Atmos. Chem. Phys., 18, 11125–11133, 2018 https://doi.org/10.5194/acp-18-11125-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. Technical note: Comparison and interconversion of pH based on different standard states for aerosol acidity characterization Shiguo Jia1,2, Xuemei Wang3, Qi Zhang1, Sayantan Sarkar4, Luolin Wu1, Minjuan Huang1,2, Jinpu Zhang5, and Liming Yang6 1School of Atmospheric Sciences, Sun Yat-sen University, Guangzhou 510275, P. R. China 2Guangdong Province Key Laboratory for Climate Change and Natural Disaster Studies, Sun Yat-sen University, Guangzhou 510275, P. R. China 3Institute for Environmental and Climate Research, Jinan University, Guangzhou 510632, P. R. China 4Department of Earth Sciences, and Centre for Climate and Environmental Studies, Indian Institute of Science Education and Research – Kolkata, Nadia 741246, West Bengal, India 5Guangzhou Environmental Monitoring Center, Guangzhou 510030, P. R. China 6Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117576, Republic of Singapore Correspondence: Xuemei Wang ([email protected]) and Liming Yang ([email protected]) Received: 26 January 2018 – Discussion started: 2 February 2018 Revised: 30 June 2018 – Accepted: 6 July 2018 – Published: 9 August 2018 Abstract. Aerosol pH is often calculated based on differ- protocol might influence some conclusions on aerosol acidity ent standard states thus making it inappropriate to compare reported by past studies, and thus a clear definition of pH and aerosol acidity parameters derived thereby. However, such a precise statement of thermodynamic model parameters are comparisons are routinely performed in the atmospheric sci- recommended to avoid bias when pH comparisons are made ence community. This study attempts to address this issue across studies. by comparing PM2:5 aerosol pH based on different scales (molarity, molality and mole fraction) on the basis of theo- retical considerations followed with a set of field data from Guangzhou, China as an example. The three most widely 1 Introduction used thermodynamic models (E-AIM-IV, ISORROPIA-II, and AIOMFAC) are employed for the comparison. Estab- Aerosol acidity is of great scientific interest due to its effects lished theory dictates that the difference between pHx (mole on human health and atmospheric chemical processes (Am- fraction based) and pHm (molality based) is always a con- dur and Chen, 1989; Xue et al., 2011). Acidic aerosols are stant (1.74, when the solvent is water) within a thermody- found to correlate with health effects including asthma, bron- namic model regardless of aerosol property. In contrast, pHm chitis, and others respiratory diseases along with reduced and pHc (molarity based) are almost identical with a minor lung function (Amdur and Chen, 1989; Ricciardolo et al., effect from temperature and pressure. However, when the 2004; Longo and Yang, 2008). Acidic aerosols can also con- activity coefficient is simplified as unity by thermodynamic tribute to the bioavailability of iron and phosphorus in open models, the difference between pHm and pHc ranges from oceans (Nenes et al., 2011; Zhu et al., 1992) and acidic sea 0.11 to 0.25 pH units, depending on the chemical composi- salts have the potential to catalyze halogens to deplete tro- tion and the density of hygroscopic aerosol. Therefore, while pospheric ozone (O3) (Keene et al., 1998; Pszenny et al., evaluating aerosol acidity (especially, trend analysis) when 2004; Simpson et al., 2007). Moreover, aerosol acidity plays the activity coefficient is simplified as 1, considering the pH a key role in the gas-particle partitioning of species such as − − C scale is important. The application of this pH standardization HCl=Cl , HNO3=NO3 and NH3=NH4 , and is thus vital for predicting lifetimes of gaseous compounds such as HCl, NH3 Published by Copernicus Publications on behalf of the European Geosciences Union. 11126 S. Jia et al.: Aerosol pH comparison for different standard states and HNO3 in the atmosphere (Nemitz et al., 2004; Oss et al., to characterize the acidity of hygroscopic aerosols (5 out of 1998). Further, aerosol acidity is known to affect the forma- 32 studies) as this approach is more convenient to describe tion of secondary organic aerosols (SOA); e.g., experimen- solutions with high concentrations (Rard et al., 2010). tal studies show that seed aerosols with acidic surfaces can It appears that the selection of the standard state of activity enhance the formation of organosulfate SOA upon reaction is arbitrary for aerosol acidity studies, and is not always de- with volatile organic compounds such as octanal, carbonyls, fined in published articles when pH is used to characterize the isoprene, limonene, and caryophyllene (Jang et al., 2002). acidity of aerosol (8 out of 32 studies as shown in Table S1). The most accurate parameter to characterize aerosol acid- This may not be problematic in the case of ISORROPIA- ity is considered to be pH. The