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Abstract. Secondary organic aerosol (SOA) generated from the photooxidationof aromatic compounds in the presence of oxides of nitrogen (NOx) isknown to efficiently absorb ultraviolet and visible radiation. With exposureto sunlight, the photodegradation of chromophoric compounds in the SOAcauses this type of SOA to slowly photobleach. These photodegradationreactions may occur in cloud droplets, which are characterized by lowconcentrations of solutes, or in aerosol particles, which can have highlyviscous organic phases and aqueous phases with high concentrations ofinorganic salts. To investigate the effects of the surrounding matrix on therates and mechanisms of photodegradation of SOA compounds, SOA was preparedin a smog chamber by photooxidation of toluene in the presence of NOx.The collected SOA was photolyzed for up to 24 h using near-UV radiation(300–400 nm) from a xenon arc lamp under different conditions: directly onthe filter, dissolved in pure water, and dissolved in 1 M ammonium sulfate.The SOA mass absorption coefficient was measured as a function ofirradiation time to determine photobleaching rates. Electrospray ionizationhigh-resolution mass spectrometry coupled to liquid chromatographyseparation was used to observe changes in SOA composition resulting from theirradiation. The rate of decrease in SOA mass absorption coefficient due tophotobleaching was the fastest in water, with the presence of 1 M ammoniumsulfate modestly slowing down the photobleaching. By contrast,photobleaching directly on the filter was slower. The high-resolutionmass spectrometry analysis revealed an efficient photodegradation ofnitrophenol compounds on the filter but not in the aqueous phases, withrelatively little change observed in the composition of the SOA irradiatedin water or 1 M ammonium sulfate despite faster photobleaching than in theon-filter samples. This suggests that photodegradation of nitrophenolscontributes much more significantly to photobleaching in the organic phasethan in the aqueous phase. We conclude that the SOA absorption coefficientlifetime with respect to photobleaching and lifetimes of individualchromophores in SOA with respect to photodegradation will depend strongly onthe sample matrix in which SOA compounds are exposed to sunlight.