Formation of nitrogen‐ and sulfur‐containing light‐absorbing compounds accelerated by evaporation of water from secondary organic aerosols

Abstract

Aqueous extracts of secondary organic aerosols (SOA) generated from the ozonolysis of d‐limonene were subjected to dissolution, evaporation, and re‐dissolution in the presence and absence of ammonium sulfate (AS). Evaporation with AS at pH 4–9 produced chromophores that were stable with respect to hydrolysis and had a distinctive absorption band at 500 nm. Evaporation accelerated the rate of chromophore formation by at least three orders of magnitude compared to the reaction in aqueous solution, which produced similar compounds. Absorption spectroscopy and high‐resolution nanospray desorption electrospray ionization (nano‐DESI) mass spectrometry experiments suggested that the molar fraction of the chromophores was small (<2%), and that they contained nitrogen atoms. Although the colored products represented only a small fraction of SOA, their large extinction coefficients (>10 5 L mol−1 cm−1 at 500 nm) increased the effective mass absorption coefficient of the residual organics in excess of 10 3 cm 2 g−1 ‐ a dramatic effect on the optical properties from minor constituents. Evaporation of SOA extracts in the absence of AS resulted in the production of colored compounds only when the SOA extract was acidified to pH ∼ 2 with sulfuric acid. These chromophores were produced by acid‐catalyzed aldol condensation, followed by a conversion into organosulfates. The presence of organosulfates was confirmed by high resolution mass spectrometry experiments. Results of this study suggest that evaporation of cloud or fog droplets containing dissolved organics leads to significant modification of the molecular composition and serves as a potentially important source of light‐absorbing compounds.