Oxidation potential and coupling effects of the fractionated components in airborne fine particulate matter.

Fine particulate matter (PM2.5) can induce the generation of reactive oxygen species (ROS) and damage human tissues. Fully understanding the generation mechanism of oxidative toxicity of PM is challenging due to the extremely complex composition. Classification methods may be helpful in understanding the ROS production mechanisms of complex PM. This study used a solvent extraction and solid phase extraction methods to separate five different components from PM2.5 includes non-extractable components that have rarely been studied before, and discussed the coupling effect and heterogeneous characteristics of oxidation activity they produced. It is found that the water-soluble component contribute about half of the PM oxidation activity, and metal ions probably contribute most of the oxidation activity. Experimental results show that oxygen molecules is the main precursor of ROS production, which depends on whether the aerosol component has catalytic conversion ability. After mixing humic-like substance (HULIS) and hydrophilic water-soluble (HP-WSM) PM, the oxidation activity increased, it is most likely to be a synergistic effect between HULIS and metal ions is dominant, but limited contribution to oxidation activity. It turns out that the non-extractable and water-insoluble components have higher oxidation activity than the water-soluble components, and the two components exhibited a more durable ability to produce 1O2. The reaction of soluble components to produce ROS is homogeneous, but it is obviously heterogeneous for these insoluble components. This study suggests that future attention should be paid to the oxidative toxicity of the non-extractable component, and that single PM component or compound cannot simply be studied independently.

Identification of species and sources of atmospheric chromophores by fluorescence excitation-emission matrix with parallel factor analysis.

It is essential to fully understand the physicochemical properties and sources of atmospheric chromophores to evaluate their impacts on environmental quality and global climate. Three-dimensional excitation-emission matrix (EEM) fluorescence spectroscopy is an important method for directly characterizing the occurrences, origins, and chemical behaviors of atmospheric chromophores. However, there is still a lack of adequate information on the sources and chemical structures of EEM-defined chromophores. This situation limits the extensive application of the EEM method in the study of atmospheric chromophores. Under these adverse conditions, this work uses the analysis of EEM data by the parallel factor (PARAFAC) analysis model and a comprehensive comparison of the types and abundances of different chromophores in different aerosol samples (combustion source samples, secondary organic aerosols, and ambient aerosols) to demonstrate that the EEM method can distinguish among different chromophore types and aerosol sources. Indeed, approximately half of all fluorescent substances can be attributed to specific chemicals and sources. These findings provide an important basis for the study of the sources and chemical processes of atmospheric chromophores by the EEM approach.

Study on the oxidation potential of the water-soluble components of ambient PM2.5 over Xi'an, China: Pollution levels, source apportionment and transport pathways.

Reactive oxygen species (ROS) are a class of substances that are of general concern in terms of human health and are used to represent the oxidation potential (OP) of the atmosphere. In this study, the ROS levels in 116 daily fine particulate matter (PM2.5) samples taken over Xi'an in 2017 were measured with the dithiothreitol (DTT) method. The sources of DTTv (volume-based DTT consumption) in PM2.5 as well as their contributions were identified by both positive matrix factorization (PMF) and multiple linear regression (MLR) based on the measured chemical species in particulate matter (PM). The results showed that the yearly average DTTv over Xi'an was 0.53 nmol/min/m3 (0.19-1.10 nmol/min/m3). The highest DTTv level occurred in winter, followed by spring, summer and autumn. DTTv was the most strongly correlated with the water-soluble organic carbon (WSOC; r = 0.85), but the effects of WSOC on DTTv were very limited. SO2, NO2, CO, elemental carbon (EC) and K+ (r > 0.64) had moderate correlations with DTTv and were moderately related to environmentally persistent free radicals (EPFRs) (r = 0.56). The linear mixed-effects model showed that pollutants originating from incomplete combustion had greater effects on DTTv than those from complete combustion. Source apportionment results from PMF showed that motor vehicle emissions (27.4%), secondary sulfates (21.6%) and coal combustion sources (18.8%) were more important contributors to the DTTv in PM2.5 than dust sources (8.4%), metal processing (4.9%), industrial emissions (11.3%) and secondary nitrates (7.5%). The PMF results for the DTTv were consistent with the MLR results, which verified that both PMF and MLR are feasible methods for source apportionment of PM2.5 as well as specific species such as ROS and EPFRs. Backward trajectory clusters showed that the dominant cluster groups were local and regional transport, while the OP of the PM2.5 over Xi'an was affected more by long-range transport than by local transport. As stated above, the improvement of atmospheric oxidation potential require not only regional efforts but also large-scale joint cooperation. Furthermore, this study on the OP of PM as well as the specific source information provides important guidance for health effect research.

