HETEROGENEOUS REACTIONS OF OH RADICALS WITH PARTICULATE-PYRENE AND 1-NITROPYRENE OF ATMOSPHERIC INTEREST

Formation of Environmentally Persistent Free Radicals from the Irradiation of Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) provide a complex matrix for environmentally persistent free radicals (EPFRs) to stabilize in particulate matter, allowing them to be transported over long distances in the atmosphere while participating in light-driven reactions and causing various cardiopulmonary diseases. In this study, four PAHs ranging from three to five rings (anthracene, phenanthrene, pyrene, and benzo[e]pyrene) were investigated for EPFR formation upon photochemical and aqueous-phase aging. Through electron paramagnetic resonance (EPR) spectroscopy, it was found that approximately 10¹⁵ to 10¹⁶ spins g^-1 of EPFRs were formed from the PAH upon aging. EPR analysis also revealed that carbon-centered and monooxygen-centered radicals were predominantly formed by irradiation. However, oxidation and fused-ring matrices have added complexity to the chemical environment of these carbon-centered radicals, as observed by their g-values. This study showed that atmospheric aging results not only in the transformation of PAH-derived EPFR but also in an increase in EPFR concentrations of up to 10¹⁷ spins g^-1. Therefore, because of their stability and photosensitivity, PAH-derived EPFRs have a major impact on the environment.

Active iron species driven hydroxyl radicals formation in oxygenation of different paddy soils: Implications to polycyclic aromatic hydrocarbons degradation

The frequently occurring redox fluctuations in paddy soil are critical to the cycling of redox-sensitive elements (e.g., iron (Fe) and carbon) due to the driving of microbial processes. However, the associated abiotic process, such as hydroxyl radical (^•OH) formation, was rarely investigated. Hence, we examined the under-appreciated role of ^•OH formation in driving polycyclic aromatic hydrocarbons (PAHs) degradation upon oxygenation of anoxic paddy slurries. Results showed that ^•OH production largely differed in different paddy slurries, in the range of 271.5-581.2 μmol kg^-1 soil after 12 h reaction. The ^•OH production was highly hinged on the contents of active Fe species, i.e., exchangeable, surface-bound Fe and Fe in low-crystalline phases rather than Fe in high-crystalline minerals or silicates. Besides, ^•OH production significantly decreased with increasing soil depth due to the declined active Fe species and abundance of functional microbes. Oxygenation also induced the transformation of these active Fe species into the low- and high-crystalline phases, which might affect the following redox process. The produced ^•OH can efficiently degrade PAHs with degradation extents depending on their physiochemical properties. Our findings highlight the key roles of active Fe species in driving ^•OH formation and organic contaminants degradation during redox fluctuations of paddy soils.

PM2.5-bound polycyclic aromatic hydrocarbons (PAHs) and nitrated PAHs (NPAHs) in rural and suburban areas in Shandong and Henan Provinces during the 2016 Chinese New Year's holiday

Eighteen polycyclic aromatic hydrocarbons (PAHs) and fourteen nitrated PAHs (NPAHs) in PM_2.5 samples were collected during the 2016 Chinese New Year's holiday (CNY) at one suburban and three rural sites in Shandong and Henan Provinces. The PAH and NPAH concentrations were highest at the suburban site. The rural PAH concentrations in Qingzhou (QZ), Heze (HZ), and Liaocheng (LC) were higher than those measured at many other urban sites, indicating that PAHs pollution was notably higher in the suburban and rural sites during this festive period. Elevated PAH concentrations were observed during fireworks periods, but fireworks burning was not a significant or direct PAHs or NPAHs source based on molecular profiles and diagnostic ratios. The measured PAHs and NPAHs at the sampling sites mainly originated from coal and biomass burning. The increased concentrations during CNY's Eve may be related to behavioural changes during the period. Secondary formation of NPAHs mainly occurred via OH radical chemistry at all four sites. Fireworks burning did not increase secondary formation of NPAHs. ∑BaP_eq concentrations exhibited strong correlations with PAHs concentrations, and the highest and lowest concentrations were observed in QZ and Xiping (XP), respectively. The incremental lifetime cancer risk (ILCR) was calculated to be between 10^-6 and 10^-4 for 1-70 years old persons, with the highest risks observed in the adult (30-70 years) and the toddler (1-6 years) groups.

