Conservation and Environment

This technique produced estimates of hydroxyl that swung wildly in the 1980s and 1990s. Researchers struggled to understand whether the ups and downs were due to errors in emissions estimates for methyl chloroform, for example, or to real swings in hydroxyl levels. The swings would be of concern: Large fluctuations in hydroxyl radicals would mean the atmosphere’s self-cleaning ability was very sensitive to human-caused or natural changes in the atmosphere.

To complicate matters, when scientists tried to measure the concentration of hydroxyl radical levels compared to other gases, such as methane, they were seeing only small variations from year to year. The same small fluctuation was occurring when scientists ran the standard global chemistry models.

An international agreement helped resolve the issue. In response to the Montreal Protocol – the international agreement to phase out chemicals that are destroying the Earth’s protective stratospheric ozone layer – production of methyl chloroform all but stopped in the mid 1990s. As a result, emissions of this potent ozone-depleting gas dropped precipitously.

Without the confounding effect of any appreciable methyl chloroform emissions, a more precise picture of hydroxyl variability emerged based on the observed decay of remaining methyl chloroform. The scientists studied hydroxyl radicals both by making measurements of methyl chloroform from NOAA’s international cooperative air sampling program and also by modeling results with state-of-the-art models.

The group’s findings improve confidence in projecting the future of Earth’s atmosphere.

“Say we wanted to know how much we’d need to reduce human-derived emissions of methane to cut its climate influence by half,” Montzka said. “That would require an understanding of hydroxyl and its variability. Since the new results suggest that large hydroxyl radical changes are unlikely, such projections become more reliable.”

Co-authors of the new paper, “Small Inter-Annual Variability of Global Atmospheric Hydroxyl,” include Maarten Krol (University of Utrecht and Wageningen University in the Netherlands); Ed Dlugokencky and Bradley Hall (NOAA Earth System Research Laboratory); Patrick Jöckel (Max-Planck-Institute for Chemistry in Mainz, Germany, and the Institut für Physik der Atmosphäre, Oberpfaffenhofen, Germany); and Jos Lelieveld (Max-Planck and the Cyprus Institute in Nicosia, Cyprus).

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