Key drivers of Western U.S. surface ozone variability during 1980-2050: From background trends to extreme transport events

Abstract:

The U.S. Environmental Protection Agency is proposing to lower the National Ambient Air Quality Standard (NAAQS) for ground-level ozone from 75 ppbv to a range of 60-70 ppbv on the basis of the latest health evidence. Rising Asian emissions, more frequent wildfires, and poorly understood deep stratospheric intrusions may pose challenges to achieving more stringent ozone standards over high-elevation Western U.S. (WUS) regions. Understanding the factors driving daily to decadal variability in sources of ozone is thus relevant to an effective implementation of the ozone NAAQS in WUS states. Here I present a detailed analysis of WUS surface ozone variability during 1980-2050, based on observations and a suite of chemistry-climate model simulations designed to isolate the role of changes in meteorology, background, STT, wildfires, methane, and Asian vs. North American anthropogenic emissions. Our analysis identifies the key role of stratospheric intrusion frequency on year-to-year variability in springtime high-ozone events measured at high-elevation WUS sites. We show more frequent late spring deep stratospheric intrusions when the polar jet stream meanders towards the southwestern United States, as occurs following strong La Niña conditions. Fewer and weaker intrusion events followed the pronounced positive phase of the Arctic Oscillation in response to volcanic aerosols from the 1991 eruption of Mt. Pinatubo.

With Asian emissions, global methane, and meteorology varying with time, the model simulates a background ozone increase of 0.20-0.45 ppb yr^-1 (P≤0.01) at high-elevation WUS sites for spring during 1988-2012. With global methane and meteorology varying with time, the model shows an increasing ozone trend of 0.1-0.2 ppb yr^-1 in spring (0.01<P≤0.05). For summer, increasing Eurasian and North American boreal fires in the 2000s as compared to the 1980s contribute to simulated increases in background ozone over the northeastern U.S., with implications for possibly more frequent fires in a warming climate. I will further discuss the extent to which projected changes in methane and Asian emissions in RCP8.5 and 4.5 scenarios contribute to lower tropospheric ozone changes over the WUS during 2005-2050.