V. Faye McNeill, PhD , Columbia University
Atmospheric aerosol particles nucleate cloud droplets, thereby influencing the radiative properties, amount and lifetime of clouds, also known as the aerosol indirect effects on climate. The relationship between an aerosol particle's chemical composition and its ability to nucleate cloud droplets (CCN ability) is complex. Organic aerosol material (OA), a common component of tropospheric aerosols, is typically more hydrophobic, and therefore less hygroscopic than inorganic salts. Surface-active OM can lower aerosol surface tension, thus enhancing CCN activation, however, an organic surface film can also act as a kinetic barrier for water uptake to the aerosol.
Heterogeneous oxidation of aerosols composed of pure oleic acid by gas-phase O3 is known to increase aerosol hygroscopicity and CCN activity. Whether this trend is preserved for wet inorganic aerosols with oleic acid surface layers is unknown. In collaboration with the Nenes group of Georgia Tech, we quantified the CCN activity of sodium salt aerosols (NaCl, Na2SO4) internally mixed with oleic acid (OA). These multicomponent aerosols showed depressed CCN activity upon oxidation with O3. The behavior after oxidation is consistent with the disappearance of the organic surface film, supported by Köhler Theory Analysis (KTA). κ-Köhler calculations show a small decrease in hygroscopicity after oxidation. The important implication of this finding is that oxidative aging may not always enhance the hygroscopicity of internally mixed inorganic-organic aerosols.
We recently showed that uptake of methylglyoxal from the gas phase driven by aqueous-phase oligomerization chemistry is a potentially significant, previously unidentified source of surface-active organic material in aerosols. Bulk aqueous solutions of methylglyoxal, with and without inorganic salts, exhibit significant surface tension depression. Evidence of aldol condensation products and oligomeric species up to 759 amu was found using chemical ionization mass spectrometry with a volatilization flow tube inlet (Aerosol-CIMS). The surface tension depression is greater when NaCl or (NH4)2SO4 is present in the solution; this is likely to be a physical effect of the salts rather than an effect of especially surface-active products formed by a chemical reaction of methylglyoxal with the salts.
We also examined the chemical reactions of two other abundant CVOCs, formaldehyde and acetaldehyde, in water and aqueous ammonium sulfate (AS) solutions mimicking tropospheric aerosols. Secondary organic products were identified using Aerosol-CIMS, and changes in surface tension were monitored using pendant drop tensiometry. Acetaldehyde depresses surface tension in pure water and in AS solutions. Surface tension depression by formaldehyde in AS solutions is attributed to the formation of surface-active secondary products via reaction with AS. Mixtures of these species were also studied in combination with methylglyoxal in order to evaluate the influence of cross-reactions on surface tension depression and product formation in these systems. We find that surface tension depression in the solutions containing mixed CVOCs exceeds that predicted by an additive model based on the single-species isotherms. The relatively low solubility of formaldehyde and acetaldehyde in water suggests that their potential to contribute to total SOA mass is low as compared to highly soluble species such as glyoxal. This is supported by the observations of Kroll et al. that AS aerosols exposed to formaldehyde in an aerosol reaction chamber did not result in significant particle volume growth. However, formaldehyde and acetaldehyde in the gas phase could adsorb at the aerosol surface (vs. bulk aqueous absorption), and this may also impact aerosol surface tension.
We conducted aerosol chamber experiments in our laboratory, in collaboration with the Nenes group at Georgia Tech, on the effects of methylglyoxal and acetaldehyde on aerosol CCN activity. Exposing deliquesced (NH4)2SO4 particles to gas-phase methylglyoxal or acetaldehyde for 3-5 hours enhances CCN activity: the critical dry diameter for activation is depressed by up to 15%. The enhancement in CCN activity is greatest at small particle sizes, typical of aerosol surface tension depression by surface-active organics. For marine aerosols, where stratocumulus clouds (supersaturation ~0.1-0.2%) are prevalent, a similar decrease in critical activation diameter can increase CCN concentrations by up to 54%. For continental aerosols, at supersaturations around 0.6-1.0%, typical of convective clouds, CCN concentrations can increase by up to 14%. No detectable particle volume growth associated with uptake was observed, consistent with the relatively low Henry's Law constants of methylglyoxal and acetaldehyde. Furthermore, the particle surface tensions inferred from the CCN data using Kohler Theory Analysis were significantly lower than those measured for bulk solutions with the equivalent aqueous phase composition predicted using Henry's Law. This suggests that surface adsorption is also important for determining aerosol surface tension and thus CCN activity.
While the effects of traditional SOA products or primary surface-active organics such as carboxylic acids on CCN activity in mixed inorganic-organic aerosols have been studied, this is the first time a gas-phase reservoir of volatile surface-active organics such as this has been studied.