Formation and photochemical investigation of brown carbon by hydroxyacetone reactions with glycine and ammonium sulfate.

Yan Gao,Yunhong Zhang

doi:10.1039/c8ra02019a

Abstract

Increasing attention has been paid to atmospheric “brown carbon” (BrC) aerosols due to their effect on the earth's climate. Aqueous BrC aerosols were formed through aqueous reactions of hydroxyacetone (HA) with nitrogen compounds such as glycine (Gly) and/or ammonium sulfate (AS). When exposed to nitrogen compounds for several days, HA, as a type of aqueous carbonyl compound, becomes absorbent and fluorescent in the blue visible and near ultraviolet regions, which have been monitored by UV/vis and fluorescence spectroscopy. In this study, we quantified absorption and excitation-emission matrix (EEM) spectra in the formation of aqueous BrCs, which was generated from HA-Gly and HA-Gly-AS mixtures, respectively. The obtained data was used to determine the base-10 absorption coefficient (α), absorption Angstrom exponent (AAE), and effective quantum yield (QY). All of the related parameters provide further evidence for the formation of aqueous BrC. The absorbances of the as-obtained BrCs follow the order HA-Gly-AS > HA-Gly > HA-AS. In other words, HA-Gly-AS mixtures displayed the most intense absorbances, whereas HA-AS mixtures barely produced visible absorbance. It is reasonable to speculate that Gly promotes the formation of HA-Gly BrC mixtures. The experimental results are consistent with previous measurements reported by Powelson et al. BrCs from HA-Gly-AS and HA-Gly exhibit stronger fluorescence between 300 and 400 nm. Glycine plays a more important role in the formation of aqueous BrC than that of AS. Furthermore, we examined the mass absorption coefficient (MAC) by photolysis of aqueous BrCs, which resulted from the oxidation of HA-Gly and HA-Gly-AS mixtures by 5 mM H2O2. An effective photolysis time induced significant changes near-UV (300–400 nm) absorption intensity of HA-Gly and HA-Gly-AS mixtures. These results emphasize the dynamic nature of the corresponding atmospheric aqueous BrC. Overall, our study provides the optical properties of the corresponding atmospheric aqueous BrC and the H2O2 oxidation photolysis process of the as-obtained BrC in detail, which may contribute to the understanding of the important effects of aqueous BrC for atmospheric chemistry and climate.

Highlights

We examined the mass absorption coefficient (MAC) by photolysis of aqueous brown carbon” (BrC), which resulted from the oxidation of HA-Gly and HA-Gly-ammonium sulfate (AS) mixtures by 5 mM H2O2
Our study provides the optical properties of the corresponding atmospheric aqueous BrC and the H2O2 oxidation photolysis process of the asobtained BrC in detail, which may contribute to the understanding of the important effects of aqueous BrC for atmospheric chemistry and climate
We provide further evidence for the formation of BrC using the related parameters of excitation-emission matrix (EEM) uorescence spectra, uorescence emission spectra and absorption spectra, such as wavelength-independent uorescence quantum yield (QY), Stokes shi, wavelength-dependent MAC, and absorption Angstrom exponent (AAE)

Summary

Introduction

Atmospheric aerosols play a key role in modulating the Earth's radiative balance, positive radiative forcing (warming) and the average surface temperatures on both global and local scales by direct and indirect mechanisms.[2,3,4] They have been linked to a signi cant adverse impact on public health, in the occupational environmental effects by airborne dispersal and transport.[5,6,7] Atmospheric “brown carbon” (BrC), as a collective term for light absorbing but poorly characterized organic compounds in the atmosphere, is an Review compounds that are routinely found in clouds and aerosol have the potential to form uorescent and absorbent BrC products.[18]Fluorescence spectroscopy that can detect and record emission spectra in real time is a rapid and sensitive technique for quantitative and qualitative analysis of BrC.[19,20,21] Fluorescent particles must contain signi cant amounts of organic compounds with large absorption coefficients and uorescence quantum yields.[22]. We quantified absorption and excitation-emission matrix (EEM) spectra in the formation of aqueous BrCs, which was generated from HA-Gly and HA-GlyAS mixtures, respectively.

Results

Conclusion