The aging behaviors of chromophoric biomass burning brown carbon during dark aqueous hydroxyl radical oxidation processes in laboratory studies

Highlights

• Dark aqueous OH radical oxidation resulted in strong degradation of BB BrC.

• Aging behaviors of BB BrC were explored by EEM-PARAFAC and 2D-COS.

• Tryptophan-like component (C2) presented pronounced degradation for BB BrC.

• Sequences of chromophores and fluorophores within BB BrC were investigated.

Abstract

This study was conducted to explore the aging behavior of biomass burning (BB) emitted brown carbon (BrC) during an aqueous hydroxyl radical (OH) oxidation process in the absence of light. The evolution characteristics of rice straw (RS) and pine wood (PW) emitted BrC were monitored by UV–vis, synchronous fluorescence and excitation-emission matrix (EEM) spectroscopies combined with two-dimensional correlation spectroscopy (2D-COS) and parallel factor (PARAFAC) analysis. The results indicated that OH radical oxidation could greatly weaken the absorption and fluorescence intensity of both BB BrC, implying the significant degradation of chromophoric BB BrC. EEM-PARAFAC analysis identified one humic-like (C2) and two protein-like (C1, C3) components from BB BrC. The intensities of these fluorescent components all steadily decreased with oxidation, in which the tryptophan-like substance (C1) presented pronounced reduction for both BB BrC. However, the C2/C1 and C3/C1 intensity ratios exhibited an increasing trend with oxidation time, suggesting that the distribution of humic-like (C2) and tyrosine-like (C3) components within BrC could be enhanced during dark OH oxidation processes. These findings suggested that the levels and types of chromophores and fluorophores within BB BrC had been transformed once they were emitted to atmosphere, resulting in the change of optical properties of BB BrC. 2D-absorption-COS demonstrated that the decomposition of chromophores within the high ultraviolet region (275–310 nm) occurred preferentially to those within the low ultraviolet region (225–260 nm). In addition, the 2D-COS combined with fluorescence spectra for both BrC showed that the oxidation of types of fulvic-like fluorophores (340–380 nm) occurred before protein-like fractions (250–300 nm). These results provide detailed knowledge of the aqueous OH radical oxidation process of BB BrC in the atmosphere and are of great significance for understanding the climate and environmental effects of BB BrC and refining the atmospheric models established on the optical properties of BrC.

Introduction

Brown carbon (BrC) is generally defined as the substantial fraction of organic carbon, which can absorb light in the ultraviolet and lower visible spectral regions (Andreae and Gelencser, 2006; Lack and Cappa, 2010; Feng et al., 2013; Laskin et al., 2015; Kim et al., 2016; Wu et al., 2016). Brown carbon is ubiquitously present in atmospheric aerosols (Liu et al., 2013; Cheng et al., 2016; Yuan et al., 2016; Yan and Kim, 2017), cloudwater (Gelencser et al., 2003), rainwater (Zhang et al., 2014; Pantelaki et al., 2018) and biomass burning aerosols (Fan et al., 2016b; Park and Yu, 2016). Because of its strong ability to absorb light, BrC plays significant roles in radiative forcing (Feng et al., 2013) and atmospheric chemical processes (Wu et al., 2016).

Increased evidence has shown that biomass burning (BB) is an important source of BrC based on both field observations (Baduel et al., 2010; Lin et al., 2010; Fan et al., 2016a) and laboratory chamber measurements (Chen et al., 2016; Fan et al., 2016b; Park and Yu, 2016). Moreover, BB has been found to directly release large amounts of primary BrC with considerable chromophores, leading to significant effects on the abundance and optical properties of atmospheric BrC (Fan et al., 2016b, 2018a; Park and Yu, 2016; Song et al., 2018). Furthermore, numerous oxidants (e.g., ozone (O₃), hydroxyl radical (OH)) which widely exist in the atmosphere might result in heterogeneous evolution of primary BB BrC effecting its abundance, light absorption and molecular characteristics (Gelencser et al., 2003; Hoffer et al., 2004; Moonshine et al., 2008; Baduel et al., 2011; Sumlin et al., 2017; Wong et al., 2017). The transformation of BB BrC under photochemical reactions has been extensively studied, and the results have illustrated that there are photo enhancement and photobleaching effects on BrC during these processes (Saleh et al., 2013; Zhong and Jang, 2014; Zhao et al., 2015; Wong et al., 2017). For example, Zhong and Jang (2014) found that the photochemical aging of BB BrC lead to chromophore formation and sunlight bleaching. Wong et al. (2017) demonstrated that large BrC molecules were rapidly photoenhanced followed by slow photolysis under aqueous photolytic aging of BB BrC.

