Abstract
Abstract. To investigate the characteristics of atmospheric brown carbon (BrC) in the semiarid region of East Asia, PM2.5 and size-resolved particles in the urban atmosphere of Xi'an, inland China, during the winter and summer of 2017 were collected and analyzed for optical properties and chemical compositions. Methanol extracts (MeOH extracts) were more light-absorbing than water extracts (H2O extracts) in the optical wavelength of 300–600 nm and well correlated with nitrophenols, polycyclic aromatic hydrocarbons (PAHs) and oxygenated PAHs (r > 0.78). The light absorptions (absλ=365 nm) of H2O extracts and MeOH extracts in winter were 28±16 and 49±32 M m−1, respectively, which are about 10 times higher than those in summer, mainly due to the enhanced emissions from biomass burning for house heating. Water-extracted BrC predominately occurred in the fine mode (< 2.1 µm) during winter and summer, accounting for 81 % and 65 % of the total absorption of BrC, respectively. The light absorption and stable carbon isotope composition measurements showed an increasing ratio of absλ=365 nm-MeOH to absλ=550 nm-EC along with an enrichment of 13C in PM2.5 during the haze development, indicating an accumulation of secondarily formed BrC (e.g., nitrophenols) in the aerosol aging process. Positive matrix factorization (PMF) analysis showed that biomass burning, fossil fuel combustion, secondary formation, and fugitive dust are the major sources of BrC in the city, accounting for 55 %, 19 %, 16 %, and 10 % of the total BrC of PM2.5, respectively.
Highlights
Brown carbon (BrC) is a vital fraction of carbonaceous aerosols and exhibits strong absorption ability from the nearultraviolet (UV) to visible light region
This study investigated the seasonality of the light absorption characteristics of BrC in Xi’an
The strong correlation of levoglucosan with BrC and the diagnostic ratios of levoglucosan / mannosan and levoglucosan / galacosan revealed that the wintertime abundant BrC in Xi’an was mainly derived from the residential biofuel combustion for house heating in the city and its surrounding region
Summary
Brown carbon (BrC) is a vital fraction of carbonaceous aerosols and exhibits strong absorption ability from the nearultraviolet (UV) to visible light region. Multiple approaches have been developed to quantify the light absorption properties of BrC (Moosmuller et al, 2009), and a common and sensitive approach is the direct measurement of spectrophotometric properties of aerosol water or filter extracts by using optical instrumentation The advantage of this method can avert interference from insoluble absorption material (e.g., black carbon) (Cheng et al, 2016; Shen et al, 2017) and supply a high-resolution spectrum over a wide wavelength coverage. Xi’an is a metropolitan city located in Guanzhong Basin of inland China, which is a typical semiarid region in East Asia and has been suffering from serious particle pollution due to the large emission of anthropogenic pollutants (Wu et al, 2018; Wang et al, 2016; Wu et al, 2019), especially intensive coal combustion and biomass burning in winter for house heating (Wang et al, 2017) In this study, both PM2.5 and sizesegregated aerosol samples in Xi’an were collected during the 2017 winter and summer and analyzed for the characteristics of BrC. We firstly investigated the seasonal variations in chemical composition and light absorption of BrC in the city, discussed the size distribution of BrC and the impact of aerosol aging process on BrC, and quantified its source contributions
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