Fine-particle water and pH in the southeastern United States

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.

The underappreciated role of nonvolatile cations on aerosol ammonium-sulfate molar ratios

“…Such kinetic limitations, if prevalent, opposes the validity of aerosol thermodynamic equilibrium and could significantly impact aerosol chemistry and acidity-mediated processes, given the large organic aerosol mass fractions worldwide (Zhang et al, 2007) and expected increasing organic mass fractions in the future due to changing emission, such as SO2 emission reduction in the eastern US (Hand et al, 2012;Attwood et al, 2014;Hidy et al, 2014). The hypothesis of organic films, however, is in stark contrast to established literature showing that NH3, water vapor, and HNO3 equilibrate with organic-rich 30 aerosols (Fountoukis et al, 2009;Guo et al, 2015;Guo et al, 2016;Guo et al, 2017a). Such a film, as proposed by Silvern et al (2017), selectively limits NH3, but not H2O and HNO3 molecules that are both larger than NH3 hence more difficult to diffuse through media.…”

“…Rindelaub et al (2016) presents an indirect method that infers particle H + activity for sizes above 10 µm and requires activity coefficient predicted by a thermodynamic modeling. This method reports the pH for a HSO4 -/SO4 2-aerosol system similar to the fine particle pH predicted by a thermodynamic modeling used in this study (Guo et al, 2015)) has led to the use of measurable aerosol properties as acidity proxies, such as aerosol ammoniumsulfate ratio or ion balances (e.g. (Paulot and Jacob, 2014;Wang et al, 2016;Silvern et al, 2017)).…”

“…Recent work has shown that 10 acidity proxies are not uniquely related to pH, which in turn strongly questions any conclusions derived from its use. There are numerous reasons why acidity proxies do not represent pH well; they do not capture the variability in particle water content, ion activity coefficients, or partial dissociation of species in the aerosol phase (Guo et al, 2015;Hennigan et al, 2015;Guo et al, 2016). The method that best constrains aerosol pH is comparison between a thermodynamic analysis and observations of gasparticle partitioning of semivolatile species that are sensitive to pH at the given environmental conditions (i.e., gas-particle 15 concentration ratios near 1:1) (Guo et al, 2015;Guo et al, 2016;Weber et al, 2016;Guo et al, 2017a).…”

“…In the 20 southeastern US, where total ammonium (NHx = NH3 + NH4 + ) is observed to be in large excess of particle sulfate and observed NH4 + /SO4 2-molar ratios are in the range of 1-2 (Hidy et al, 2014;Guo et al, 2015;Kim et al, 2015), thermodynamic models often predict very low pH (0.5 to 2) (Guo et al, 2015) and molar ratios close to 2 (Kim et al, 2015;Weber et al, 2016;Silvern et al, 2017). The molar ratio discrepancy has led to the hypothesis that thermodynamic predictions are incorrect, and particles are coated by organic films that inhibit the condensation of NH3 from the gas phase and give rise to the molar ratio discrepancy 25 (Silvern et al, 2017).…”

Validation of LIRIC aerosol concentration retrievals using airborne measurements during a biomass burning episode over Athens

“…ISORROPIA-II (Fountoukis and Nenes, 2007) calculates the particle water associated with inorganics for the PM1 aerosol fraction based on a thermodynamic equilibrium between an inorganic aerosol (NH4-SO4-NO3-Cl-Na-Ca-K-Mg-water) and its gas phase precursors (Guo et al, 2015;Bougiatioti et al, 2016). Here, ISORROPIA-II was operated in the "forward mode" assuming a metastable aerosol state.…”