Reactive processing of formaldehyde and acetaldehyde in aqueous aerosol mimics: Surface tension depression and secondary organic products
The reactive uptake of carbonyl-containing volatile organic compounds (cVOCs) by aqueous atmo-spheric aerosols is a likely source of particulate organic mate-rial. The aqueous-phase secondary organic products of some cVOCs are surface-active. Therefore, cVOC uptake can lead to organic film formation at the gas-aerosol interface and changes in aerosol surface tension. We examined the chemi-cal reactions of two abundant cVOCs, formaldehyde and ac-etaldehyde, in water and aqueous ammonium sulfate (AS) solutions mimicking tropospheric aerosols. Secondary or-ganic products were identified using Aerosol Chemical Ion-ization Mass Spectrometry (Aerosol-CIMS), and changes in surface tension were monitored using pendant drop tensiom-etry. Hemiacetal oligomers and aldol condensation prod-ucts were identified using Aerosol-CIMS. Acetaldehyde de-presses surface tension to 65(±2) dyn cm −1 in pure water (a 10 % surface tension reduction from that of pure water) and 62(±1) dyn cm −1 in AS solutions (a 20.6 % reduction from that of a 3.1 M AS solution). Surface tension depression by formaldehyde in pure water is negligible; in AS solutions, a 9 % reduction in surface tension is observed. Mixtures of these species were also studied in combination with methyl-glyoxal 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 solu-tions containing mixed cVOCs exceeds that predicted by an additive model based on the single-species isotherms.