Fine-particle pH for Beijing winter haze as inferred from different thermodynamic equilibrium models

Abstract

pH is an important property of aerosol particles but is difficult to measure directly. Several studies have estimated the pH values for fine particles in northern China winter haze using thermodynamic models (i.e., E-AIM and ISORROPIA) and ambient measurements. The reported pH values differ widely, ranging from close to 0 (highly acidic) to as high as 7 (neutral). In order to understand the reason for this discrepancy, we calculated pH values using these models with different assumptions with regard to model inputs and particle phase states. We find that the large discrepancy is due primarily to differences in the model assumptions adopted in previous studies. Calculations using only aerosol-phase composition as inputs (i.e., reverse mode) are sensitive to the measurement errors of ionic species, and inferred pH values exhibit a bimodal distribution, with peaks between −2 and 2 and between 7 and 10, depending on whether anions or cations are in excess. Calculations using total (gas plus aerosol phase) measurements as inputs (i.e., forward mode) are affected much less by these measurement errors. In future studies, the reverse mode should be avoided whereas the forward mode should be used. Forward-mode calculations in this and previous studies collectively indicate a moderately acidic condition (pH from about 4 to about 5) for fine particles in northern China winter haze, indicating further that ammonia plays an important role in determining this property. The assumed particle phase state, either stable (solid plus liquid) or metastable (only liquid), does not significantly impact pH predictions. The unrealistic pH values of about 7 in a few previous studies (using the standard ISORROPIA model and stable state assumption) resulted from coding errors in the model, which have been identified and fixed in this study.

Role of Ammonia on the Feedback Between AWC and Inorganic Aerosol Formation During Heavy Pollution in the North China Plain

“…Using the reverse mode of ISORROPIA II model, Wang et al (2016) estimated the aerosol acidity and found that the neutralization of NH 3 could result in a neutral aerosol with pH≈7. On the contrary, Song et al (2018) stated the forward mode calculations would be superior to the reverse one and obtained a moderate acidity (pH≈4-5) for fine particles under the winter haze conditions in the NCP, which is consistent with that reported by Liu et al (2017). The forward mode can solve the total (gas + aerosol) concentration of each species, while the reverse mode only use the concentration of each species in the aerosol phase as input.…”

Particle acidity and sulfate production during severe haze events in China cannot be reliably inferred by assuming a mixture of inorganic salts

“…Noticeably, the pH values estimated by the ISORROPIA-II model under the two modes are significantly different, with the values of 4.57 ± 0.40 under the metastable mode and 6.96 ± 1.33 under the stable mode. Most recently, it was suggested that the large discrepancy in predicting pH is attributable to the model code errors (Song et al, 2018).…”

Contribution of Hydroxymethane Sulfonate to Ambient Particulate Matter: A Potential Explanation for High Particulate Sulfur During Severe Winter Haze in Beijing

“…Measurements from fog/low cloud water in Beijing showed a pH of 5.2-6.2 between 1999 and 2006 (Jiang et al, 2009). More recent observations infer an aerosol pH of~4-5 during haze events Liu et al, 2017;Song et al, 2018), which implies a somewhat higher pH in cloud droplets due to ion exchange between cloud and aerosol during cloud processing (Ervens, 2015;Liu et al, 2017). In addition, precipitation in Beijing reveals pH values of~5.5-6 (Huo et al, 2010;Tang et al, 2005;Zhu et al, 2016).…”