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ACID IN THE AIR

When acidic materials are spilled, clean-up involves adding a base chemical to neutralize the acid. Up to a point, the more base added, the more neutral and less toxic the spill becomes.

photo - three researchers standing on roof with Atlanta skyline in background

Researchers have found that the pH of atmospheric sulfate particles is lower than expected. Shown are Georgia Tech graduate student Hongyu Guo, Professor Rodney Weber from the School of Earth and Atmospheric Sciences, and Professor Armistead Russell from the School of Civil and Environmental Engineering. Photo: Rob Felt

Something similar happens in the atmosphere. Acidic sulfur emissions from power plants have been rapidly declining over the past decade, and the neutralizing base — ammonia — emitted from a different source, hasn’t declined. This has led many atmospheric scientists to assume that the ambient sulfate particles we all breathe are becoming less acidic and therefore less toxic.

But a new study shows this intuitive expectation hasn’t happened, at least not in the Southeastern United States, where the remaining sulfate particles appear to be as acidic as ever. Beyond human health, the research has broader implications for atmospheric pollution and global climate change modeling.

Sponsored by the National Science Foundation and the U.S. Environmental Protection Agency, the research was reported in the journal Nature Geoscience. The conclusions are based on observed gas and aerosol composition as well as humidity and temperature data collected at a site in rural Alabama as part of the Southern Oxidant and Aerosol Study.

“Sulfates are the major source of acidity in the atmosphere, and gas-phase ammonia — mostly from agriculture — had been expected to react with the remaining particles to reduce their acidity,” explained Rodney Weber, a professor in Georgia Tech’s School of Earth and Atmospheric Sciences. “But what we found is that the system that forms the sulfate particles isn’t very sensitive to the amounts of ammonia neutralizer. This has implications because the acidity of these particles affects other important atmospheric reactions.” — John Toon

Factoid

The pH of PM 2.5 particles can’t be directly measured, so scientists must infer their acidity by studying the distribution of atmospheric species that can be measured and are highly sensitive to the value of the particle pH.

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