Stable Isotopes Reveal Photoreduction of Particle-Bound Mercury Driven by Water-Soluble Organic Carbon during Severe Haze

Haze with high loading of particles may result in significant enrichment of particle-bound Hg (PBM), potentially impacting the atmospheric Hg transformation and transport. However, the dynamics of Hg transformation and the relative environmental effect during severe haze episodes remain unclear. Here, we report Hg isotopic compositions of atmospheric particles (PM_2.5, PM₁₀, and TSP) collected during a severe haze episode in Tianjin, China, to investigate the transformation and fate of Hg during haze events. All severe haze samples display significantly higher Δ¹⁹⁹Hg (up to 1.50‰) than global urban PBM, which cannot be explained by primary anthropogenic emissions. The high Δ¹⁹⁹Hg is likely caused by photoreduction of PBM promoted by water-soluble organic carbon (WSOC) during the particle accumulation period, as demonstrated by the positive correlations of Δ¹⁹⁹Hg with WSOC and relative humidity and confirmed by our laboratory-controlled photoreduction experiment. The results show that, on average, 21% of PBM are likely photoreduced and re-emitted back to the atmosphere as Hg(0), potentially requiring revision of atmospheric Hg budgeting and modeling. This study highlights the release of large portions of PBM back to the gas phase through photoreduction, which needs to be taken into account while evaluating the atmospheric Hg cycle and the relative ecological effects.

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.

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.

Vertical profile of particle hygroscopicity and CCN effectiveness during winter in Beijing: insight into the hygroscopicity transition threshold of black carbon

The hygroscopicity and ability of aerosol particles to act as cloud condensation nuclei (CCN) is important in determining their lifetime and role in aerosol-cloud interactions, thereby influencing cloud formation and climate. Previous studies have used the aerosol hygroscopic properties measured at the ground to evaluate the influence on cloud formation in the atmosphere, which may introduce uncertainty associated with aerosol hygroscopicity variability with altitude. In this study, the CCN behaviour and hygroscopic properties of daily filter collections of PM_2.5 from three different heights (8, 120, 260 m) on a tower in Beijing were determined in the laboratory using water, water/methanol and methanol as the atomization solvents. Whilst there was substantial temporal variability in particle concentration and composition, there was little obvious difference in aerosol CCN and hygroscopic behaviour at different heights, although the planetary boundary layer height (PBLH) reduced to below the tower height during the nighttime, suggesting that use of surface hygroscopicity measurements is sufficient for the estimation of aerosol particle activation in clouds. Additionally, the critical coating thickness (in terms of mass ratio of coating/refractory BC, MRc) defining the BC transition between being hydrophobic to hydrophilic, was determined by combining hygroscopic tandem differential mobility analyser (H-TDMA), centrifugal particle mass analyzer (CPMA) and single particle soot photometer (SP2) measurements. The MRc of 250 nm BC-containing particles increased from a background value of between 0.8 and 1.6 to around 4.6 at the onset of the growth event of nanoparticles, decreasing monotonically back to the background level as the event progressed. This indicates that large particles do not act as an effective pre-existing condensation sink of the hygroscopic vapours during the nanoparticle growth events, leading to the 250 nm BC particles requiring more coating materials to transition between being hydrophobic and hydrophilic. These findings show that large particles may be less important in suppressing the new particle formation and subsequent growth in the atmosphere.

Significant Contribution of Primary Sources to Water-Soluble Organic Carbon During Spring in Beijing, China

