Detailed Source-Specific Molecular Composition of Ambient Aerosol Organic Matter Using Ultrahigh Resolution Mass Spectrometry and 1H NMR

Size-Resolved Molecular Characterization of Water-Soluble Organic Matter in Atmospheric Particulate Matter from Northern China

Atmospheric particulate matter (PM) affects visibility, climate, biogeochemical cycles and human health. Water-soluble organic matter (WSOM) is an important component of PM. In this study, PM samples with size-resolved measurements at aerodynamic cut-point diameters (Dp) of 0.01-18μm were collected in the rural area of Baoding and the urban area of Dalian, Northern China. Non-targeted analysis was adopted for the characterization of the molecule constitutes of WSOM in different sized particles using Fourier transform-ion cyclotron resonance mass spectrometry. Regardless of the location, the composition of WSOM in Aitken mode particles (aerodynamic diameter < 0.05 μm) was similar. The WSOM in accumulation mode particles (0.05-2 μm) in Baoding was predominantly composed of CHO compounds (84.9%), which were mainly recognized as lignins and lipids species. However, S-containing compounds (64.2%), especially protein and carbohydrates species, accounted for most of the WSOM in the accumulation mode particles in Dalian. The CHO compounds (67.6%-79.7%) contributed the most to the WSOM in coarse mode particles (> 2 μm) from both sites. Potential sources analysis indicated the WSOM in Baoding were mainly derived from biomass burning and oxidation reactions, while the WSOM in Dalian arose from coal combustion, oxidation reactions, and regional transport.

Development of an automatic measurement system using atmospheric pressure photoionization ultrahigh-resolution mass spectrometry and application for on-line analysis of particulate matter

On-line chemical characterization of atmospheric particulate matter (PM) with soft ionization technique and ultrahigh-resolution Mass Spectrometry (UHRMS) provides molecular information of organic constituents in real time. Here we describe the development and application of an automatic measurement system that incorporates PM2.5 sampling, thermal desorption, atmospheric pressure photoionization, and UHRMS analysis. Molecular formulas of detected organic compounds were deducted from the accurate (±10 ppm) molecular weights obtained at a mass resolution of 100,000, allowing the identification of small organic compounds in PM2.5. Detection efficiencies of 28 standard compounds were determined and we found a high sensitivity and selectivity towards organic amines with limits of detection below 10 pg. As a proof of principle, PM2.5 samples collected off-line in winter in the urban area of Beijing were analyzed using the Ionization Module and HRMS of the system. The automatic system was then applied to conduct on-line measurements during the summer time at a time resolution of 2 hr. The detected organic compounds comprised mainly CHON and CHN compounds below 350 m/z. Pronounced seasonal variations in elemental composition were observed with shorter carbon backbones and higher O/C ratios in summer than that in winter. This result is consistent with stronger photochemical reactions and thus a higher oxidation state of organics in summer. Diurnal variation in signal intensity of each formula provides crucial information to reveal its source and formation pathway. In summary, the automatic measurement system serves as an important tool for the on-line characterization and identification of organic species in PM2.5.

Soil Organic Matter Molecular Composition Shifts Driven by Forest Regrowth or Pasture after Slash-and-Burn of Amazon Forest

Slash-and-burn of Amazon Forest (AF) for pasture establishment has increased the occurrence of AF wildfires. Recent studies emphasize soil organic matter (SOM) molecular composition as a principal driver of post-fire forest regrowth and restoration of AF anti-wildfire ambience. Nevertheless, SOM chemical shifts caused by AF fires and post-fire vegetation are rarely investigated at a molecular level. We employed pyrolysis-gas chromatography-mass spectrometry to reveal molecular changes in SOM (0-10, 40-50 cm depth) of a slash-burn-and-20-month-regrowth AF (BAF) and a 23-year Brachiaria pasture post-AF fire (BRA) site compared to native AF (NAF). In BAF (0-10 cm), increased abundance of unspecific aromatic compounds (UACs), polycyclic aromatic hydrocarbons (PAHs) and lipids (Lip) coupled with a depletion of polysaccharides (Pol) revealed strong lingering effects of fire on SOM. This occurs despite fresh litter deposition on soil, suggesting SOM minimal recovery and toxicity to microorganisms. Accumulation of recalcitrant compounds and slow decomposition of fresh forest material may explain the higher carbon content in BAF (0-5 cm). In BRA, SOM was dominated by Brachiaria contributions. At 40-50 cm, alkyl and hydroaromatic compounds accumulated in BRA, whereas UACs accumulated in BAF. UACs and PAH compounds were abundant in NAF, possibly air-transported from BAF.

