Measurement report: Online measurement of gas-phase nitrated phenols utilizing a CI-LToF-MS: primary sources and secondary formation

Abstract

To investigate the composition, variation, and sources of nitrated phenols (NPs) in the winter of Beijing, gas-phase NPs were measured by a chemical ionization long time-of-flight mass spectrometer (CI-LToF-MS). A box model was applied to simulate the secondary formation process of NPs. In addition, the primary sources of NPs were resolved by a non-negative matrix factorization (NMF) model. Our results showed that secondary formation contributed 38 %, 9 %, 5 %, 17 %, and almost 100 % of the nitrophenol (NP), methyl-nitrophenol (MNP), dinitrophenol (DNP), methyl-dinitrophenol (MDNP or DNOC), and dimethyl-nitrophenol (DMNP) concentrations. The phenol–OH reaction was the predominant loss pathway (46.7 %) during the heavy pollution episode, which produced the phenoxy radical (C6H5O). The phenoxy radical consequently reacted with NO2 and produced nitrophenol. By estimating the primarily emitted phenol from the ratio of phenol/CO from freshly emitted vehicle exhaust, this study proposed that oxidation of primary phenol contributes much more nitrophenol (37 %) than that from benzene oxidation (<1 %) in the winter of Beijing. The latter pathway was widely used in models and might lead to great uncertainties. The source apportionment results by NMF indicated the importance of combustion sources (>50 %) to the gas-phase NPs. The industry source contributed 30 % and 9 % to DNP and MDNP, respectively, which was non-negligible. The concentration weighted trajectory (CWT) analysis demonstrated that regional transport from provinces that surround the Yellow and Bohai seas contributed more primary NPs to Beijing. Both primary sources and secondary formation at either local or regional scale should be considered when making control policies of NPs.