Abstract
Abstract. Secondary organic aerosol (SOA) particles, formed from gas-phase biogenic volatile organic compounds (BVOCs), contribute large uncertainties to the radiative forcing that is associated with aerosols in the climate system. Reactive uptake of surface-active organic oxidation products of BVOCs at the gas–aerosol interface can potentially decrease the overall aerosol surface tension and therefore influence their propensity to act as cloud condensation nuclei (CCN). Here, we synthesize and measure some climate-relevant physical properties of SOA particle constituents consisting of the isoprene oxidation products α-, δ-, and cis- and trans-β-IEPOX (isoprene epoxide), as well as syn- and anti-2-methyltetraol. Following viscosity measurements, we use octanol–water partition coefficients to quantify the relative hydrophobicity of the oxidation products while dynamic surface tension measurements indicate that aqueous solutions of α- and trans-β-IEPOX exhibit significant surface tension depression. We hypothesize that the surface activity of these compounds may enhance aerosol CCN activity, and that trans-β-IEPOX may be highly relevant for surface chemistry of aerosol particles relative to other IEPOX isomers.
Highlights
Surface tension is expected to be of particular importance for Secondary organic aerosol (SOA) formation and growth as it involves processes occurring at the interface between the SOA particle phase and the gas phase (Wang and Wexler, 2013; Djikaev and Tabazadeh, 2003)
These compounds displayed the longest GC retention times (∼ 16.5 min) with nearly identical fragmentation patterns (Fig. S1 in the Supplement). δ-isoprene epoxide (IEPOX) (3), with its secondary and primary hydroxyl groups, had a slightly higher partition coefficient. αIEPOX (4) proved to be the most hydrophobic epoxide with the least negative Kow value of all the epoxides. These results are consistent with α-IEPOX (4) having the least accessible hydroxyl groups of the epoxides due to the placement of the methyl group and possibly indicate that α-IEPOX (4) would be the isomer most likely to partition into the organicrich phase of particles
We experimentally determined octanol–water partitioning coefficients (Kow) and viscosities of these compounds. Results these experiments revealed that α-IEPOX (4) is the most hydrophobic and surface active of the compounds studied here; the hydrophobicity of these compounds did not coincide with surface activity for all compounds
Summary
Secondary organic aerosol (SOA) particles make up a substantial fraction of tropospheric aerosol and are known to lead to negative radiative forcing (Kanakidou et al, 2005; Carlton et al, 2009; Williams et al, 2011), yet their formation ranks among the least understood processes in the atmosphere (Kanakidou et al, 2005; Goldstein and Galbally, 2007; Galbally et al, 2007; Riipinen et al, 2011; Hallquist et al, 2009). We report here surface tension values, measured using pen- 348 dant drop tensiometry, of suspended drops of deionized water and 1.0 M ammonium sulfate solutions containing 0.1 to 30 mM concentrations of synthetically prepared isoprene- 2.2 Partition experiments derived SOA particle constituents. Due to the limited solubiloctanol–water partitioning coefficients (Kow) and viscosities ity of the 2-methyltetraol compounds in octanol, stock soof the compounds under investigation These studies reveal lutions (∼ 45 mM) of the tetraol compounds were prepared that α-IEPOX (4) significantly decreases surface tension in in deionized water. Surface tension measurements performed in this work are most likely insensitive to impurities below the detection limit of NMR spectroscopy due to the higher concentrations of IEPOX and tetraols used in this study (above micromolar amounts). Upshur et al.: Climate-relevant physical properties of molecular constituents the compound originally introduced and the quantity in the octanol phase determined using the mass balance technique
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