Volatility Basis Set Distributions and Viscosity of Organic Aerosol Mixtures: Insights from Chemical Characterization Using Temperature-Programmed Desorption-Direct Analysis in Real-Time High-Resolution Mass Spectrometry.

Quantitative assessment of gas-particle partitioning of individual components within complex atmospheric organic aerosol (OA) mixtures is critical for predicting and comprehending the formation and evolution of OA particles in the atmosphere. This investigation leverages previously documented data obtained through a temperature-programmed desorption-direct analysis in real-time, high-resolution mass spectrometry (TPD-DART-HRMS) platform. This methodology facilitates the bottom-up construction of volatility basis set (VBS) distributions for constituents found in three biogenic secondary organic aerosol (SOA) mixtures produced through the ozonolysis of α-pinene, limonene, and ocimene. The apparent enthalpies (ΔH*, kJ mol-1) and saturation mass concentrations (CT*, µg·m-3) of individual SOA components, determined as a function of temperature (T, K), facilitated an assessment of changes in VBS distributions and gas-particle partitioning with respect to T and atmospheric total organic mass loadings (tOM, µg·m-3). The VBS distributions reveal distinct differences in volatilities among monomers, dimers, and trimers, categorized into separate volatility bins. At the ambient temperature of T = 298 K, only monomers efficiently partition between gas and particle phases across a broad range of atmospherically relevant tOM values of 1-100 µg·m-3. Partitioning of dimers and trimers becomes notable only at T > 360 K and T > 420 K, respectively. The viscosity of SOA mixtures is assessed using a bottom-up calculation approach, incorporating the input of elemental formulas, ΔH*, CT*, and particle-phase mass fractions of the SOA components. Through this approach, we are able to accurately estimate the variations in SOA viscosity that result from the evaporation of its components. These variations are, in turn, influenced by atmospherically relevant changes in tOM and T. Comparison of the calculated SOA viscosity and diffusivity values with literature reported experimental results shows close agreement, thereby validating the employed calculation approach. These findings underscore the significant potential for TPD-DART-HRMS measurements in enabling the untargeted analysis of organic molecules within OA mixtures. This approach facilitates quantitative assessment of their gas-particle partitioning and allows for the estimation of their viscosity and condensed-phase diffusion, thereby contributing valuable insights to atmospheric models.

Dominant contribution of combustion-related ammonium during haze pollution in Beijing.

Aerosol ammonium (NH4+), mainly produced from the reactions of ammonia (NH3) with acids in the atmosphere, has significant impacts on air pollution, radiative forcing, and human health. Understanding the source and formation mechanism of NH4+ can provide scientific insights into air quality improvements. However, the sources of NH3 in urban areas are not well understood, and few studies focus on NH3/NH4+ at different heights within the atmospheric boundary layer, which hinders a comprehensive understanding of aerosol NH4+. In this study, we perform both field observation and modeling studies (the Community Multiscale Air Quality, CMAQ) to investigate regional NH3 emission sources and vertically resolved NH4+ formation mechanisms during the winter in Beijing. Both stable nitrogen isotope analyses and CMAQ model suggest that combustion-related NH3 emissions, including fossil fuel sources, NH3 slip, and biomass burning, are important sources of aerosol NH4+ with more than 60% contribution occurring on heavily polluted days. In contrast, volatilization-related NH3 sources (livestock breeding, N-fertilizer application, and human waste) are dominant on clean days. Combustion-related NH3 is mostly local from Beijing, and biomass burning is likely an important NH3 source (∼15%-20%) that was previously overlooked. More effective control strategies such as the two-product (e.g., reducing both SO2 and NH3) control policy should be considered to improve air quality.

Sedimentary organic molecular compositions reveal the influence of glacier retreat on ecology on the Tibetan Plateau.

Global warming and glacier retreat have significant impacts on the structure and function of natural ecosystems. However, little is known about how glacier retreat affects the long-term evolution of ecosystems at high-altitude regions. In this study, we explored the possible effects of glacier retreat on catchment vegetation and lake productivity in Lake Puma Yumco, southeastern Tibetan Plateau, based on detailed organic molecular compositions determined by an ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and combined with various sedimentary geochemical indicators. The glaciers in the catchment keep retreating since 1870 CE, as inferred from the multiple indices of total organic carbon content (TOC), total nitrogen content (TN), C/N ratios, and carbonate contents. Accompanying modern global warming and glacier shrinkage, the relative abundance of soil- and vegetation-derived large molecular compounds (e.g., vascular plant-derived polyphenols, highly unsaturated and phenolic compounds, and condensed aromatics) increased gradually in lake sediments, suggesting that ice-covered land was exposed under warming condition, and gradually revegetation occurred. Both increases in relative abundance of nitrogen-containing compounds (e.g., CHNO) and chlorophyll derivative contents in the lake sediments were observed since 1870 CE, suggesting that stronger catchment weathering and increasing terrestrial nutrient loads enhanced the downstream lake productivity after glacier retreat. Our results imply that continued global warming and alpine glacier retreat in the future may further promote vegetation expansion and increases in lake productivity on the Tibetan Plateau.

