Abstract. Secondary organic aerosol (SOA) generated from the photooxidation of aromatic compounds in the presence of oxides of nitrogen (NOx) is known to efficiently absorb ultraviolet and visible radiation. With exposure to sunlight, the photodegradation of chromophoric 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 near-UV 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. By contrast, photobleaching directly on the filter was slower. 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 than in the on-filter samples. This suggests that photodegradation of nitrophenols 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.
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.
<p>Brown carbon (BrC) in aerosol particles and cloud droplets can contribute to climate warming by absorbing solar radiation in the visible region of the solar spectrum. Large uncertainties remain in our parameterization of this warming, in part due to a lack of knowledge about atmospheric lifetimes for the chromophores (the light absorbing structures in BrC molecules). An important removal pathway includes chemical transformations that fragment the chromophore, thus removing its ability to absorb visible light. However, the photochemical loss rates measured in the laboratory do not generally match what is observed in ambient measurements. There are also different amounts of photo-resistant BrC, which is a fraction of the mixture that does not rapidly bleach. An important BrC source in the atmosphere is biomass burning and the overall photochemical decay rates for these emissions are important to quantify to improve our parameterizations of their radiative effects. Here we show results for laboratory studies of FIREX filter samples probing the role of water vapor in photolysis of aerosol particles irradiated on a filter. Kinetic analysis of photo-bleaching in aqueous solutions demonstrates that an intermediate photolysis rate should be included to improve predictions for BrC lifetimes in the atmosphere.</p>
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