Direct detection of polycyclic aromatic hydrocarbons on a molecular composition level in summertime ambient aerosol via proton transfer reaction mass spectrometry

Abstract

Condensed particulate polycyclic aromatic hydrocarbons (PAHs) are a group of toxic organic compounds that are produced by the incomplete combustion of organic material, for example, via biomass burning or traffic emissions. Even at low long-term exposure levels, such as 1 ng m−3 of benzo(a)pyrene, PAHs are recognized to be detrimental to human health. Therefore, a quantitative characterization of PAHs at sub-nanogram-per-cubic-meter levels is important to examine precise long-term exposure. A new ultrasensitive generation of proton transfer reaction mass spectrometry (PTR-MS) instruments coupled to the CHARON particle inlet are capable of quantitatively detecting this toxic class of compounds at a molecular composition level while also offering a high temporal resolution of < 1 min and sub-nanogram-per-cubic-meter limits of detection. To demonstrate the capabilities of this new CHARON FUSION PTR-TOF 10k instrument, we present a thorough characterization of summertime ambient condensed PAHs in Innsbruck, Austria. With a mass resolution of > 14 000 (m Δm−1 at full width at half maximum) and sensitivities of up to 40 cps ng−1 m3 (where cps represents counts per second), a series of nine condensed PAHs of four (C16H10) to six aromatic rings (C26H16) are identified among a plethora of organic compounds in ambient organic aerosol. With 1 min limits of detection between 19 and 46 pg m−3, quantitative time series of these PAHs at the lowermost mass concentrations are determined. To understand the sources and processes associated with these condensed summertime PAHs in greater detail, a matrix factorization including the ∼ 4000 ionic signals detected by the CHARON FUSION PTR-TOF 10k is performed, representing the vast majority of the mass concentration of ambient organic aerosol. A total of 10 factors and their corresponding time series can be identified. Known tracer compounds like levoglucosan, pinonic acid or nicotine consequently allow the assignment to individual organic aerosol sources and physicochemical processes. PAH emissions from traffic are found to be minor contributors during this summertime sampling period. The highest concentrations of PAHs are identified in a mixed aged oxygenated organic aerosol, followed by a biomass burning and a cigarette smoke organic aerosol.