Large contribution of non-priority PAHs in atmospheric fine particles: Insights from time-resolved measurement and nontarget analysis.

Polycyclic aromatic hydrocarbons (PAHs), detrimental to human health, are key components contributing to the carcinogenicity of fine particles. The 16 priority PAHs listed by the United States Environment Protection Agency have been studied extensively. However, other than them, there is a large diversity of PAH species, whose atmospheric concentrations, risks, and variations remain elusive. Here, we carried out a time-resolved nontarget measurement in atmospheric PM2.5 using an improved comprehensive two-dimensional gas chromatography mass spectrometry. The measurement conducted during a 5-day pollution episode at an urban site of Beijing with a time resolution of 2 h. The nontarget analysis of time-resolved chromatographic data was performed for screening PAHs. A total number of 85 PAHs were identified and quantified. We found that other than 16 EPA PAHs, other screened PAHs contributed 43.3% of the total PAH mass concentration and 40.8% poential health risks. Dynamic variations of mass concentrations and their potential health risks of the screened PAHs were captured during a short-term heavy pollution episode, during which the instantaneous PAHs concentrations were much higher than their average concentrations. This study shows the potential for application of nontarget analysis for online comprehensive two-dimensional gas chromatography mass spectrometry and highlights the importance of time-resolved measurement of PAHs in PM2.5 and attention on extended PAHs species other than 16 EPA PAHs.

Dynamic variations of phthalate esters in PM2.5 during a pollution episode.

Phthalate esters (PAEs) as hazardous air pollutants can be easily released during the life cycle of plastic products. In this study, a thermal desorption aerosol comprehensive two-dimensional gas chromatography mass spectrometer coupled with a dual-trap was developed and used to measure the hourly-resolved PAEs characteristics in atmospheric PM2.5 at an urban site. Dimethyl phthalate (DMP), diethyl (DEP), dibutyl (DnBP), benzyl butyl (BBP), di(2-ethylhexyl) (DEHP), and di-n-octyl phthalate (DnOP) in PM2.5 were analyzed. The most abundant compounds were DEHP and DMP, followed by DnBP and DEP. The mass concentrations of the detected PAEs are comparable to those at other urban sites measured using offline methods with a lower time resolution. The concentrations of PAEs showed intense change with the variation of PM2.5 mass concentration. The proportion of DEHP increased while that of DMP decreased with the increase in PM2.5 pollution. Positive correlations between PAEs and PM2.5, organic carbon, and elemental carbon were observed, while PAEs had negative correlation with the ambient temperature. Our observation provides evidences on understanding the volatile and semi-volatile PAEs in the ambient aerosols.

Comprehensive two-dimensional gas chromatography mass spectrometry with a solid-state thermal modulator for in-situ speciated measurement of organic aerosols.

Thermal desorption aerosol gas chromatography mass spectrometry is capable of online measuring speciated organics in atmospheric aerosols. Compared to the one-dimensional gas chromatography, comprehensive two-dimensional gas chromatography increases the resolution and the sensitivity, mitigates the unresolved complex mixture and co-elution occurred in one-dimensional gas chromatography. In this study, we report a quartz filter-based thermal desorption aerosol comprehensive two-dimensional gas chromatography mass spectrometry (2D-Q-TAG). It combines a solid-state thermal modulator with a quartz filter-based thermal desorption aerosol gas chromatography mass spectrometry. The solid-state thermal modulator conducts modulation independently from the chromatographic oven without using cryogens or compressed air, which makes the system readily adaptive for field measurement. The 2D-Q-TAG was evaluated using C7-C40 n-alkanes and 16 polycyclic aromatic hydrocarbons (PAHs). It has low limits of detection from 0.001 to 0.104 ng. The instrument was then deployed to measure atmospheric PM2.5 (particulate matter with an equivalent aerodynamic diameter  ≤  2.5 µm) in urban Beijing. It allows in-situ detection of speciated organics in atmospheric aerosols with hourly time resolution. Organic classes including alkanes, furanones, alcohols, aldehydes, ketones, acids, PAHs, oxy-PAHs, and alkyl-naphthalenes were well separated and detected. The total mass concentration of n-alkanes ranged from 122.1 to 629.9 ng/m3. Diurnal variation of n-alkanes was captured due to the high time resolution of the instrument.

Quartz filter-based thermal desorption gas chromatography mass spectrometry for in-situ molecular level measurement of ambient organic aerosols.

Thermal desorption aerosol gas chromatography (TAG) is an effective technique for online chemical characterization of organics in atmospheric fine particulate matter (PM2.5) on the molecular level. Unlike the previously reported impactor- or metal filter-based TAG, in this study, a replaceable quartz filter-based TAG (Q-TAG) was developed to achieve online measurement of organic constituents in ambient PM2.5. A homemade device for automatic online sampling and pretreatment of aerosols was developed and coupled with gas chromatography-mass spectrometry (GC-MS) by a 4-port valve. Performance of the Q-TAG system was evaluated using C7 - C40 n-alkanes to cover a wide range of volatility. C11 - C40 could be measured by Q-TAG. The response of their peaks depends on their volatility and thermal desorption conditions (the desorption time and the flow rate). Under the optimized conditions, good precision (<12%), acceptable linearity (R2> 0.98) and high sensitivity (detection limits from 0.02 to 0.01 ng) of C13 - C40 were obtained. The developed Q-TAG system was applied for online analysis of organic species in ambient PM2.5. The Q-TAG is suitable for detection of semi and low volatile organic species in particulate matter, and its filter can be easily changed, making it a useful tool in molecular characterization of ambient organic aerosols.

Improving thermal desorption aerosol gas chromatography using a dual-trap design.

Thermal desorption aerosol gas chromatography (TAG) is an effective tool for in situ analysis of particulate organic molecules. However, the performance of current TAG is limited by the detectability of low volatile compounds and the matrix effect. In this study, a dual-trap TAG system was developed to address these issues. Thermally desorbed effluent is focused by a weakly retained trap (for low volatile compounds) in a 1 m capillary column conditioned in the GC oven, followed by a strongly retained trap (for high volatile compounds). Then, the focused analytes are desorbed in a reverse flow into the GC column for analysis. Detection over a wide volatility range from C10 to C40 n-alkanes is achieved using the dual-trap TAG. We show that it has lower discrimination of injection, better linearity and higher detectability of n-alkanes. The dual-trap TAG was applied for in-situ measurement of ambient fine particles (PM2.5) in Beijing. Repeatable retention time of n-alkanes was demonstrated during a continuous measurement over two weeks.