Improvements in identification and quantitation of alkylated PAHs and forensic ratio sourcing

Parent and alkylated polycyclic aromatic hydrocarbons (PAHs) are present in a number of different sources in varying proportions depending on the source material and weathering. This range of PAH sources can make it difficult to determine the origin of exposure(s). Ratios of alkylated and parent PAHs have been applied as a forensic tool to distinguish between different sources. However, few studies have examined PAH ratios comprehensively as indicators for sourcing beyond a single study area or matrix type. In this paper, we introduce an expanded analytical method based on ASTM D7363-13a which we adapted for a gas chromatography triple quadrupole mass spectrometry instrument. The modifications increase selectivity and sensitivity compared to the ASTM method. We added five alkylated series to the method. This method has then been applied to 22 independent forensic ratios. We evaluated the method and the forensic ratios with certified reference materials and known environmental samples. This analytical method and thirteen PAH ratios were found to accurately predict sources of PAHs. Graphical abstract

A principal component analysis of polycyclic aromatic hydrocarbon emission in NGC 2023

ABSTRACT We use the measured fluxes of polycyclic aromatic hydrocarbon (PAH) emission features at 6.2, 7.7, 8.6, 11.0, and 11.2 μm in the reflection nebula NGC 2023 to carry out a principal component analysis (PCA) as a means to study previously reported variations in the PAH emission. We find that almost all of the variations (99 per cent) can be explained with just two parameters – the first two principal components (PCs). We explore the characteristics of these PCs and show that the first PC (PC1), which is the primary driver of the variation, represents the amount of emission of a mixture of PAHs with ionized species dominating over neutral species. The second PC (PC2) traces variations in the ionization state of the PAHs across the nebula. Correlations of the PCs with various PAH ratios show that the 6.2 and 7.7 μm bands behave differently than the 8.6 and 11.0 μm bands, thereby forming two distinct groups of ionized bands. We compare the spatial distribution of the PCs to the physical conditions, in particular to the strength of the radiation field, G0, and the G0/nH ratio and find that the variations in PC2, i.e. the ionization state of PAHs are strongly affected by G0 whereas the amount of PAH emission (as traced by PC1) does not depend on G0.

Cold molecular gas and PAH emission in the nuclear and circumnuclear regions of Seyfert galaxies

We investigate the relation between the detection of the 11.3 μm polycyclic aromatic hydrocarbon (PAH) feature in the nuclear (∼24−230 pc) regions of 22 nearby Seyfert galaxies and the properties of the cold molecular gas. For the former we use ground-based (0.3−0.6″ resolution) mid-infrared (mid-IR) spectroscopy. The cold molecular gas is traced by ALMA and NOEMA high (0.2−1.1″) angular resolution observations of the CO(2–1) transition. Galaxies with a nuclear detection of the 11.3 μm PAH feature contain more cold molecular gas (median 1.6 × 107 M⊙) and have higher column densities (N(H2) = 2 × 1023 cm−2) over the regions sampled by the mid-IR slits than those without a detection. This suggests that molecular gas plays a role in shielding the PAH molecules in the harsh environments of Seyfert nuclei. Choosing the PAH molecule naphthalene as an illustration, we compute its half-life in the nuclear regions of our sample when exposed to 2.5 keV hard X-ray photons. We estimate shorter half-lives for naphthalene in nuclei without a 11.3 μm PAH detection than in those with a detection. The Spitzer/IRS PAH ratios on circumnuclear scales (∼4″ ∼ 0.25−1.3 kpc) are in between model predictions for neutral and partly ionized PAHs. However, Seyfert galaxies in our sample with the highest nuclear H2 column densities are not generally closer to the neutral PAH tracks. This is because in the majority of our sample galaxies, the CO(2–1) emission in the inner ∼4″ is not centrally peaked and in some galaxies traces circumnuclear sites of strong star formation activity. Spatially resolved observations with the MIRI medium-resolution spectrograph on the James Webb Space Telescope will be able to distinguish the effects of an active galactic nucleus (AGN) and star formation on the PAH emission in nearby AGN.

