Isomeric Identification of Particle-Phase Organic Nitrates through Gas Chromatography and Time-of-Flight Mass Spectrometry Coupled with an Electron Capture Negative Ionization Source.

Abstract

Organic nitrates (ONs) are an important component of secondary organic aerosols that play significant roles in atmospheric chemical processes such as ozone formation and as a reservoir of nitrogen oxides (NOx). However, hindered by the availability of analytical techniques, characteristics of ON molecules remain unclear in regions influenced by anthropogenic volatile organic compounds (VOCs) and pollution. In this study, we achieved isomeric identification of particle-phase ONs in such regions. Using gas chromatography and time-of-flight mass spectrometry with an electron capture negative ionization source, we established a systematic procedure for screening unknown ONs in fine particulate matter (PM) collected in Beijing based primarily on the characteristic fragment ions of NO2- and [M-NO2]-/[M-NO2-H2]-. We found 78 ON candidates, 12 of which were confirmed using synthesized standards. Seventy-three of these detected ONs might originate from anthropogenic VOC precursors especially alkenes. Significantly, we observed two isomers generated from straight-chain 1-alkenes, namely, 2-hydroxy-1-nitrate and 1-hydroxy-2-nitrate. The signal ratios of the two isomers suggested that these hydroxy nitrates are mainly produced photochemically rather than through nighttime reactions. This study provides a promising method for identifying ONs in atmospheric PM and elucidating their formation pathways.