Effect of organic coatings derived from the OH-initiated oxidation of amines on soot morphology and cloud activation

Abstract

The presence of coatings on soot particles may significantly influence their environmental impacts. In this study, the processing of soot by OH-initiated photooxidation of amines was investigated in a series of chamber experiments. Monodisperse soot aerosol was exposed to oxidation products of monoethanolamine (MEA), triethylamine (TEA) and aniline, and changes in particle size, mass, effective density, shape factor, and activation as cloud condensation nuclei (CCN) were simultaneously measured by a suite of instruments. Different initial concentrations of amine precursor were applied, and in selected experiments, organic coatings were removed by thermal denuding to investigate the change of the soot core structure. Aniline generated the largest amount of low volatility condensable oxidation products, which induced minimal soot restructuring even at a high aniline concentration. On the contrary, the high volatility of MEA oxidation products led to reversible condensation on soot while inducing significant aggregate collapse for a smaller coating mass than with aniline. The products from TEA photooxidation showed volatility intermediate between aniline and MEA and induced the most significant restructuring. It can be concluded that restructuring depended on the physical state of the coating material and its distribution over the aggregate surface. Soot aged with the three different amine precursors produced distinct CCN activity profiles that depended on measured coating mass, and there was an indication that intermediate volatility oxidation products of MEA co-condensed with water during the droplet activation process.