Individual Atmospheric Particles Research Articles

Glass Transition Temperatures of Individual Submicrometer Atmospheric Particles: Direct Measurement via Heated Atomic Force Microscopy Probe.

The phase (solid, semisolid, or liquid) of atmospheric aerosols is central to their ability to take up water or undergo heterogeneous reactions. In recent years, the unexpected prevalence of viscous organic particles has been shown through field measurements and global atmospheric modeling. The aerosol phase has been predicted using glass transition temperatures (Tg), which were estimated based on molecular weight, oxygen:carbon ratio, and chemical formulae of organic species present in atmospheric particles via studies of bulk materials. However, at the most important sizes for cloud nucleation (∼50-500 nm), particles are complex mixtures of numerous organic species, inorganic salts, and water with substantial particle-to-particle variability. To date, direct measurements of Tg have not been feasible for individual atmospheric particles. Herein, nanothermal analysis (NanoTA), which uses a resistively heated atomic force microscopy (AFM) probe, is combined with AFM photothermal infrared (AFM-PTIR) spectroscopy to determine the Tg and composition of individual particles down to 76 nm in diameter at ambient temperature and pressure. Laboratory-generated proxies for organic aerosol (sucrose, ouabain, raffinose, and maltoheptaose) had similar Tg values to bulk Tg values measured with differential scanning calorimetry (DSC) and the Tg predictions used in atmospheric models. Laboratory-generated phase-separated particles and ambient particles were analyzed with NanoTA + AFM-PTIR showing intraparticle variation in composition and Tg. These results demonstrate the potential for NanoTA + AFM-PTIR to increase our understanding of viscosity within submicrometer atmospheric particles with complex phases, morphologies, and compositions, which will enable improved modeling of aerosol impacts on clouds and climate.

Solid organic-coated ammonium sulfate particles at high relative humidity in the summertime Arctic atmosphere

SignificancePhysical and chemical properties of individual atmospheric particles determine their climate impacts. Hygroscopic inorganic salt particles mixed with trace amounts of organic material are predicted to be liquid under typical tropospheric conditions in the summertime Arctic. Yet, we unexpectedly observed a significant concentration of solid particles composed of ammonium sulfate with an organic coating under conditions of high relative humidity and low temperature. These particle properties are consistent with marine biogenic-derived new particle formation and growth, with particle collision hypothesized to result in the solid phase. This particle source is predicted to have increasing relevance in the context of declining Arctic sea ice and increasing open water, with impacts on clouds, and therefore climate.

Using Micro-Raman Spectroscopy to Investigate Chemical Composition, Mixing States, and Heterogeneous Reactions of Individual Atmospheric Particles.

Measuring the chemical composition of individual atmospheric aerosol particles can provide direct evidence of their heterogeneous reactions and mixing states in the atmosphere. In this study, micro-Raman spectroscopy was used to measure the chemical composition of 1200 individual atmospheric particles in 11 samples collected in Beijing air. (NH4)2SO4, NH4NO3, various minerals, carbonaceous species (soot and organics), and NaNO3 were identified in the measured particles according to their characteristic Raman peaks. These species represented the main components of aerosol particles. In individual particles, NH4NO3 and (NH4)2SO4 either existed separately or were internally mixed. Possible reaction pathways of CaCO3 particles in the atmosphere were proposed based on the results of this study and laboratory simulations on heterogeneous reactions in the literature. CaCO3 reacted with N- and S-containing (nitrogen- and sulfur-containing) acidic gases to produce Ca(NO3)2 and CaSO4. Ca(NO3)2 further reacted with S-containing acidic gases and oxidants to produce CaSO4. Of the soot-containing particles, 23% were internal mixtures of soot and inorganic material. Of the organics-containing particles, 57% were internal mixtures of organic and inorganic materials. Micro-Raman spectroscopy directly identified functional groups and molecules in individual atmospheric particles under normal ambient conditions, rendering it a powerful tool for measuring the chemical composition of individual atmospheric particles with a diameter of ≥1.0 μm.

