Effect of Photolysis on Absorption and Fluorescence Spectra of Light-Absorbing Secondary Organic Aerosols

Excitation-emission matrices (EEMs) constructed from fluorescence measurements are increasingly used for the characterization of chromophoric dissolved organic matter (CDOM) and light-absorbing atmospheric organic aerosols known as brown carbon (BrC). There is a high uncertainty in the effect of BrC aerosols on climate because their optical properties depend on the amount of time they spent in the atmosphere. In order to aid in the quantification of BrC aerosols' contribution to radiative forcing, we investigated the effect of solar radiation on the fluorescence, expressed as EEMs, and absorption spectra of the water-soluble fraction of BrC species formed by the high-NOx photooxidation of benzene, toluene, p-xylene, and naphthalene. The BrC samples were prepared in a smog chamber, extracted in water, and irradiated in a solar simulator at a fixed pH of 3, representative of aerosol liquid water, or at a fixed pH of 6, representative of cloudwater. Semicontinuous fluorescence and absorbance measurements were carried out during the irradiation at 20 min intervals for 44 h. The absorption coefficients depended on the solution pH, with the solutions at pH 6 absorbing stronger than solutions at pH 3. All samples underwent a decrease in absorption coefficient at all visible wavelengths, whereas fluorescence intensities showed both increases and decreases in different regions of the EEMs. Upon comparison with CDOM samples, the fluorescence intensity of all secondary organic aerosol (SOA) samples decreased in the region of the EEMs where the characteristic terrestrial humic-like C peak occurs. These experimental observations suggest that (i) this type of BrC will have different effects on climate depending on whether it ends up in an acidic or neutral environment; (ii) exposure to UV radiation will diminish the ability of this type of BrC to affect climate on a time scale of about a day; (iii) fluorescence by BrC compounds has a minimal effect on aerosol radiative forcing; (iv) photooxidized aromatics may be closely related, in terms of optical properties, to CDOM found in fresh waters.