Despite the significant role water-soluble organic carbon (WSOC) plays in climate and human health, sources and formation mechanisms of atmospheric WSOC are still unclear; especially in some heavily polluted areas. In this study, near real-time WSOC measurement was conducted in Beijing for the first time with a particle-into-liquid-sampler coupled to a total organic carbon analyzer during the springtime, together with collocated online measurements of other chemical components in fine particulate matter with a 1 h time resolution, including elemental carbon (EC), organic carbon (OC), multiple metals, and water-soluble ions. Good correlations of WSOC with primary OC, as well as carbon monoxide, indicated that major sources of WSOC were primary instead of secondary during the study period. The positive matrix factorization model-based source apportionment results quantified that 68 ± 19% of WSOC could be attributed to primary sources, with predominant contributions by biomass burning during the study period. This finding was further confirmed by the estimate with the modified EC-tracer method, suggesting significant contribution of primary sources to WSOC. However, the relative contribution of secondary source to WSOC increased during haze episodes. The WSOC/OC ratio exhibited similar diurnal distributions with O3 and correlated well with secondary WSOC, suggesting that the WSOC/OC ratio might act as an indicator of secondary formation when WSOC was dominated by primary sources. This study provided evidence that primary sources could be major sources of WSOC in some polluted megacities, such as Beijing. From this study, it can be seen that WSOC cannot be simply used as a surrogate of secondary organic aerosol, and its major sources could vary by season and location.

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.

Comparative study of atmospheric water-soluble organic aerosols composition in contrasting suburban environments in the Iberian Peninsula Coast

This study investigates the structural composition and major sources of water-soluble organic matter (WSOM) from PM_2.5 collected, in parallel, during summer and winter, in two contrasting suburban sites at Iberian Peninsula Coast: Aveiro (Portugal) and Coruña (Spain). PM₁₀ samples were also collected at Coruña for comparison. Ambient concentrations of PM_2.5, total nitrogen (TN), and WSOM were higher in Aveiro than in Coruña, with the highest levels found in winter at both locations. In Coruña, concentrations of PM₁₀, TN, and WSOM were higher than those from PM_2.5. Regardless of the season, stable isotopic δ¹³C and δ¹⁵N in PM_2.5 suggested important contributions of anthropogenic fresh organic aerosols (OAs) at Aveiro. In Coruña, δ¹³C and δ¹⁵N of PM_2.5 and PM₁₀ suggests decreased anthropogenic input during summer. Although excitation-emission fluorescence profiles were similar for all WSOM samples, multi-dimensional nuclear magnetic resonance (NMR) spectroscopy confirmed differences in their structural composition, reflecting differences in aging processes and/or local sources between the two locations. In PM_2.5 WSOM in Aveiro, the relative distribution of non-exchangeable proton functional groups was in the order: HC (40-43%) > HCC (31-39%) > HCO (12-15%) > Ar-H (5.0-13%). However, in PM_2.5 and PM₁₀ WSOM in Coruña, the relative contribution of HCO groups (24-30% and 23-29%, respectively) equals and/or surpasses that of HCC (25-26% and 25-29%, respectively), being also higher than those of Aveiro. In both locations, the highest aromatic contents were observed during winter due to biomass burning emissions. The structural composition of PM_2.5 and PM₁₀ WSOM in Coruña is dominated by oxygenated aliphatic compounds, reflecting the contribution of secondary OAs from biogenic, soil dust, and minor influence of anthropogenic emissions. In contrast, the composition of PM_2.5 WSOM in Aveiro appears to be significantly impacted by fresh and secondary anthropogenic OAs. Marine and biomass burning OAs are important contributors, common to both sites.

Online wet oxidation/isotope ratio mass spectrometry method for determination of stable carbon isotope ratios of water-soluble organic carbon in particulate matter

RATIONALE

Water-soluble organic carbon (WSOC) is formed by oxidation of organic compounds in particulate matter (PM) and accounts for 25-80% of the organic carbon in PM. Stable carbon isotope ratio (δ¹³ C) analysis is widely used to identify the sources of PM, but determining the δ¹³ C values of WSOC is complicated and requires a time-consuming pretreatment process.

METHODS

We have developed an online wet oxidation/isotope ratio mass spectrometry method with a reduced pretreatment time. We have measured the δ¹³ C values of WSOC by using this method.

RESULTS

The method showed high accuracy (0.1‰) and precision (0.1‰) for levoglucosan, and the limit of detection was sufficiently low for WSOC analysis. Using this method, we determined δ¹³ C values of WSOC in PM_2.5 samples collected in Japan during the period from July to November 2017 and found that the values ranged from -26.5‰ to -25.0‰ (average, -25.8‰).