Coexistence of three liquid phases in individual atmospheric aerosol particles

Individual atmospheric particles can contain mixtures of primary organic aerosol (POA), secondary organic aerosol (SOA), and secondary inorganic aerosol (SIA). To predict the role of such complex multicomponent particles in air quality and climate, information on the number and types of phases present in the particles is needed. However, the phase behavior of such particles has not been studied in the laboratory, and as a result, remains poorly constrained. Here, we show that POA+SOA+SIA particles can contain three distinct liquid phases: a low-polarity organic-rich phase, a higher-polarity organic-rich phase, and an aqueous inorganic-rich phase. Based on our results, when the elemental oxygen-to-carbon (O:C) ratio of the SOA is less than 0.8, three liquid phases can coexist within the same particle over a wide relative humidity range. In contrast, when the O:C ratio of the SOA is greater than 0.8, three phases will not form. We also demonstrate, using thermodynamic and kinetic modeling, that the presence of three liquid phases in such particles impacts their equilibration timescale with the surrounding gas phase. Three phases will likely also impact their ability to act as nuclei for liquid cloud droplets, the reactivity of these particles, and the mechanism of SOA formation and growth in the atmosphere. These observations provide fundamental information necessary for improved predictions of air quality and aerosol indirect effects on climate.

Sequential SEM-EDS, PLM, and MRS Microanalysis of Individual Atmospheric Particles: A Useful Tool for Assigning Emission Sources.

In this work, the particulate matter (PM) from three different monitoring stations in the Monterrey Metropolitan Area in Mexico were investigated for their compositional, morphological, and optical properties. The main aim of the research was to decipher the different sources of the particles. The methodology involved the ex situ sequential analysis of individual particles by three analytical techniques: scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), polarized light microscopy (PLM), and micro-Raman spectroscopy (MRS). The microanalysis was performed on samples of total suspended particles. Different morphologies were observed for particles rich in the same element, including prismatic, spherical, spheroidal, and irregular morphologies. The sequential microanalysis by SEM-EDS/PLM/MRS revealed that Fe-rich particles with spherical and irregular morphologies were derived from anthopogenic sources, such as emissions from the metallurgical industry and the wear of automobile parts, respectively. In contrast, Fe-rich particles with prismatic morphologies were associated with natural sources. In relation to carbon (C), the methodology was able to distinguish between the C-rich particles that came from different anthopogenic sources—such as the burning of fossil fuels, biomass, or charcoal—and the metallurgical industry. The optical properties of the Si-rich particles depended, to a greater extent, on their chemical composition than on their morphology, which made it possible to quickly and accurately differentiate aluminosilicates from quartz. The methodology demonstrated in this study was useful for performing the speciation of the particles rich in different elements. This differentiation helped to assign their possible emission sources.

Atomic Force Microscopy: An Emerging Tool in Measuring the Phase State and Surface Tension of Individual Aerosol Particles.

Atmospheric aerosols are suspended particulate matter of varying composition, size, and mixing state. Challenges remain in understanding the impact of aerosols on the climate, atmosphere, and human health. The effect of aerosols depends on their physicochemical properties, such as their hygroscopicity, phase state, and surface tension. These properties are dynamic with respect to the highly variable relative humidity and temperature of the atmosphere. Thus, experimental approaches that permit the measurement of these dynamic properties are required. Such measurements also need to be performed on individual, submicrometer-, and supermicrometer-sized aerosol particles, as individual atmospheric particles from the same source can exhibit great variability in their form and function. In this context, this review focuses on the recent emergence of atomic force microscopy as an experimental tool in physical, analytical, and atmospheric chemistry that enables such measurements. Remaining challenges are noted and suggestions for future studies are offered.

Photochemical reactions on aerosols at West Antarctica: A molecular case-study of nitrate formation among sea salt aerosols

