Abstract
Abstract. Aerosol acidity is a key parameter in atmospheric aqueous chemistry and strongly influences the interactions of air pollutants and the ecosystem. The recently proposed multiphase buffer theory provides a framework to reconstruct long-term trends and spatial variations in aerosol pH based on the effective acid dissociation constant of ammonia (Ka,NH3∗). However, non-ideality in aerosol droplets is a major challenge limiting its broad applications. Here, we introduced a non-ideality correction factor (cni) and investigated its governing factors. We found that besides relative humidity (RH) and temperature, cni is mainly determined by the molar fraction of NO3- in aqueous-phase anions, due to different NH4+ activity coefficients between (NH4)2SO4- and NH4NO3-dominated aerosols. A parameterization method is thus proposed to estimate cni at a given RH, temperature and NO3- fraction, and it is validated against long-term observations and global simulations. In the ammonia-buffered regime, with cni correction, the buffer theory can reproduce well the Ka,NH3∗ predicted by comprehensive thermodynamic models, with a root-mean-square deviation ∼ 0.1 and a correlation coefficient ∼ 1. Note that, while cni is needed to predict Ka,NH3∗ levels, it is usually not the dominant contributor to its variations, as ∼ 90 % of the temporal or spatial variations in Ka,NH3∗ are due to variations in aerosol water and temperature.
Highlights
Aerosol acidity strongly influences the thermodynamics and chemical kinetics of atmospheric aerosols and is one essential parameter in evaluating their environmental, health and climate effects (Pye et al, 2020; Zheng et al, 2020)
We thereby provided a way for pH retrieval when chemical measurements are unavailable for the ammonia-buffered regions and periods
The ratio of mean activity coefficients is expected to differ when they are mainly combined with SO24− or NO−3
Summary
Aerosol acidity strongly influences the thermodynamics and chemical kinetics of atmospheric aerosols and is one essential parameter in evaluating their environmental, health and climate effects (Pye et al, 2020; Zheng et al, 2020). Direct measurements of aerosol pH in the real atmosphere are not available so far (Pye et al, 2020; Li et al, 2020). Craig et al (2018) and Li et al (2020) developed colorimetric analyses on pH-indicator papers for aerosol pH measurement, which exhibit uncertainties of around 0.4–0.5 pH units. Wei et al (2018) developed an in situ Raman microscopy method for pH measurements in microdroplets (diameter ∼ 20 μm), with an uncertainty of ∼ 0.5 pH units. These currently available techniques, still need to be developed further for real atmospheric applications
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