Abstract. 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.
The acidity of atmospheric particles and clouds