Unexpected molecular diversity of brown carbon formed by Maillard-like reactions in aqueous aerosols

Atmospheric brown carbon (BrC) exerts a key impact on the global radiative balance due to its light-absorbing properties. Maillard-like reactions between carbonyl and amino compounds have been identified as an important pathway for forming secondary BrC. Although optical properties have been widely studied, the molecular composition of secondary BrC generated in Maillard chemistry remains unclear, resulting in a knowledge gap to understand its formation and light-absorbing mechanism. In this study, a combination of optical spectroscopy, ¹H nuclear magnetic resonance (NMR), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was employed to comprehensively characterize the chemical and light-absorbing characteristics of secondary BrC. The results indicate that both the light-absorbing and molecular characteristics of secondary BrC were highly related to the structures of their precursors. Organic amine precursors consistently result in enhanced light-absorbing capacities of BrC compared to ammonium, but have inconsistent effects on the molecular diversity of BrC. Compared to amino precursors (i.e., glycine, ethylamine, propylamine, and ammonium), carbonyl precursors play a more important role in determining the molecular diversity of BrC. Different from black carbon, the light-absorbing products from Maillard-like reactions are mainly nitrogen-containing heterocycles. Unexpectedly, 35–64% of molecular formulae detected in real atmospheric samples were found in simulated Maillard reaction products, implying a potentially important contribution of Maillard chemistry to the atmospheric organic molecular pool. These results will improve our understanding of the formation and molecular diversity of BrC, and further help to manage emissions of secondary aerosol precursors.

We found unexpected molecular diversity of brown carbon formed by Maillard-like reactions in aqueous aerosols, and carbonyl precursors play a more important role in determining the molecular diversity of brown carbon.

On the Water-Soluble Organic Matter in Inhalable Air Particles: Why Should Outdoor Experience Motivate Indoor Studies?

The current understanding of water-soluble organic aerosol (OA) composition, sources, transformations, and effects is still limited to outdoor scenarios. However, the OA is also an important component of particulate matter indoors, whose complexity impairs a full structural and molecular identification. The current limited knowledge on indoor OA, and particularly on its water-soluble organic matter (WSOM) fraction is the basis of this feature paper. Inspired by studies on outdoor OA, this paper discusses and prioritizes issues related to indoor water-soluble OA and their effects on human health, providing a basis for future research in the field. The following three main topics are addressed: (1) what is known about the origin, mass contribution, and health effects of WSOM in outdoor air particles; (2) the current state-of-the-art on the WSOM in indoor air particles, the main challenges and opportunities for its chemical characterization and cytotoxicity evaluation; and (3) why the aerosol WSOM should be considered in future indoor air quality studies. While challenging, studies on the WSOM fraction in air particles are highly necessary to fully understand its origin, fate, toxicity, and long-term risks indoors.

Direct noninvasive 1 H NMR analysis of stream water DOM: Insights into the effects of lyophilization compared with whole water

NMR spectroscopy is widely used in the field of aquatic biogeochemistry to examine the chemical structure of dissolved organic matter (DOM). Most aquatic DOM analyzed by proton NMR (¹ H NMR) is concentrated mainly by freeze-drying prior to analysis to combat low concentrations, frequently <100 μM C, and eliminate interference from water. This study examines stream water with low dissolved organic carbon content by ¹ H NMR with a direct noninvasive analysis of whole water using a water-suppression technique. Surface waters, collected from the headwaters of the Rio Tempisquito, Costa Rica, were examined directly, and the spectral characteristics were compared with those of the traditional preanalysis freeze-drying approach revealing significant differences in the relative intensity of peaks between the whole water and freeze-dried DOM. The freeze-dried DOM required less time to obtain quality spectra, but several peaks were missing compared with the spectra of whole water DOM; notably the most dominant peak in the spectrum constituting roughly 10% of the DOM. The stream water DOM showed an increase in the relative intensity of aliphatic methyl and methylene groups and a decrease in carbonyl, carboxyl, and carbohydrate functionalities after freeze-drying. The results of this study show that freeze-drying alters the original composition of DOM and thus freeze-dried DOM may not represent the original DOM. The information gained from whole water analysis of stream water DOM in a noninvasive fashion outweighs the attraction of reduced analysis times for preconcentrated samples, particularly for studies interested in investigating the low molecular weight fraction of DOM.