Additionally, dark oxidation is also an important aging pathway of BrC, with OH radical oxidation drawing a great deal of attention (Gelencser et al., 2003; Hoffer et al., 2004; Vione et al., 2006; Moonshine et al., 2008; Santos and Duarte, 2015; Santos et al., 2016a, 2016b). For example, Gelencser et al. (2003) and Hoffer et al. (2004) demonstrated that the oligomerization occurred during dark aqueous OH radical oxidation of 3,5-dihydroxylbenzoic acid, which resulted in the aged one to be more light absorption. Santos and Duarte, 2015, Santos et al., 2016a, Santos et al., 2016b reported a conflicting result, in which the aged aromatic acids (i.e. benzoic, 4-hydroxybenzoic, 3,5-dihydroxy-benzoic, vanillic and syringic acids) were found to be less absorptive than fresh ones under similar reaction conditions. However, these studies primarily focused on the transformation of individual BB-derived organic compounds (i.e., aromatic acids, levoglucosan). Notably, the BB aerosols generally comprise substantial amounts of organic compounds with multiple functional groups, such as aromatic carboxyls and hydroxyls (Fan et al., 2016b, 2018a; Song et al., 2018), all of which could be potential active components in atmospheric chemical processes. Therefore, the heterogeneous aging behaviors of bulk BB BrC under aqueous OH radical oxidation need to be explored.

The light absorption of BrC has been investigated by UV–vis and fluorescence absorption, even when present at low concentrations in BB and ambient aerosols (Fan et al., 2012, 2016a, b; Matos et al., 2015; Pantelaki et al., 2018; Qin et al., 2018). Importantly, the developed protocols, such as excitation-emission matrix (EEM)-parallel factor analysis (EEM-PARAFAC) and two-dimensional correlation spectroscopy (2DCOS), were able to enhance resolution of absorption and withdraw more information regarding chromophoric characteristics. For example, EEM-PARAFAC analysis can effectively separate and quantify the individual fluorophores from matrix chromophores (Stedmon and Bro, 2008; Matos et al., 2015; Chen et al., 2016; Yan and Kim, 2017), and 2D-COS analysis allows exploration of the dynamic spectral behavior of a heterogeneous mixture with changes in external factors such as pH, temperature, concentration, and reaction time (Hur and Lee, 2014; Duarte et al., 2015; Noda, 2016; Aftab et al., 2018; Jin et al., 2018). Therefore, EEM-PARAFAC and 2D-absorption/fluorescence-COS are expected to be able to determine the heterogeneous evolution characteristics of bulk BB BrC under oxidation by OH radical with respect to variations in optical properties and individual fluorophore levels.

In this study, the dark aqueous OH radical oxidation of fresh BrC from rice straw (RS) and pine wood (PW) smoke particles was simulated in the laboratory, and the resulting BrC were investigated by UV–vis and fluorescence spectroscopy in combination of EEM-PARAFAC and 2DCOS. The results presented herein improve our understanding of the aging behaviors of primary BB BrC in the atmosphere. Moreover, it is of great importance for refining climate models and remote sensing measurement, which are established on parametrizing the optical properties of aerosols, especial for BB BrC aerosols (Omar et al., 2009; Feng et al., 2013; Tan et al., 2015).