Abstract
Brown carbon (BrC) aerosols exert vital impacts on climate change and atmospheric photochemistry due to their light absorption in the wavelength range from near-ultraviolet (UV) to visible light. However, the optical properties and formation mechanisms of ambient BrC remain poorly understood, limiting the estimation of their radiative forcing. In the present study, fine aerosols (PM2.5) were collected during 2016–2017 on a day/night basis over urban Tianjin, a megacity in North China, to obtain seasonal and diurnal patterns of atmospheric water-soluble BrC. There were obvious seasonal but no evident diurnal variations in light absorption properties of BrC. In winter, BrC showed much stronger light absorbing ability since mass absorption efficiency at 365 nm (MAE365) (1.54 ± 0.33 m2 g−1), which was 1.8 times larger than that (0.84 ± 0.22 m2 g−1) in summer. Direct radiative effects by BrC absorption relative to black carbon in the UV range were 54.3 ± 16.9 % and 44.6 ± 13.9 %, respectively. In addition, five fluorescent components in BrC, including three humic-like fluorophores and two protein-like fluorophores were identified with excitation-emission matrix fluorescence spectrometry and parallel factor (PARAFAC) analysis. The lowly-oxygenated components contributed more to winter and nighttime samples, while more-oxygenated components increased in summer and daytime samples. The higher humification index (HIX) together with lower biological index (BIX) and fluorescence index (FI) suggest that the chemical compositions of BrC were associated with a high aromaticity degree in summer and daytime due to photobleaching. Fluorescent properties indicate that wintertime BrC were predominantly affected by primary emissions and fresh secondary organic aerosol (SOA), while summer ones were more influenced by aging processes. Results of source apportionments using organic molecular compositions of the same set of aerosols reveal that fossil fuel combustion and aging processes, primary bioaerosol emission, biomass burning, and biogenic and anthropogenic SOA formation were the main sources of BrC. Biomass burning contributed much larger to BrC in winter and at nighttime, while biogenic SOA contributed more in summer and at daytime. Especially, our study highlights that primary bioaerosol emission is an important source of BrC in urban Tianjin in summer.
Highlights
Brown carbon (BrC) is light absorbing organic carbon (OC) in the atmosphere, which can absorb radiation in the range from 35 near-ultraviolet (UV) to visible and show strong wavelength dependence (Andreae and Gelencsér, 2006; Bahadur et al, 2012). its light absorbing ability is generally weaker than that of black carbon (BC), BrC exerts considerable impacts on atmospheric radiative balance and global climate due to their large abundance and strong light absorption in the near-UV spectrum (Feng et al, 2013; Jo et al, 2016; Zhang et al, 2017; Zhang et al, 2020a)
Fluorescent properties indicate that wintertime BrC were predominantly affected by primary emissions and fresh secondary organic aerosol (SOA), while summer ones were more influenced by aging processes
BrC in winter may be significantly affected by primary emissions of fossil fuel combustion, since high AAE coefficients are often associated with biomass burning (Desyaterik et al, 2013) and coal combustion (Li et al, 2019)
Summary
Brown carbon (BrC) is light absorbing organic carbon (OC) in the atmosphere, which can absorb radiation in the range from 35 near-ultraviolet (UV) to visible and show strong wavelength dependence (Andreae and Gelencsér, 2006; Bahadur et al, 2012). its light absorbing ability is generally weaker than that of black carbon (BC), BrC exerts considerable impacts on atmospheric radiative balance and global climate due to their large abundance and strong light absorption in the near-UV spectrum (Feng et al, 2013; Jo et al, 2016; Zhang et al, 2017; Zhang et al, 2020a). It is quite a challenge to understand the extremely complex chemical composition, sources, and formation and 45 evolution mechanisms of BrC (Laskin et al, 2015; Yan et al, 2018; Wu et al, 2021). Despite the progresses reported in recent years, it is needed to further characterize the sources and formation mechanisms of atmospheric BrC, from the perspectives of chromophore and molecular composition (Laskin et al, 2015; Yan et al, 2018). 55 To identify the chromophores in BrC will be benefit for probing the sources, dynamic optical properties, and aging processes of atmospheric BrC (Laskin et al, 2015; Yan et al, 2020). Despite numerous studies on chemical compositions, source apportionment and formation mechanisms of atmospheric aerosols, current understanding of the optical properties and sources of BrC aerosols over the NCP are still inadequate. This study provides a comprehensive view on the temporal variability in optical properties and sources of BrC, helping to deepen the understanding in its climatic effects
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