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

• Published in: Nature geoscience Vol. 9; no. 4; pp. 282 - 285
• Main Authors: Weber, Rodney J., Guo, Hongyu, Russell, Armistead G., Nenes, Athanasios
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01-04-2016; Nature Publishing Group
• Subjects: 704/172/169/209; 704/172/169/824; Acidity; Aerosols; Air sampling; Ammonia; Ammonium; Ammonium nitrate; Anthropogenic factors; Earth Sciences; Earth System Sciences; Geochemistry; Geology; Geophysics/Geodesy; letter; Particulate matter; Sulfates; Sulfur; USA, Southeast
• ISSN: 1752-0894; 1752-0908
• DOI: 10.1038/ngeo2665

Atmospheric sulfate levels are thought to determine the pH of small aerosol particles. Thermodynamic analysis of field aerosol data reveals that fine particles remain acidic in the southeastern United States despite large sulfate reductions.
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, PM
2.5
. 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 PM
2.5
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.