Direct Sampling and Analysis of Atmospheric Particulate Organic Matter by Proton-Transfer-Reaction Mass Spectrometry

Advances in analysis of atmospheric ultrafine particles and application in air quality, climate, and health research

There is growing interest in the contribution of ultrafine particles to air quality, climate, and human health. Ultrafine particles are of central significance for the influence of radiative forcing of climate change by involving in the formation of clouds and precipitation. Moreover, exposure to ultrafine particles can enhance the disease burden. The determination of those effects of ultrafine particles strongly depends on their chemical composition and physicochemical properties. This review focuses on the advanced techniques for the characterization of chemical composition and physicochemical properties of ultrafine particles in the past five years. The current analytical methodologies are broadly classified into electron and X-ray microscopy, optical spectroscopy and microscopy, electrical mobility, and mass spectrometry, and then described and discussed its operation principle, advantages, and limitations. Besides measurements, application of the state-of-the-art techniques is briefly reviewed to help us to promote a better understanding of atmospheric ultrafine particles relevant to air quality, climate, and health.

Clair JM, Trost B, Flynn JH, Savarino J, Conner LD, Kettle N, Heeringa KM, Albertin S, Baccarini A, Barret B, Battaglia MA, Bekki S, Brado T, Brett N, Brus D, Campbell JR, Cesler-Maloney M, Cooperdock S, Cysneiros de Carvalho K, Delbarre H, DeMott PJ, Dennehy CJ, Dieudonné E, Dingilian KK, Donateo A, Doulgeris KM, Edwards KC, Fahey K, Fang T, Guo F, Heinlein LMD, Holen AL, Huff D, Ijaz A, Johnson S, Kapur S, Ketcherside DT, Levin E, Lill E, Moon AR, Onishi T, Pappaccogli G, Perkins R, Pohorsky R, Raut JC, Ravetta F, Roberts T, Robinson ES, Scoto F, Selimovic V, Sunday MO, Temime-Roussel B, Tian X, Wu J, Yang Y. Overview of the Alaskan Layered Pollution and Chemical Analysis (ALPACA) Field Experiment

The Alaskan Layered Pollution And Chemical Analysis (ALPACA) field experiment was a collaborative study designed to improve understanding of pollution sources and chemical processes during winter (cold climate and low-photochemical activity), to investigate indoor pollution, and to study dispersion of pollution as affected by frequent temperature inversions. A number of the research goals were motivated by questions raised by residents of Fairbanks, Alaska, where the study was held. This paper describes the measurement strategies and the conditions encountered during the January and February 2022 field experiment, and reports early examples of how the measurements addressed research goals, particularly those of interest to the residents. Outdoor air measurements showed high concentrations of particulate matter and pollutant gases including volatile organic carbon species. During pollution events, low winds and extremely stable atmospheric conditions trapped pollution below 73 m, an extremely shallow vertical scale. Tethered-balloon-based measurements intercepted plumes aloft, which were associated with power plant point sources through transport modeling. Because cold climate residents spend much of their time indoors, the study included an indoor air quality component, where measurements were made inside and outside a house to study infiltration and indoor sources. In the absence of indoor activities such as cooking and/or heating with a pellet stove, indoor particulate matter concentrations were lower than outdoors; however, cooking and pellet stove burns often caused higher indoor particulate matter concentrations than outdoors. The mass-normalized particulate matter oxidative potential, a health-relevant property measured here by the reactivity with dithiothreiol, of indoor particles varied by source, with cooking particles having less oxidative potential per mass than pellet stove particles.

Application of Secondary Electrospray Ionization Coupled with High-Resolution Mass Spectrometry in Chemical Characterization of Thermally Generated Aerosols

Inhalation as a route for administering drugs and dietary supplements has garnered significant attention over the past decade. We performed real-time analyses of aerosols using secondary electrospray ionization (SESI) technology interfaced with high-resolution mass spectrometry (HRMS), primarily developed for exhaled breath analysis with the goal to detect the main aerosol constituents. Several commercially available inhalation devices containing caffeine, melatonin, cannabidiol, and vitamin B12 were tested. Chemical characterization of the aerosols produced by these devices enabled detection of the main constituents and screening for potential contaminants, byproducts, and impurities in the aerosol. In addition, a programmable syringe pump was connected to the SESI-HRMS system to monitor aerosolized active pharmaceutical ingredients (APIs) such as chloroquine, hydroxychloroquine, and azithromycin. This setup allowed us to detect caffeine, melatonin, hydroxychloroquine, chloroquine, and cannabidiol in the produced aerosols. Azithromycin and vitamin B12 in the aerosols could not be detected; however, our instrument setup enabled the detection of vitamin B12 breakdown products that were generated during the aerosolization process. Positive control was realized by liquid chromatography-HRMS analyses. The compounds detected in the aerosol were confirmed by exact mass measurements of the protonated and/or deprotonated species, as well as their respective collision-induced dissociation tandem mass spectra. These results reveal the potential wide application of this technology for the real-time monitoring of aerosolized active pharmaceutical ingredients that can be administered through the inhalation route.

