Atmospheric Analytical Chemistry

Regional differences in molecular characteristics of atmospheric water-soluble organic carbon over northern China: Comparison of remote, rural, and urban environments

Atmospheric water-soluble organic carbon (WSOC) is a critical component of airborne particulates. It significantly affects the Earth's energy balance, air quality, and human health. Despite its importance, the molecular composition and sources of WSOC remain unclear, particularly in non-urban areas. In this study, we collected total suspended particulate (TSP) samples from three sites in northern China: Erenhot (remote site), Zhangbei (rural site), and Jinan (urban site). The WSOC components were analyzed using high-performance liquid chromatography coupled with high-resolution mass spectrometry. The results showed that the formula numbers of identified compounds exhibited a decreasing trend of Jinan (2647) > Zhangbei (2046) > Erenhot (1399). Among the assigned formulas, CHO compounds were the most abundant category for all three sites, accounting for 33 %-38 % of the identified compounds, followed by the CHON compounds with contributions of 27 %-30 %. In the remote site of Erenhot, CHO compounds were dominated by oxidized unsaturated organic compounds, and CHON compounds were mainly low-oxygenated aliphatic compounds, suggesting a significant influence of primary emissions. In contrast, the urban site of Jinan showed higher contributions of CHO and CHON compounds with elevated oxidation degrees, indicating the influence of more extensive secondary oxidation processes. Atmospheric WSOC in Erenhot and Zhangbei had abundant reduced sulfur-containing species, likely from coal or diesel combustion, while that in Jinan was characterized by aliphatic organosulfates and nitrooxy-organosulfates, which are mainly associated with traffic emissions and biogenetic sources, respectively. These findings reveal significant differences in the molecular composition of WSOC in different atmospheric environments and improve our understanding of the chemical properties, potential sources, and transformations of organic aerosols.

Evidence of Surface-Tension Lowering of Atmospheric Aerosols by Organics from Field Observations in an Urban Atmosphere: Relation to Particle Size and Chemical Composition

Surface-active organics lower the aerosol surface tension (σs/a), leading to enhanced cloud condensation nuclei (CCN) activity and potentially exerting impacts on the climate. Quantification of σs/a is mainly limited to laboratory or modeling work for particles with selected sizes and known chemical compositions. Inferred values from ambient aerosol populations are deficient. In this study, we propose a new method to derive σs/a by combining field measurements made at an urban site in northern China with the κ-Köhler theory. The results present new evidence that organics remarkably lower the surface tension of aerosols in a polluted atmosphere. Particles sized around 40 nm have an averaged σs/a of 53.8 mN m-1, while particles sized up to 100 nm show σs/a values approaching that of pure water. The dependence curve of σs/a with the organic mass resembles the behavior of dicarboxylic acids, suggesting their critical role in reducing the surface tension. The study further reveals that neglecting the σs/a lowering effect would result in lowered ultrafine CCN (diameter <100 nm) concentrations by 6.8-42.1% at a typical range of supersaturations in clouds, demonstrating the significant impact of surface tension on the CCN concentrations of urban aerosols.

Nontarget Screening Exhibits a Seasonal Cycle of PM2.5 Organic Aerosol Composition in Beijing

The molecular composition of atmospheric particulate matter (PM) in the urban environment is complex, and it remains a challenge to identify its sources and formation pathways. Here, we report the seasonal variation of the molecular composition of organic aerosols (OA), based on 172 PM_2.5 filter samples collected in Beijing, China, from February 2018 to March 2019. We applied a hierarchical cluster analysis (HCA) on a large nontarget-screening data set and found a strong seasonal difference in the OA chemical composition. Molecular fingerprints of the major compound clusters exhibit a unique molecular pattern in the Van Krevelen-space. We found that summer OA in Beijing features a higher degree of oxidation and a higher proportion of organosulfates (OSs) in comparison to OA during wintertime, which exhibits a high contribution from (nitro-)aromatic compounds. OSs appeared with a high intensity in summer-haze conditions, indicating the importance of anthropogenic enhancement of secondary OA in summer Beijing. Furthermore, we quantified the contribution of the four main compound clusters to total OA using surrogate standards. With this approach, we are able to explain a small fraction of the OA (∼11-14%) monitored by the Time-of-Flight Aerosol Chemical Speciation Monitor (ToF-ACSM). However, we observe a strong correlation between the sum of the quantified clusters and OA measured by the ToF-ACSM, indicating that the identified clusters represent the major variability of OA seasonal cycles. This study highlights the potential of using nontarget screening in combination with HCA for gaining a better understanding of the molecular composition and the origin of OA in the urban environment.

