Time‐Resolved Measurements of PM2.5 Chemical Composition and Brown Carbon Absorption in Nanjing, East China: Diurnal Variations and Organic Tracer‐Based PMF Analysis

• Published in: Journal of geophysical research. Atmospheres 2023-09, Vol.128 (18), p.n/a
• Main Authors: Feng, Wei, Wang, Xinyue, Shao, Zhijuan, Liao, Hong, Wang, Yuhang, Xie, Mingjie
• Format: Article
• Language: English
• Subjects: Absorption; Absorption coefficient; Absorptivity; Aerosol absorption; Aerosols; Atmospheric processes; Biomass; Biomass burning; brown carbon; Burning; Carbon; Chemical composition; Chromophores; Climate models; Coefficients; Components; Diameters; diurnal variation; Diurnal variations; Electromagnetic absorption; Geophysics; Levoglucosan; Light; Light absorption; Methanol; Organic carbon; Organic chemistry; organic molecular marker; Particulate emissions; Particulate matter; Particulate matter sources; Receptors; source apportionment; Speciation; Summer; Suspended particulate matter; Time measurement; Tracers; Valleys; Water; Winter
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1029/2023JD039092

To understand diurnal variations in PM2.5 composition and aerosol extract absorption, PM2.5 samples were collected at intervals of 2 hr from 8:00 to 20:00 and 6 hr from 20:00 to 8:00 (the next day) in northern Nanjing, China, during the winter and summer of 2019–2020 and analyzed for bulk components, organic tracers, and light absorption of water and methanol extracts—a proxy measure of brown carbon (BrC). Diurnal patterns of measured species reflected the influences of primary emissions and atmospheric processes. Light absorption coefficients of water (Abs365,w) and methanol extracts (Abs365,m) at 365 nm shared a similar diurnal profile peaking at 18:00–20:00, generally following changes in biomass burning tracers. However, Abs365,w, Abs365,m, and their normalizations to organic aerosols increased at 14:00–16:00, earlier than that of levoglucosan in the late afternoon, which was attributed to secondarily formed BrC. The methanol extracts showed a less drastic decrease in light absorption at night than the water extracts and elevated absorption efficiency during 2:00–8:00. This is due to the fact that the water‐insoluble OC has a longer lifetime and stronger light absorption than the water‐soluble OC. According to the source apportionment results solved by positive matrix factorization (PMF), biomass burning and secondary formation were the major BrC sources in northern Nanjing, with an average total relative contribution of about 90%. Compared to previous studies, diurnal source cycles were added to the PMF simulations in this work by using time‐resolved speciation data, which avoided misclassification of BrC sources. Plain Language Summary Light‐absorbing organic carbon (OC), also known as “brown carbon”, plays an important role in influencing the Earth's radiative balance. Filter samples of particulate matter with an aerodynamic diameter of less than 2.5 μm (PM2.5) were collected at a resolution of 2 hr from 8:00 to 20:00 and a resolution of 6 hr from 20:00 to 8:00 (the next day) in northern Nanjing, China, during the winter and summer of 2019 and 2020. To understand the diurnal variations of BrC and its sources, each sample was analyzed for bulk components (OC, elemental carbon, and water‐soluble ions), organic molecular markers, and light absorption by BrC, and a receptor model was used to identify potential BrC sources based on PM2.5 composition data. We found that diurnal variations of BrC generally followed changes in concentrations of levoglucosan