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

Abstract

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.

Chromophores and chemical composition of brown carbon characterized at an urban kerbside by excitation–emission spectroscopy and mass spectrometry

“…Therefore, we consider that LO-HULIS may be oxidized into HO-HULIS. This is consistent with other studies (Deng et al, 2022;.…”

“…In contrast, HO-HULIS and phenol-and naphthalene-like chromophores showed an anti-correlation with this diurnal evolution. Deng et al (2022) also found that LO-HULIS components contributed more at nighttime and HO-HULIS obviously increased in daytime during the winter in Tianjin, northern China. The characteristics of LO-HULIS are a low oxidizing state and high double bond equivalent.…”

Continuous Measurement and Molecular Compositions of Atmospheric Water‐Soluble Brown Carbon in the Nearshore Marine Boundary Layer of Northern China: Secondary Formation and Influencing Factors

“…Yuan et al, 2021), Yulin (Lei et al, 2018), and Tianjin (J. Deng et al, 2022) at the same wavelength. The time-series of hourly light absorption of WS-BrC is shown in Figure 3b.…”

“…For example, J. Deng et al (2022) revealed that wintertime BrC in Tianjin was predominately affected by equivalent contribution of fossil fuels combustion (30%) and biomass burning (30.7%), while the contribution of bioaerosol emission and biogenic Secondary Organic Aerosol (SOA) formation accounted for more than 60% of BrC in summer. Sources of BrC in northwest China also exhibited similar seasonal variations as those in Tianjin (Yuan et al, 2020).…”

Measurement report: Changes in light absorption and molecular composition of water-soluble humic-like substances during a winter haze bloom-decay process in Guangzhou, China

“…Therefore, the HULIS fractions exhibit relatively high absorption at UV and short visible wavelengths and relatively low absorption at long visible wavelengths, which results in relatively high AAE values. Moreover, the AAE values of HULIS did not present significant variation during the entire haze process, which could be related to the evolution of HULIS chromophores in different stages (Jiang et al, 2020;Huang et al, 2018;Deng et al, 2022). At first, the enhanced oxidation of aromatic species on haze days could lead to the bleaching or degradation of chromophores (a detailed explanation is provided in Sect.…”