Rural West Sees More Smog; Now We Know Why

Brian Donegan
Published: March 2,2017
The rural West has seen more-persistent smog over the last 20 to 25 years, despite laws limiting the emission of smog-forming chemicals from cars, airplanes and factories, and now we know why.
A study published March 1 in the journal Atmospheric Chemistry and Physics determined the culprits are an intrusion of pollution from Asia in the western U.S. and more-frequent heat waves in the eastern U.S.
The study, led by Meiyun Lin and assisted by other researchers at Princeton University and NOAA's Geophysical Fluid Dynamics Laboratory (GFDL), suggests that continued implementation of U.S. nitrogen-oxide emission controls will guard against rising pollution levels triggered either by climate change or global-emission growth.
Lin looked at the sources of smog from the 1980s through today. Smog, or ground-level ozone, is harmful to human health and can increase the risk of asthma attacks or cause difficulty breathing. Sensitive trees and crops can also be harmed from too much exposure to ozone.
Smog-forming chemicals, such as nitrogen oxides, have been cut in half in the U.S. over the past quarter century. This has led to an overall decrease in ozone in the East – though it typically increases during heat waves, which have become more frequent in the past few decades – but ozone levels have actually climbed in the rural West.

Surface ozone has decreased during pollution episodes in the eastern U.S. but is on the increase in the rural western U.S. during the spring, partly due to rising Asian nitrogen-oxide emissions. Shown above are the 1988-2014 trends in springtime daily maximum 8-hour average ozone at surface sites for the 95th and 50th percentiles from observations (left) and GFDL-AM3 model simulations (right). Larger circles indicate sites where there is greater certainty about the ozone trend. (Meiyun Lin, Princeton University Program in Atmospheric and Oceanic Sciences and NOAA's Geophysical Fluid Dynamics Laboratory)

Asian countries – China, North Korea, South Korea, Japan and India – have collectively tripled their nitrogen-oxide emissions since 1990, and this is traveling across the Pacific Ocean in the spring, when winds are generally out of a westerly direction, and settling into western U.S. national parks, including Yellowstone.
Springtime ozone levels in the national parks have risen by 5 to 10 parts per billion (ppb) in the last 25 years. The study's authors say this increased pollution from Asia could make it difficult for affected areas to comply with the federal ozone standard, which is 70 ppb.
Climate models developed at GFDL were combined with actual ozone measurements in this study, and the results showed that even after the springtime spike in ozone, the levels still remain above average during the summer despite a shift in weather pattern and the U.S.'s efforts to protect against pollution levels.
It's been known for over a decade that Asian pollution contributes to U.S. ozone levels, but Lin says this is one of the first studies to categorize the extent of how much U.S. ozone is actually caused by rising Asian emissions.
This study's findings suggest a global perspective is necessary when developing a strategy to meet U.S. ozone air quality objectives, Lin told Princeton University.

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