Reactive processing of formaldehyde and acetaldehyde in aqueous aerosol mimics: Surface tension depression and secondary organic products

TL;DR

This study investigates how formaldehyde and acetaldehyde react in aqueous aerosols, producing surface-active organic compounds that lower surface tension and potentially influence aerosol properties and secondary organic aerosol formation.

Contribution

It provides new insights into the chemical reactions and surface tension effects of formaldehyde and acetaldehyde in model aerosol systems, highlighting non-additive effects in mixed cVOC systems.

Findings

Acetaldehyde significantly depresses surface tension in water and ammonium sulfate solutions.

Formaldehyde causes negligible surface tension change in pure water but reduces it in ammonium sulfate solutions.

Mixed cVOC systems show greater surface tension depression than predicted by additive models.

Abstract

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.