Environmentally Persistent Free Radical (EPFR) Formation by Visible-Light Illumination of the Organic Matter in Atmospheric Particles.

A secondary process may be an important source of environmentally persistent free radicals (EPFRs) in atmospheric particulates; yet, this process remains to be elucidated. This study demonstrated that secondary EPFRs could be generated by visible-light illumination of atmospheric particulate matter (PM), and their lifetimes were only 30 min to 1 day, which were much shorter than the lifetimes of the original EPFRs in PM. The yields of secondary EPFRs produced by PM could reach 15-60% of those of the original EPFRs. The extractable organic matter contributed to the formation of secondary EPFRs (∼55%), and a humic-like substance was the main precursor of the secondary EPFRs and was also the most productive precursor compared to the other aerosol components. The results of simulation experiments showed that the secondary EPFRs generated by the extractable and nonextractable PM components were similar to those produced by phenolic compounds and polycyclic aromatic hydrocarbons, respectively. We have found that oxygen molecules play an important role in the photochemical generation and decay of EPFRs. The reactive oxygen capture experiments showed that the original EPFRs may contribute to singlet oxygen generation, while the secondary EPFRs generated by photoexcitation may not produce singlet oxygen or hydroxyl radicals.

Source apportionment of environmentally persistent free radicals (EPFRs) in PM2.5 over Xi'an, China.

Environmentally persistent free radicals (EPFRs) have recently attracted considerable attention as a new type of environmental risk substance due to their potential health effects. However, the sources and contributions of EPFRs in PM2.5 are not yet clear. Therefore, this study reports the sources of EPFRs in PM2.5 based on chemical analysis and positive matrix factorization (PMF). Daily PM2.5 samples (116) were collected in Xi'an city from April 4 to December 29, 2017, and were quantitatively analyzed for EPFRs and other chemical constituents. The PMF model revealed contributions from five main sources of EPFRs in PM2.5 (dust sources, coal combustion, secondary nitrates, industrial emissions and motor vehicle emissions). Coal combustion, motor vehicle emissions and dust sources are the top three contributors to EPFRs (76.12% in total). Coal combustion is highly important for PM2.5 (35.10%) and EPFRs (16.75%). A high dust source contribution to EPFRs in spring may be due to dust storm events. Motor vehicle emissions are the top contributor to EPFRs, with a mean percentage of 32.13%. Secondary nitrates barely contributes to EPFRs (3.42%), indicating an EPFR origin from primary emissions rather than secondary inorganic reactions. Industrial emissions contribute less to PM2.5 (4.31%) than to EPFRs (11.71%), which implies that fossil fuels contains many high-molecular-weight organics that could emit EPFRs. Integrating the PMF results with meteorological data revealed that atmospheric pollutants emitted in Xi'an city center could be transported to the sampling site by southern winds. These results suggest the need for further studies on the public health effects of EPFRs and can be used to help formulate source control measures to reduce the potential health risks posed by EPFRs in PM2.5.

Oxidative Potential of Water-Soluble Matter Associated with Chromophoric Substances in PM2.5 over Xi'an, China.