Molecular transformation of organic nitrogen in Antarctic penguin guano-affected soil.

Organic nitrogen (ON) is an important participant in the Earth's N cycle. Previous studies have shown that penguin feces add an abundance of nutrients including N to the soil, significantly changing the eco-environment in ice-free areas in Antarctica. To explore the molecular transformation of ON in penguin guano-affected soil, we collected guano-free weathered soil, modern guano-affected soil from penguin colonies, ancient guano-affected soil from abandoned penguin colonies, and penguin feces from the Ross Sea region, Antarctica, and Fourier transform ion cyclotron mass spectrometry (FT-ICR MS) was used to investigate the chemical composition of water-extractable ON. By comparing the molecular compositions of ON among different samples, we found that the number of ON compounds (>4,000) in weathered soil is minimal, while carboxylic-rich alicyclic-like molecules (CRAM-like) are dominant. Penguin feces adds ON into the soil with > 10,000 CHON, CHONS and CHN compounds, including CRAM-like, lipid-like, aliphatic/ peptide-like molecules and amines in the guano-affected soil. After the input of penguin feces, macromolecules continue to degrade, and other ON compounds tend to be oxidized into relatively stable CRAM-like molecules, this is an important transformation process of ON in guano-affected soils. We conclude the roles of various forms of ON in the N cycle are complex and diverse. Combined with previous studies, ON eventually turns into inorganic N and is lost from the soil. The lost N ultimately returns to the ocean and the food web, thus completing the N cycle. Our study preliminarily reveals the molecular transformation of ON in penguin guano-affected soil and is important for understanding the N cycle in Antarctica.

Secondary Mineral Formation and Carbon Dynamics during FeS Oxidation in the Presence of Dissolved Organic Matter.

Iron (Fe) minerals constitute a major control on organic carbon (OC) storage in soils and sediments. While previous research has mainly targeted Fe (oxyhydr)oxides, the impact of Fe sulfides and their subsequent oxidation on OC dynamics remains unresolved in redox-fluctuating environments. Here, we investigated the impact of dissolved organic matter (DOM) on FeS oxidation and how FeS and its oxidation may alter the retention and nature of DOM. After the anoxic reaction of DOM with FeS, FeS preferentially removed high-molecular-weight and nitrogen-rich compounds and promoted the formation of aqueous sulfurized organic molecules, according to Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) analysis. When exposed to O2, FeS oxidized to nanocrystalline lepidocrocite and additional aqueous sulfurized organic compounds were generated. The presence of DOM decreased the particle size of the resulting nano-lepidocrocite based on Mössbauer spectroscopy. Following FeS oxidation, most solid-phase OC remained associated with the newly formed lepidocrocite via a monodentate chelating mechanism (based on FTIR analysis), and FeS oxidation caused only a slight increase in the solubilization of solid-phase OC. Collectively, this work highlights the under-appreciated role of Fe sulfides and their oxidation in driving OC transformation and preservation.

High contribution from outdoor air to personal exposure and potential inhaled dose of PM2.5 for indoor-active university students.

People spend most of their time indoors, isolated from the outdoor environment where serious air pollution usually occurs. To what extent outdoor air pollution contributes to their daily personal exposure and inhaled dose? To fill this knowledge gap, an exposure assessment study was conducted for indoor-active university students during a wintertime period of hazy and non-hazy (clear) days in Beijing. Indoor and outdoor fine particulate matter (PM2.5) samples were collected at six indoor microenvironments, and two outdoor environments representing traffic and ambient exposure in the university, respectively, to estimate the personal exposure of students. The average daily personal exposure and poteantial inhaled dose on hazy days (124.8 ± 72.3 µg m-3 and 2.74 ± 1.53 mg) were much higher than that on clear days (57.5 ± 31.9 µg m-3 and 1.26 ± 0.59 mg), indicating a significant influence from the ambient air quality. The indoor PM2.5 concentrations were significantly and positively correlated with the outdoor ones (r = 0.67-0.96) with an FINF (infiltration factor) range of 0.44-0.81 during sampling periods. The outdoor-origin air contributed 68%-95% to the total indoor PM2.5, the average of which was higher during haze events (87%) than clear periods (73%). Correspondingly, outdoor-origin PM2.5 contributed around 105.4 µg m-3 and 2.41 mg (85% and 89%) to the daily exposure and inhaled dose of college students on hazy days, respectively, compared to just 39.2 µg m-3 and 0.95 mg (68% and 75%) on clear days. Our results highlight the significant contribution of outdoor-origin PM2.5 occurred indoor to both the daily personal exposure and inhaled dose due to air pollution filtration between outdoor and indoor environments. These also suggest a continuous effort not only on ambient air quality improvements, but also on environmental friendly building for public health protection with lower exposure.

