August 7th, 2024
Key Findings
- The authors analyzed surface and airborne measurements alongside a new variable resolution global chemistry-climate model to better understand the physical and chemical processes that emissions from wildfires undergo as they travel.
- In the case of nearby wildfires, nitrogen oxide emissions convert to peroxyacetyl
nitrate and other more oxidized forms, reducing the impact of these wildfires on local ozone levels. - Conversely, when smoke plumes travel long distances southward from Canada, higher air temperatures cause peroxyacetyl nitrate to decompose and interact with organic gases in smoke to create higher levels of ozone.
- Through ozone produced remotely during plume transport and locally via interactions of smoke plumes with urban emissions, wildfire plumes can contribute 5-25 parts per billion by volume of ozone to U.S. cities.
Meiyun Lin, Larry W. Horowitz, Lu Hu, Wade Permar. Geophysical Research Letters. DOI: 10.1029/2024GL109369
Large, damaging wildfires are becoming a common occurrence in Canada, the Pacific Northwest, and California. Five of the most destructive wildfire seasons of the last half-century have occurred in the past seven years. These wildfires can cause significant air pollution: burning biomass emits hundreds of reactive gases, including nitrogen oxides, carbon monoxide, ammonia, and an array of volatile organic compounds.
Wildfire smoke can travel long distances, affecting air quality in cities far from the source of the wildfire. Quantifying how this smoke contributes to the production of secondary air pollutants such as ozone is critical for U.S. air pollution policy. But the complex composition of wildfire smoke and the variable ways it interacts with light, temperature, and urban pollution makes it difficult to assess its impacts on ozone.
The authors analyzed how smoke from nearby fires (i.e. smoke that had taken less than a day to arrive at the study location) affects ozone formation. To assess this, the authors used data on western smoke plumes sampled by aircraft measurements during summer 2018. They found that near-fire smoke was associated with lesser production of ozone, thanks to the rapid conversion of nitrogen oxide emissions from wildfires to peroxyacetyl nitrate (PAN) and particulate nitrate.
Aged wildfire smoke, on the other hand, can increase levels of ozone in locations thousands of kilometers from the wildfire. The authors found that the long distances wildfire smoke travels from Canada to U.S. cities allows PAN to thermally decompose and increase the production of ozone. For cities in the path of this traveling smoke, O3 levels can increase by 5–25 parts per billion by volume (ppbv) on days when observed O3 levels exceed the EPA limit (70 ppbv).
To conduct this research, the authors used AM4VR, a new GFDL variable resolution global chemistry-climate model that provides detailed air quality and climate information for targeted U.S. regions. AM4VR provides spatial resolution that is more than 10 times finer than the global models used in the latest Intergovernmental Panel on Climate Change Report. This allows the authors to investigate interactions between urban pollution and smoke plumes from wildfires thousands of kilometers away.
