Application and Progress of Single Particle Aerosol Time-of-Flight Mass Spectrometry in Fine Particulate Matter Research

Hollow Cathode Discharge Ionization Mass Spectrometry: Detection, Quantification and Gas Phase Ion-Molecule Reactions of Explosives and Related Compounds

Mass spectrometry (MS) has become an essential analytical method in every sector of science and technology. Because of its unique ability to provide direct molecular structure information on analytes, an extra method is rarely required. This review describes fabrication of a variable-pressure hollow cathode discharge (HCD) ion source for MS in detection, quantification and investigation of gas-phase ion molecule reactions of explosives and related compounds using air as a carrier gas. The HCD ion source has been designed in such a way that by altering the ion source pressures, the system can generate both HCD and conventional GD. This design enables for the selective detection and quantification of explosives at trace to ultra-trace levels. The pressure-dependent HCD ion source has also been used to investigate ion-molecule reactions in the gas phase of explosives and related compounds. The mechanism of ion formation in explosive reactions is also discussed.

Deposition of ambient particles in the human respiratory system based on single particle analysis: A case study in the Pearl River Delta, China

It is important to evaluate how ambient particles are deposited in the human respiratory system in view of the adverse effects they pose to human health. Traditional methods of investigating human exposure to ambient particles suffer from drawbacks related either to the lack of chemical information from particle number-based measurements or to the poor time resolution of mass-based measurements. To address these issues, in this study, human exposure to ambient particulate matter was investigated using single particle analysis, which provided chemical information with a high time resolution. Based on single particle measurements conducted in the Pearl River Delta, China, nine particle types were identified, and EC (elemental carbon) particles were determined to be the most dominant type of particle. In general, the submicron size mode was dominant in terms of the number concentration for all of the particle types, except for Na-rich and dust particles. On average, around 34% of particles were deposited in the human respiratory system with 13.9%, 7.9%, and 12.6% being distributed in the head, tracheobronchial, and pulmonary regions, respectively. The amount of Na-rich particles deposited was the highest, followed by EC. The overall deposition efficiencies of the Na-rich and dust particles were higher than those of the other particle types due to their higher efficiencies in the head region, which could be caused by the greater sedimentation and impaction rates of larger particles. In the head region, the Na-rich particles made the largest contribution (30.5%) due to their high deposition efficiency, whereas in the tracheobronchial and pulmonary regions, EC made the largest contribution due to its high concentration. In summary, the findings of this initial trial demonstrate the applicability of single particle analysis to the assessment of human exposure to ambient particles and its potential to support traditional methods of analysis.

Review of online source apportionment research based on observation for ambient particulate matter

Particulate matter source apportionment (SA) is the basis and premise for preventing and controlling haze pollution scientifically and effectively. Traditional offline SA methods lack the capability of handling the rapid changing pollution sources during heavy air pollution periods. With the development of multiple online observation techniques, online SA of particulate matter can now be realized with high temporal resolution, stable and reliable continuous observation data on particle compositions. Here, we start with a summary of online measuring instruments for monitoring particulate matters that contains both online mass concentration (online MC) measurement, and online mass spectrometric (online MS) techniques. The former technique collects ambient particulate matter onto filter membrane and measures the concentrations of chemical components in the particulate matter subsequently. The latter technique could be further divided into two categories: bulk measurement and single particle measurement. Aerosol Mass Spectrometers (AMS) could provide mass spectral information of chemical components of non-refractory aerosols, especially organic aerosols. While the emergence of single-particle aerosol mass spectrometer (SPAMS) technology can provide large number of high time resolution data for online source resolution. This is closely followed by an overview of the methods and results of SA. However, online instruments are still facing challenges, such as abnormal or missing measurements, that could impact the accuracy of online dataset. Machine leaning algorithm are suited for processing the large amount of online observation data, which could be further considered. In addition, the key research challenges and future directions are presented including the integration of online dataset from different online instruments, the ensemble-trained source apportionment approach, and the quantification of source-category-specific human health risk based on online instrumentation and SA methods.

Understanding sources of fine particulate matter in China

Fine particulate matter has been a major concern in China as it is closely linked to issues such as haze, health and climate impacts. Since China released its new national air quality standard for fine particulate matter (PM2.5) in 2012, great efforts have been put into reducing its concentration and meeting the standard. Significant improvement has been seen in recent years, especially in Beijing, the capital city of China. This paper reviews how China understands its sources of fine particulate matter, the major contributor to haze, and the most recent findings by researchers. It covers the characteristics of PM2.5 in China, the major methods to understand its sources such as emission inventory and measurement networks, the major research programmes in air quality research, and the major measures that lead to successful control of fine particulate matter pollution. A great example of linking scientific findings to policy is the control of coal combustion from the residential sector in northern China. This review not only provides an overview of the fine particulate matter pollution problem in China, but also its experience of air quality management, which may benefit other countries facing similar issues. This article is part of a discussion meeting issue 'Air quality, past present and future'.

