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

Abstract

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, pxylene, 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.

Supplementary material to "Dynamic changes of optical and chemical properties of tar ball aerosols by atmospheric photochemical aging"

“…They suggested that photolysis, rather than hydrolysis, bleached SOA absorption due to degradation of nitrogen-containing chromophores. This conclusion was also confirmed by similar studies by Lee et al (2014) and Aiona et al (2018). In our study, the changes in the optical properties as a function of O/C ratio for tar balls upon photolysis are shown in Figure S15.…”

Photo-degradation of atmospheric chromophores: type conversion and changes in photochemical reactivity

“…The decrease of absorbance confirm that COM are photo-bleached (Duarte et al, 2005). The subduction function of photolysis on absorbance is significant (Aiona et al, 2018). In POA (Fig.…”

Chemical transfer of dissolved organic matter from surface seawater to sea spray water-soluble organic aerosol in the marine atmosphere

“…Three-dimensional excitation emission matrix (EEM) spectroscopy, which yields fluorescence intensities as a function of both excitation and emission wavelengths, has been widely applied to characterize chromophoric dissolved organic matter in terrestrial and oceanic systems 17 , 18 . However, the method has rarely been used for the analysis of complex organic matter in atmospheric aerosols 19 – 23 , particularly for marine aerosols. Classifications of organic matter with EEM spectra in SSA need to be employed in comparison with that in SSW, because their sources, chemical compositions, and transformation pathways are expected to be different from those in the marine aqueous environment.…”