Organic compounds are important contributors to the oxidative potential (OP) of atmospheric aerosols. This study is the first to report the OP of water-soluble organic matter (WSOM) related to the chromophoric substances in PM2.5 over Xi'an, China. The dithiothreitol (DTT) activity levels in PM2.5 extracted by water were quantified as well as the relationships between DTT activity and light absorption and fluorescence properties. The results show that the DTT activity has significantly correlated with colored WSOM, in which we identified three light absorbing substances (BrC1-3) and eight fluorescent substances (C1-8). It is further found that BrC3 and C7 accounted for almost all of the DTT activity by colored WSOM, although these two factors contributed only a small fraction of light absorption and fluorescence. BrC3 and C7 are clearly distinguished from other chromophoric substances because of their long absorption wavelength (λmax = 475 nm) and fluorescence emission wavelength (λmax = 462 nm), respectively. This discovery will help to better interpret and understand the mechanism of oxidation activity generation by light absorbing organic aerosols and provide guidance for predicting the OPs of light absorbing organic aerosols based on their optical properties.

[Characteristics of Carbonaceous Aerosol Pollution in PM2.5 in Xi'an].

Mass concentration, seasonal variation and sources of organic carbon (OC), element carbon (EC), methanol-soluble organic carbon (MSOC), and seven carbon components (OC1-4, EC1-3) were detected by thermal-optical analysis of 353 PM2.5 samples in Xi'an in 2017. The results show that the average mass concentrations of OC, EC, and MSOC were (17.56±11.83), (4.08±2.95) and (11.10±6.77) µg·m-3, respectively. The seasonal trend of the OC concentration follows the order winter > spring > summer > autumn. The seasonal trend in EC concentration follows the order winter > spring≈autumn > summer. The average MSOC/OC value is 0.64±0.20. The highest value is observed in winter and the lowest in summer. The correlation between OC and EC is good in spring (r2=0.76), but the correlation is poor in winter (r2=0.43). These results indicate that the source of the carbon aerosols was different. The content of secondary organic aerosols was estimated by the EC tracing method. The average mass concentration of SOC accounted for 51.9%, 38.4%, 37.3% and 44.0% of OC in spring, summer, autumn, and winter, respectively. The main sources of carbonaceous aerosols were analyzed by principal component analysis. The results show that carbonaceous aerosols originate mainly from coal and vehicle emissions in Xi'an.

Characteristics of environmentally persistent free radicals in PM2.5: Concentrations, species and sources in Xi'an, Northwestern China.

Environmentally persistent free radicals (EPFRs) are a new class of environmental risk substances that can stably exist in atmospheric particles and pose a potential threat to human health. In this study, electron paramagnetic resonance (EPR) spectroscopy was used to study the concentration levels, species characteristics, and sources of EPFRs in PM2.5 in Xi'an in 2017. The results showed that the concentrations of EPFRs in PM2.5 in Xi'an in 2017 ranged from 9.8 × 1011 to 6.9 × 1014 spins/m3. The highest concentration of EPFRs occurred in winter when the average concentration was 2.1 × 1014 spins/m3. The lowest concentration of EPFRs occurred in autumn when the average concentration was 7.0 × 1013 spins/m3. According to the annual average atmospheric concentration of EPFRs, the amount of EPFRs inhaled by people in Xi'an is equivalent to approximately 5 cigarettes per person per day and approximately 23 cigarettes per person per day in winter when haze occurs. The results of the study on the EPFR characteristics show that the EPFRs in PM2.5 in Xi'an are mainly C-center organic radicals that are primarily non-decaying types, accounting for approximately 75% and 85% of total concentration of EPFRs in autumn and winter, respectively. Finally, a correlation analysis was used to explore the origins of EPFRs in PM2.5. Significant positive correlations were found between EPFRs and SO2, NO2 and the thermally derived OC3 and OC4 carbonaceous components. The results suggested that coal-fired and traffic may be important sources of EPFRs in PM2.5 in Xi'an. In addition, EPFRs are significantly positively correlated with O3 in summer, suggesting that some EPFRs may also originate from secondary processes. This study provides important basic data and evidence for further assessments of the potential health risks of EPFRs in PM2.5 and the development of effective air pollution control measures.

