The evolutionary behavior of chromophoric brown carbon during ozone aging of fine particles from biomass burning

Abstract

<p><strong>Abstract.</strong> Biomass burning (BB) emits large amounts of brown carbon (BrC), however, little is known about the evolutionary behavior of BrC produced by BB (BB-BrC) in atmospheric processes. In this study, the transformation of levels and the chromophoric characteristics of BrC in smoke particles emitted by the burning of rice straw (RS), corn straw (CS) and pine wood (PW) under O<sub>3</sub> aging are investigated. The O<sub>3</sub> aging induced the reduction of light absorption and fluorescence for the BB-BrC, suggesting the decomposition of chromophores and fluorophores. These changes were accompanied by a decrease of aromaticity, average molecular weight and the light absorption capacity for the chromophores, and an increase of humification for the fluorophores. The excitation emission matrix combined with a parallel factor analysis revealed that protein-like components (C3) were predominantly decomposed by O<sub>3</sub> aging, while the relative distribution of a humic-like component with highly oxygenated chromophores (C4) gradually increased. In general, the humic-like substances (C1+C2+C4) were transformed to be the most abundant fluorophores for all the BB-BrC samples, which accounted for 84 %–87 % of the total fluorophores in final O<sub>3</sub>-aged BB-BrC. Two dimensional correlation spectroscopy (2D-COS) was performed on the synchronous fluorescence, which suggested that the RS- and CS- BrC exhibits the same susceptible fluorophores changes upon O<sub>3</sub> aging. It showed that O<sub>3</sub> firstly reacted with protein-like fractions (263–289 nm), and then with fulvic-like fractions (333–340 nm). In comparison, the changing sequence of susceptible fluorophores in the PW-BrC to O<sub>3</sub> were in the order of fulvic-like with shorter wavelengths (309 nm) > protein-like fluorophores (276 nm) > fulvic-like fluorophores with longer wavelengths (358 nm). The 2D-FTIR-COS analysis showed conjugated C=O and aromatic C=C and C=O groups were the most susceptible functional groups to O<sub>3</sub> aging for all BB-BrC. Moreover, it also revealed a consistent sequential changes, which is in the order of aromatic OH, conjugated C=O groups and aromatic C=O, aromatic COO<sup>−</sup>, and finally lignin-derived C-C, C-H and C-O groups. Our results provide new insights into the evolutionary behavior of the chromophoric and fluorescent properties of BB-BrC during O<sub>3</sub> aging. They have important implications for the heterogeneous oxidation of BB emission to form BrC, and are of great significance for improving the accuracy of climate models and source apportionment models parameterized by the optical properties of BrC.</p>

Measurement report: Investigation of pH- and particle-size-dependent chemical and optical properties of water-soluble organic carbon: implications for its sources and aging processes

“…The overall fluorescence intensity per TOC (FI / TOC) decreased steadily with increasing particle size (Table 1). The fluorescence intensity was higher for fresh than aged brown carbon (BrC), as previously observed by Fan et al (2020). Therefore, the FI / TOC further highlights that WSOC may have undergone a cascade of aging processes (e.g., photochemical aging and oxidative aging) with increasing particle size and thus has weaken its fluorescence intensity (Kuang et al, 2021;Wu et al, 2021).…”

Characteristics and evolution of brown carbon in western United States wildfires

“…A fraction of BrC is semi-volatile (Devi et al, 2016) and may be a component of the secondary organic aerosol (SOA). This secondary BrC can be formed from a range of species and processes, such as reactions between aromatic volatile organic compounds (VOCs) with ozone (O 3 ) or the hydroxyl (OH) or nitrate (NO 3 ) radicals (Lee et al, 2014;Jiang et al, 2019;Fan et al, 2020), aqueous reactions involving carbonyl function groups with ammonium sulfate or PAHs with illumination (Nguyen et al, 2012;Haynes et al, 2019), and heterogeneous reactions of isoprene on acidic particles (Limbeck et al, 2003).…”

“…Functionalization of nitrophenol molecules through oxidation by aqueous OH radicals and fragmentation of aromatic structures to smaller oxygenated molecules by direct photolysis was observed to first produce a photo enhancement, followed by photobleaching (Hems and Abbatt, 2018). Oxidation by O 3 has been observed to bleach BrC (Sareen et al, 2013;Fan et al, 2020), but BrC absorption can also increase at the beginning of the O 3 oxidation process (Kuang and Shang, 2020). The chemical processing of individual chromophores has also been studied.…”

“…Fan et al (2020); Fleming et al (2020); Harrison et al (2020); Schnitzler et al (2020) Aqueous reaction involving or forming carbonyl compounds Enhancement Nguyen et al (2013); Powelson et al (2014); Kasthuriarachchi et al (2020) High NO x or NO 3 , associated with night chemistry Enhancement Lin et al (2017); Jiang et al (2019); Cheng et al (2020); Li et al (2020); He et al (2021); Mayorga et al (2021) Dilution-driven evaporation Bleaching Palm et al (2020)…”