  The reactive uptake of carbonyl-containing volatile organic compounds (cVOCs)
by aqueous atmospheric aerosols is a likely source of particulate organic
material. 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 chemical reactions of two abundant cVOCs, formaldehyde and acetaldehyde, in
water and aqueous ammonium sulfate (AS) solutions mimicking tropospheric
aerosols. Secondary organic products were identified using Aerosol Chemical
Ionization Mass Spectrometry (Aerosol-CIMS), and changes in surface tension
were monitored using pendant drop tensiometry. Hemiacetal oligomers and aldol
condensation products were identified using Aerosol-CIMS. Acetaldehyde
depresses surface tension to 65(\pm2) dyn/cm in pure water (a 10% surface
tension reduction from that of pure water) and 62(\pm1) dyn/cm 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 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.
