Where’s the atmosphere’s self-cleaning power?

In a finding that could alter how scientists quantify emissions of certain pollutants, a new study in Nature concludes that the self-cleaning power of the atmosphere does not differ substantially between the northern and southern hemispheres. The finding was surprising, as model simulations generally show that the hydroxyl molecule (OH)—the dominant “detergent” of the atmosphere that removes many pollutants by oxidizing them—is more common in the Northern Hemisphere.

“This suggests we still have more to learn about this aspect of atmospheric chemistry,” said NCAR scientist Britton Stephens, a co-author of the paper.

Because massive amounts of nitrogen oxides are emitted in the Northern Hemisphere by motor vehicles, industry, and other human-related activities concentrated there, OH has been thought to be more common in that region. The nitrogen oxides promote the formation of ozone, which in turn can react with water vapor molecules in the presence of sunlight to form OH.

OH is highly difficult to measure because it exists in low concentrations and lasts for just about a second on average before reacting with pollutants and other gases.

The new study, led by Prabir Patra of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC), inferred OH concentrations by using measurements of methyl chloroform, a human-made chemical that is chiefly removed from the atmosphere by OH.

The measurements came from two long-term, ground-based observation networks as well as from a 2009–2011 field project that deployed an advanced research aircraft owned by the National Science Foundation and operated by NCAR. The HIAPER Pole-to Pole Observations project, or HIPPO, flew from the Arctic to Antarctic, taking detailed observations of the atmosphere at a range of altitudes.

The new study also drew on an advanced JAMSTEC atmospheric chemistry model.

The results indicate that the northern/southern hemisphere ratio of OH is nearly equivalent at about .97, plus or minus .12. In contrast, state-of-the-art models have predicted an OH concentration that is 13-42% higher in the Northern Hemisphere.

The ratio is significant as scientists have relied on OH concentration to estimate emissions of certain gases, such as methane and carbon monoxide. The estimates of these emissions in the Northern Hemisphere may need to be revised if concentrations of OH are approximately the same as in the Southern Hemisphere, the authors said.

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