Evaluating Computational and Structural Approaches to Predict Transformation Products of Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) undergo transformation reactions with atmospheric photochemical oxidants, such as hydroxyl radicals (OH•), nitrogen oxides (NOx), and ozone (O3). The most common PAH-transformation products (PAH-TPs) are nitrated, oxygenated, and hydroxylated PAHs (NPAHs, OPAHs, and OHPAHs, respectively), some of which are known to pose potential human health concerns. We sampled four theoretical approaches for predicting the location of reactive sites on PAHs (i.e., the carbon where atmospheric oxidants attack), and hence the chemoselectivity of the PAHs. All computed results are based on density functional theory (B3LYP/6-31G(d) optimized structures and energies). The four approaches are (1) Clar's prediction of aromatic resonance structures, (2) thermodynamic stability of all OHPAH adduct intermediates, (3) computed atomic charges (Natural Bond order, ChelpG, and Mulliken) at each carbon on the PAH, and (4) average local ionization energy (ALIE) at atom or bond sites. To evaluate the accuracy of these approaches, the predicted PAH-TPs were compared to published laboratory observations of major NPAH, OPAH, and OHPAH products in both gas and particle phases. We found that the Clar's resonance structures were able to predict the least stable rings on the PAHs but did not offer insights in terms of which individual carbon is most reactive. The OHPAH adduct thermodynamics and the ALIE approaches were the most accurate when compared to laboratory data, showing great potential for predicting the formation of previously unstudied PAH-TPs that are likely to form in the atmosphere.

Effect of the blocked-sites phenomenon on the heterogeneous reaction of pyrene with N2O5/NO3/NO2

To clarify whether the blocking reaction sites problem has a significant impact on heterogeneous reactions, experiments contrasting the order of pyrene (PY) particles' exposure to N₂O₅–O₃ or O₃–N₂O₅ in a heterogeneous process were conducted.

Application of Direct Analysis in Real Time-Mass Spectrometry (DART-MS) to the study of gas-surface heterogeneous reactions: focus on ozone and PAHs

A novel analytical method is presented whereby Direct Analysis in Real Time-Mass Spectrometry (DART-MS) is applied to the study of gas-surface heterogeneous reactions. To illustrate the capabilities of the approach, the kinetics of a well-studied reaction of surface-bound polycyclic aromatic hydrocarbons with ozone are presented. Specifically, using helium as the reagent gas and with the DART heater temperature of 500 °C, nanogram quantities of benzo[e]pyrene (BeP) deposited on the outside of glass melting point capillary tubes were analyzed in positive ion mode with a limit of detection of 40 pg. Using bis(2-ethylhexyl) sebacate as an internal standard, the kinetics of the ozone-BeP reaction were assessed by determining the surface-bound BeP decays, after oxidation in an off-line reaction cell. The reaction is demonstrated to follow the Langmuir-Hinshelwood mechanism, known to prevail for heterogeneous reactions of this type. In addition, a wide array of oxygenated, condensed-phase products has been observed. The present work demonstrates the capability of the DART-MS technique to investigate the heterogeneous chemistry taking place on a wide range of surfaces, such as those that form in both outdoor and indoor environments.

