Abstract
Abstract. Particle water and pH are predicted using meteorological observations (relative humidity (RH), temperature (T)), gas/particle composition, and thermodynamic modeling (ISORROPIA-II). A comprehensive uncertainty analysis is included, and the model is validated. We investigate mass concentrations of particle water and related particle pH for ambient fine-mode aerosols sampled in a relatively remote Alabama forest during the Southern Oxidant and Aerosol Study (SOAS) in summer and at various sites in the southeastern US during different seasons, as part of the Southeastern Center for Air Pollution and Epidemiology (SCAPE) study. Particle water and pH are closely linked; pH is a measure of the particle H+ aqueous concentration and depends on both the presence of ions and amount of particle liquid water. Levels of particle water, in turn, are determined through water uptake by both the ionic species and organic compounds. Thermodynamic calculations based on measured ion concentrations can predict both pH and liquid water but may be biased since contributions of organic species to liquid water are not considered. In this study, contributions of both the inorganic and organic fractions to aerosol liquid water were considered, and predictions were in good agreement with measured liquid water based on differences in ambient and dry light scattering coefficients (prediction vs. measurement: slope = 0.91, intercept = 0.5 μg m−3, R2 = 0.75). ISORROPIA-II predictions were confirmed by good agreement between predicted and measured ammonia concentrations (slope = 1.07, intercept = −0.12 μg m−3, R2 = 0.76). Based on this study, organic species on average contributed 35% to the total water, with a substantially higher contribution (50%) at night. However, not including contributions of organic water had a minor effect on pH (changes pH by 0.15 to 0.23 units), suggesting that predicted pH without consideration of organic water could be sufficient for the purposes of aqueous secondary organic aerosol (SOA) chemistry. The mean pH predicted in the Alabama forest (SOAS) was 0.94 ± 0.59 (median 0.93). pH diurnal trends followed liquid water and were driven mainly by variability in RH; during SOAS nighttime pH was near 1.5, while daytime pH was near 0.5. pH ranged from 0.5 to 2 in summer and 1 to 3 in the winter at other sites. The systematically low pH levels in the southeast may have important ramifications, such as significantly influencing acid-catalyzed reactions, gas–aerosol partitioning, and mobilization of redox metals and minerals. Particle ion balances or molar ratios, often used to infer pH, do not consider the dissociation state of individual ions or particle liquid water levels and do not correlate with particle pH.
Highlights
This is due to two factors: first, an ion balance assumes all ions are completely dissociated, but multiple forms are possible, depending on pH; second, pH depends on the particle liquid water content (LWC), as pH is the concentration of H+ in an aqueous solution
Centreville site (CTR) is a rural site within a large forested region dominated by biogenic volatile organic compound (VOC) emissions, with minor local anthropogenic emissions and some plumes transported from other locations
It is representative of background conditions in the southeastern US and chosen to investigate biogenic secondary organic aerosol (SOA) formation and its interaction with anthropogenic pollution transported from other locations
Summary
The concentration of the hydronium ion (H+) in aqueous aerosols, or pH, is an important aerosol property that drives many processes related to particle composition and gas– aerosol partitioning (Jang et al, 2002; Meskhidze et al, 2003; Gao et al, 2004; Iinuma et al, 2004; Tolocka et al, 2004; Edney et al, 2005; Czoschke and Jang, 2006; Kleindienst et al, 2006; Surratt et al, 2007; Eddingsaas et al, 2010; Surratt et al, 2010). As part of the Southern Oxidant and Aerosol Study (SOAS), we made detailed measurements of particle organic and inorganic composition (Xu et al, 2015), and aerosol hygroscopicity (Cerully et al, 2014), and indirect measurements of particle LWC. These data are used to first determine the particle water mass concentrations, which are utilized in a thermodynamic model for predicting pH. The fineparticle LWC and pH data from this analysis are used in our other studies of secondary aerosol formation as part of SOAS and discussed in companion papers to this work (Cerully et al, 2014; Xu et al, 2015)
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