Investigating PAH relative reactivity using congener profiles, quinone measurements and back trajectories

Vapour and particle-associated concentrations of 15 polycyclic aromatic hydrocarbons (PAH) and 11 PAH quinones have been measured in winter and summer campaigns at the rural site, Weybourne in eastern England. Concentrations of individual PAH are relatively smaller than average concentrations measured previously at urban sites in the UK. The concentrations of PAH of the air masses originating from southern England and mainland UK are significantly larger than those from Eastern Europe and the North Atlantic, while quinone to parent PAH ratios show an inverse behaviour, being highest in the more aged North Atlantic polar air masses. While concentrations of 1,2-naphthoquinone decline from winter to summer, those of 1,4-naphthoquinone and anthraquinone increase suggesting a photochemical formation pathway. A comparison of congener concentration profiles measured at Weybourne with those from an urban source area (Birmingham) reveals differential losses at the rural site, especially evident in fluoranthene : pyrene ratios and consistent with the known rates of vapour phase reactions of 3 and 4 ring compounds with hydroxyl radical. The ratios of quinones to their parent PAH at Weybourne are greater than those in the urban source area indicating either more rapid loss processes for PAH, or formation of quinones during advection of the air mass, or probably both.

Investigating PAH relative reactivity using congener profiles, quinone measurements and back trajectories

Vapour and particle-associated concentrations of 15 polycyclic aromatic hydrocarbons (PAH) and 11 PAH quinones have been measured in winter and summer campaigns at the rural site, Weybourne in eastern England. Concentrations of individual PAH are 20–140 times smaller than average concentrations at an English urban site. The concentrations of PAH are greatest in air masses originating from southern England relative to those from Scandinavia and the North Atlantic, while quinone to parent PAH ratios show an inverse behaviour, being highest in the more aged North Atlantic polar air masses. While concentration of 1,2-naphthoquinone decline from summer to winter, those of 1,4-naphthoquinone and anthraquinone increase suggesting a photochemical formation pathway. A comparison of congener concentration profiles measured at Weybourne with those from an urban source area (Birmingham) reveals differential losses at the rural site, especially evident in fluoranthene: pyrene ratios and consistent with the known rates of vapour phase reactions of 3 and 4 ring compounds with hydroxyl radical. The ratios of quinones to their parent PAH at Weybourne are greater than those in the urban source area indicating either more rapid loss processes for PAH, or formation of quinones during advection of the air mass, or probably both.

Polycyclic aromatic hydrocarbons in the atmosphere of two French alpine valleys: Temporal trends and examination of sources

Alpine valleys represent some of the most important crossroads for international heavy duty traffic in Europe, but impact of this traffic on air quality is not well-known due to a lack of data in these complex systems. As part of the program "Pollution des Vallées Alpines'' (POVA), we performed two sampling surveys of PAHs in two sensitive valleys: the Chamonix and the Maurienne Valleys, between France and Italy. Sampling campaigns were performed during summer 2000 and winter 2001, both periods taking place during the closure of the "Tunnel du Mont-Blanc''. The objectives of this paper are first to describe the relations between PAH concentrations, external parameters (sampling site localization, meteorological parameters, sources), and the aerosol characteristics, including its carbonaceous fraction (OC and EC). The second objective is to study the capacity of the PAH profiles to accurately discriminate the different emissions sources. Temporal evolution of relative concentration of individual PAH (CHR), and PAH groups (BghiP+COR and BbF+BkF) are studied in order to discriminate wood combustion, gasoline, and diesel emissions, respectively. The results show that during both seasons the total particulate PAH concentrations were higher in the valley of Chamonix, despite the stopping of the international traffic through the Chamonix valley. Seasonal cycles, with higher concentrations in winter are also stronger in this valley. During winter, particulate PAH concentration can reach very high levels (155 ng.m−3) in this valley during cold anticyclonic periods. Sources examination shows the impact of heavy duty traffic in the Maurienne valley and of gasoline vehicles in the Chamonix Valley during summer. During winter, Chamonix is characterized by the strong influences wood combustion in residential fireplaces, even if temporal evolution of specific PAH ratios are uneasy to interpret. Information on sources given by PAH profiles can only be considered as qualitative.