Identification of sources contributing to PM2.5 and ozone at elevated sites in the western U.S. by receptor analysis: Lassen Volcanic National Park, California, and Great Basin National Park, Nevada.

The proposed revision of the United States (US) air quality standard for ozone will result in violations in sparsely populated remote rural areas in the Western US. Replicating air quality as measured at surface monitoring sites by modeling is particularly difficult in this region due to complex terrain, poorly represented in regional and global models, and uncertainties in emission rates and timing at all scales (locally as well as hundreds to thousands of km upwind). As an alternative method, a fully empirical, receptor-based scheme using in situ aerosol composition and simple meteorological variables to simulate ozone (O3) measurements was tested and found to produce O3 simulation results comparable in uncertainty to regional modeling, and supporting trajectory-based identification of O3 source regions. This approach was tested using two widely-separated (650 km) high altitude (approx. 2 km above sea level) monitoring sites, Lassen Volcanic National Park, in northern California (LAVO) and Great Basin National Park in eastern Nevada (GRBA). Comparing correlations between observed O3 and aerosols, and examining back-trajectories associated with peak concentrations for the two sites permitted distinguishing among local, distant North American, and Asian sources of particulate matter (PM2.5) and O3. This analysis indicates that anthropogenic enhancement of O3 at LAVO is primarily due to transport from Asia. Asia is also the dominant source of anthropogenic O3 at GRBA in spring, but regional North American sources of O3 appear to drive additional ozone peaks in late summer and fall at this more interior site.