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Multiphase buffer theory explains contrasts in atmospheric aerosol acidity

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Science  11 Sep 2020:
Vol. 369, Issue 6509, pp. 1374-1377
DOI: 10.1126/science.aba3719

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A multiphasic effect

Aerosols exert a primary influence on atmospheric chemistry. One of the main controls on their internal chemistry is their acidity, so understanding what determines aerosol pH is fundamental for determining their environmental effects. Zheng et al. considered how buffering capacity in a multiphase aerosol system differs from bulk solution and found an important role for water content in determining pH in ammonia-buffered regions. Their conclusions underscore the important influence of ammonia emissions in the Anthropocene.

Science, this issue p. 1374

Abstract

Aerosol acidity largely regulates the chemistry of atmospheric particles, and resolving the drivers of aerosol pH is key to understanding their environmental effects. We find that an individual buffering agent can adopt different buffer pH values in aerosols and that aerosol pH levels in populated continental regions are widely buffered by the conjugate acid-base pair NH4+/NH3 (ammonium/ammonia). We propose a multiphase buffer theory to explain these large shifts of buffer pH, and we show that aerosol water content and mass concentration play a more important role in determining aerosol pH in ammonia-buffered regions than variations in particle chemical composition. Our results imply that aerosol pH and atmospheric multiphase chemistry are strongly affected by the pervasive human influence on ammonia emissions and the nitrogen cycle in the Anthropocene.

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