Aging of atmospheric organic aerosol during summertime in Nanjing: insights from on-line measurement

Real-time characterization of the aging of organic aerosol (OA) in the atmosphere is challenging in aerosol chemistry research. In this study, a continuous on-line monitoring instrument, the Aerosol Chemical Speciation Monitor (ACSM), was employed to measure non-refractory submicron aerosol (NR-PM1) characteristics during summertime in Nanjing. Ion fractions obtained from ACSM were used to evaluate the aging of atmospheric hydrocarbon-like OA (HOA) as a function of f 44 (the ratio of m/z 44 to total OA mass spectrometry signal) versus f 57 (the ratio of m/z 57 to total OA mass spectrometry signal) ( f 44 vs f 57). Results show that the mean mass concentration of NR-PM1 was 19.87 ± 8.46 μg m-3 for the entire study. OA constituted the largest component, on average accounting for 51.8% of the total NR-PM1. The oxygenated OA (OOA) on average accounted for the largest fraction (72% ± 0.14%) of OA. The diurnal trends of both the mass concentration of OOA and O x (O x = O3 + NO2) closely correlated with OOA/ΔCO ratios (ΔCO is the CO minus the background CO), indicating that OOA was largely effected by atmospheric photochemical activity. In the f 44 vs f 57 space, the OOA/ΔCO ratios and the mass concentration of O x increased with increasing f 44, while HOA/ΔCO and HOA/OA ratios decreased with increasing f 44, suggesting that HOA was gradually transformed to OOA by photochemical aging. Furthermore, the trend of f 57 implies that the aging time of HOA in the atmosphere was approximately 5-10 hours. This study not only provides a new method for long-lasting studies on the evolution of atmospheric organic aerosols, but also provides a new idea for the application of ACSM.