Biological and Nonbiological Sources of Fluorescent Aerosol Particles in the Urban Atmosphere.

Online detection of bioaerosols based on the light-induced fluorescence (LIF) technique is still challenging due to the complexity of bioaerosols and the external/internal mixing with nonbiological fluorescent compositions. Although many lab studies have measured the fluorescence properties of the biological and nonbiological materials, there is still a scarcity of knowledge of the sources of fluorescent aerosol particles (FAP) in the ambient atmosphere. Here, we fill this gap by combining the online measurement of an LIF-based instrument (wideband integrated bioaerosol sensor, WIBS, 0.8-20 µm) with the measurements of typical biological matter and the compositions related to major nonbiological FAP from May to July in the megacity Beijing. We find that fungal spores and pollen are widely observed in all types of FAP using a WIBS. Bacteria are suggested to be associated with the fine mode FAP (excitation/emission: 280 nm/310-400 nm; 0.8-3 µm). The FL-B and -BC particles (emission in 420-650 nm) contributing the most to FAP are strongly associated with humic-like substances, dust, burning and combustion emissions, and secondary organic aerosols (SOA). This study provides a guide for interpreting individual FAP measured by LIF instruments and points to the applicability of online LIF instruments to characterize nonbiological compositions including SOA.

Dwindling aromatic compounds in fine aerosols from chunk coal to honeycomb briquette combustion.

With the implementation of clean coal policy in China, the chunk coal has been gradually replaced by honeycomb briquette in domestic energies. In this study, the molecular composition of fine particles (PM2.5) from chunk coal and honeycomb briquette combustion is characterized using the Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS). More than 6000 molecular formulae were detected in each PM2.5 sample. A remarkable decrease in unsaturation and aromatic compounds was found from chunk coal to honeycomb briquette derived aerosols. Around 73.6% of the unique CHON compounds in chunk coal are considered to have aromatic structures, while it decreased to 7.3% in honeycomb briquette. Most of these nitroaromatics detected only in chunk coal are highly carcinogenic and mutagenic with 4-6 rings. Moreover, the aromatic compounds in sulfur-containing compounds also showed a significant decrease. Meanwhile, because of the perforated shape and the additives added during the production of honeycomb briquettes, there are more heteroatoms-containing molecules released from honeycomb briquette combustion, which are highly functional compounds with high molecular weight, high degree of oxidation, and low volatility. Our results provide molecular level evidence that the transformation from chunk coal to honeycomb briquette can effectively reduce the emission of aromatic compounds, which is beneficial to assessing and reducing the impacts to climate change as well as human health.

Source and formation process impact the chemodiversity of rainwater dissolved organic matter along the Yangtze River Basin in summer.

Rainwater dissolved organic matter (DOM) plays an important role in the biogeochemical cycle and evolution of organic matter in the land-atmosphere interface. To better understand their sources and molecular composition in the atmosphere, rainwater samples were collected at six different locations along the Yangtze River Basin. Based on the application of a combined approach including excitation-emission matrix (EEM) fluorescence and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), various sources (terrestrial, anthropogenic, and autochthonous sources) of rainwater DOM were revealed. Results show that the derivatives of biogenic volatile organic compounds were widely distributed and contributed to rainwater DOM along the Yangtze River Basin. In the up-river city Batang, rainwater DOM was affected by the long-range atmospheric transport due to the Indian summer monsoon. Lijiang, a city on the southeastern edge of Tibetan plateau, was related to strong local biomass burning. The industrial cities of Panzhihua and Luzhou showed large differences in organic composition due to distinct industrial types. Fuling, a district in Chongqing Municipality, was significantly contributed by aged organics from biomass burning. While rainwater DOM in Shanghai, a coastal megacity, contained a high fraction of sea spray organics. Further, more than 70% of rainwater DOM molecules are associated with 36 typical transformation mechanisms during rainwater-scavenging processes, e.g., oxidation reactions, dealkylation and decarboxylation. Our study demonstrates that local natural and anthropogenic emissions and climatic conditions strongly shaped the chemodiversity and possible precursor-product pairs of rainwater DOM along the Yangtze River Basin, which helps to better understand the biogeochemical cycles of organic matter in a large-scale watershed under the influence of human activities.

