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The Acidity of Atmospheric Particles and Clouds
Citation:
Pye, H., A. Nenes, B. Alexander, A. Ault, M. Barth, S. Clegg, J. Collett, K. Fahey, C. Hennigan, H. Herrmann, M. Kanakidou, J. Kelly, I. Ku, V. McNeill, N. Riemer, T. Schaefer, G. Shi, A. Tilgner, J. Walker, T. Wang, R. Weber, J. Xing, R. Zaveri, AND A. Zuend. The Acidity of Atmospheric Particles and Clouds. Atmospheric Chemistry and Physics. Copernicus Publications, Katlenburg-Lindau, Germany, 20(8):4809–4888, (2020). https://doi.org/10.5194/acp-20-4809-2020
Impact/Purpose:
Acid rain has been largely mitigated in the US through regulation of sulfur and nitrogen containing precursors. However, acidity remains an important indicator of condensed phase (particle and cloud droplet) chemistry. In addition, although acid rain and cloud acidity (pH) have been affected by decreases in precursor abundance, PM2.5 particles remain largely acidic (pH often below 2). This review summarizes our state of knowledge on the acidity of particles and clouds in the atmosphere and highlights the utility of examining pH to better understand atmospheric process as well as evaluate regulatory models.
Description:
Acidity, defined as pH, is a central component of aqueous chemistry. In the atmosphere, the acidity of condensed phases (aerosol particles, cloud water, and fog droplets) governs the phase partitioning of semivolatile gases such as HNO3, NH3, HCl, and organic acids and bases as well as chemical reaction rates. It has implications for the atmospheric lifetime of pollutants, deposition, and human health. Despite its fundamental role in atmospheric processes, only recently has this field seen a growth in the number of studies on particle acidity. Even with this growth, many fine-particle pH estimates must be based on thermodynamic model calculations since no operational techniques exist for direct measurements. Current information indicates acidic fine particles are ubiquitous, but observationally constrained pH estimates are limited in spatial and temporal coverage. Clouds and fogs are also generally acidic, but to a lesser degree than particles, and have a range of pH that is quite sensitive to anthropogenic emissions of sulfur and nitrogen oxides, as well as ambient ammonia. Historical measurements indicate that cloud and fog droplet pH has changed in recent decades in response to controls on anthropogenic emissions, while the limited trend data for aerosol particles indicate acidity may be relatively constant due to the semivolatile nature of the key acids and bases and buffering in particles. This paper reviews and synthesizes the current state of knowledge on the acidity of atmospheric condensed phases, specifically particles and cloud droplets. It includes recommendations for estimating acidity and pH, standard nomenclature, a synthesis of current pH estimates based on observations, and new model calculations on the local and global scale.