Tabletop extreme ultraviolet time-of-flight spectrometry for trace analysis of high ionization energy samples

RATIONALE

Species with ionization energies beyond what is accessible using state-of-the-art lab sources are affected by poor detection limits in ordinary mass spectrometry setups, whose throughput is also often limited. Extreme ultraviolet (XUV) photoionization mass spectrometry, in combination with linear time-of-flight (TOF), is necessary for the sensitive detection of high ionization energy compounds at trace level. XUV photoionization is available at beamlines, although with limited access. A tabletop setup may fill such a gap.

METHODS

A self-developed tabletop system, based on a plasma discharge with extreme ultraviolet emission (λ = 5-50 nm) coupled to a TOF mass spectrometer, was used in this study. Simultaneous validation measurements with a reference electron ionization quadrupole mass filter were carried out. An in-house developed hollow toroidal coil (HTC) induction detector was used for concomitant photoelectron detection.

RESULTS

Straightforward XUV mass spectra without fragmentation, thanks to the single-photon ionization, were acquired. The measurements with the reference quadrupole were in agreement with the spectra acquired by XUV-TOF. The resolution obtained for N₂ was at least factor of 2 higher than that measured with the reference quadrupole. Initial energy distributions of photoelectrons were retrieved by cross-correlation that gave access to the photoionization distribution.

CONCLUSIONS

The system allows XUV single-photon ionization of elements and molecules with IE >10 eV that are of fundamental interest e.g. for water splitting and catalysis research. The demonstrated performance is now suitable for a prototype platform.

Characterization of Nanoparticles: Advances

Over the past two decades, the unique properties of engineered nanoparticles (NPs) have placed them at the centre of revolutionary advancements in many sectors of science, technology and commerce. Multi-technique and multi-disciplinary analytical approaches are required to identify, quantify, and characterize the chemical composition, size and size distribution, surface properties and the number and concentration of NPs. In this chapter, an overview of the recent advances in the characterization of NPs will be presented.

Oxalate Formation Enhanced by Fe-Containing Particles and Environmental Implications

We used a single particle mass spectrometry to online detect chemical compositions of individual particles over four seasons in Guangzhou. Number fractions (Nfs) of all the measured particles that contained oxalate were 1.9%, 5.2%, 25.1%, and 15.5%, whereas the Nfs of Fe-containing particles that were internally mixed with oxalate were 8.7%, 23.1%, 45.2%, and 31.2% from spring to winter, respectively. The results provided the first direct field measurements for the enhanced formation of oxalate associated with Fe-containing particles. Other oxidized organic compounds including formate, acetate, methylglyoxal, glyoxylate, purivate, malonate, and succinate were also detected in the Fe-containing particles. It is likely that reactive oxidant species (ROS) via Fenton reactions enhanced the formation of these organic compounds and their oxidation product oxalate. Gas-particle partitioning of oxalic acid followed by coordination with Fe might also partly contribute to the enhanced oxalate. Aerosol water content likely played an important role in the enhanced oxalate formation when the relative humidity is >60%. Interactions with Fe drove the diurnal variation of oxalate in the Fe-containing particles. The study could provide a reference for model simulation to improve understanding on the formation and fate of oxalate, and the evolution and climate impacts of particulate Fe.

Trends in analytical techniques applied to particulate matter characterization: A critical review of fundaments and applications

Epidemiological studies have shown the association of airborne particulate matter (PM) size and chemical composition with health problems affecting the cardiorespiratory and central nervous systems. PM also act as cloud condensation nuclei (CNN) or ice nuclei (IN), taking part in the clouds formation process, and therefore can impact the climate. There are several works using different analytical techniques in PM chemical and physical characterization to supply information to source apportionment models that help environmental agencies to assess damages accountability. Despite the numerous analytical techniques described in the literature available for PM characterization, laboratories are normally limited to the in-house available techniques, which raises the question if a given technique is suitable for the purpose of a specific experimental work. The aim of this work consists of summarizing the main available technologies for PM characterization, serving as a guide for readers to find the most appropriate technique(s) for their investigation. Elemental analysis techniques like atomic spectrometry based and X-ray based techniques, organic and carbonaceous techniques and surface analysis techniques are discussed, illustrating their main features as well as their advantages and drawbacks. We also discuss the trends in analytical techniques used over the last two decades. The choice among all techniques is a function of a number of parameters such as: the relevant particles physical properties, sampling and measuring time, access to available facilities and the costs associated to equipment acquisition, among other considerations. An analytical guide map is presented as a guideline for choosing the most appropriated technique for a given analytical information required.

