In laboratory studies of forest fires, nitrous acid appears to be a minor actor, often underrepresented in atmospheric models. But in the real atmosphere, during forest fires, this chemical plays a leading role – it rises sharply to critical and unpredictable levels. This leads to increased ozone pollution and degraded air quality, according to a new study by the University of Colorado at Boulder (CU Boulder) and the Royal Belgian Institute for Space Aeronomy. The results are published by the journal Nature Geoscience.
“We have found that levels of nitrous acid in wildfire plumes around the world are now two to four times higher than expected,” said Rainer Volkamer, Research Fellow at CIRES (Cooperative Institute for Research in Environmental Sciences), professor of chemistry at CU Boulder and co-author of the study. “This chemical may eventually cause the formation of ozone pollution downwind from fires. It damages the lungs of humans, animals, and all plants. ” Earlier it was reported that the smoke from wildfires in the United States reached Europe.
Nitrous acid in forest fire smoke accelerates the formation of an oxidizer, hydroxyl radical, or OH. The group estimated that nitrous acid was responsible for 60% of the formation of OH in plumes worldwide – by far the main precursor of the hydroxyl radical in fresh plumes of fire. Thus, OH can decompose greenhouse gases as well as accelerate the chemical production of ozone pollution – up to 7 parts per billion in some places. This is enough for ozone levels to exceed US and European regulatory levels.
“The size of the fire and the burning conditions in the real world show a higher nitrous acid content than can currently be explained on the basis of laboratory data, and this added nitrous acid accelerates the chemical process to produce ozone,” explains Volkamer.
Nitrous acid, although present in high quantities after forest fires, decomposes quickly in the sun and is therefore extremely difficult to study. The CU Boulder team worked with European counterparts to combine two datasets:
- Global measurements with the TROPOMI satellite instrument, which observed nitrous acid in wildfire plumes around the world;
- Dedicated instrumentation used on aircraft during the 2018 Pacific Northwest fire survey during the BB-FLUX campaign.
“Simultaneous measurements at different time and space scales helped us understand and use the first global measurements of nitrous acid by our Belgian colleagues,” said Volkamer. With the new data, Volkamer and his colleagues, including Nicholas Theiss, lead author of the study at BIRA, were able to scrutinize satellite data on large numbers of wildfires in all major ecosystems across the planet to estimate nitrous acid emissions.
“Emissions of nitrous acid compared to other gases involved in ozone production vary by ecosystem, with the lowest in savannas and grasslands and highest in extratropical evergreen forests,” explains Kyle Zarzana, a chemical scientist at CU Boulder, who supervised the deployment of aircraft measurements and co-authored a new paper.
“Wildfire smoke contains many contaminants and aerosols that adversely affect visibility and public health over long distances, as we see with wildfires raging in the western United States that affect air quality and the east coast. Our results show that this smoke is a highly reactive chemical and helps us track better as photochemistry rapidly changes downwind emissions, ”the scientists conclude.