Measurement of Gaseous Nitrophenols and Their Precursors Using a Vocus Proton-Transfer-Reaction Time-of-Flight Mass Spectrometer (PTR-ToF-MS)

Atmospheric nitrophenol compounds (NPs) affect air quality, radiation, and health. Real-time measurements of NPs are still limited, preventing an in-depth understanding of the dynamics of their emission and formation. In this study, we used a Vocus proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) to quantify gaseous NPs and related aromatic compounds. Calibrations using a permeation-tube method showed that NPs, phenols, and nitroaromatics had protonated ions as the dominating ion, except for 3-methyl-2-nitrophenol. The combined effect of the methyl, nitro, and phenolic functional groups in this compound may lead to its steady fragmentation (dehydration) in the PTR-ToF-MS. The 5-s limits of detection (LODs) of these nitrated and/or oxygenated aromatic species ranged from 0.0001 to 0.0015 ppbv, demonstrating the capability of PTR-ToF-MS in the quantification of these compounds. In a campaign conducted in a coastal urban area in South China (Macau), NPs had average concentrations of 0.005 to 0.016 ppbv, lower than most reported concentrations in other urban areas. In addition to possible primary sources such as burning of coal or biomass in some periods, secondary formation of NPs was revealed to be limited by the availability of NO2 and could outweigh photolysis loss. This study provides new insights into the quantification and sources of NPs and their precursors in urban areas for a better understanding of their effects on the ecosystem, climate, and health.