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 particles (PM2.5) were collected during 2016–2017 on a day/night basis over urban Tianjin, a megacity in northern China. Light absorption and fluorescence properties of water extracts of PM2.5 were investigated to obtain seasonal and diurnal patterns of atmospheric water-soluble BrC. There were obvious seasonal, but no evident diurnal, variations in the light absorption properties of BrC. In winter, BrC showed much stronger light-absorbing ability, with a mass absorption efficiency at 365 nm (MAE365) in winter (1.54±0.33 m2 gC−1) that was 1.8 times larger than MAE365 in summer (0.84±0.22 m2 gC−1). Direct radiative effects by BrC absorption relative to black carbon in the UV range were 54.3±16.9 % and 44.6±13.9 % in winter and summer, 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 less 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), suggests 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 more to BrC in winter and at nighttime, while biogenic SOA contributed more in summer and during the daytime. In particular, our study highlights that primary bioaerosol emission is an important source of BrC in urban Tianjin in summer.

Stable Isotopes Reveal Photoreduction of Particle-Bound Mercury Driven by Water-Soluble Organic Carbon during Severe Haze

“…In addition, the fluorescent components of WSOC were measured utilizing an excitation-emission matrix (EEM) fluorometer (Aqualog, HORIBA) within the excitation and emission wavelength ranges of 240−550 nm and 246−828 nm, respectively. 52,53 The EEM fluorescence spectra data were analyzed by the parallel factor (PARAFAC) model performed with the software package Solo (Eigenvector Inc.). 52,54 Water-soluble species including SO 4 2− , NO 3 − , and NH 4 + were analyzed using an ion chromatography system (ICS-5000+, Thermo).…”

“…The analytical errors for OC, EC, and WSOC concentrations were within ±10% through a duplicate analysis of each filter. In addition, the fluorescent components of WSOC were measured utilizing an excitation-emission matrix (EEM) fluorometer (Aqualog, HORIBA) within the excitation and emission wavelength ranges of 240–550 nm and 246–828 nm, respectively. , The EEM fluorescence spectra data were analyzed by the parallel factor (PARAFAC) model performed with the software package Solo (Eigenvector Inc.). , Water-soluble species including SO 4 2– , NO 3 – , and NH 4 + were analyzed using an ion chromatography system (ICS-5000+, Thermo). The relative water content (RWC) of particles was calculated as RWC (%) = 100% × ( W I – W D )/ W I , where W I (g) and W D (g) are the initial weight of the collected particles and the dry weight of the particles after being freeze-dried, respectively.…”

Enhanced behaviors of optical properties and the radiative effects of molecular-specific brown carbon from dung combustion in the Tibetan Plateau

“…In Fig. S6b, our SFEBrC values from burning residential solid fuels over the TPL region were lower than paddy-residue burning over the Indo-Gangetic Plain (Choudhary et al, 2021) and rice straw burning in Anhui Province, China (Zhao et al, 2022), but they were comparable with those from the burning of maize straw in Shaanxi province (Lei et al, 2018b) and from ambient aerosols over heavily polluted northern Chinese cities, including Tianjin, Erenhot, Zhangbei and Jinan Deng et al, 2022). These phenomena confirmed that the heavy BrC loadings from residential fuels can cause high radiative forcing effects over TPL regions.…”

“…The method based on "SFE" can effectively assess the potential direct radiative forcing of BrC light absorption emitted from solid fuel combustion in this study. However, the radiation effects caused by aerosol scattering were ignored in the estimation and only the absorption was considered (Deng et al, 2022). Therefore, the formula for calculating radiative forcing in the range of 280-700 nm can be simplified as follows (Lei et al, 2018b;Zhang et al, 2020):…”

Impact of biogenic SOA loading on the molecular composition of wintertime PM2.5 in urban Tianjin: an insight from Fourier transform ion cyclotron resonance mass spectrometry

“…These results indicate that secondary CHO organic aerosols have obvious bias in the formation processes. At low SOA load, especially monoterpene derivatives, secondary CHO organic aerosols are dominated by conjugated polyene compounds with low oxygen content and high saturation, while with the increased of SOA load, multi-step oxidation formed the O-rich compounds containing monophenyl ring, which may be important light-absorbing chromophores in the atmosphere (Deng et al, 2022).…”