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Abstract. Acidity, defined as pH, is a central component of aqueouschemistry. In the atmosphere, the acidity of condensed phases (aerosolparticles, cloud water, and fog droplets) governs the phase partitioning ofsemivolatile gases such as HNO3, NH3, HCl, and organic acids andbases as well as chemical reaction rates. It has implications for theatmospheric lifetime of pollutants, deposition, and human health. Despiteits fundamental role in atmospheric processes, only recently has this fieldseen a growth in the number of studies on particle acidity. Even with thisgrowth, many fine-particle pH estimates must be based on thermodynamic modelcalculations since no operational techniques exist for direct measurements.Current information indicates acidic fine particles are ubiquitous, butobservationally constrained pH estimates are limited in spatial and temporalcoverage. Clouds and fogs are also generally acidic, but to a lesser degreethan particles, and have a range of pH that is quite sensitive toanthropogenic emissions of sulfur and nitrogen oxides, as well as ambientammonia. Historical measurements indicate that cloud and fog droplet pH haschanged in recent decades in response to controls on anthropogenicemissions, while the limited trend data for aerosol particles indicateacidity may be relatively constant due to the semivolatile nature of thekey acids and bases and buffering in particles. This paper reviews andsynthesizes the current state of knowledge on the acidity of atmosphericcondensed phases, specifically particles and cloud droplets. It includesrecommendations for estimating acidity and pH, standard nomenclature, asynthesis of current pH estimates based on observations, and new modelcalculations on the local and global scale. 

To tackle the severe fine particle (PM_2.5) pollution in China, the government has implemented stringent control policies mainly on power plants, industry, and transportation since 2005, but estimates of the effectiveness of the policy and the temporal trends in health impacts are subject to large uncertainties. By adopting an integrated approach that combines chemical transport simulation, ambient/household exposure evaluation, and health-impact assessment, we find that the integrated population-weighted exposure to PM_2.5(IPWE) decreased by 47% (95% confidence interval, 37–55%) from 2005 [180 (146–219) μg/m³] to 2015 [96 (83–111) μg/m³]. Unexpectedly, 90% (86–93%) of such reduction is attributed to reduced household solid-fuel use, primarily resulting from rapid urbanization and improved incomes rather than specific control policies. The IPWE due to household fuels for both cooking and heating decreased, but the impact of cooking is significantly larger. The reduced household-related IPWE is estimated to avoid 0.40 (0.25–0.57) million premature deaths annually, accounting for 33% of the PM_2.5-induced mortality in 2015. The IPWE would be further reduced by 63% (57–68%) if the remaining household solid fuels were replaced by clean fuels, which would avoid an additional 0.51 (0.40–0.64) million premature deaths. Such a transition to clean fuels, especially for heating, requires technology innovation and policy support to overcome the barriers of high cost of distribution systems, as is recently being attempted in the Beijing–Tianjin–Hebei area. We suggest that household-fuel use be more highly prioritized in national control policies, considering its effects on PM_2.5exposures.