Atmospheric Sciences

The acidity of atmospheric particles and clouds

The acidity of atmospheric particles and clouds

Many of us learned about acidity, or pH, in high school chemistry. We learned that acids like HCl could dissociate into H+ and Cl- and the activity of those H+ ions defined the acidity. In the atmosphere, the same basic definition of acidity, or pH on the molality scale, applies to aqueous phases like suspended particles and cloud droplets. Atmospheric acidity regulates what kinetic reactions are favorable as well as partitioning of semivolatile species with implications for climate and human health.

Unlike the high school chemistry lab, pH measurements are challenging for clouds and particles, and model predictions have historically been limited by the availability of measurements. Because of the importance of pH and rapid growth in literature on the topic, a team of researchers from 7 countries and 8 U.S. states worked from summer 2018 to early 2020 to review and synthesize the current state of knowledge on the acidity of atmospheric condensed phases. The review was a team effort with me and Athanasios Nenes designing the overall project scope and John Walker, Jim Kelly, Andreas Zuend, Chris Hennigan, Athanasios Nenes, Faye McNeill, Rahul Zaveri, Maria Kanakidou, Jeff Collett, and me leading sections. Other coauthors contributed figures, text, and model calculations. The resulting manuscript, highlighted in Atmospheric Chemistry and Physics (Pye et al., 2020), includes a full discussion of the acidity of atmospheric particles and clouds. Here, three unique features of particles and cloud droplets, that influence how we as a community determine pH, are highlighted.


1. Small water volumes. The amount of water in atmospheric condensed phases is in equilibrium with ambient humidity and extremely small compared to a traditional laboratory setting. Fine-particle liquid water content (LWC) in 1 cubic meter of air typically ranges from 1 to 10s of µL (e.g., in the eastern U.S.) to 100s of µL in polluted conditions. Cloud LWC is more abundant (where clouds exist) and can be 1 mL per m3 (Herrmann et al., 2015). At the individual particle and droplet level, LWC is even smaller and generally prohibits routine measurement. However, for cloud water, bulk collection produces enough sample that is sufficiently dilute to allow for reliable pH measurements.


Droplets of cloud water in Thailand. Bulk cloud water, such as that accessed from a mountaintop location, can be collected for measurement with traditional pH probes. Image: istockphoto


2. Concentrated conditions. Fine particles are extremely concentrated solutions consisting of inorganic species such as sulfate, nitrate, ammonium, and chloride as well as water and organic compounds. Even if enough particle LWC was collected for measurement, any measurement technique would have to be calibrated for the very high ionic strength. As a result, the main way in which fine particle pH is determined is using thermodynamic models which account for deviations in ideality. With few exceptions, all fine particle pH values for the atmosphere have been created with thermodynamic models.


Los Angeles skyline obscured by light scattering and absorption of small particles. Fine particles in summertime Los Angles have pH below 3 as estimated by a thermodynamic model (Guo et al., 2017). Image: istockphoto.


3. Semivolatile gas behavior. In a traditional laboratory beaker, common atmospheric acids such as nitric acid are expected to be predominantly in the liquid-water phase. In the atmosphere, many acids actively partition between the gas and particle phase. At very low pH (generally below 2), nitric acid is almost exclusively in the gas-phase but can partition appreciably to the particle if pH rises. Conversely, ammonia partitions to the particle under low pH conditions and volatilizes to a greater degree in higher pH environments. As a result, many acids and bases behave as semivolatile when interacting with fine particles (see also the work of Zheng et al., 2020).

Small water volumes, concentrated conditions, and the semivolatile nature of acids and bases are important factors determining atmospheric pH values and how we as a community determine them. pH impacts how efficiently chemical species are deposited to surfaces and the lifetime of various constituents which ultimately determine the impacts of particles and clouds on human health and the environment. See the complete review in Atmospheric Chemistry and Physics for recommendations on estimating pH, standard nomenclature, a synthesis of current pH estimates based on observations, and new model calculations on the local and global scale. In addition, a companion paper by Tilgner et al. (2021) on “Acidity and the multiphase chemistry of atmospheric aqueous particles and clouds” is now available as a preprint.

Disclaimer: The U.S. Environmental Protection Agency collaborated in the research described here. The views expressed in this article are those of the authors and do not represent the views or policies of the U.S. Environmental Protection Agency.

Edited by Athanasios Nenes and Mengze Li

The Author

Dr. Havala O. T. Pye is a research scientist at the U.S. Environmental Protection Agency (EPA) Office of Research and Development where her work focuses on fine particles and other airborne pollutants that can impact human health and climate change. Specifically, she develops the representation of fine particles and organic species in the Community Multiscale Air Quality modeling system ( allowing for improved quantification of air pollution impacts in regulatory analysis. She serves as a topical editor for Geoscientific Model Development and is the recipient of a Presidential Early Career Award for Scientists and Engineers, the highest honor bestowed by the U.S. government on scientists and engineers beginning their independent careers. More information about her work can be found at




Guo, H., Liu, J., Froyd, K. D., Roberts, J. M., Veres, P. R., Hayes, P. L., Jimenez, J. L., Nenes, A., and Weber, R. J.: Fine particle pH and gas–particle phase partitioning of inorganic species in Pasadena, California, during the 2010 CalNex campaign, Atmos. Chem. Phys., 17, 5703-5719, 10.5194/acp-17-5703-2017, 2017.

Herrmann, H., Schaefer, T., Tilgner, A., Styler, S. A., Weller, C., Teich, M., and Otto, T.: Tropospheric Aqueous-Phase Chemistry: Kinetics, Mechanisms, and Its Coupling to a Changing Gas Phase, Chem. Rev., 115, 4259-4334, 10.1021/cr500447k, 2015.

Pye, H. O. T., Nenes, A., Alexander, B., Ault, A. P., Barth, M. C., Clegg, S. L., Collett Jr, J. L., Fahey, K. M., Hennigan, C. J., Herrmann, H., Kanakidou, M., Kelly, J. T., Ku, I. T., McNeill, V. F., Riemer, N., Schaefer, T., Shi, G., Tilgner, A., Walker, J. T., Wang, T., Weber, R., Xing, J., Zaveri, R. A., and Zuend, A.: The acidity of atmospheric particles and clouds, Atmos. Chem. Phys., 20, 4809-4888, 10.5194/acp-20-4809-2020, 2020.

Tilgner, A., Schaefer, T., Alexander, B., Barth, M., Collett Jr, J. L., Fahey, K. M., Nenes, A., Pye, H. O. T., Herrmann, H., and McNeill, V. F.: Acidity and the multiphase chemistry of atmospheric aqueous particles and clouds, Atmos. Chem. Phys. Discuss., 2021, 1-82, 10.5194/acp-2021-58, 2021.

Zheng, G., Su, H., Wang, S., Andreae, M. O., Pöschl, U., and Cheng, Y.: Multiphase buffer theory explains contrasts in atmospheric aerosol acidity, Science, 369, 1374, 10.1126/science.aba3719, 2020.

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