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High aerosol acidity despite declining atmospheric sulfate concentrations over the past 15 years


Particle acidity affects aerosol concentrations, chemical composition and toxicity. Sulfate is often the main acid component of aerosols, and largely determines the acidity of fine particles under 2.5 μm in diameter, PM2.5. Over the past 15 years, atmospheric sulfate concentrations in the southeastern United States have decreased by 70%, whereas ammonia concentrations have been steady. Similar trends are occurring in many regions globally. Aerosol ammonium nitrate concentrations were assumed to increase to compensate for decreasing sulfate, which would result from increasing neutrality. Here we use observed gas and aerosol composition, humidity, and temperature data collected at a rural southeastern US site in June and July 2013 (ref. 1), and a thermodynamic model that predicts pH and the gas–particle equilibrium concentrations of inorganic species from the observations to show that PM2.5 at the site is acidic. pH buffering by partitioning of ammonia between the gas and particle phases produced a relatively constant particle pH of 0–2 throughout the 15 years of decreasing atmospheric sulfate concentrations, and little change in particle ammonium nitrate concentrations. We conclude that the reductions in aerosol acidity widely anticipated from sulfur reductions, and expected acidity-related health and climate benefits, are unlikely to occur until atmospheric sulfate concentrations reach near pre-anthropogenic levels.

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Figure 1: Evaluation of the thermodynamic model.
Figure 2: Sensitivity of PM2.5 pH and R SO 4 to gas-phase ammonia (NH3) and PM2.5 sulfate (SO42−) concentrations.
Figure 3: Mean summer (June–August) trends in PM2.5 composition, NH3, R SO 4 and predicted PM2.5 pH at the SEARCH-CTR site.


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This research was supported by a grant from the National Science Foundation through contract number 1242258, US Environmental Protection Agency Grants RD835410 and RD834799. Its contents are solely the responsibility of the grantee and do not necessarily represent the official views of the US government. Further, the US government does not endorse the purchase of any commercial products or services mentioned in the publication. Atmospheric Research Associates (ARA) and the SOAS team provided observational data and logistical support.

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R.J.W. initiated the investigation. H.G. and A.N. performed the thermodynamic modelling analyses. A.G.R. provided the mass balance analysis. All authors extensively discussed the concepts, commented on the manuscript and contributed to writing the manuscript.

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Correspondence to Rodney J. Weber.

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The authors declare no competing financial interests.

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Weber, R., Guo, H., Russell, A. et al. High aerosol acidity despite declining atmospheric sulfate concentrations over the past 15 years. Nature Geosci 9, 282–285 (2016).

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