Molecular Absorption and Evolution Mechanisms of PM_2.5 Brown Carbon Revealed by Electrospray Ionization Fourier Transform–Ion Cyclotron Resonance Mass Spectrometry During a Severe Winter Pollution Episode in Xi'an, China

Abstract

Knowledge of the molecular-level chemistry of brown carbon (BrC) is important in reducing the uncertainties in aerosol radiative forcing. Time-resolved ambient PM 2.5 samples were collected during a severe pollution episode in January 2017 over Xi'an, China for a comprehensive nontarget and full scanning of BrC molecules and their absorption properties using electrospray ionization Fourier transform-ion cyclotron resonance mass spectrometry combined with partial least squares regression analysis, which apportioned the overall ultraviolet absorption to individual molecules. The estimated absorption of CHNO and CHNOS molecules exhibited nighttime prevalence, whereas CHOS, CHNS, CHN, CHO, CHS, and CH molecules presented a dynamic trend. Carbon conjugation was positively correlated with estimated absorption by CHO and CHNO molecules, while exhibiting a mixed relationship with CHNOS. Higher nitrogen content was associated with enhanced light-absorption properties of BrC molecules, while higher oxygen and sulfur content appeared to be associated with photobleaching during secondary transformation.Plain Language Summary Individual molecular absorption has significant implications for ambient brown carbon analysis, but existing studies mostly focused on bulk absorption properties for the sample as a whole. A novel method is presented here to evaluate individual brown carbon absorption at the molecular level through high resolution mass spectrometry, and partial least squares regression analysis was applied to PM 2.5 samples collected during a heavy pollution episode in Xi'an, China. Nitrogen-containing compounds were found to be the strongest contributor to brown carbon absorption, especially during the nighttime, while processes involving oxidation and sulfur addition appeared to weaken the molecular absorption. This study bridges the gap between real atmospheric PM 2.5 absorption and knowledge from controlled laboratory studies, which support these findings.

The Roles of N, S, and O in Molecular Absorption Features of Brown Carbon in PM2.5 in a Typical Semi‐Arid Megacity in Northwestern China

“…The gases of SO 2 , O 3 , NO 2 , and NH 3 were active in secondary BrC transformation, but this important point was not elucidated in our previous study on BrCM absorption (Zeng et al., 2020). During the sampling periods, concentrations of O 3 , NO 2 , and NH 3 were simultaneously measured by an O 3 monitor (Ecotech, EC9810, Knoxfield, Australia), nitrogen oxides analyzer (Thermal, 42i, Franklin, MA, USA), and NH 3 analyzer (Thermal, 17i, Franklin, MA, USA), respectively, at a time resolution of 5 min.…”

“…Compared with the winter data presented in Zeng et al. (2020), sampling in summer was added to gain a full understanding of BrC in Xi'an because the large discrepancies of emissions, diffusion, and transformation between the two seasons (Du et al., 2013; Shen et al., 2008; Xu et al., 2015). Highly time‐resolved PM 2.5 samples were collected on each sampling day with time‐intervals of 00:00–03:00, 03:00–07:00, 07:00–10:00, 10:00–13:00, 13:00–18:00, 18:00–21:00, and 21:00–00:00 LST.…”

“…Secondary BrC calculation was published in our previous study and the details are listed in Text S2. Secondary BrC was formed through atmospheric chemical reactions and reflected the dynamic nature of BrC, and such analysis was neglected in our previous study (Zeng et al, 2020).…”

“…PM 2.5 samples were collected on pre-combusted quartz filters (780°C for 5 h) using a mid-volume (50 L min −1 ) air sampler (HVS-PM 2.5 , Thermo-Anderson Inc., Waltham, MA, USA) from January 1 to January 6 (winter) and from July 24 to August 5 (summer), 2017. Compared with the winter data presented in Zeng et al (2020), sampling in summer was added to gain a full understanding of BrC in Xi'an because the large discrepancies of emissions, diffusion, and transformation between the two seasons (Du et al, 2013;Shen et al, 2008;Xu et al, 2015). Highly time-resolved PM 2.5 samples were collected on each sampling day with time-intervals of 00:00-03:00, 03:00-07:00, 07:00-10:00, 10:00-13:00, 13:00-18:00, 18:00-21:00, and 21:00-00:00 LST.…”

“…It has been successfully applied to coefficient obtainment in organic carbon, ammonium, and organic functional groups quantifications in aerosols (Takahama & Dillner, 2015; Weakley et al., 2016). In addition, PLSR was used to reveal the primary statistical features of molecular absorbance in winter ambient BrC (Zeng et al., 2020).…”