CONCLUSIONS

Our simple, low-blank method could be used for rapid quantitative analysis of the δ¹³ C values of WSOC in PM_2.5 . We propose that this online method be used as a standard method for δ¹³ C analysis of WSOC.

Evidence of a reduction in cloud condensation nuclei activity of water-soluble aerosols caused by biogenic emissions in a cool-temperate forest

Biogenic organic aerosols can affect cloud condensation nuclei (CCN) properties, and subsequently impact climate change. Large uncertainties exist in how the difference in the types of terrestrial biogenic sources and the abundance of organics relative to sulfate affect CCN properties. For the submicron water-soluble aerosols collected for two years in a cool-temperate forest in northern Japan, we show that the hygroscopicity parameter κ_CCN (0.44 ± 0.07) exhibited a distinct seasonal trend with a minimum in autumn (κ_CCN = 0.32-0.37); these κ_CCN values were generally larger than that of ambient particles, including water-insoluble fractions. The temporal variability of κ_CCN was controlled by the water-soluble organic matter (WSOM)-to-sulfate ratio (R² > 0.60), where the significant reduction of κ_CCN in autumn was linked to the increased WSOM/sulfate ratio. Positive matrix factorization analysis indicates that α-pinene-derived secondary organic aerosol (SOA) substantially contributed to the WSOM mass (~75%) in autumn, the majority of which was attributable to emissions from litter/soil microbial activity near the forest floor. These findings suggest that WSOM, most likely α-pinene SOA, originated from the forest floor can significantly suppress the aerosol CCN activity in cool-temperate forests, which have implications for predicting climate effects by changes in biogenic emissions in future.

Structural signatures of water-soluble organic aerosols in contrasting environments in South America and Western Europe

This study describes and compares the key structural units present in water-soluble organic carbon (WSOC) fraction of atmospheric aerosols collected in different South American (Colombia - Medellín and Bogotá, Peru - Lima, Argentina - Buenos Aires, and Brazil - Rio de Janeiro, São Paulo, and Porto Velho, during moderate (MBB) and intense (IBB) biomass burning) and Western European (Portugal - Aveiro and Lisbon) locations. Proton nuclear magnetic resonance (¹H NMR) spectroscopy was employed to assess the relative distribution of non-exchangeable proton functional groups in aerosol WSOC of diverse origin, for the first time to the authors' knowledge in South America. The relative contribution of the proton functional groups was in the order H-C > H-C-C= > H-C-O > Ar-H, except in Porto Velho during MBB, Medellín, Bogotá, and Buenos Aires, for which the relative contribution of H-C-O was higher than that of H-C-C=. The ¹H NMR source attribution confirmed differences in aging processes or regional sources between the two geographic regions, allowing the differentiation between urban combustion-related aerosol and biological particles. The aerosol WSOC in Aveiro, Lisbon, and Rio de Janeiro during summer are more oxidized than those from the remaining locations, indicating the predominance of secondary organic aerosols. Fresh emissions, namely of smoke particles, becomes important during winter in Aveiro and São Paulo, and in Porto Velho during IBB. The biosphere is an important source altering the chemical composition of aerosol WSOC in South America locations. The source attribution in Medellín, Bogotá, Buenos Aires, and Lima confirmed the mixed contributions of biological material, secondary formation, as well as urban and biomass burning emissions. Overall, the information and knowledge acquired in this study provide important diagnostic tools for future studies aiming at understanding the water-soluble organic aerosol problem, their sources and impact at a wider geographic scale.

