NASA - National Aeronautics and Space Administration

Ozone is one of the six “criteria” pollutants identified by the U.S. Clean Air Act
Amendment of 1970 as particularly harmful to human health. Concentrations have decreased
markedly across the United States over the past 50 years in response to regulatory efforts,
but continuing research on its deleterious effects have spurred further reductions in the legal
threshold. The South Coast and San Joaquin Valley Air Basins of California remain the only two
“extreme” ozone nonattainment areas in the United States. Further reductions of ozone in the
West are complicated by significant background concentrations whose relative importance increases
as domestic anthropogenic contributions decline and the national standards continue to
be lowered. These background concentrations derive largely from uncontrollable sources including
stratospheric intrusions, wildfires, and intercontinental transport. Taken together the exogenous
sources complicate regulatory strategies and necessitate a much more precise understanding of
the timing and magnitude of their contributions to regional air pollution. The California Baseline
Ozone Transport Study was a field campaign coordinated across Northern and Central California
during spring and summer 2016 aimed at observing daily variations in the ozone columns crossing
the North American coastline, as well as the modification of the ozone layering downwind across
the mountainous topography of California to better understand the impacts of background ozone
on surface air quality in complex terrain.