA8) concentration. Trend analyses for GBNP data collected from 1993 to 2013 indicate that MDA8 O 3 increased significantly for November to February, and May. The greatest increase was for May at 0.38, 0.35, and 0.46 ppb yr − 1 for the 95th, 50th, and 5th percentiles of MDA8 O 3 values, respectively. With the exception of GBNP, continuous O 3 monitoring in Nevada has been limited to the greater metropolitan areas. Due to the limited spatial detail of O 3 measurements in rural Nevada, a network of rural monitoring sites was established beginning in July 2011. For a period ranging from July 2011 to June 2013, maximum MDA8 O 3 at 6 sites occurred in the spring and summer, and ranged from 68 to 80 ppb. Our analyses indicate that GBNP, in particular, is ideally positioned to intercept air containing elevated O 3 derived from regional and global sources. For the 2 year period considered here, MDA8 O 3 at GBNP was an average of 3.1 to 12.6 ppb higher than at other rural Nevada sites. Measured MDA8 O 3 at GBNP exceeded the current regulatory threshold of 75 ppb on 7 occasions. Analyses of synoptic conditions, model tracers, and air mass back-trajectories on these days indicate that stratospheric intrusions, interstate pollution transport, wildfires, and Asian pollution contributed to elevated O 3 observed at GBNP. We suggest that regional and global sources of ozone may pose challenges to achieving a more stringent O 3 NAAQS in rural Nevada.