Two-Dimensional Offline Chromatographic Fractionation for the Characterization of Humic-Like Substances in Atmospheric Aerosol Particles

Organic carbon in atmospheric particles comprises a large fraction of chromatographically unresolved compounds, often referred to as humic-like substances (HULIS), which influence particle properties and impact climate, human health, and ecosystems. To better understand its composition, a two-dimensional (2D) offline method combining size-exclusion (SEC) and reversed-phase liquid chromatography (RP-HPLC) using a new spiked gradient profile is presented. It separates HULIS into 55 fractions of different size and polarity, with estimated ranges of molecular weight and octanol/water partitioning coefficient (log P) from 160-900 g/mol and 0.2-3.3, respectively. The distribution of HULIS within the 2D size versus polarity space is illustrated with heat maps of ultraviolet absorption at 254 nm. It is found to strongly differ in a small example set of samples from a background site near Leipzig, Germany. In winter, the most intense signals were obtained for the largest molecules (>520 g/mol) with low polarity (log P ∼ 1.9), whereas in summer, smaller (225-330 g/mol) and more polar (log P ∼ 0.55) molecules dominate. The method reveals such differences in HULIS composition in a more detailed manner than previously possible and can therefore help to better elucidate the sources of HULIS in different seasons or at different sites. Analyzing Suwannee river fulvic acid as a common HULIS surrogate shows a similar polarity range, but the sizes are clearly larger than those of atmospheric HULIS.

Laboratory Studies of the Cloud Droplet Activation Properties and Corresponding Chemistry of Saline Playa Dust

Playas emit large quantities of dust that can facilitate the activation of cloud droplets. Despite the potential importance of playa dusts for cloud formation, most climate models assume that all dust is nonhygroscopic; however, measurements are needed to clarify the role of dusts in aerosol-cloud interactions. Here, we report measurements of CCN activation from playa dusts and parameterize these results in terms of both κ-Köhler theory and adsorption activation theory for inclusion in atmospheric models. κ ranged from 0.002 ± 0.001 to 0.818 ± 0.094, whereas Frankel-Halsey-Hill (FHH) adsorption parameters of A_FHH = 2.20 ± 0.60 and B_FHH = 1.24 ± 0.14 described the water uptake properties of the dusts. Measurements made using aerosol time-of-flight mass spectrometry (ATOFMS) revealed the presence of halite, sodium sulfates, and sodium carbonates that were strongly correlated with κ underscoring the role that mineralogy, including salts, plays in water uptake by dust. Predictions of κ made using bulk chemical techniques generally showed good agreement with measured values. However, several samples were poorly predicted suggesting that chemical heterogeneities as a function of size or chemically distinct particle surfaces can determine the hygroscopicity of playa dusts. Our results further demonstrate the importance of dust in aerosol-cloud interactions.

Hygroscopic behavior of water-soluble matter in marine aerosols over the East China Sea

In this study, we investigated hygroscopic properties of water-soluble matter (WSM) in marine aerosols over the East China Sea, which were collected during a Natural Science Foundation of China (NSFC) sharing cruise in 2014. Hygroscopic growth factors (g) of WSM were measured by a hygroscopicity tandem differential mobility analyzer (H-TDMA) with an initial dry particle mobility diameter of 100nm. The observed g at 90% relative humidity (RH), g(90%)_WSM, defined as the ratio of the particle diameter at 90% RH to that at RH<5% (initial dry diameter), ranged from 1.67 to 2.41 (mean±std: 1.99±0.23). The g values were lower than that of seawater (2.1) but comparable with those reported for marine aerosols (1.79-2.08). The H-TDMA retrieved hygroscopicity parameter of WSM, κ_WSM, ranged from 0.46 to 1.56 (0.88±0.35). The observed g(90%)_WSM during the daytime ranged from 1.67 to 2.40 (1.95±0.21) versus 1.71 to 2.41 (2.03±0.26) during the nighttime. κ_WSM was 0.81±0.32 in the daytime and 0.95±0.40 in the nighttime. The day/night differences of g(90%)_WSM and κ_WSM indicated that nighttime marine aerosols were more hygroscopic than those in daytime, which was likely related to enhanced heterogeneous reaction of ammonium nitrate in nighttime and the higher Cl^-/Na⁺ molar ratios obtained (0.80) in nighttime than those (0.47) in daytime. Inorganic ions accounted for 72-99% of WSM with SO₄^2- being the dominant species, contributing to 47% of the total inorganic ion mass. The declined g(90%) comparing with sea water was likely due to the transport of anthropogenic aerosols, chemical aging of dust particles, the contribution of biomass burning products, and the aerosol hygroscopic growth inhibition of organics.

