ABSTRACT
Nitroaromatic compounds (NACs) are important nitrogen organics in aerosol with strong light-absorbing and chemically reactive properties. In this study, NACs in six Chinese megacities, including Harbin (HB), Beijing (BJ), Xi'an (XA), Wuhan (WH), Chengdu (CD), and Guangzhou (GZ), were investigated for understanding their sources, gas-particle partitioning, and impact on BrC absorption properties. The concentrations of ΣNACs in PM2.5 in the six cities ranged from 9.15 to 158.8 ng/m3 in winter and from 2.02 to 9.39 ng/m3 in summer. Nitro catechols (NCs), nitro phenols (NPs), and nitro salicylic acids (NSAs) are the main components in ΣNACs, with NCs being dominant in particulate phase and NPs being dominant in the gas phase. Correlation analysis between different pollutant species revealed that coal and biomass combustions were the major sources of NACs in the northern cities during wintertime, while secondary formation dominated NACs in the southern cities during summertime. The contribution of ΣNACs to brown carbon (BrC) light absorption ranged from 0.85 to 7.98 % during the wintertime and 2.07-6.44 % during the summertime. The mass absorption efficiency at 365 nm (MAE365) were highest for 4-nitrocatechol (4NC, 17.4-89.0 m2/g), 4-methyl-5-nitrocatechol (4M5NC, 15.0-76.9 m2/g), and 4-nitroguaiacol (4NG, 11.7-59.8 m2/g). The formation of NCs and NG through oxidation and nitration of catechol and guaiacol led to a significant increase in aerosol light absorption. In contrast, NPs and NSAs formed by the photonitration and photooxidation in liquid phase showed high polarity but low light absorption ability, and the proportions of (NPs + NSAs) in the light absorption of ΣNACs were lower than 15.3 % in the six megacities.
ABSTRACT
Particulate brown carbon (BrC) plays a crucial role in the global radiative balance due to its ability to absorb light. However, the effect of molecular formation on the light absorption properties of BrC remains poorly understood. In this study, atmospheric BrC samples collected from six Chinese megacities in winter and summer were characterized through ultrahigh-performance liquid chromatography coupled with Orbitrap mass spectrometry (UHPLC-Orbitrap MS) and light absorption measurements. The average values of BrC light absorption coefficient at a wavelength of 365 nm (babs365) in winter were approximately 4.0 times higher than those in summer. Nitrogen-containing organic molecules (CHNO) were identified as critical components of light-absorbing substances in both seasons, underscoring the importance of N-addition in BrC. These nitrogen-containing BrC chromophores were more closely related to nitro-containing compounds originating from biomass burning and nitrogen oxides (NOx)/nitrate (NO3-) reactions in winter. In summer, they were related to reduced N-containing compounds formed in ammonia (NH3)/ammonium (NH4+) reactions. The NH3/NH4+-mediated reactions contributed more to secondary BrC in summer than winter, particularly in southern cities. Compared with winter, the higher O/Cw, lower molecule conjugation indicator (double bond equivalent, DBE), and reduced BrC babs365 in summer suggest a possible bleaching mechanism during the oxidation process. These findings strengthen the connection between molecular composition and the light-absorbing properties of BrC, providing insights into the formation mechanisms of BrC chromophores across northern and southern Chinese cities in different seasons.