Multidimensional Analytical Characterization of Water-Soluble Organic Aerosols: Challenges and New Perspectives

Water-soluble organic aerosols (OA) are an important component of air particles and one of the key drivers that impact both climate and human health. Understanding the processes involving water-soluble OA depends on how well the chemical composition of this aerosol component is decoded. Yet, obtaining detailed information faces several challenges, including water-soluble OA collection, extraction, and chemical complexity. This review highlights the multidimensional non-targeted analytical strategies that have been developed and employed for providing new insights into the structural and molecular features of water-soluble organic components present in air particles. First, the most prominent high-resolution mass spectrometric methods for near real-time measurements of water-soluble OA and their limitations are discussed. Afterward, a special emphasis is given to the degree of compositional information provided by offline multidimensional analytical techniques, namely excitation–emission (EEM) fluorescence spectroscopy, high-resolution mass spectrometry and two-dimensional nuclear magnetic resonance (NMR) spectroscopy and their hyphenation with chromatographic systems. The major challenges ahead on the application of these multidimensional analytical strategies for OA research are also addressed so that they can be used advantageously in future studies.

Effect of a wildfire and of post-fire restoration actions in the organic matter structure in soil fractions

The impact of wildfires and of restoration actions on soil organic matter (SOM) content and structure was studied in a soil under pine (Pinus pinea) from Doñana National Park (SW Spain). Samples were collected from burnt areas before (B) and after post-fire restoration (BR) and compared with an unburnt (UB) site. Analytical pyrolysis (Py-GC/MS) was used to investigate SOM molecular composition in whole soil samples and in coarse (CF) and fine (FF) fractions. The results were interpreted using a van Krevelen graphical-statistical method. Highest total organic carbon (TOC) was found in UB soil and no differences were found between B and BR soils. The CF had the highest TOC values and FF presented differences among the three scenarios. Respect to SOM structure, the B soil was depleted in lignin and enriched in unspecific aromatics and polycyclic aromatic hydrocarbons, and in all scenarios, CF SOM consisted mainly of lignocellulose derived compounds and fatty acids. In general, FF SOM was found more altered than CF. High contribution of unspecific aromatic compounds and polycyclic aromatic hydrocarbons was observed in B-FF whereas BR-FF samples comprised considerable proportions of compounds from labile biomass, possibly due to soil mixing during rehabilitation actions. The fire caused a defunctionalisation of lignin-derived phenolics and the formation of pyrogenic compounds. The van Krevelen diagram was found useful to-at first sight-differentiate between chemical processes caused by fire and of the rehabilitation actions. Fire exerted SOM demethoxylation, dealkylation and dehydration. Our results indicate that soil management actions after the fire lead to an increase in aromaticity corresponding to the accumulation of lignin and polycyclic aromatic compounds. This suggests additional inputs from charred lignocellulosic biomass, including black carbon, that was incorporated into the soil during rehabilitation practices.

Validation and Evaluation of High-Resolution Orbitrap Mass Spectrometry on Molecular Characterization of Dissolved Organic Matter

Molecular composition of dissolved organic matter (DOM) is a hot topic in subjects such as environmental science and geochemistry. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) has been applied to molecular composition characterization of DOM successfully. However, high instrument and maintenance costs have constrained its wider application. A high-resolution Orbitrap mass spectrometer (Orbitrap MS) can provide approximately 500,000 resolving power (at m/z 200), which is potentially capable of characterizing the molecular composition of DOM. In this paper, the application of high-resolution Orbitrap MS was evaluated by comparing with FT-ICR MS in the aspect of resolution, mass distribution, detection dynamic range, and isotopic peak intensity ratio. The impact of instrument parameters of Orbitrap MS was further investigated, which includes ionization, ion transfer, and mass detection. The result shows that the high-resolution Orbitrap MS is capable or even preferable for molecular characterization of DOM. However, the peak intensity distributions are dependent on the instrument parameters, which could affect the environmental impact assessment caused by the sample itself. The result indicates that development of a universal and comparable method is of great demand.

