22 Apr 2022
22 Apr 2022
Status: this preprint is currently under review for the journal ACP.

Effects of the sample matrix on the photobleaching and photodegradation of toluene-derived secondary organic aerosol compounds

Alexandra Klodt1, Marley Adamek2, Monica Dibley2, Sergey Nizkorodov1, and Rachel O’Brien2 Alexandra Klodt et al.
  • 1Department of Chemistry, University of California Irvine, Irvine, CA, 92697, USA
  • 2Department of Chemistry, William & Mary, Williamsburg, VA, 23187, USA

Abstract. Secondary organic aerosol (SOA) generated from the photooxidation of aromatic compounds in the presence of oxides of nitrogen is known to efficiently absorb ultraviolet and visible radiation. With exposure to sunlight, the photodegradation of chormophoric compounds in the SOA causes this type of SOA to slowly photobleach. These photodegradation reactions may occur in cloud droplets, which are characterized by low concentrations of solutes, or in aerosol particles, which can have highly viscous organic phases and aqueous phases with high concentrations of inorganic salts. To investigate the effects of the surrounding matrix on the rates and mechanisms of photodegradation of SOA compounds, SOA was prepared in a smog chamber by photooxidation of toluene in the presence of NOx. The collected SOA was photolyzed for up to 24 h using UV-B radiation (300–400 nm) from a Xenon arc lamp under different conditions: directly on the filter, dissolved in pure water, and dissolved in 1 M ammonium sulfate. The SOA mass absorption coefficient was measured as a function of irradiation time to determine photobleaching rates. Electrospray ionization high resolution mass spectrometry coupled to liquid chromatography separation was used to observe changes in SOA composition resulting from the irradiation. The rate of decrease in SOA mass absorption coefficient due to photobleaching was the fastest in water, with the presence of 1 M ammonium sulfate modestly slowing down the photobleaching. Photobleaching directly on the filter was much slower, and approximately 50 % of the absorption persisted even after 5 h of irradiation. The high-resolution mass spectrometry analysis revealed an efficient photodegradation of nitrophenol compounds on the filter but not in the aqueous phases, with relatively little change observed in the composition of the SOA irradiated in water or 1 M ammonium sulfate despite faster photobleaching. This suggests that nitrophenol photodegradation contributes much more significantly to photobleaching in the organic phase than in the aqueous phase. We conclude that the SOA absorption coefficient lifetime with respect to photobleaching and lifetimes of individual chromophores in SOA with respect to photodegradation will depend strongly on the sample matrix in which SOA compounds are exposed to sunlight.

Alexandra Klodt et al.

Status: open (until 03 Jun 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on acp-2022-278', Anonymous Referee #1, 13 May 2022 reply
  • RC2: 'Comment on acp-2022-278', Anonymous Referee #2, 17 May 2022 reply

Alexandra Klodt et al.

Alexandra Klodt et al.


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Short summary
We investigated photochemistry of a secondary organic aerosol under three different conditions: in a dilute aqueous solution mimicking cloud droplets, in a solution of concentrated ammonium sulfate mimicking deliquesced aerosol, and in an organic matrix mimicking dry organic aerosol. We find that rate and mechanisms of photochemistry depend sensitively on these conditions, suggesting that the same organic aerosol compounds will degrade at different rates depending on their local environment.