Molecular characterization of organic aerosols in Taiyuan, China: Seasonal variation and source identification

Fine particulate matter (PM_2.5) samples collected in 2018 in Taiyuan, a typical industrial and mining city in North China Plain (NCP), were characterized based on ultrahigh-performance liquid chromatography (UHPLC) coupled with orbitrap mass spectrometry. Potential molecular identifications based on precise molecular weight were conducted to obtain the compositional and source information of organic aerosols (OAs) in this city. Evident variation trends were observed during the sampling period in the composition, degree of oxidation and saturation of the obtained molecules. The proportion of CHOS- and CHO+ were increased from winter to summer and then decreased, conversely the proportion of CHN+ was decreased from winter to summer and then increased. By reclassifying the molecules, OA molecules were observed to be more saturated and oxidized in summer. It was caused by the high abundance of organosulfates (OSs) in summer, and aromatic amines/N-heterocycle aromatic hydrocarbons (PANHs) in winter. Molecular identification indicated that the OSs were basically originated from biogenic source isoprene or monoterpene, while the aromatic amines and PANHs were related to anthropogenic emissions of fossil fuel combustion, like other cities in the NCP area. The prevailing northwesterlies in winter may bring coal-burning pollutants from other cities, while the high abundance of organosulfates in summer may be related to the PM_2.5 transportation from Shijiazhuang. This study firstly demonstrates the molecular composition characteristics, potential sources, and geographical origins of PM_2.5 in Taiyuan, which gives a comprehensive understanding of PM_2.5 in a typical industrial and mining city.

The maximum carbonyl ratio (MCR) as a new index for the structural classification of secondary organic aerosol components

RATIONALE

Organic aerosols (OA) account for a large fraction of atmospheric fine particulate matter and thus are affecting climate and public health. Elucidation of the chemical composition of OA is the key for addressing the role of ambient fine particles at the atmosphere-biosphere interface and mass spectrometry is the main method to achieve this goal.

METHODS

High-resolution mass spectrometry (HRMS) is on its way to becoming one of the most prominent analytical techniques, also for the analysis of atmospheric aerosols. The combination of high mass resolution and accurate mass determination allows the elemental compositions of numerous compounds to be easily elucidated. Here a new parameter for the improved classification of OA is introduced - the maximum carbonyl ratio (MCR) - which is directly derived from the molecular composition and is particularly suitable for the identification and characterization of secondary organic aerosols (SOA).

RESULTS

The concept is exemplified by the analysis of ambient OA samples from two measurement sites (Hyytiälä, Finland; Beijing, China) and of laboratory-generated SOA based on ultrahigh-performance liquid chromatography (UHPLC) coupled to Orbitrap MS. To interpret the results, MCR-Van Krevelen (VK) diagrams are generated for the different OA samples and the individual compounds are categorized into specific areas in the diagrams. The results show that the MCR index is a valuable parameter for representing atmospheric SOA components in composition and structure-dependent visualization tools such as VK diagrams.

CONCLUSIONS

The MCR index is suggested as a tool for a better characterization of the sources and the processing of atmospheric OA components based on HRMS data. Since the MCR contains information on the concentration of highly electrophilic organic compounds in particulate matter (PM) as well as on the concentration of organic (hydro)peroxides, the MCR could be a promising metric for identifying health-related particulate matter parameters by HRMS.

Detection of organosulfates and nitrooxy-organosulfates in Arctic and Antarctic atmospheric aerosols, using ultra-high resolution FT-ICR mass spectrometry

Organosulfates (OSs) are recognized as important secondary organic aerosols (SOAs) in recent years. Due to their amphipathy and light absorptive capacity, OSs may potentially impact climate. Moreover, OSs can serve as molecular tracers for precursors and multiple processes leading to the generation of SOA. However, studies on OSs are lacking in the polar regions which limits our understanding of both their formation pathways and impacts on the polar environment. Here we present the first investigation into OSs in both the Arctic and Antarctic. Organic compounds in aerosol samples collected from the polar regions during the 2013/2014 Chinese National Arctic/Antarctic Research Expedition (CHINARE) were analyzed by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) coupled with negative ion mode electrospray ionization (ESI(-)). Hundreds to thousands of OSs were detected at the polar sampling sites. The estimated total concentrations of OSs were in the range of 46-670 ng/m³ in the Arctic sampling area, and 47-260 ng/m³ in the Antarctic sampling area, accounting for 1-16% of total OM. OSs were found to have undergone a high degree of oxidation in the aerosol samples, which might be due to the combined effects of enhanced photo-oxidation in summertime or continuous oxidation during transport to the polar region. The potential appointment of OS precursors highlights the important role of long-range air-mass transport on the OSs derived from biogenic precursors and a notably large contribution from anthropogenic emissions, suggesting that human activities have significant impacts in remote polar environments. The results of this study provide important insights into the characteristics of OSs in the polar atmosphere. However, the need for further research focusing on the quantification, formation mechanisms and impacts of OSs on climate is emphasized.