Atmospheric Chemistry of N-Methylmethanimine (CH3N═CH2): A Theoretical and Experimental Study

The OH-initiated photo-oxidation of N-methylmethanimine, CH₃N=CH₂, was investigated in the 200 m³ EUPHORE atmospheric simulation chamber and in a 240 L stainless steel photochemical reactor employing time-resolved online FTIR and high-resolution PTR-ToF-MS instrumentation and in theoretical calculations based on quantum chemistry results and master equation modeling of the pivotal reaction steps. The quantum chemistry calculations forecast the OH reaction to primarily proceed via H-abstraction from the =CH₂ group and π-system C-addition, whereas H-abstraction from the −CH₃ group is a minor route and forecast that N-addition can be disregarded under atmospheric conditions. Theoretical studies of CH₃N=CH₂ photolysis and the CH₃N=CH₂ + O₃ reaction show that these removal processes are too slow to be important in the troposphere. A detailed mechanism for OH-initiated atmospheric degradation of CH₃N=CH₂ was obtained as part of the theoretical study. The photo-oxidation experiments, obstructed in part by the CH₃N=CH₂ monomer–trimer equilibrium, surface reactions, and particle formation, find CH₂=NCHO and CH₃N=CHOH/CH₂=NCH₂OH as the major primary products in a ratio 18:82 ± 3 (3σ-limit). Alignment of the theoretical results to the experimental product distribution results in a rate coefficient, showing a minor pressure dependency under tropospheric conditions and that can be parametrized k(T) = 5.70 × 10^–14 × (T/298 K)^3.18 × exp(1245 K/T) cm³ molecule^–1 s^–1 with k₂₉₈ = 3.7 × 10^–12 cm³ molecule^–1 s^–1. The atmospheric fate of CH₃N=CH₂ is discussed, and it is concluded that, on a global scale, hydrolysis in the atmospheric aqueous phase to give CH₃NH₂ + CH₂O will constitute a dominant loss process. N₂O will not be formed in the atmospheric gas phase degradation, and there are no indications of nitrosamines and nitramines formed as primary products.

Online Aerosol Chemical Characterization by Extractive Electrospray Ionization-Ultrahigh-Resolution Mass Spectrometry (EESI-Orbitrap)

Current mass spectrometry techniques for the online measurement of organic aerosol (OA) composition are subjected to either thermal/ionization-induced artifacts or limited mass resolving power, hindering accurate molecular characterization. Here, we combined the soft ionization capability of extractive electrospray ionization (EESI) and the ultrahigh mass resolution of Orbitrap for real-time, near-molecular characterization of OAs. Detection limits as low as tens of ng m^-3 with linearity up to hundreds of μg m^-3 at 0.2 Hz time resolution were observed for single- and mixed-component calibrations. The performance of the EESI-Orbitrap system was further evaluated with laboratory-generated secondary OAs (SOAs) and filter extracts of ambient particulate matter. The high mass accuracy and resolution (140 000 at m/z 200) of the EESI-Orbitrap system enable unambiguous identification of the aerosol components' molecular composition and allow a clear separation between adjacent peaks, which would be significantly overlapping if a medium-resolution (20 000) mass analyzer was used. Furthermore, the tandem mass spectrometry (MS²) capability provides valuable insights into the compound structure. For instance, the MS² analysis of ambient OA samples and lab-generated biogenic SOAs points to specific SOA precursors in ambient air among a range of possible isomers based on fingerprint fragment ions. Overall, this newly developed and characterized EESI-Orbitrap system will advance our understanding of the formation and evolution of atmospheric aerosols.

Bulk Organic Aerosol Analysis by Proton-Transfer-Reaction Mass Spectrometry: An Improved Methodology for the Determination of Total Organic Mass, O:C and H:C Elemental Ratios, and the Average Molecular Formula

We have recently shown in this journal (Müller et al. Anal. Chem. 2017 , 89 , 10889 - 10897 ) how a proton-transfer-reaction mass spectrometry (PTR-MS) analyzer measured particulate organic matter in urban atmospheres using the "Chemical Analysis of Aerosol Online" (CHARON) inlet. Our initial CHARON studies did not take into account fragmentation of protonated analyte molecules, which introduced a small but significant negative bias in the determination of bulk organic aerosol parameters. Herein, we studied the ionic fragmentation of 26 oxidized organic compounds typically found in atmospheric particles. This allowed us to derive a correction algorithm for the determination of the bulk organic mass concentration, m_OA, the bulk-average hydrogen-to-carbon ratio, (H:C)_bulk, the bulk-average oxygen-to-carbon ratio, (O:C)_bulk, and the bulk-average molecular formula, MF_bulk. The correction algorithm was validated against AMS data using two sets of published data. Finally, we determined MF_bulk of particles generated from the reaction of α-pinene and ozone and compared and discussed the results in relation to the literature.