Morphology of mixed primary and secondary organic particles and the adsorption of spectator organic gases during aerosol formation

Primary organic aerosol (POA) and associated vapors can play an important role in determining the formation and properties of secondary organic aerosol (SOA). If SOA and POA are miscible, POA will significantly enhance SOA formation and some POA vapor will incorporate into SOA particles. When the two are not miscible, condensation of SOA on POA particles forms particles with complex morphology. In addition, POA vapor can adsorb to the surface of SOA particles increasing their mass and affecting their evaporation rates. To gain insight into SOA/POA interactions we present a detailed experimental investigation of the morphologies of SOA particles formed during ozonolysis of α-pinene in the presence of dioctyl phthalate (DOP) particles, serving as a simplified model of hydrophobic POA, using a single-particle mass spectrometer. Ultraviolet laser depth-profiling experiments were used to characterize two different types of mixed SOA/DOP particles: those formed by condensation of the oxidized α-pinene products on size-selected DOP particles and by condensation of DOP on size-selected α-pinene SOA particles. The results show that the hydrophilic SOA and hydrophobic DOP do not mix but instead form layered phases. In addition, an examination of homogeneously nucleated SOA particles formed in the presence of DOP vapor shows them to have an adsorbed DOP coating layer that is ∼4 nm thick and carries 12% of the particles mass. These results may have implications for SOA formation and behavior in the atmosphere, where numerous organic compounds with various volatilities and different polarities are present.     

Rapid dark aging of biomass burning as an overlooked source of oxidized organic aerosol

Oxidized organic aerosol (OOA) is a major component of ambient particulate matter, substantially impacting climate, human health, and ecosystems. OOA is readily produced in the presence of sunlight, and requires days of photooxidation to reach the levels observed in the atmosphere. High concentrations of OOA are thus expected in the summer; however, our current mechanistic understanding fails to explain elevated OOA during wintertime periods of low photochemical activity that coincide with periods of intense biomass burning. As a result, atmospheric models underpredict OOA concentrations by a factor of 3 to 5. Here we show that fresh emissions from biomass burning exposed to NO₂ and O₃ (precursors to the NO₃ radical) rapidly form OOA in the laboratory over a few hours and without any sunlight. The extent of oxidation is sensitive to relative humidity. The resulting OOA chemical composition is consistent with the observed OOA in field studies in major urban areas. Additionally, this dark chemical processing leads to significant enhancements in secondary nitrate aerosol, of which 50 to 60% is estimated to be organic. Simulations that include this understanding of dark chemical processing show that over 70% of organic aerosol from biomass burning is substantially influenced by dark oxidation. This rapid and extensive dark oxidation elevates the importance of nocturnal chemistry and biomass burning as a global source of OOA.

Highly oxidized organic aerosol (OOA) is a dominant component of particulate matter air pollution globally (1–3); however, sources of OOA remain uncertain, limiting the ability of models to accurately represent OOA and thus predict the associated climate, ecosystem, and health implications (4, 5). The current conceptual model of OOA formation suggests that anthropogenic OOA predominantly originates from the oxidation of volatile (VOCs), intermediate volatility (IVOCs), and semivolatile (SVOCs) organic compounds by the OH radical, resulting in lower-volatility products that condense to the particle phase (6). As the OH radical is formed through photolysis and has a very short atmospheric lifetime [less than a second (7)], this oxidation mechanism only occurs in the presence of sunlight. Further, the time scale for OOA formation through oxidation with OH in models is on the order of a few days (8). While this understanding is sufficient in explaining OOA concentrations in summer or periods with high solar radiation, atmospheric models fail to reproduce the observed concentration of OOA in the ambient atmosphere during winter and low-light conditions (9, 10). Fountoukis et al. (9) found simulated OOA concentrations significantly underestimated in wintertime Paris. Tsimpidi et al. (10) also reported an underprediction of simulated OOA globally in winter, suggesting missing sources of both primary OA (POA) and secondary formation pathways. This underproduction suggests a possible overlooked conversion pathway of organic vapors or particles to OOA that is not accounted for in current chemical transport and climate models.As stricter controls on fossil fuel combustion are implemented, residential biomass burning (BB) as a source of heating or cooking is becoming an increasingly important source of OA in urban environments (1, 11, 12). Further, increasing rates of wildfires from climate change are increasing the frequency of smoke-impacted days in urban areas (12–14). BB emissions include high concentrations of POA, SVOCs, IVOCs, and VOCs (15, 16), thus making BB a key source of OOA. Previous research has focused on quantifying the concentration of OOA formed through photochemical oxidation reactions (i.e., OH) with BB emissions (17, 18). However, oxidation of BB emissions in low or no sunlight is less well understood and is not included in chemical transport models. As opposed to OH, the NO₃ radical is formed through reactions with NO₂ and O₃ and is rapidly lost in the presence of sunlight (19). Thus, the NO₃ radical is only available in significant concentrations at night or other low-light conditions (20, 21). Previous research has established that biogenic VOCs may undergo oxidation at night when mixed with anthropogenic emissions containing NO₂ and O₃ (19, 22–27). There have been only a few studies that consider that nighttime oxidation of residential wood combustion may proceed through similar pathways (28–31); however, the magnitude and relevance to observed OOA in the ambient atmosphere has not yet been established. By combining laboratory experiments and ambient observations to inform a chemical transport model, we present strong evidence that nighttime oxidation of BB plumes (proceeding through reactions with O₃ and the NO₃ radical) is an important source of OOA.     

