Atmospheric aerosol has received much attention due to its role in global climate change, visibility degradation, and adverse health effects. Chemical composition provides crucial information for the understanding of physicochemical properties, behaviors, and sources of ambient aerosol. Ultra high resolution mass spectrometry is being widely used to analyze complex organic matter in various environments. In this study, its application to the detailed chemical speciation and associated physicochemical properties were conducted for of organic aerosol formed secondarily in the laboratory and collected in the atmosphere.
The molecular composition of secondary organic aerosol (SOA), obtained from the ozonolysis and photooxidation of α-pinene, was investigated using an ultrahigh-resolution Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) in negative ion mode electrospray ionization (ESI) mode. SOA formation was performed in KNU indoor smog chamber. The molecular formulae of individual species were identified on the basis of the measured ionic mass using general principles, such as number of atoms, elemental ratios, and the nitrogen rule. In each of the SOAs obtained, 815-3501 monomeric and oligomeric (mainly dimeric) species were identified below m/z 800. From ozonolysis, mainly 95% of the typical oxygenated species (CHO) were detected, whereas from photooxidation under high NOx concditions, 32% of nitrogen-containing species (CHON) were detected. Several common abundant species (e.g., C9H14O6, C10H14O6, C10H16O5, C17H26O7, C19H28O9, C10H15NO8, and C10H15NO9) could be listed as candidate tracers for the conventional tracers for α-pinene SOA. The increased percentage of CHON as a primary effect of NOx on the SOA composition evidently affected other physicochemical parameters, such as elemental ratios (i.e., O/C, H/C, and N/C), the double-bond equivalent (DBE), the carbon oxidation state (COS), and the organic-mass-to-carbon ratio (OM/OC). The O/C, OM/OC, and COS for CHON were apparently greater than those observed for CHO, indicating that nitrogen preferentially exists in the oxidized form (e.g., -ONO2). The complexity of oligomerization was observed in DBE and OM/OC according to the number of carbon atoms.
Additional SOA formation experiments were performed using KNU flow reactor. The effect of NH3 on the composition of α-pinene SOA under ozonolysis and photooxidation was studied. SOA was analyzed using a combination of 15 T FT-ICR MS, Orbitap, and qudrupole-time of flight mass spectrometry. Terebic and pinonic acids were major species found among 54 species. Several other chemical species were also identified by MS/MS. During MS/MS analysis, cleaved structures were H2O, CO, CO2, and CH2CO2. The concentration of individual species ranged from 0.10 to 21 mg/L. Recovery by LC-MS analysis was in the range between 19.69 and 100.69%. Among these, the recovery of OH radical oxidation time for 3 days was the highest at 100.69%. In the quantification of CHON higher uncertainties were present due to the limitation of authentic standard compounds.
PM2.5 samples were daily collected using a Hi-Vol sampler at Olympic Park site of Seoul and Baengnyeongdo site during the KORUS-AQ field campaign in 2016. The organic fraction partitioned in water and n-hexane termed as high polarity (HPOM) and low polarity organic matter (LPOM), respectively, were analyzed using 15 T FT-ICR MS equipped with an atmospheric pressure photoionization. Each identified species were classified based on atomic constituents. In Seoul, CH, CHO, CHON, CHOS, and CHONS group was 0.56%, 50.5%, 48.6%, 0.19%, and 0.12% in HPOM and 4.57%, 84.0%, 10.0%, 0.79%, and 0.57% in LPOM, respectively. In Baengnyeongdo, CH, CHO, CHON, CHOS, and CHONS group was 0.51%, 61.4%, 37.8%, 0.18%, and 0.14% in HPOM and 5.95%,79.5%, 12.7%, 0.54%, and 1.35% in LPOM, respectively. In Seoul, H/C, O/C, and N/C ratios were 1.42, 0.36, and 0.04 for HPOM and 1.68, 0.13, and 0.01 for LPOM, respectively. In Baengnyeongdo, H/C, O/C, and N/C ratios were 1.31, 0.35, and 0.03 for HPOM and 1.63, 0.11, and 0.01 for LPOM, respectively. The identified molecular formulae were also used to determine chemical properties of carbon oxidation state and OM/OC ratio. Furthermore UV-vis absorption and fluorescene were measured for those samples, mass absorption efficiency (MAE) of Seoul was lower than that of Baekryeong Island. Compared with Japan and Colorado Rocky Mountains, MAE and AAE300-400 were low, and absorption Angstrom exponent (AAE)300-400 was similar to Beijing, China. The compounds measured in Seoul and Baekryeong Island are believed to be derived from aromatic compounds (e.g. benzene, toluene, and xylene), and aromatic derivatives with double bonds, carbonyl, and carboxylic acids typically absorbing UV radiation in the range the samples were FI, HIX, BIX, peak b, M, A, and C. Contribution of protein, fulvic acid, humic acid, and meaty humic acid organic compounds were accounted using PARAFAC analysis of excitation emission matrix with the help of fluorescence characteristics (i.e., FI, HIX, BIX, and paak b, M, A, and C).