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The State of Acidity in the Atmosphere: Particles and Clouds ; American Geophysical Union (AGU) Fall Meeting
Citation:
Pye, H. The State of Acidity in the Atmosphere: Particles and Clouds ; American Geophysical Union (AGU) Fall Meeting. American Geophysical Union (AGU) Fall Meeting, San Francisco, California, December 09 - 13, 2019.
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 or the negative logarithm of the H+ ion activity, is a central concept of natural water chemistry. In the atmosphere, acidity governs gas-particle partitioning of semivolatile gases as well as chemical reaction rates. It has implications for the lifetime of pollutants, deposition, and human health. Our work is a review and synthesis of the current state of knowledge on the acidity of condensed phases, specifically particles and cloud droplets, in the atmosphere. While pH is rigorously defined by thermodynamics and operationally by scientific organizations based on the activity of H+ ions, a number of approximations to pH are required for the calculation and communication of atmospheric aerosol acidity. This is because aerosol pH estimates are generally based on observationally-constrained model calculations and are thus limited by the thermodynamic treatment of the aqueous phase within the models used. Nevertheless, these approximations of pH are preferred over proxies of particle pH, such as molar ratios and charge balance estimates, as proxies lack modulation by liquid water abundance and can be overwhelmed by limitations in measurement precision. Results from two regional and two global models, as well as over 80 estimates from literature, indicate that acidic fine particles are ubiquitous. Clouds and fogs experience higher liquid water content than particles (and lower ionic strengths), and they have greater but still generally acidic pH that is quite sensitive to anthropogenic emissions of sulfur and nitrogen oxides as well as ammonia. Historical measurements indicate that cloud droplet pH has changed in recent decades in response to controls on anthropogenic emissions, while the limited trend data for particles indicates current emission reductions have not been aggressive enough to substantially change acidity in regions such as the southeast US or Canada. An examination of six current chemical transport models indicates acidity over large areas of the globe is driven by nonvolatile cations and/or the assumptions regarding the mixing state of fine particles, but limited observational data over large portions of the globe limits our ability to properly evaluate current large-scale models.