Investigation of molar volume and surfactant characteristics of water-soluble organic compounds in biomass burning aerosol

Abstract

Abstract. In this study, we characterize the CCN activity of the water-soluble organics in biomass burning aerosol. The aerosol after collection upon filters is dissolved in water using sonication. Hydrophobic and hydrophilic components are fractionated from a portion of the original sample using solid phase extraction, and subsequently desalted. The surface tension and CCN activity of these different samples are measured with a KSV CAM 200 goniometer and a DMT Streamwise Thermal Gradient CCN Counter, respectively. The measurements show that the strongest surfactants are isolated in the hydrophobic fraction, while the hydrophilics exhibit negligible surface tension depression. The presence of salts (primarily (NH 4 ) 2 SO 4 ) in the hydrophobic fraction substantially enhances surface tension depression; their synergistic effects considerably enhance CCN activity, exceeding that of pure (NH 4 ) 2 SO 4 . From our analysis, average thermodynamic properties (i.e, molar volume) are determined for samples using our newly developed Köhler Theory Analysis (KTA) method. The molar mass of the hydrophilic and hydrophobic aerosol components is estimated to be 87±26 g mol −1 and 780±231 g mol −1 , respectively. KTA also suggests that the relative proportion (in moles) of hydrophobic to hydrophilic compounds in the original sample to be 1:3. For the first time, KTA is applied to an aerosol with this level of complexity and displays its potential for providing physically-based constraints for GCM parameterizations of the aerosol indirect effect.

CCN Properties of Organic Aerosol Collected Below and within Marine Stratocumulus Clouds near Monterey, California

“…The activation experiments were repeated (a minimum of four times) for each s level, which varied from 0.2% to 1.2%. The compositional data, aerosol surfactant behavior, and the dependence of d with respect to sc, were used to infer the molar volume and surfactant characteristics of the organic fraction with Köhler Theory Analysis [33][34][35]43] (Section 3). The CFSTGC was calibrated using (NH4)2SO4 (density = 1.77 g·cm −3 , and molar mass of 132 g·mol…”

“…Two measures of molar volume uncertainty are used: (i) the standard deviation of all the inferences (Equation (2)), and, (ii) estimates determined from, Δ = ∑ (Φ Δ ) , where ∆x is the uncertainty of each of the measured parameters x, (i.e., any of σ, ω, εi, εj, νi, and νj) and Φx is the sensitivity of molar volume to x, = ( ), using the formulas of [33,34,43]. The maximum of both estimates is the reported uncertainty of .…”

“…The maximum of both estimates is the reported uncertainty of . KTA has been shown to constrain molecular weight estimates for laboratory aerosol (having organic mass fraction between 20 and 50%) with an average error of 20% [33], 40% for complex biomass burning aerosol [43], 30% for secondary organic matter [35], and to within 25% for primary marine organic matter [34].…”

“…Hence, Equation (4) is used to infer σ at carbon concentrations relevant for CCN activation (roughly 1000 mg·C·L −1 ). WSOC from biomass burning aerosol and marine aerosol have been shown to contain strong surfactants that depress surface tension by 25%-42% [43,47,54,55] at similar concentration. The inferred σ values for both SC and CR aerosol (Figure 3) exhibit weak surface tension depression (−Δσ/σ ≃ 5%) at concentrations >1000 mg·C·L .…”

“…Finding the average organic molar mass from KTA will help constrain if the SC and CR samples indeed have different organic aerosol compositions that affect aerosol solubility and surfactant properties. [43], marine organic aerosol (grey dash-dot; [54]) and dissolved marine organic matter (solid red; [34]). (e.g., (NH4)2SO4 data in Figure 4).…”