Optical properties and molecular compositions of water-soluble and water-insoluble brown carbon (BrC) aerosols in northwest China

Abstract

Abstract. Brown carbon (BrC) contributes significantly to aerosol light absorption and thus can affect the Earth's radiation balance and atmospheric photochemical processes. In this study, we examined the light absorption properties and molecular compositions of water-soluble (WS-BrC) and water-insoluble (WI-BrC) BrC in PM2.5 collected from a rural site in the Guanzhong Basin – a highly polluted region in northwest China. Both WS-BrC and WI-BrC showed elevated light absorption coefficients (Abs) in winter (4–7 times those in summer) mainly attributed to enhanced emissions from residential biomass burning (BB) for heating of homes. While the average mass absorption coefficients (MACs) at 365 nm (MAC365) of WS-BrC were similar between daytime and nighttime in summer (0.99±0.17 and 1.01±0.18 m2 g−1, respectively), the average MAC365 of WI-BrC was more than a factor of 2 higher during daytime (2.45±1.14 m2 g−1) than at night (1.18±0.36 m2 g−1). This difference was partly attributed to enhanced photochemical formation of WI-BrC species, such as oxygenated polycyclic aromatic hydrocarbons (OPAHs). In contrast, the MACs of WS-BrC and WI-BrC were generally similar in winter and both showed few diel differences. The Abs of wintertime WS-BrC correlated strongly with relative humidity, sulfate and NO2, suggesting that aqueous-phase reaction is an important pathway for secondary BrC formation during the winter season in northwest China. Nitrophenols on average contributed 2.44±1.78 % of the Abs of WS-BrC in winter but only 0.12±0.03 % in summer due to faster photodegradation reactions. WS-BrC and WI-BrC were estimated to account for 0.83±0.23 % and 0.53±0.33 %, respectively, of the total down-welling solar radiation in the ultraviolet (UV) range in summer, and 1.67±0.72 % and 2.07±1.24 %, respectively, in winter. The total absorption by BrC in the UV region was about 55 %–79 % relative to the elemental carbon (EC) absorption.

Chemical composition, optical properties, and oxidative potential of water- and methanol-soluble organic compounds emitted from the combustion of biomass materials and coal

“…In this study, 1,4-phenanthraquinone was used to conduct a positive control, of which the DTT consumption rate was 0.46 ± 0.03 µM DTT/min (n = 10). The rate was similar to those reported in the previous studies (Fan et al, 2018;Lin and Yu, 2019). Finally, principal component analysis (PCA) was performed to investigate the key factors that may affect the DTT activities from a series of characteristics of the BrC fraction.…”

Photochemical Degradation of Organic Matter in the Atmosphere

“…Their optical properties could be modified following photochemical processing with both photobleaching (the loss of light absorptivity) and photo enhancement possibly occurring. [77,79] The formation and degra dation of BrC by photochemical processes may complicate the understanding of atmospheric BrC. [79] A series of researches has focused on changes in light absorptivity and oxidation state of BrC under illumination and O 3 and •OH radicals (abundant during the daytime).…”

“…[77,79] The formation and degra dation of BrC by photochemical processes may complicate the understanding of atmospheric BrC. [79] A series of researches has focused on changes in light absorptivity and oxidation state of BrC under illumination and O 3 and •OH radicals (abundant during the daytime). Dasari et al [35] found that atmospheric photooxidation reduced the light absorptivity (i.e., photobleaching) of WSBrC by ≈84% during longrange transport over the altitude of 6000 km in the IndoGangetic Plain, with a firstorder bleaching rate of 0.20 ± 0.05 per day during overocean travel to an Indian Ocean receptor site across Bay of Bengal.…”

Hygroscopicity of Water‐Soluble PM2.5 in Rural Northwest China: Contrasting Contributors Between Summer and Winter

“…Detailed information of the sampling campaign was described by J. Li et al (2020). Briefly, daytime (08:00-20:00) and nighttime (20:00-08:00 next morning) PM 2.5 samples were collected on pre-combusted (450°C for 6 h) quartz filters (Whatman QM/A) using a high-volume air sampler (TISCH, TE6070DV-BL, 1.13 m 3 •min −1 ) during August 3-23, 2016 (summer, 42 samples) and January 20-February.…”

“…The details for the sample extraction procedures and chemical analysis were provided in previous publications (J. Li et al, 2014Li et al, , 2020Steven et al, 2016;T. Zhang et al, 2011).…”