2D Liquid Chromatographic Fractionation with Ultra-high Resolution MS Analysis Resolves a Vast Molecular Diversity of Tropospheric Particle Organics

A 2D-liquid chromatographic fractionation method was combined with direct infusion electrospray ionization Fourier transform-ion cyclotron resonance mass spectrometry to better resolve the high complexity of the organic material in atmospheric particles. The number of assigned molecular formulas increased by a factor of 2.3 for the fractionated sample (18 144) compared to a bulk sample analysis without fractionation (7819), while simultaneously allowing the identification of 71 240 isomeric compounds. Accounting for these isomers has an impact on the means and distributions of different descriptive sample parameters. More than 15 000 compounds were exclusively identified in the fractionated sample providing insights regarding the formation of organosulfates, reduced N-containing compounds, and polyaromatic compounds. Further, a new method for assigning organonitrates and poly-organonitrates based on Kendrick mass defect analysis is presented. The current study implicates that analytical separation leads to much more detailed insights into particle organics composition, while more commonly applied direct infusion MS studies can strongly underestimate composition complexity and lead to biased assignments of bulk organic properties. Overall, the particle organics composition is far more complex than previously shown, while separation through better chromatographic techniques helps to understand formation processes of atmospheric particle constituents.

Ultra-high resolution mass spectrometry of physical speciation patterns of organic matter in fire-affected soils

Fire is one of the most important modulating factors of the environment and the forest inducing chemical and biological changes on the most reactive soil component, the soil organic matter (SOM). Assuming the complex composition of the SOM, we used an ultra-high resolution mass spectrometry analysis technique to assess the chemical composition and fire-induced alterations in soil particle size fractions (coarse and fine) from a sandy soil in a Mediterranean oak forest at Doñana National Park (Southwest Spain). Electrospray ionization-Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICRMS) showed that the coarse fraction of soils not affected by fires consisted mainly of polyphenolic compounds consistent with little-transformed SOM and fresh biomass, whereas the fine fraction was enriched in protein and lipid like homologues suggesting microbially reworked SOM. In fire-affected SOM, the coarse fraction contained a high proportion of aromatic compounds, consistent with inputs of charred litter or in situ chemical transformation of the SOM. Analysis of the fine fraction revealed two differentiated chemical families pointing to the existence of two carbon pools; a native microbial-derived moiety composed of lipids and polypeptide compounds, and a secondary, pyrogenic or thermally-altered moiety rich in aromatic compounds. This work represents the first application of ultra-high resolution mass spectrometry to study the chemical composition of SOM in different particle size fractions.

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

We report on a new method for analyzing atmospheric submicrometer particulate organic matter which combines direct particle sampling and volatilization with online chemical ionization mass spectrometric analysis. Technically, the method relies on the combined use of a CHARON ("Chemical Analysis of Aerosol Online") particle inlet and a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS). Laboratory studies on target analytes showed that the ionization conditions in the PTR-ToF-MS lead to extensive fragmentation of levoglucosan and cis-pinonic acid, while protonated oleic acid and 5α-cholestane molecules remain intact. Potential problems and biases in quantitative and qualitative analyses are discussed. Side-by-side atmospheric comparison measurements of total particulate organic mass and levoglucosan with an aerosol mass spectrometer (AMS) were in good agreement. Complex and clearly distinct organic mass spectra were obtained from atmospheric measurements in three European cities (Lyon, Valencia, Innsbruck). Data visualization in reduced-parameter frameworks (e.g., oxidation state of carbon vs carbon number) revealed that the CHARON-PTR-ToF-MS technique adds significant analytical capabilities for characterizing particulate organic carbon in the Earth's atmosphere. Positive matrix factorization (PMF) was used for apportioning sources of atmospheric particles in late fall in Innsbruck. The m/z signatures of known source marker compounds (levoglucosan and resin acids, polycyclic aromatic hydrocarbons, nicotine) in the mass spectra were used to assign PMF factors to biomass burning, traffic, and smoking emission sources.