Characterization and parameterization of aerosol cloud condensation nuclei activation under different pollution conditions

To better understand the cloud condensation nuclei (CCN) activation capacity of aerosol particles in different pollution conditions, a long-term field experiment was carried out at a regional GAW (Global Atmosphere Watch) station in the Yangtze River Delta area of China. The homogeneity of aerosol particles was the highest in clean weather, with the highest active fraction of all the weather types. For pollution with the same visibility, the residual aerosol particles in higher relative humidity weather conditions were more externally mixed and heterogeneous, with a lower hygroscopic capacity. The hygroscopic capacity (κ) of organic aerosols can be classified into 0.1 and 0.2 in different weather types. The particles at ~150 nm were easily activated in haze weather conditions. For CCN predictions, the bulk chemical composition method was closer to observations at low supersaturations (≤0.1%), whereas when the supersaturation was ≥0.2%, the size-resolved chemical composition method was more accurate. As for the mixing state of the aerosol particles, in haze, heavy haze, and severe haze weather conditions CCN predictions based on the internal mixing assumption were robust, whereas for other weather conditions, predictions based on the external mixing assumption were more accurate.

Unraveling the structural features of organic aerosols by NMR spectroscopy: a review

Our limited understanding of the effect of organic aerosols (OAs) on the climate and human health is largely because of the vast array of formation processes and sources that produce a multitude of molecular structures and physical properties. The need to unravel the enormous complexity and heterogeneity of OAs and thus understand their effects on the climate and human health has led to the development of different off-line methods based on the use of advanced analytical techniques. Within this context, nuclear magnetic resonance (NMR) spectroscopy has become essential for acquiring detailed structural characterization of the complex natural organic matter contained in atmospheric aerosols. In this article, we present a critical review on the application of NMR spectroscopy in OAs (primary and secondary) studies, focusing mainly on the water-soluble organic fraction, and how NMR has impacted our knowledge on atmospheric organic matter. A major emphasis is given on the wealth of chemical information that solid-state and multi-dimensional solution-state NMR can provide, including the sources, formation pathways, seasonal, and regional characterization of atmospheric OAs. Finally, major challenges are discussed and recommendations for future research directions are proposed.

Surfactants from the gas phase may promote cloud droplet formation

Clouds, a key component of the climate system, form when water vapor condenses upon atmospheric particulates termed cloud condensation nuclei (CCN). Variations in CCN concentrations can profoundly impact cloud properties, with important effects on local and global climate. Organic matter constitutes a significant fraction of tropospheric aerosol mass, and can influence CCN activity by depressing surface tension, contributing solute, and influencing droplet activation kinetics by forming a barrier to water uptake. We present direct evidence that two ubiquitous atmospheric trace gases, methylglyoxal (MG) and acetaldehyde, known to be surface-active, can enhance aerosol CCN activity upon uptake. This effect is demonstrated by exposing acidified ammonium sulfate particles to 250 parts per billion (ppb) or 8 ppb gas-phase MG and/or acetaldehyde in an aerosol reaction chamber for up to 5 h. For the more atmospherically relevant experiments, i.e., the 8-ppb organic precursor concentrations, significant enhancements in CCN activity, up to 7.5% reduction in critical dry diameter for activation, are observed over a timescale of hours, without any detectable limitation in activation kinetics. This reduction in critical diameter enhances the apparent particle hygroscopicity up to 26%, which for ambient aerosol would lead to cloud droplet number concentration increases of 8-10% on average. The observed enhancements exceed what would be expected based on Köhler theory and bulk properties. Therefore, the effect may be attributed to the adsorption of MG and acetaldehyde to the gas-aerosol interface, leading to surface tension depression of the aerosol. We conclude that gas-phase surfactants may enhance CCN activity in the atmosphere.