Environmental implications of climate change are complex and exhibit regional variations both within and between the polar regions. The increase of solar UV radiation flux over Antarctica due to stratospheric ozone depletion creates the optimal conditions for photochemical reactions on the snow. Modeling, laboratory, and indirect field studies suggest that snowpack process release gases to the atmosphere that can react on sea salt particles in remote regions such as Antarctica, modifying aerosol composition and physical properties of aerosols. Here, we present evidence of photochemical processing in West Antarctica aerosols using microscopic and chemical speciation of individual atmospheric particles. Individual aerosol particles collected at the Brazilian module Criosfera 1 were analyzed by scanning transmission X-ray microscopy with near edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) combined with computer-controlled scanning electron microscopy (CCSEM) with energy-dispersive X-ray (EDX) microanalysis. The displacement of chlorine relative to sodium was observed over most of the sea salt particles. Particles with a chemical composition consistent with NaCl-NO3 contributed up to 30% of atmospheric particles investigated. Overall, this study provides evidence that the snowpack and particulate nitrate photolysis should be considered in dynamic partition equilibrium in the troposphere. These findings may assist in reducing modeling uncertainties and present new insights into the aerosol chemical composition in the polar environment.

Emerging investigator series: influence of marine emissions and atmospheric processing on individual particle composition of summertime Arctic aerosol over the Bering Strait and Chukchi Sea.

The Arctic is rapidly transforming due to sea ice loss, increasing shipping activity, and oil and gas development. Associated marine and combustion emissions influence atmospheric aerosol composition, impacting complex aerosol-cloud-climate feedbacks. To improve understanding of the sources and processes determining Arctic aerosol composition, atmospheric particles were collected aboard the Korean icebreaker R/V Araon cruising within the Bering Strait and Chukchi Sea during August 2016. Offline analyses of individual particles by microspectroscopic techniques, including scanning electron microscopy with energy dispersive X-ray spectroscopy and atomic force microscopy with infrared spectroscopy, provided information on particle size, morphology, and chemical composition. The most commonly observed particle types were sea spray aerosol (SSA), comprising ∼60-90%, by number, of supermicron particles, and organic aerosol (OA), comprising ∼50-90%, by number, of submicron particles. Sulfate and nitrate were internally mixed within both SSA and OA particles, consistent with particle multiphase reactions during atmospheric transport. Within the Bering Strait, SSA and OA particles were more aged, with greater number fractions of particles containing sulfate and/or nitrate, compared to particles collected over the Chukchi Sea. This is indicative of greater pollution influence within the Bering Strait from coastal and inland sources, while the Chukchi Sea is primarily influenced by marine sources.

Moving beyond Fine Particle Mass: High-Spatial Resolution Exposure to Source-Resolved Atmospheric Particle Number and Chemical Mixing State.

Background:Most epidemiological studies address health effects of atmospheric particulate matter (PM) using mass-based measurements as exposure surrogates. However, this approach ignores many critical physiochemical properties of individual atmospheric particles. These properties control the deposition of particles in the human lung and likely their toxicity; in addition, they likely have larger spatial variability than PM mass.Objectives:This study was designed to quantify the spatial variability in number, size, source, and chemical mixing state of individual particles in a populous urban area. We quantified the population exposure to these detailed particle properties and compared them to mass-based exposures.Methods:We performed mobile sampling using an advanced single-particle mass spectrometer to measure the spatial variability of number concentration of source-resolved particles and particle mixing state in Pittsburgh, Pennsylvania. We built land-use regression (LUR) models to estimate their spatial patterns and coupled them with demographic data to estimate population exposure.Results:Particle number concentration had a much larger spatial variability than mass concentration within the city. Freshly emitted particles from traffic and cooking drive the variability in particle number, but mass concentrations are dominated by aged background particles composed of secondary materials. In addition, people exposed to elevated number concentrations of atmospheric particles are also exposed to more externally mixed particles.Conclusions:Our advanced measurement technique provides a new exposure picture that resolves the large intra-city spatial heterogeneity in traffic and cooking particle number concentrations in the populous urban area. Our results provide a complementary and more detailed perspective compared with bulk measurements of composition. In addition, given the influence of particle mixing state on properties such as particle deposition in the lung, the large spatial gradients of chemical mixing state may significantly influence the health effects of fine PM. https://doi.org/10.1289/EHP5311

Characteristics of Single Aerosol Particles during Pollution in Winter in an Urban Area of Ningbo, China