Enhanced health risks from exposure to environmentally persistent free radicals and the oxidative stress of PM2.5 from Asian dust storms in Erenhot, Zhangbei and Jinan, China.

Asian dust storms can increase the level of atmospheric pollution over regions downwind of dust storms and may have adverse health effects on residents along the sandstorm transmission route. This study was the first to report the concentration levels, properties and possible sources of environmentally persistent free radicals (EPFRs) and oxidative potential in atmospheric PM2.5 at the three sites of Erenhot, Zhangbei, and Jinan along the transport route of Asian dust storms during the occurrence of Asian dust storms in the spring of 2016. Under non-sandstorm weather conditions, the average EPFR concentrations at the three sites were Zhangbei>Jinan>Erenhot, while the PM-induced oxidative potential levels were Erenhot>Jinan>Zhangbei. The PM2.5 concentration increased significantly during dust storm events, and the total atmospheric concentration of EPFRs (spins/m3) and total oxidation potential (a.u./m3) of PM2.5 simultaneously increased. However, the EPFR concentration in PM2.5 (spins/g) and the unit mass of the PM oxidation potential (a.u./g) were significantly reduced. Electron paramagnetic resonance analysis combined with backward trajectory analysis and MODIS products showed that Asian dust storms can carry EPFRs over long distances. Correlation analysis showed that the atmospheric concentrations of EPFRs were positively correlated with elemental carbon (EC) for the Zhangbei and Jinan samples but were not significantly correlated with EC for the Erenhot samples, indicating that combustion may be an important source of EPFRs for the Zhangbei and Jinan samples. In contrast, the EPFRs in the Erenhot samples were more affected by dust/sand. The EPFR concentration levels showed a significant positive correlation with the oxidation potentials for the Erenhot and Zhangbei samples and showed negative correlations for the Jinan samples, suggesting that the EPFRs in the Erenhot and Zhangbei samples may provide an important contribution to the oxidative stress in PM2.5. In contrast, the oxidation potential for the Jinan samples was mainly caused by substances other than EPFRs. This study presents a basic understanding of the potential health effects of Asian dust storms, and this information can be used to assess the health risks of Asian dust storms in future studies.

Dominant Fraction of EPFRs from Nonsolvent-Extractable Organic Matter in Fine Particulates over Xi'an, China.

To understand the nature and possible sources of environmentally persistent free radicals (EPFRs) in atmospheric aerosols, the present study used a solvent extraction method to fractionate aerosol components with different polarities and solvent resistance in fine particulate matter (PM2.5) from Xi'an, China. The characteristics of EPFRs, that is., their concentration, type and lifetime, were obtained based on their electron paramagnetic resonance spectra. The results showed that the EPFRs in the PM2.5 samples were carbon-centered with a nearby heteroatom ( g = 2.0031) and had a long half-life of more than 3 years. Nearly all of the extractable EPFRs were detected in the water-insoluble organic fraction and showed characteristics indicating that may contain oxygen-centered radical ( g = 2.0038). Most of the total EPFRs in the PM2.5 were derived from solvent-resistant organic matter (88%), which likely consisted of graphene oxide analogues. The results suggest that previous studies may have missed the major proportion of EPFRs in atmospheric particulates if they only focused on solvent-extractable or metallic oxide-formed EPFRs. Our results showed that the EPFR concentration was significantly and positively correlated with the elemental carbon and NO2 concentrations, suggesting that traffic emissions may be an important source of EPFRs in PM2.5 over Xi'an.