Effects of humidity and [NO3]/[N2O5] ratio on the heterogeneous reaction of fluoranthene and pyrene with N2O5/NO3/NO2

Atmospheric 2-nitrofluoranthene (2-NFL) and 2-nitropyrene (2-NPY) were two important nitro-polycyclic aromatic hydrocarbons (NPAHs). Especially, 2-NFL was recognized to be the most abundant particle-associated NPAH (Ramdahl et al., 1986). In previous studies, these two products were observed in the gas-phase reaction between N2O5/NO3/NO2 and their parent polycyclic aromatic hydrocarbons (PAHs), while the heterogeneous reaction generated other nitro-PAH isomers (1, 3, 7, 8-NFL and 1-NPY) (Atkinson et al. 1990). To clarify the possible reasons for this difference, the heterogeneous reactions of suspended fluoranthene (FL) and pyrene (PY) particles under different relative humidity (RH; 0.5%-43%) and [NO3]/[N2O5] ratios were carried out. Under low humidity (0.5% RH) or a relatively high ratio of [NO3]/[N2O5], 2-NFL and 2-NPY were observed as the major nitro-FL isomers for the first time in the heterogeneous reaction. Decreasing the humidity or increasing the [NO3]/[N2O5] ratio in the reaction essentially increases the concentration radio of [NO3(g)]/[NO2(+)(aq)] on the particle surface (NO2(+) is derived from the ionization of N2O5). Thus, it can be concluded that under different atmospheric conditions, the change of [NO3(g)]/[NO2(+)(aq)] in the particle surface has an influence on the product distribution of FL and PY in the atmosphere. The experimental results provide evidence for the heterogeneous formations of particle-bound 2-NFL and 2-NPY. However, relative to the gas-phase formation, they will be negligible in the real atmosphere. 2-NFL and 2-NPY observed in the ambient particles should mainly derive from deposition of gas-phase reactions. Additionally, this study also clarifies the reason for different nitro-PAHs isomers observed between gas and particulate reactions.

Atmospheric reactions of 9,10-anthraquinone

The probably carcinogenic compound 9,10-anthraquinone is mainly existing in the atmosphere in the particulate phase and is often detected and measured among other oxygenated PAHs in atmospheric samples. Its fate, once released or formed in the atmosphere, still remains unknown. In this work, heterogeneous chemical oxidation processes of 9,10-anthraquinone were investigated with ozone (O3), nitrogen dioxide (NO2) and hydroxyl radical (OH). The study of 9,10-anthraquinone adsorbed on silica particles showed no reactivity with O3 and NO2. On the other hand, the reaction with OH radicals was observed and led to the formation of 1-hydroxy-9,10-anthraquinone, another oxidation product recognized as possibly carcinogenic to humans. This study showed that reactions with ozone and nitrogen dioxide are unlikely to contribute to atmospheric degradation of 9,10-anthraquinone, whereas reactions with OH radicals could be involved in 9,10-anthraquinone degradation processes, even if such reaction is probably very slow under ambient conditions.

Factors affecting atmospheric 1-, 2-nitropyrenes and 2-nitrofluoranthene in winter at Noto peninsula, a remote background site, Japan

Airborne particulates were collected at a background site (Wajima Air Monitoring Station; WAMS) on the Noto Peninsula, Japan from January 2006 to December 2007. 1-, 2-nitropyrenes (1-, 2-NPs) and 2-nitrofluoranthene (2-NFR), in the particulates were determined with a sensitive HPLC method with chemiluminescence detection. The average concentrations were higher in winter than in summer. A meteorological analysis indicated that the air samples collected in winter were transported mainly from Northeast China over the Japan Sea. Both the concentration ratios of 2-NFR to 1-NP and 1-NP to pyrene were similar to those in Shenyang in Northeast China which located along the air transportation route to WAMS, but not in Kanazawa which near WAMS. These results strongly suggest that most of the atmospheric 1-, 2-NPs and 2-NFR at WAMS in winter were long range transported from Northeast China.