One-year characterization of organic aerosol markers in urban Beijing: Seasonal variation and spatiotemporal comparison

Organic aerosol (OA) is a major component of fine particulate matter (PM); however, only 10%-30% of OA have been identified as individual compounds, and some are used as markers to trace the sources and formation mechanisms of OA. The temporal and spatial coverage of these OA markers nonetheless remain inadequately characterized. This study presents a year-long measurement of 92 organic markers in PM_2.5 samples collected at an urban site in Beijing from 2014 to 2015. Saccharides were the most abundant (340.1 ng m^-3) species detected, followed by phthalic acids (283.4 ng m^-3). In summer, high proportions (8%-24%) of phthalic acids, n-alkanes, fatty acids, and n-alcohols indicate dominant contributions of biogenic emission and atmospheric oxidation to OA in Beijing. In winter, when anthropogenic sources prevail, saccharides, polycyclic aromatic hydrocarbons, and hopanes are more prominent (4%-25%). The spatial distributions of these OA markers in China show higher concentrations in northern cities (mainly from coal combustion and biomass burning) than in southern cities (mainly from vehicular emission). The inter-annual variations of OA markers, except for hopanes, from 2001 to 2015 suggest significant alleviation of the primary OA pollution in Beijing, with an average reduction of 35%-89% compared with those before 2008. The diagnostic ratio analyses between OA markers indicate that contributions from coal combustion and biomass burning decreased, whereas those from vehicular emission increased. Increasingly large vehicle fleets have increased hopane concentrations since 2008, but the levels were 35% lower in 2015 than those in 2010-2011 because of the tightening of emission controls for vehicles. This study provides a long-term and geographical comparison (from Beijing to other locations in China and beyond) of OA markers, demonstrating the temporal and spatial variations in primary OA, and calls for more studies on secondary OA.

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.

Mass spectral chemical fingerprints reveal the molecular dependence of exhaust particulate matters on engine speeds

Particulate matters (PMs) emitted by automobile exhaust contribute to a significant fraction of the global PMs. Extractive atmospheric pressure chemical ionization mass spectrometry (EAPCI-MS) was developed to explore the molecular dependence of PMs collected from exhaust gases produced at different vehicle engine speeds. The mass spectral fingerprints of the organic compounds embedded in differentially sized PMs (e.g., 0.22-0.45, 0.45-1.00, 1.00-2.00, 2.00-3.00, 3.00-5.00, and 5.00-10.00μm) generated at different engine speeds (e.g., 1000, 1500, 2000, 2500, and 3000r/min) were chemically profiled in the mass range of mass to charge ratio (m/z) 50-800. Organic compounds, including alcohols, aldehydes, and esters, were detected in all the PMs tested, with varied concentration levels for each individual PM sample. At relatively low engine speeds (≤1500r/min), the total amount of organic species embedded in PMs of 0.22-1.00μm was greater than in PMs of other sizes, while more organic species were found in PMs of 5.00-10.00μm at high engine speeds (≥3000r/min), indicating that the organic compounds distributed in different sizes of PMs strongly correlated with the engine speed. The experimental data showed that the EAPCI-MS technique enables molecular characterization of PMs in exhaust, revealing the chemical dependence of PMs on the engine speeds (i.e., the combustion conditions) of automobiles.

Use of an Open Port Sampling Interface Coupled to Electrospray Ionization for the On-Line Analysis of Organic Aerosol Particles

A simple design for an open port sampling interface coupled to electrospray ionization (OPSI-ESI) is presented for the analysis of organic aerosols. The design uses minimal modifications to a Bruker electrospray (ESI) emitter to create a continuous flow, self-aspirating open port sampling interface. Considerations are presented for introducing aerosol to the open port sampling interface including aerosol gas flow and solvent flow rates. The device has been demonstrated for use with an aerosol of nicotine as well as aerosol formed in the pyrolysis of biomass. Upon comparison with extractive electrospray ionization (EESI), this device has similar sensitivity with increased reproducibility by nearly a factor of three. The device has the form factor of a standard Bruker/Agilent ESI emitter and can be used without any further instrument modifications. Graphical Abstract ᅟ.

