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

Kai Song,Zichao Wan,Shiyong Xia,Rui Tan,Ruizhe Shen,Xin Li,Hui Wang,Haichao Wang,Wenfei Zhu,Rongzhi Tang,Song Guo,Xiaofeng Huang,Ying Yu,Xuena Yu,Shiyi Chen,Liming Zeng,Daqi Lv,Yuanzheng Gong

doi:10.5194/acp-21-7917-2021

Abstract

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.

Highlights

Nitrated phenols (NPs) refer to aromatic compounds with at least a hydroxyl (–OH) group and a nitro (–NO2) group
The measurement started with a heavy pollution episode from 1 to 2 December, with an average wind speed of 0.61 m s−1, an average Relative humidity (RH) of 63 %, and average concentration of PM2.5, NOy, and CO of 166 μg m−3, 118 ppb, and 1912 ppb, respectively
A box model was utilized to simulate the secondary formation of nitrated phenols (NPs). 38 %, 9 %, 5 %, 17 %, and almost 100 % of the ambient nitrophenol (NP), MNP, DNP, MDNP, and DMNP could be explained by the oxidation of aromatic precursors

Summary

Introduction

Nitrated phenols (NPs) refer to aromatic compounds with at least a hydroxyl (–OH) group and a nitro (–NO2) group. They are crucial species in forest decline (Grosjean and Williams, 1992; Qingguo Huang et al, 1995). Song et al.: Gas-phase nitrated phenols in Beijing since the 1980s (Harrison et al, 2005) Among these studies, gas-phase NPs were detected in urban, suburban, and remote regions (Mohr et al, 2013; Morville et al, 2006; Priestley et al, 2018). Most of the studies in Beijing focused on particle-phase NPs (or so-called nitro-aromatic compounds, NACs) (Li et al, 2020; Wang et al, 2019). It is of vital importance to identify the concentration and sources of NPs in Beijing

Methods

Results

Conclusion