ABSTRACT The size and composition of individual atmospheric particles were assessed using a single particle aerosol mass spectrometer in Ningbo, China, from 30 December 2016 to 12 January 2017. The particles were primarily carbonaceous and inorganic, with the majority (60%) being carbonaceous. All of the major particle types contained internally mixed secondary species, such as nitrate and sulfate. The temporal trends of the particle number concentrations and PM2.5 (particulate matter with an aerodynamic diameter < 2.5 µm) mass concentrations indicated that secondary formation in Ningbo severely affected the air quality. The sampling period was divided into three subperiods according to the PM2.5 mass concentration. During Period I, local emission and secondary formation were the primary contributors of the pollution, and during Periods II and III, the abundance of particles occurring with high-speed winds indicated that regional transport was a primary factor in these two pollution processes. Additionally, high PM2.5 mass concentrations were often observed at night during high relative humidity and low temperatures across the entire sampling period. These results demonstrate that stagnant meteorological conditions increase pollution during winter. Overall, this study enhances our understanding of particulate pollution in the southern Yangtze River Delta region and provides useful information on the formation and growth of atmospheric aerosols.

Environmental Transmission Electron Microscopy of Individual Atmospheric Particles from the North Atlantic

Environmental Transmission Electron Microscopy of Individual Atmospheric Particles from the North Atlantic

Heating-Induced Transformations of Atmospheric Particles: Environmental Transmission Electron Microscopy Study.

Environmental transmission electron microscopy was employed to probe transformations in the size, morphology, and composition of individual atmospheric particles as a function of temperature. Two different heating devices were used and calibrated in this work: a furnace heater and a Micro Electro Mechanical System heater. The temperature calibration used sublimation temperatures of NaCl, glucose, and ammonium sulfate particles, and the melting temperature of tin. Volatilization of Suwanee River Fulvic Acid was further used to validate the calibration up to 800 °C. The calibrated furnace holder was used to examine both laboratory-generated secondary organic aerosol particles and field-collected atmospheric particles. Chemical analysis by scanning transmission X-ray microscopy and near-edge fine-structure spectroscopy of the organic particles at different heating steps showed that above 300 °C particle volatilization was accompanied by charring. These methods were then applied to ambient particles collected in the central Amazon region. Distinct categories of particles differed in their volatilization response to heating. Spherical, more-viscous particles lost less volume during heating than particles that spread on the imaging substrate during impaction, due to either being liquid upon impaction or lower viscosity. This methodology illustrates a new analytical approach to accurately measure the volume fraction remaining for individually tracked atmospheric particles at elevated temperatures.

Physicochemical analysis of individual atmospheric fine particles based on effective surface-enhanced Raman spectroscopy

Fine particles associated with haze pollution threaten the health of more than 400 million people in China. It is therefore of great importance to thoroughly investigate and understand their composition. To determine the physicochemical properties in atmospheric fine particles at the micrometer level, we described a sensitive and feasible surface-enhanced Raman scattering (SERS) method using Ag foil as a substrate. This novel method enhanced the Raman signal intensities up to 10,000 a.u. for ν(NO3−) in fine particles. The SERS effect of Ag foil was further studied experimentally and theoretically and found to have an enhancement factor of the order of ~104. Size-fractionated real particle samples with aerodynamic diameters of 0.4–2.5 μm were successfully collected on a heavy haze day, allowing ready observation of morphology and identification of chemical components, such as soot, nitrates, and sulfates. These results suggest that the Ag-foil-based SERS technique can be effectively used to determine the microscopic characteristics of individual fine particles, which will help to understand haze formation mechanisms and formulate governance policies.

Recognition of the Source and Nature of Atmospheric Aerosols in Tehran, Iran

The size, morphology and chemical characterization of individual atmospheric particles have received significant attention due to their effects on radiative and chemical properties. Tehran is one of the most polluted cities not only in Iran but in the world, which is continuously affected by air pollution. In this study, 25 sites were sampled in urban and non-urban areas. The sampling was performed with a high-volume sampler for a period of 14–24 hours. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) were used to characterize the aerosols in the urban and non-urban areas during each season. The SEM results indicate that during summer, flaky, irregularly shaped aggregate particles dominated the urban areas, whereas non-urban areas were relatively clean. During winter, the non-urban sites were observed to be dominated by spherical and irregularly shaped particles, suggesting combustion sources, whereas particles were observed in a more concentrated form at the urban sites. EDS analysis shows varying percentages of C, O, Zn, Si, Ca and Fe at both the urban and non-urban sites for all seasons. Furthermore, this study reveals the variability of the morphological and elemental composition of PM in different seasons at urban and non-urban locations and highlights the various probable sources associated with them.