Lab on a tip: atomic force microscopy – photothermal infrared spectroscopy of atmospherically relevant organic/inorganic aerosol particles in the nanometer to micrometer size range

AFM-PTIR is utilized to analyze atmospherically relevant multicomponent substrate deposited aerosol particles.

Quantification of the sources and composition of particulate matter by field-deployable mass spectrometry: implications for air quality and public health

Aerosol mass spectrometry has become an indispensable technique for characterizing the chemistry of one of the world's deadliest air pollutants.

Atmospheric chemistry of bioaerosols: heterogeneous and multiphase reactions with atmospheric oxidants and other trace gases

Once airborne, biologically-derived aerosol particles are prone to reaction with various atmospheric oxidants such as OH, NO₃, and O₃.

Advances in analytical techniques and instrumentation have now established methods for detecting, quantifying, and identifying the chemical and microbial constituents of particulate matter in the atmosphere. For example, recent cryo-TEM studies of sea spray have identified whole bacteria and viruses ejected from ocean seawater into air. A focal point of this perspective is directed towards the reactivity of aerosol particles of biological origin with oxidants (OH, NO₃, and O₃) present in the atmosphere. Complementary information on the reactivity of aerosol particles is obtained from field investigations and laboratory studies. Laboratory studies of different types of biologically-derived particles offer important information related to their impacts on the local and global environment. These studies can also unravel a range of different chemistries and reactivity afforded by the complexity and diversity of the chemical make-up of these particles. Laboratory experiments as the ones reviewed herein can elucidate the chemistry of biological aerosols.

Real-Time Analysis of Ambient Organic Aerosols Using Aerosol Flowing Atmospheric-Pressure Afterglow Mass Spectrometry (AeroFAPA-MS)

Organic compounds contribute to a major fraction of atmospheric aerosols and have significant impacts on climate and human health. However, because of their chemical complexity, their measurement remains a major challenge for analytical instrumentation. Here we present the development and characterization of a new soft ionization technique that allows mass spectrometric real-time detection of organic compounds in aerosols. The aerosol flowing atmospheric-pressure afterglow (AeroFAPA) ion source is based on a helium glow discharge plasma, which generates excited helium species and primary reagent ions. Ionization of the analytes occurs in the afterglow region after thermal desorption and produces mainly intact quasimolecular ions, facilitating the interpretation of the acquired mass spectra. We illustrate that changes in aerosol composition and concentration are detected on the time scale of seconds and in the ng m(-3) range. Additionally, the successful application of AeroFAPA-MS during a field study in a mixed forest region is presented. In general, the observed compounds are in agreement with previous offline studies; however, the acquisition of chemical information and compound identification is much faster. The results demonstrate that AeroFAPA-MS is a suitable tool for organic aerosol analysis and reveal the potential of this technique to enable new insights into aerosol formation, growth, and transformation in the atmosphere.

Analysis of organic aerosols using a micro-orifice volatilization impactor coupled to an atmospheric-pressure chemical ionization mass spectrometer

We present the development and characterization of a combination of a micro-orifice volatilization impactor (MOVI) and an ion trap mass spectrometer (IT/MS) with an atmospheric-pressure chemical ionization (APCI) source. The MOVI is a multi-jet impactor with 100 nozzles, allowing the collection of aerosol particles by inertial impaction on a deposition plate. The pressure drop behind the nozzles is approximately 5%, resulting in a pressure of 96kPa on the collection surface for ambient pressures of 101.3 kPa. The cut-point diameter (diameter of 50% collection efficiency) is at 0.13 microm for a sampling flow rate of 10 L min(-1). After the collection step, aerosol particles are evaporated by heating the impaction surface and transferred into the APCI-IT/MS for detection of the analytes. APCI was used in the negative ion mode to detect predominantly mono- and dicarboxylic acids, which are major oxidation products of biogenic terpenes. The MOVI-APCI-IT/MS instrument was used for the analysis of laboratory-generated secondary organic aerosol (SOA), which was generated by ozonolysis of alpha-pinene in a 100 L continuous-flow reactor under dark and dry conditions. The combination of the MOVI with an APCI-IT/MS improved the detection Limits for small dicarboxylic acids, such as pinic acid, compared to online measurements by APCI-IT/MS. The Limits of detection and quantification for pinic acid were determined by external calibration to 4.4 ng and 13.2 ng, respectively. During a field campaign in the southern Rocky Mountains (USA) in summer 2011 (BEACHON-RoMBAS), the MOVI-APCI-IT/MS was applied for the analysis of ambient organic aerosols and the quantification of individual biogenic SOA marker compounds. Based on a measurement frequency of approximately 5 h, a diurnal cycle for pinic acid in the sampled aerosol particles was found with maximum concentrations at night (median: 10.1 ngm(-3)) and minimum concentrations during the day (median: 8.2 ng m(-3)), which is likely due to the partitioning behavior of pinic acid and the changing phase state of the organic aerosol particles with changing relative humidity.

