Abstract
Abstract. Chromophores represent an important portion of light-absorbing species, i.e., brown carbon. Yet knowledge of what and how chromophores contribute to aerosol light absorption is still sparse. To address this problem, we examined soluble independent chromophores in a set of year-round aerosol samples from Bangkok. The water-soluble fluorescent chromophores identified via excitation–emission matrix (EEM) spectroscopy and follow-up parallel factor analysis could be mainly assigned as humic-like substances and protein-like substances, which differed in their EEM pattern from that of the methanol-soluble fraction. The emission wavelength of fluorescent chromophores in environmental samples tended to increase compared with that of the primary combustion emission, which could be attributed to secondary formation or the aging process. Fluorescent indices inferred that these light-absorbing chromophores were not significantly humified and comprised a mixture of organic matter of terrestrial and microbial origin, which exhibited a different characteristic from primary biomass burning and coal-combustion results. A multiple linear regression analysis revealed that larger fluorescent chromophores that were oxygen-rich and highly aromatic with high molecular weights were the key contributors of light absorption, preferably at longer emission wavelengths (λmax > 500 nm). Positive matrix factorization analysis further suggested that up to 50 % of these responsible chromophores originated from biomass burning emissions.
Highlights
Atmospheric aerosols play a substantial role in climate change through radiative forcing (Alexander et al, 2008)
Fluorescence indices based on intensity ratios provide insight into the origins of dissolved Brown carbon (BrC), such as the humification index (HIX) (Zsolnay et al, 1999), the biological index (BIX) (Huguet et al, 2009), and the fluorescence index (FI) (Lee et al, 2013; Murphy et al, 2018)
excitation–emission matrix (EEM) combined with Parallel factor (PARAFAC) analysis showed that the identified fluorescent components were humic-like and protein-like substances but with different patterns in the water-soluble OC (WSOC) and methanol-soluble OC (MSOC), indicating different chemical compositions
Summary
Atmospheric aerosols play a substantial role in climate change through radiative forcing (Alexander et al, 2008). Parallel factor (PARAFAC) analysis has been widely used to decompose the EEM spectral signature into independent underlying components (Han et al, 2020; Yue et al, 2019; Wu et al, 2019; Chen et al, 2019b), adding valuable information to absorbance-based measurements (Yan and Kim, 2017) This technique helps to categorize groups of similar fluorophores or chromophores or similar optical properties, thereby allowing a better understanding of the chemical properties of BrC, but it should be noted that not all chromophores in BrC compounds are fluorescent (Chen et al, 2019a). Water-soluble and methanol-soluble BrC in the aerosol samples was characterized by EEM, followed by statistical analyses to retrieve information on the contributions of fluorescent chromophores to BrC light absorption and their emission sources. This study provides a comprehensive dataset on seasonal variability in the light absorption properties, sources, and chemical components of BrC, which may be useful for improving further modeling and field observation
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