Quantification of complex polycyclic aromatic hydrocarbon mixtures in standard reference materials using comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry

This research is the first to quantify complex PAH mixtures in NIST SRMs using comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC×GC/ToF-MS), with and without extract cleanup, and reports previously unidentified PAH congeners in the NIST SRMs. We tested a novel, high orthogonality GC column combination (LC-50×NSP-35), as well as with a commonly used column combination (Rtx-5ms×Rxi-17) for the quantification of a complex mixture of 85 different PAHs, including parent (PAHs), alkyl- (MPAHs), nitro- (NPAHs), oxy- (OPAHs), thio- (SPAHs), bromo- (BrPAHs), and chloro-PAHs (ClPAHs) in extracts from two standard reference materials: NIST SRM1650b (diesel particulate matter), with cleanup and NIST SRM1975 (diesel particulate extract), with and without extract cleanup. The LC-50×NSP-35 column combination resulted in an average absolute percent difference of 33.8%, 62.2% and 30.8% compared to the NIST certified PAH concentrations for NIST SRM1650b, NIST SRM1975 with cleanup and NIST SRM1975 without cleanup, while the Rtx-5ms×Rxi-17 resulted in an absolute percent difference of 38.6%, 67.2% and 79.6% for NIST SRM1650b, NIST SRM1975 with cleanup and NIST SRM1975 without cleanup, respectively. This GC×GC/ToF-MS method increases the number of PAHs detected and quantified in complex environmental extracts using a single chromatographic run. Without clean-up, 7 additional compounds were detected and quantified in NIST SRM1975 using the LC-50×NSP-35 column combination. These results suggest that the use of the LC-50×NSP-35 column combination in GC×GC/ToF-MS not only results in better chromatographic resolution and greater orthogonality for the separation of complex PAH mixtures, but can also be used for the accurate quantification of complex PAH mixtures in environmental extracts, such as diesel particulate matter, without silica gel cleanup.

Homogeneous and heterogeneous reactions of anthracene with selected atmospheric oxidants

The reactions of gas-phase anthracene and suspended anthracene particles with O3 and O3-NO were conducted in a 200-L reaction chamber, respectively. The secondary organic aerosol (SOA) formations from gas-phase reactions of anthracene with O3 and O3-NO were observed. Meanwhile, the size distributions and mass concentrations of SOA were monitored with a scanning mobility particle sizer (SMPS) during the formation processes. The rapid exponential growths of SOA reveal that the atmospheric lifetimes of gas-phase anthracene towards O3 and O3-NO are less than 20.5 and 4.34 hr, respectively. The particulate oxidation products from homogeneous and heterogeneous reactions were analyzed with a vacuum ultraviolet photoionization aerosol time-of-flight mass spectrometer (VUV-ATOFMS). Gas chromatograph/mass spectrometer (GC/MS) analyses of oxidation products of anthracene were carried out for assigning the time-of-flight (TOF) mass spectra of products from homogeneous and heterogeneous reactions. Anthrone, anthraquinone, 9,10-dihydroxyanthracene, and 1,9,10-trihydroxyanthracene were the ozonation products of anthracene, while anthrone, anthraquinone, 9-nitroanthracene, and 1,8-dihydroxyanthraquinone were the main products of anthracene with O3-NO.

Diurnal/nocturnal concentrations and sources of particulate-bound PAHs, OPAHs and NPAHs at traffic and suburban sites in the region of Paris (France)

Particulate concentrations of polycyclic aromatic compounds (PACs) including, 17 polycyclic aromatic hydrocarbons (PAHs), 9 oxygenated PAHs (OPAHs) and 18 nitrated PAHs (NPAHs) were determined at traffic and suburban sites located in the region of Paris. A 12 h sampling basis time resolution was applied in order to study their diurnal and nocturnal variations. Observed concentrations were about 10 times higher at the traffic site for all compounds and were higher during the night-time for both sites (except for NPAHs at the traffic site). No significant differences in PAH and OPAH profiles were observed at both sites whereas, for NPAHs, 1-nitropyrene (diesel source) was the most abundant at the traffic site and 2+3-nitrofluoranthene (secondary formed by gas-phase reaction) was predominant at the suburban site. The study of the specific ratio 2-nitrofluoranthene/1-nitropyrene (2-NFlt/1-NP) showed a local formation of NPAHs in gaseous phase at the suburban site. A detailed analysis showed that atmospheric humidity and rainfalls modified differently PAH and NPAH profiles, in comparison to OPAH. A difference of the scale variability of water solubility between, light (MW≤228 g mol(-1)) and heavy compounds (MW≥273 g mol(-1)), could explain these observations. The specific study of the relationships between PACs and other measured pollutants highlighted that particle resuspension could constitute a significant source of PM on the traffic site. Even if NPAH formation seemed clearly evident at the suburban site during periods characterised by high O(3) and NO(2) concentration levels, results showed also that the primary and/or secondary origins of OPAHs and NPAHs were strongly dependent on the sampling site and on sampling conditions. Finally, we conclude that higher time sampling resolutions would be helpful in investigating the atmospheric chemistry and behaviours of PACs in correlation with the local meteorological variations and the daily cycle of human activities.