Single particle mass spectral signatures from vehicle exhaust particles and the source apportionment of on-line PM2.5 by single particle aerosol mass spectrometry

In order to accurately apportion the many distinct types of individual particles observed, it is necessary to characterize fingerprints of individual particles emitted directly from known sources. In this study, single particle mass spectral signatures from vehicle exhaust particles in a tunnel were performed. These data were used to evaluate particle signatures in a real-world PM_2.5 apportionment study. The dominant chemical type originating from average positive and negative mass spectra for vehicle exhaust particles are EC species. Four distinct particle types describe the majority of particles emitted by vehicle exhaust particles in this tunnel. Each particle class is labeled according to the most significant chemical features in both average positive and negative mass spectral signatures, including ECOC, NaK, Metal and PAHs species. A single particle aerosol mass spectrometry (SPAMS) was also employed during the winter of 2013 in Guangzhou to determine both the size and chemical composition of individual atmospheric particles, with vacuum aerodynamic diameter (d_va) in the size range of 0.2-2μm. A total of 487,570 particles were chemically analyzed with positive and negative ion mass spectra and a large set of single particle mass spectra was collected and analyzed in order to identify the speciation. According to the typical tracer ions from different source types and classification by the ART-2a algorithm which uses source fingerprints for apportioning ambient particles, the major sources of single particles were simulated. Coal combustion, vehicle exhaust, and secondary ion were the most abundant particle sources, contributing 28.5%, 17.8%, and 18.2%, respectively. The fraction with vehicle exhaust species particles decreased slightly with particle size in the condensation mode particles.

Source apportionment of Pb-containing particles in Beijing during January 2013

Although leaded gasoline has been banned in some megacities in China since 1997 and nationally since 2000, atmospheric lead (Pb) pollution is still an important issue in China, as its concentration in megacities such as Beijing remains high. To measure the Pb concentration and identify sources of Pb-containing particles in Beijing during January 2013, both an online Single Particle Aerosol Mass Spectrometer (SPAMS) and offline filters analyzed by inductively coupled plasma-mass spectrometer (ICP-MS) were used at a monitoring site on the Peking University (PKU) campus. The average Pb concentration in PM_2.5 was 370 ng/m³ in January 2013 and the highest daily concentration was as high as 1.3 μg/m³ during our sampling period. Based on the mass spectra from the SPAMS, these particles were classified into 4 major types, including NO₃-rich (61%), ECOC-rich (18%), Fe-rich (14%), and SO₄-rich (7%). Results from this study suggest that combustion processes and the iron/steel industry were the major primary sources of Pb in Beijing. On clean days, the importance of the primary combustion particle type (ECOC-rich) increased, while during severe haze episodes, Pb-containing particles mixed with secondary ions and Fe were dominant. Based on estimates from the CMAQ model, on average 45% of Pb in PM_2.5 in urban Beijing was transported in January 2013, with a much higher percent transported during the haze episodes. The percentage of transported Pb increased with the concentration of Pb and PM_2.5, indicating that emissions from the surrounding areas need to be controlled during high Pb episodes in Beijing in winter.

Review of receptor-based source apportionment research of fine particulate matter and its challenges in China

As the key for haze control, atmospheric fine particulate matter with aerodynamic diameter <2.5μm (or PM_2.5) is of great concern lately in China. It is closely linked to fast pace of urbanization, industrialization and economic development, especially in eastern China. A good understanding of its sources is required for effective pollution abatement. Receptor models are one of the major methods for source apportionment used in China. The major objective of this study is to understand sources that contribute to fine particulate matter in China and key challenges in this area. Spatial distribution of fine particulate matter concentration, chemical composition and dominant sources in North and South China are summarized. Based on chemical speciation results from 31 cities and source apportionment results from 21 cities, it is found that secondary sources and traffic emission have higher contribution in South China while the percentage of coal combustion, dust and biomass burning to total PM_2.5 are higher in North China. Source profiles established in China from 44 cities and areas are also summarized as references for future source apportionment studies. Suggestions for future research are also provided including methods for evaluating source apportionment results, ways for integrating multiple source apportionment methods, the need for standardizing protocols and developing tools for high-time resolution source apportionment.

Review of receptor modeling methods for source apportionment

Efforts have been made to relate measured concentrations of airborne constituents to their origins for more than 50 years. During this time interval, there have been developments in the measurement technology to gather highly time-resolved, detailed chemical compositional data. Similarly, the improvements in computers have permitted a parallel development of data analysis tools that permit the extraction of information from these data. There is now a substantial capability to provide useful insights into the sources of pollutants and their atmospheric processing that can help inform air quality management options. Efforts have been made to combine receptor and chemical transport models to provide improved apportionments. Tools are available to utilize limited numbers of known profiles with the ambient data to obtain more accurate apportionments for targeted sources. In addition, tools are in place to allow more advanced models to be fitted to the data based on conceptual models of the nature of the sources and the sampling/analytical approach. Each of the approaches has its strengths and weaknesses. However, the field as a whole suffers from a lack of measurements of source emission compositions. There has not been an active effort to develop source profiles for stationary sources for a long time, and with many significant sources built in developing countries, the lack of local profiles is a serious problem in effective source apportionment. The field is now relatively mature in terms of its methods and its ability to adapt to new measurement technologies, so that we can be assured of a high likelihood of extracting the maximal information from the collected data.

IMPLICATIONS

Efforts have been made over the past 50 years to use air quality data to estimate the influence of air pollution sources. These methods are now relatively mature and many are readily accessible through publically available software. This review examines the development of receptor models and the current state of the art in extracting source identification and apportionments from ambient air quality data.