Isoprene-Derived Organosulfates: Vibrational Mode Analysis by Raman Spectroscopy, Acidity-Dependent Spectral Modes, and Observation in Individual Atmospheric Particles.

Isoprene, the most abundant biogenic volatile organic compound (BVOC) in the atmosphere, and its low-volatility oxidation products lead to secondary organic aerosol (SOA) formation. Isoprene-derived organosulfates formed from reactions of isoprene oxidation products with sulfate in the particle phase are a significant component of SOA and can hydrolyze forming polyols. Despite characterization by mass spectrometry, their basic structural and spectroscopic properties remain poorly understood. Herein, Raman microspectroscopy and density functional theory (DFT) calculations (CAM-B3LYP level of theory) were combined to analyze the vibrational modes of key organosulfates, 3-methyltetrol sulfate esters (racemic mixture of two isomers), and racemic 2-methylglyceric acid sulfate ester, and hydrolysis products, 2-methyltetrols, and 2-methylglyceric acid. Two intense vibrational modes were identified, ν(RO-SO3) (846 ± 4 cm-1) and νs(SO3) (1065 ± 2 cm-1), along with a lower intensity δ(SO3) mode (586 ± 2 cm-1). For 2-methylglyceric acid and its sulfate esters, deprotonation of the carboxylic acid at pH values above the pKa decreased the carbonyl stretch frequency (1724 cm-1), while carboxylate modes grew in for νs(COO-) and νa(COO-) at 1413 and 1594 cm-1, respectively. The ν(RO-SO3) and νs(SO3) modes were observed in individual atmospheric particles and can be used in future studies of complex SOA mixtures to distinguish organosulfates from inorganic sulfate or hydrolysis products.

Elemental Mixing State of Aerosol Particles Collected in Central Amazonia during GoAmazon2014/15

Two complementary techniques, Scanning Transmission X-ray Microscopy/Near Edge Fine Structure spectroscopy (STXM/NEXAFS) and Scanning Electron Microscopy/Energy Dispersive X-ray spectroscopy (SEM/EDX), have been quantitatively combined to characterize individual atmospheric particles. This pair of techniques was applied to particle samples at three sampling sites (ATTO, ZF2, and T3) in the Amazon basin as part of the Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) field campaign during the dry season of 2014. The combined data was subjected to k-means clustering using mass fractions of the following elements: C, N, O, Na, Mg, P, S, Cl, K, Ca, Mn, Fe, Ni, and Zn. Cluster analysis identified 12 particle types across different sampling sites and particle sizes. Samples from the remote Amazon Tall Tower Observatory (ATTO, also T0a) exhibited less cluster variety and fewer anthropogenic clusters than samples collected at the sites nearer to the Manaus metropolitan region, ZF2 (also T0t) or T3. Samples from the ZF2 site contained aged/anthropogenic clusters not readily explained by transport from ATTO or Manaus, possibly suggesting the effects of long range atmospheric transport or other local aerosol sources present during sampling. In addition, this data set allowed for recently established diversity parameters to be calculated. All sample periods had high mixing state indices (χ) that were &gt;0.8. Two individual particle diversity (Di) populations were observed, with particles &lt;0.5 µm having a Di of ~2.4 and &gt;0.5 µm particles having a Di of ~3.6, which likely correspond to fresh and aged aerosols, respectively. The diversity parameters determined by the quantitative method presented here will serve to aid in the accurate representation of aerosol mixing state, source apportionment, and aging in both less polluted and more developed environments in the Amazon Basin.