Acidification impacts on the molecular composition of dissolved organic matter revealed by FT-ICR MS.

Solid-phase extraction (SPE) is a traditional pretreatment procedure widely used for dissolved organic matter (DOM) desalination and enrichment prior to the Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis, and the extracts are usually acidified to pH = 2 with hydrochloric acid (HCl) before passing through the cartridge. However, little is known about the effects of acidification on the integrity and molecular composition of DOM. Here, the differences in the molecular compositions in acidified and nonacidified DOM samples of soil, seawater and atmospheric aerosol were performed by FT-ICR MS. The results showed that the quantity and intensity of aromatic compounds with high oxygen content (e.g., polyphenols, tannin-like and highly oxygenated organic compounds) were greatly enhanced after acidification, while highly saturated compounds (lipid-like and aliphatic/peptide-like) were absent. The underlying reason is the variation of solubility and hydrolysis of DOM under acidic conditions. Meanwhile, the effect of acidification on the molecular composition of DOM was also dependent on their original environmental media. Based on these results, we suggest that the extracts of soil samples are selectively acidified according to the focus of research, while the extract is acidified for seawater samples and the pH of the extract can be unadjusted for aerosol samples before the SPE procedure. These findings provide a reference for the selection of suitable pretreatment methods for different experimental purposes and for the comprehensive characterization of samples with different properties.

High Molecular Diversity of Organic Nitrogen in Urban Snow in North China.

Snow serves as a vital scavenging mechanism to gas-phase and particle-phase organic nitrogen substances in the atmosphere, providing a significant link between land-atmosphere flux of nitrogen in the surface-earth system. Here, we used optical instruments (UV-vis and excitation-emission matrix fluorescence) and a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) to elucidate the molecular composition and potential precursors of snow samples collected simultaneously at four megacities in North China. The elemental O/N ratio (≥3), together with the preference in the negative ionization mode, indicates that the one and two nitrogen atom-containing organics (CHON1 and CHON2) in snow were largely in the oxidized form (as organic nitrates, -ONO2). This study assumed that scavenging of particle-phase and gas-phase organic nitrates might be significant sources of CHON in precipitation. A gas-phase oxidation process and a particle-phase hydrolysis process, at a molecular level, were used to trace the potential precursors of CHON. Results show that more than half of the snow CHON molecules may be related to the oxidized and hydrolyzed processes of atmospheric organics. Potential formation processes of atmospheric organics on a molecular level provide a new concept to better understand the sources and scavenging mechanisms of organic nitrogen species in the atmosphere.

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 PM2.5 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.

Molecular characterization of size-segregated organic aerosols in the urban boundary layer in wintertime Beijing by FT-ICR MS.

Organic aerosols, complicated mixtures of organic compounds, are important constituents of atmospheric particulate matter. However, little is known about the size distributions and vertical profiles of these constituents at a molecular level in the urban boundary layer. Here, we characterized the molecular compositions of size-segregated samples collected simultaneously at two heights (8 m and 260 m above ground level) in urban Beijing during the winter of 2018. The CHO, CHNO, CHOS, and CHNOS subgroups in water-soluble organic carbon were characterized using a 15-T ultrahigh-resolution Fourier transform-ion cyclotron resonance (FT-ICR) mass spectrometer. We found that both their numbers and magnitudes increased with a decrease in the particle size, especially for high molecular weight (HMW) compounds, except CHNOS. The number of CHNOS species also increased in the coarse mode, presumably because the alkalinity could inhibit their hydrolysis in the coarse mode. The compounds in small particles with higher O/C ratios and carbon oxidation state were possibly more aged, while the coarse particles with more lipid- and peptide-like compounds should originate from fresh emissions. Moreover, as the oxidation state increases in small particles, functionalization is enhanced for sulfur-containing compounds with fracturing of the benzene ring, while CHO and CHNO are potentially dominated by demethylation with ring-retaining products. It is worth noting that common compounds with the same molecular characteristics accounted for more than 86% of the total compounds between 260 m and ground level (8 m), demonstrating that the aerosols were well mixed in the urban boundary layer. Nonetheless, the relative content of the compounds was higher at ground level due to the impact of primary emissions, which increased with the particle size. In addition, the compounds in submicron particles were more oxidized at 260 m, while the opposite was observed in the coarse mode.

Molecular composition and sources of water-soluble organic aerosol in summer in Beijing.