Characterizing an extractive electrospray ionization (EESI) source for the online mass spectrometry analysis of organic aerosols

Organic compounds comprise a major fraction of tropospheric aerosol and understanding their chemical complexity is a key factor for determining their climate and health effects. We present and characterize here a new online technique for measuring the detailed chemical composition of organic aerosols, namely extractive electrospray ionization mass spectrometry (EESI-MS). Aerosol particles composed of soluble organic compounds were extracted into and ionized by a solvent electrospray, producing molecular ions from the aerosol with minimal fragmentation. We demonstrate here that the technique has a time resolution of seconds and is capable of making stable measurements over several hours. The ion signal in the MS was linearly correlated with the mass of aerosol delivered to the EESI source over the range tested (3-600 μg/m(3)) and was independent of particle size and liquid water content, suggesting that the entire particle bulk is extracted for analysis. Tandem MS measurements enabled detection of known analytes in the sub-μg/m(3) range. Proof-of-principle measurements of the ozonolysis of oleic acid aerosol (20 μg/m(3)) revealed the formation of a variety of oxidation products in good agreement with previous offline studies. This demonstrates the technique's potential for studying the product-resolved kinetics of aerosol-phase chemistry at a molecular level with high sensitivity and time resolution.

Molecular composition of boreal forest aerosol from Hyytiälä, Finland, using ultrahigh resolution mass spectrometry

Organic compounds are important constituents of fine particulate matter (PM) in the troposphere. In this study, we applied direct infusion nanoelectrospray (nanoESI) ultrahigh resolution mass spectrometry (UHR-MS) and liquid chromatography LC/ESI-UHR-MS for the analysis of the organic fraction of PM1 aerosol samples collected over a two week period at a boreal forest site (Hyytiälä), southern Finland. Elemental formulas (460-730 in total) were identified with nanoESI-UHR-MS in the negative ionization mode and attributed to organic compounds with a molecular weight below 400. Kendrick Mass Defect and Van Krevelen approaches were used to identify compound classes and mass distributions of the detected species. The molecular composition of the aerosols strongly varied between samples with different air mass histories. An increased number of nitrogen, sulfur, and highly oxygenated organic compounds was observed during the days associated with continental air masses. However, the samples with Atlantic air mass history were marked by a presence of homologous series of unsaturated and saturated C12-C20 fatty acids suggesting their marine origin. To our knowledge, we show for the first time that the highly detailed chemical composition obtained from UHR-MS analyses can be clearly linked to meteorological parameters and trace gases concentrations that are relevant to atmospheric oxidation processes. The additional LC/ESI-UHR-MS analysis revealed 29 species, which were mainly attributed to oxidation products of biogenic volatile compounds BVOCs (i.e., α,β-pinene, Δ3-carene, limonene, and isoprene) supporting the results from the direct infusion analysis.

Chemistry and Composition of Atmospheric Aerosol Particles

For more than two decades a cadre of physical chemists has focused on understanding the formation processes, chemical composition, and chemical kinetics of atmospheric aerosol particles and droplets with diameters ranging from a few nanometers to ∼10,000 nm. They have adapted or invented a range of fundamental experimental and theoretical tools to investigate the thermochemistry, mass transport, and chemical kinetics of processes occurring at nanoscale gas-liquid and gas-solid interfaces for a wide range of nonideal, real-world substances. State-of-the-art laboratory methods devised to study molecular spectroscopy, chemical kinetics, and molecular dynamics also have been incorporated into field measurement instruments that are deployed routinely on research aircraft, ships, and mobile laboratories as well as at field sites from megacities to the most remote jungle, desert, and polar locations. These instruments can now provide real-time, size-resolved aerosol particle physical property and chemical composition data anywhere in Earth's troposphere and lower stratosphere.