Contributions of transported Prudhoe Bay oil field emissions to the aerosol population in Utqiaġvik, Alaska

Abstract. Loss of sea ice is opening the Arctic to increasing development involving oil and gas extraction and shipping. Given the significant impacts of absorbing aerosol and secondary aerosol precursors emitted within the rapidly warming Arctic region, it is necessary to characterize local anthropogenic aerosol sources and compare to natural conditions. From August to September 2015 in Utqiaġvik (Barrow), AK, the chemical composition of individual atmospheric particles was measured by computer-controlled scanning electron microscopy with energy-dispersive X-ray spectroscopy (0.13–4 µm projected area diameter) and real-time single-particle mass spectrometry (0.2–1.5 µm vacuum aerodynamic diameter). During periods influenced by the Arctic Ocean (70 % of the study), our results show that fresh sea spray aerosol contributed ∼ 20 %, by number, of particles between 0.13 and 0.4 µm, 40–70 % between 0.4 and 1 µm, and 80–100 % between 1 and 4 µm particles. In contrast, for periods influenced by emissions from Prudhoe Bay (10 % of the study), the third largest oil field in North America, there was a strong influence from submicron (0.13–1 µm) combustion-derived particles (20–50 % organic carbon, by number; 5–10 % soot by number). While sea spray aerosol still comprised a large fraction of particles (90 % by number from 1 to 4 µm) detected under Prudhoe Bay influence, these particles were internally mixed with sulfate and nitrate indicative of aging processes during transport. In addition, the overall mode of the particle size number distribution shifted from 76 nm during Arctic Ocean influence to 27 nm during Prudhoe Bay influence, with particle concentrations increasing from 130 to 920 cm−3 due to transported particle emissions from the oil fields. The increased contributions of carbonaceous combustion products and partially aged sea spray aerosol should be considered in future Arctic atmospheric composition and climate simulations.

Morphology, Mineralogy and Mixing of Individual Atmospheric Particles Over Kanpur (IGP): Relevance of Homogeneous Equivalent Sphere Approximation in Radiative Models

Estimation of the direct radiative forcing (DRF) by atmospheric particles is uncertain to a large extent owing to uncertainties in their morphology (shape and size), mixing states, and chemical composition. A region-specific database of the aforementioned physico-chemical properties (at individual particle level) is necessary to improve numerically-estimated optical and radiative properties. Till date, there is no detailed observation of the above mentioned properties over Kanpur in the Indo-Gangetic Plain (IGP). To fill this gap, an experiment was carried out at Kanpur (IITK; 26.52°N, 80.23°E, 142 m msl), India from April to July, 2011. Particle types broadly classified as (a) Cu-rich particles mixed with carbon and sulphur (b) dust and clays mixed with carbonaceous species (c) Fe-rich particles mixed with carbon and sulfur and (d) calcite (CaCO3) particles aged with nitrate, were observed. The frequency distributions of aspect ratio (AR; indicator of extent of particle non-sphericity) of total 708 particles from April to June reveal that particles with aspect ratio range >1.2 to ≤1.4 were abundant throughout the experiment except during June when it was found to shift to high AR range, >1.4 to ≤1.6 (followed with another peak of AR i.e. >2 to ≤2.4) due to dust storm conditions enhancing the occurrence of more non-spherical particles over the sampling site. The spherical particles (and close to spherical shape; AR range, 1.0 to ≤1.2) were found to be <20% throughout the experiment with a minimum (11.5%) during June. Consideration of Homogeneous Equivalent Sphere Approximation (HESA) in the optical/radiative model over the study region is found to be irrelevant during the campaign.

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 PM2.5 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 (dva) 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.

Air pollution-aerosol interactions produce more bioavailable iron for ocean ecosystems.

It has long been hypothesized that acids formed from anthropogenic pollutants and natural emissions dissolve iron (Fe) in airborne particles, enhancing the supply of bioavailable Fe to the oceans. However, field observations have yet to provide indisputable evidence to confirm this hypothesis. Single-particle chemical analysis for hundreds of individual atmospheric particles collected over the East China Sea shows that Fe-rich particles from coal combustion and steel industries were coated with thick layers of sulfate after 1 to 2 days of atmospheric residence. The Fe in aged particles was present as a "hotspot" of (insoluble) iron oxides and throughout the acidic sulfate coating in the form of (soluble) Fe sulfate, which increases with degree of aging (thickness of coating). This provides the "smoking gun" for acid iron dissolution, because iron sulfate was not detected in the freshly emitted particles and there is no other source or mechanism of iron sulfate formation in the atmosphere.