Water-soluble organic aerosol (WSOA) constitutes a large fraction of OA and plays an important role in formation of secondary OA (SOA). Here we characterized the sources and molecular composition of WSOA in summer in Beijing using high-resolution aerosol mass spectrometer and orbitrap mass spectrometer equipped with electrospray ionization. Our results showed that WSOA was the major fraction of OA on average accounting for 69% in summer, which is much higher than that (47%) in winter. However, the oxidation degree of WSOA was comparable between summer and winter (O/C = 0.62 vs. 0.63). Positive matrix factorization analysis showed that SOA contributed dominantly to WSOA (72%) indicating that WSOA was mainly from secondary formation. The two water-soluble SOA factors that are associated with regional processing (OOA-1) and photochemical production (OOA-2), respectively, showed very different behaviors throughout the study. OOA-2 showed much enhanced contribution during polluted periods with low relative humidity (RH), while OOA-1 played a more important role during high RH periods. Molecular composition analysis of WSOA revealed a high diversity of CHO (compounds only containing carbon, hydrogen and oxygen) and CHOS (sulfur-containing organics) in WSOA in summer. Particularly, the relative intensity fraction of CHOS- compounds was increased by 42% from clean to polluted days which was associated with large increases (20%) in organosulfates (OSs) with lower O∗/C (0.1-0.4), and OOA-1. These results suggest the formation of more unsaturated OSs in OOA-1 during polluted days in summer. Comparatively, the biogenic-derived OSs remained relatively stable (24-31%) for the entire study highlighting the ubiquitous importance of biogenic SOA in summer.

Water-soluble low molecular weight organics in cloud water at Mt. Tai Mo Shan, Hong Kong.

Cloud-water samples collected at the summit of Mt. Tai Mo Shan (Mt. TMS, 957 m, a.s.l.), Hong Kong in autumn 2016 and spring 2017 were measured for molecular compositions and stable carbon isotope ratios (δ13C) of dicarboxylic acids, oxoacids and α-dicarbonyls. Oxalic acid (C2, 253-1680 µg L-1) was found as the most abundant diacid, followed by succinic acid (C4, 24-656 µg L-1) in autumn and phthalic acid (Ph, 27-363 µg L-1) in spring. Higher concentrations of Ph (192 ±â€¯197 µg L-1) and terephthalic acid (tPh, 31 ±â€¯15 µg L-1) were observed in autumn than those in spring, illustrating the enhanced contribution from fossil fuel combustion and plastic wastes burning. Stronger correlations for the shorter chain diacids (C2-C4) with NO3-, nss-SO42- and nss-K+ in autumn (R2 ≥ 0.7) than spring suggested that these diacids were mainly produced via atmospheric photooxidation following anthropogenic emissions. The δ13C values of C2 (mean - 14.7‰), glyoxylic acid (ωC2, -12.2‰), pyruvic acid (Pyr, -15.5‰), glyoxal (Gly, -13.5‰) were much higher than those in atmospheric aerosols from isoprene and other precursors, indicating that diacids, oxoacids and α-dicarbonyls in cloud at Mt. TMS were significantly influenced by photochemical formation during the long-range atmospheric transport.

The organic molecular composition, diurnal variation, and stable carbon isotope ratios of PM2.5 in Beijing during the 2014 APEC summit.

Organic tracers are useful for investigating the sources of carbonaceous aerosols but there are still no adequate studies in China. To obtain insights into the diurnal variations, properties, and the influence of regional emission controls on carbonaceous aerosols in Beijing, day-/nighttime PM2.5 samples were collected before (Oct. 15th - Nov. 2nd) and during (Nov. 3rd - Nov. 12th) the 2014 Asia-Pacific Economic Cooperation (APEC) summit. Eleven organic compound classes were analysed using gas chromatography/mass spectrometry (GC/MS). In addition, the stable carbon isotope ratios (δ13CTC) of total carbon (TC) were detected using an elemental analyser/isotope ratio mass spectrometry (EA/irMS). Most of the organic compounds were more abundant during the night than in the daytime, and their concentrations generally decreased during the APEC. These features were associated with the strict regional emission controls and meteorological conditions. The day/night variations of δ13CTC were smaller during the APEC than those before the APEC the summit, suggesting that regionally transported aerosols are potentially played an important role in the loading of organic aerosols in Beijing before the APEC summit. The source apportionment based on the organic tracers suggested that biomass burning, plastic and microbial emissions, and fossil fuel combustion were important sources of organic aerosols in Beijing. Furthermore, a similar contribution of biomass burning to OC before and during the APEC suggests biomass burning was a persistent contributor to PM2.5 in Beijing and its surroundings.