other parameters often used II where the default output pH is always molality-based; as proxies of aerosol acidity do not offer information on how however, confusion is possible when E-AIM or AIOMFAC acidic the particles are when they are present as aqueous are used as these models provide both molality- and mole droplets (Pathak et al., 2004). For example, strong acidity fraction-based concentrations as output. In fact, pH based on (defined as nmol of total HC per m3 of air measured in wa- different definitions have sometimes been used in the same ter extracts of particles using the USEPA Reference Method, study; e.g., Hennigan et al. (2015) defined pH based on the USEPA, 1992) and ion charge balance are unable to distin- mole fraction of hydrogen; however, the authors used pHD7 guish between free and undissociated HC (e.g., protons as- as the critical point when [HC] D [OH−], which actually is an sociated with bisulfate) (Pathak et al., 2004; Hennigan et al., elaboration of molarity (or molality) based pH. Some studies 2015). Ammonium-to-sulfate ratio and cation-to-anion ratio have employed molarity and molality of HC interchangeably are unable to provide any measure of the degree of aerosol in terms of defining and calculating pH (defined as mol dm−3 acidity even qualitatively (Hennigan et al., 2015). Lastly, free of HC but calculated as mol kg−1 of HC, e.g., Guo et al., acidity (defined as the actual concentration of free HC per 2016), which is not ideal for the sake of consistency even m3 of air, not including the HC released from bisulfate ions though the resultant estimates are comparable. Additionally, in aqueous extracts) represents the quantity of HC in a spe- pH values obtained via different definitions are sometimes cific volume of air while neglecting the concentration of HC cross-compared, e.g., Squizzato et al. (2013) observed that in liquid water (Pathak et al., 2004). pH of PM2:5 in the Po Valley, Italy (mole fraction-based) was As per the International Union of Pure and Applied Chem- much higher than those in megacities in China (Pathak et al., istry (IUPAC), pH is defined as the negative log (base 10) 2009) (molarity-based). Such comparisons need to be reeval- activity of hydrogen ions (https://goldbook.iupac.org/html/P/ uated given the different definitions of pH adopted in these P04524.html, last access: 7 July 2018). It is immeasurable studies. because its definition involves a single ion quantity, the hy- Despite apparent incongruities in such cross-comparisons, drogen ion activity (Baucke, 2002). Therefore, the value of this issue has not been addressed with sufficient care by the pH is not an absolute one but depends on either how it is atmospheric science community. The main objective of this measured or the model used to calculate it. Especially, for study is thus to compare PM2:5 aerosol pH based on different aerosol pH, a commonly accepted measurement method is scales (molarity, molality and mole fraction) on the basis of lacking despite some recent developments (Rindelaub et al., theoretical considerations followed with a set of field data as 2016), and it is usually calculated from thermodynamic mod- an example. Further, in order to enable other researchers to els in practice. easily compare pH based on different scales, the use of an One issue in comparing aerosol pH across studies even inter-scale conversion factor has been demonstrated for the when calculated using the same model in actual practice is three most commonly used thermodynamic models, i.e., E- that different standard states can be used while defining the AIM-IV, ISORROPIA-II and AIOMFAC. activity of HC ions. Although it is recommended that pH be defined based on the standard state of 1 mol HC kg−1 solvent (molality based) (https://goldbook.iupac.org/html/P/P04524. 2 Materials and methods html), other standard states such as 1 mol HC dm−1 solu- tion (molarity based) and a hypothetical pure HC solution 2.1 Evaluation data set (mole fraction based) are also often used when quantifying aerosol acidity. Supplement Table S1 provides a brief sum- A set of field data collected in Guangzhou, China was used mary of studies reporting aerosol pH calculated using ther- to demonstrate the interconversion of pH based on differ- modynamic models with different definition of pH. Molality ent scales. The sampling site was located at the rooftop of based pH, as suggested by IUPAC, is used in 12 out of 32 a building, 15 m above the ground, in the Guangzhou Envi- studies. Molarity-based pH is the most commonly used scale ronmental Monitoring Center (23◦0705900 N, 113◦1503500 E) in aquatic chemistry as the equilibrium constant is often de- (refer to Chen et al., 2016a for details). Hourly ionic species termined based on molarity (Stumm and Morgan, 1996); it of PM2:5 were measured using an AIM-IC 9000D (URG, is also widely used for characterizing aerosol acidity (7 out Chapel Hill, NC) (refer to Chen et al., 2016b for details).