Improved separation of complex polycyclic aromatic hydrocarbon mixtures using novel column combinations in GC × GC/ToF-MS

Complex mixtures of polycyclic aromatic hydrocarbons (PAHs) are difficult to resolve because of the high degree of overlap in compound vapor pressures, boiling points, and mass spectral fragmentation patterns. The objective of this research was to improve the separation of complex PAH mixtures (including 97 different parent, alkyl-, nitro-, oxy-, thio-, chloro-, bromo-, and high molecular weight PAHs) using GC × GC/ToF-MS by maximizing the orthogonality of different GC column combinations and improving the separation of PAHs from the sample matrix interferences, including unresolved complex mixtures (UCM). Four different combinations of nonpolar, polar, liquid crystal, and nanostationary phase columns were tested. Each column combination was optimized and evaluated for orthogonality using a method based on conditional entropy that considers the quantitative peak distribution in the entire 2D space. Finally, an atmospheric particulate matter with diameter <2.5 μm (PM(2.5)) sample from Beijing, China, a soil sample from St. Maries Creosote Superfund Site, and a sediment sample from the Portland Harbor Superfund Site were analyzed for complex mixtures of PAHs. The highest chromatographic resolution, lowest synentropy, highest orthogonality, and lowest interference from UCM were achieved using a 10 m × 0.15 mm × 0.10 μm LC-50 liquid crystal column in the first dimension and a 1.2 m × 0.10 mm × 0.10 μm NSP-35 nanostationary phase column in the second dimension. In addition, the use of this column combination in GC × GC/ToF-MS resulted in significantly shorter analysis times (176 min) for complex PAH mixtures compared to 1D GC/MS (257 min), as well as potentially reduced sample preparation time.

Kinetic studies of heterogeneous reactions of polycyclic aromatic hydrocarbon aerosols with NO₃ radicals

Polycyclic aromatic hydrocarbons (PAHs) and their derivates are mutagenic and carcinogenic substances widely distributed in the atmospheric environment. In this study, effective rate constants for heterogeneous reactions of NO(3) radicals with five 4-ring PAHs [benzo[a]anthracene (BaA), chrysene (Ch), pyrene (Py), 1-nitropyrene (1-NP), and 1-hydroxypyrene (1-OHP)] adsorbed on suspended azelaic acid particles are investigated by a mixed-phase relative rate method with gas-phase isoprene as the reference substance. The concentrations of suspended PAH particles and gas-phase isoprene are monitored concurrently by a real-time vacuum ultraviolet photoionization aerosol time-of-flight mass spectrometer (VUV-ATOFMS) and an online atmospheric gas analysis mass spectrometer, respectively. The obtained effective rate constants for the reactions of BaA, Ch, Py, 1-NP, and 1-OHP particles with NO(3) radicals are 4.3 × 10(-12), 4.0 × 10(-12), 6.4 × 10(-12), 1.3 × 10(-12), and 1.0 × 10(-11) cm(3)·molecule(-1)·s(-1), respectively, and their corresponding atmospheric lifetimes range from several minutes to half an hour at the NO(3) radical concentration of 5 × 10(8) molecules·cm(-3). In addition, the NO(3) uptake coefficients on particulate PAHs are estimated according to the consumption of PAHs under the exposure of NO(3) radicals. The experimental results of these heterogeneous reactions in the aerosol state provide supplementary knowledge for kinetic behaviors of airborne PAHs particles.