Abstract
Abstract. Analysing the composition of ambient ultrafine particles (UFPs) is a challenging task due to the low mass and chemical complexity of small particles, yet it is a prerequisite for the identification of particle sources and the assessment of potential health risks. Here, we show the molecular characterization of UFPs, based on cascade impactor (Nano-MOUDI) samples that were collected at an air quality monitoring station near one of Europe's largest airports, in Frankfurt, Germany. At this station, particle-size-distribution measurements show an enhanced number concentration of particles smaller than 50 nm during airport operating hours. We sampled the lower UFP fraction (0.010–0.018, 0.018–0.032, 0.032–0.056 µm) when the air masses arrived from the airport. We developed an optimized filter extraction procedure using ultra-high-performance liquid chromatography (UHPLC) for compound separation and a heated electrospray ionization (HESI) source with an Orbitrap high-resolution mass spectrometer (HRMS) as a detector for organic compounds. A non-target screening detected ∼200 organic compounds in the UFP fraction with sample-to-blank ratios larger than 5. We identified the largest signals as homologous series of pentaerythritol esters (PEEs) and trimethylolpropane esters (TMPEs), which are base stocks of aircraft lubrication oils. We unambiguously attribute the majority of detected compounds to jet engine lubrication oils by matching retention times, high-resolution and accurate mass measurements, and comparing tandem mass spectrometry (MS2) fragmentation patterns between both ambient samples and commercially available jet oils. For each UFP stage, we created molecular fingerprints to visualize the complex chemical composition of the organic fraction and their average carbon oxidation state. These graphs underline the presence of the homologous series of PEEs and TMPEs and the appearance of jet oil additives (e.g. tricresyl phosphate, TCP). Targeted screening of TCP confirmed the absence of the harmful tri-ortho isomer, while we identified a thermal transformation product of TMPE-based lubrication oil (trimethylolpropane phosphate, TMP-P). Even though a quantitative determination of the identified compounds is limited, the presented method enables the qualitative detection of molecular markers for jet engine lubricants in UFPs and thus strongly improves the source apportionment of UFPs near airports.
Highlights
Ultrafine particles (UFPs) are particles with a diameter of less than 100 nm that are strongly influenced by primary emissions in the urban environment (Allan et al, 2010)
Studies indicate that UFPs have an influence on humans via different mechanisms compared to PM2.5 and are able to create synergetic effects with transition metals (Costa and Dreher, 1997; Campen et al, 2001; World Health Organization, 2013)
We found that Compound Discoverer falsely interprets ion signals as [M + H]+ of the measured ions [M + Na]+, [M + K]+, and [M + NH4]+ of the pentaerythritol esters and suggests up to three likely false molecular formulas per ester molecule
Summary
Ultrafine particles (UFPs) are particles with a diameter of less than 100 nm that are strongly influenced by primary emissions in the urban environment (Allan et al, 2010). Current observations indicate that airports are major sources of UFPs (Yu et al, 2012; Keuken et al, 2015; Yu et al, 2017; Fushimi et al, 2019), and road traffic and (biogenic) secondary organic aerosol are possible sources for nanoparticles contributing to the overall ultrafine-particle mass in urban areas (Zhu et al, 2002a, b; Morawska et al, 2008; Paasonen et al, 2016, Rönkkö et al, 2017). Studies indicate that UFPs have an influence on humans via different mechanisms compared to PM2.5 and are able to create synergetic effects with transition metals (Costa and Dreher, 1997; Campen et al, 2001; World Health Organization, 2013). The molecular composition of ambient UFPs and their effects on human health are only poorly understood, primarily because of a lack of the deployment of specific and sensitive chemical measurement approaches
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