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

Abstract

Abstract. Biomass burning (BB) emits large amounts of brown carbon (BrC); however, the evolutionary behavior of BrC in BB emissions (BB BrC) resulting from complex atmospheric processes is poorly understood. In this study, the transformation of contents and the chromophoric characteristics of BrC in smoke particles emitted by the burning of rice straw (RS), corn straw (CS), and pinewood (PW) under O3 aging are investigated. The O3 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 in aromaticity, average molecular weight, and the light absorption capacity for the chromophores, as well as an increase in humification for the fluorophores. The excitation emission matrix combined with a parallel factor analysis revealed that protein-like components (C3) were predominantly decomposed by O3 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 O3-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 O3 aging. It showed that O3 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 O3 was in the order of fulvic-like fluorophores with shorter wavelengths (309 nm), protein-like fluorophores (276 nm), and fulvic-like fluorophores with longer wavelengths (358 nm). The 2D-FTIR-COS (2D-COS combined with FTIR) analysis showed conjugated C=O and aromatic C=C and C=O groups were the most susceptible functional groups to O3 aging for all BB BrC. Moreover, it also revealed a consistent sequential change, which is in the order of aromatic OH; conjugated C=O groups and aromatic C=O; aromatic COO−; 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 O3 aging, which are of great significance for better understanding the heterogeneous oxidation pathways of BB-derived BrC in the atmospheric environment.

Characteristics and Evolution of Brown Carbon in Western United States Wildfires

“…These two possible divergent behaviors when BrC is oxidized by O3 have been observed in other studies. Sareen et al (2013) observed this behavior for secondary BrC (formed with methylglyoxal and ammonium) and Fan et al (2020) for aerosols from biomass burning. O3 could also just being acting as a tracer for other oxidation processes.…”

“…A fraction of BrC is semi-volatile (Jai 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 VOCs with ozone (O3) or the hydroxyl (OH) or nitrate (NO3) radicals 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).…”

“…Oxidation by O3 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 O3 oxidation process (Kuang and Shang, 2020). The chemical processing of individual chromophores has also been studied.…”

Measurement Report: Optical properties and sources of water-soluble brown carbon in Tianjin, North China: insights from organic molecular compositions

“…The fluorophore C2 exhibiting a peak at Ex/Em of ~250 nm/ 465 nm is also a humic-like fluorophore. The longer wavelength of C2 suggests that compared with C1, C2 is more aromatic with higher molecular weight, containing more conjugated and unsaturated chemical structures due to condensation reactions (Matos et al, 2015;Fan et al, 2020a;Dey et al, 2021). C3 showing a peak at Ex/Em of ~250 nm/ 385 nm is also a humic-like fluorophore (Fan et al, 2020a;Li et al, 2020b).…”

Source-specific light absorption by carbonaceous components in the complex aerosol matrix from yearly filter-based measurements

“…While it is typically assumed that primary biomass burning dominates the BrC absorption, the importance of other sources with different absorption properties including secondary OA, formed through the oxidation of precursors, may be overlooked. Certain studies suggest that aging leads to bleaching, whereas others suggest formation of secondary BrC (Saleh et al, 2013;Zhong and Jang, 2014;Zhao et al, 2014;Kumar et al, 2018;Dasari et al, 2019;Fan et al, 2020). Further, it is not yet established whether BrC from common urban sources is extractable, or else insoluble, refractory BrC can also contribute to total absorption.…”