Technical note: Comparison and interconversion of pH based on different standard states for aerosol acidity characterization

Abstract

Abstract. Aerosol pH is often calculated based on different standard states thus making it inappropriate to compare aerosol acidity parameters derived thereby. However, such comparisons are routinely performed in the atmospheric science community. This study attempts to address this issue by comparing PM2.5 aerosol pH based on different scales (molarity, molality and mole fraction) on the basis of theoretical considerations followed with a set of field data from Guangzhou, China as an example. The three most widely used thermodynamic models (E-AIM-IV, ISORROPIA-II, and AIOMFAC) are employed for the comparison. Established theory dictates that the difference between pHx (mole fraction based) and pHm (molality based) is always a constant (1.74, when the solvent is water) within a thermodynamic model regardless of aerosol property. In contrast, pHm and pHc (molarity based) are almost identical with a minor effect from temperature and pressure. However, when the activity coefficient is simplified as unity by thermodynamic models, the difference between pHm and pHc ranges from 0.11 to 0.25 pH units, depending on the chemical composition and the density of hygroscopic aerosol. Therefore, while evaluating aerosol acidity (especially, trend analysis) when the activity coefficient is simplified as 1, considering the pH scale is important. The application of this pH standardization protocol might influence some conclusions on aerosol acidity reported by past studies, and thus a clear definition of pH and a precise statement of thermodynamic model parameters are recommended to avoid bias when pH comparisons are made across studies.

The acidity of atmospheric particles and clouds

“…the deposition of which may promote phytoplankton blooms in nutrient-limited regions of the oceans (Meskhidze et al, 2005). The same process occurs for P (e.g., Nenes et al, 2011;Stockdale et al, 2016); however, the extent to which particle pH may similarly increase the solubility and amount of organic forms of nitrogen and phosphorus, a potentially large source to ecosystems (Jickells et al, 2013), is not well known . Deposition of trace nutrients from acid-promoted dissolution into regions of the ocean where the nutrients are not limiting to biological productivity may enhance productivity in nutrient-limited regions by means of long-range transport by ocean currents.…”

The Acidity of Atmospheric Particles and Clouds

“…Because the density 0 depends weakly on temperature, the exact relation between pH and pH is non-linear. In the usual case of water as the reference solvent ( 0 close to 1000 kg m -3 ), the logarithmic difference is small (typically < 0.02 pH units), resulting in pH ≈ pH (Jia et al, 2018).…”

“…7.1 for a discussion of pH as a function of particle size). Guo et al (2016) For mainland China, fine aerosol pHF estimates vary, but tend to be mildly acidic (average pHF approximately 4) and span from negative values (in Chengdu) to as high as 6.1 for PM2.5 aerosol (Liu et al, 2017;Jia et al, 2018;Song et al 2018;Tian et al, 2018;Cheng et al, 2015;He et al, 2018;Tan et al, 2018;Shi et al, 2017;Guo et al, 2017a;Ding et al, 2019;Jia et al, 2018b;, while in Southeast Asia (Singapore and Hong Kong) fine aerosol is highly acidic (average pHF approximately 1) (Behera et al, 2013;Yao et al, 2007). Ding et al (2019) estimated coarse particles were generally neutral or alkaline, based on observations in Beijing and modeling with ISORROPIA II.…”

Supplementary material to &quot;The Acidity of Atmospheric Particles and Clouds&quot;

“…The analogous conversion between molarity and molality-based pH is given by (Jia et al, 2018) pH = pH − log 10 ( ⦵ ⦵ 0…”

“…However, in the case of water near room temperature ( 0 close to 1000 kg m -3 ), the logarithm in Eq. (S13) yields a small number, resulting in pH ≈ pH pH pH (Jia et al, 2018).…”