Identifying organic aerosol sources by comparing functional group composition in chamber and atmospheric particles

Measurements of submicron particles by Fourier transform infrared spectroscopy in 14 campaigns in North America, Asia, South America, and Europe were used to identify characteristic organic functional group compositions of fuel combustion, terrestrial vegetation, and ocean bubble bursting sources, each of which often accounts for more than a third of organic mass (OM), and some of which is secondary organic aerosol (SOA) from gas-phase precursors. The majority of the OM consists of alkane, carboxylic acid, hydroxyl, and carbonyl groups. The organic functional groups formed from combustion and vegetation emissions are similar to the secondary products identified in chamber studies. The near absence of carbonyl groups in the observed SOA associated with combustion is consistent with alkane rather than aromatic precursors, and the absence of organonitrate groups can be explained by their hydrolysis in humid ambient conditions. The remote forest observations have ratios of carboxylic acid, organic hydroxyl, and nonacid carbonyl groups similar to those observed for isoprene and monoterpene chamber studies, but in biogenic aerosols transported downwind of urban areas the formation of esters replaces the acid and hydroxyl groups and leaves only nonacid carbonyl groups. The carbonyl groups in SOA associated with vegetation emissions provides striking evidence for the mechanism of esterification as the pathway for possible oligomerization reactions in the atmosphere. Forest fires include biogenic emissions that produce SOA with organic components similar to isoprene and monoterpene chamber studies, also resulting in nonacid carbonyl groups in SOA.     

Quantification of organic aerosol and brown carbon evolution in fresh wildfire plumes

The evolution of organic aerosol (OA) and brown carbon (BrC) in wildfire plumes, including the relative contributions of primary versus secondary sources, has been uncertain in part because of limited knowledge of the precursor emissions and the chemical environment of smoke plumes. We made airborne measurements of a suite of reactive trace gases, particle composition, and optical properties in fresh western US wildfire smoke in July through August 2018. We use these observations to quantify primary versus secondary sources of biomass-burning OA (BBPOA versus BBSOA) and BrC in wildfire plumes. When a daytime wildfire plume dilutes by a factor of 5 to 10, we estimate that up to one-third of the primary OA has evaporated and subsequently reacted to form BBSOA with near unit yield. The reactions of measured BBSOA precursors contribute only 13 ± 3% of the total BBSOA source, with evaporated BBPOA comprising the rest. We find that oxidation of phenolic compounds contributes the majority of BBSOA from emitted vapors. The corresponding particulate nitrophenolic compounds are estimated to explain 29 ± 15% of average BrC light absorption at 405 nm (BrC Abs₄₀₅) measured in the first few hours of plume evolution, despite accounting for just 4 ± 2% of average OA mass. These measurements provide quantitative constraints on the role of dilution-driven evaporation of OA and subsequent radical-driven oxidation on the fate of biomass-burning OA and BrC in daytime wildfire plumes and point to the need to understand how processing of nighttime emissions differs.

Biomass burning (BB) is a major global source of atmospheric trace gases (1, 2), fixed nitrogen (3–6), and primary organic carbonaceous fine particles, known as BB organic aerosol (BBOA) (7). These emissions and their subsequent atmospheric transformations play a major role in affecting air quality, atmospheric composition, and climate.Questions remain about the magnitude of BBOA emissions and evolution, particularly the relative contributions that are primary, i.e., directly emitted (BBPOA), versus secondary, i.e., formed from gas-to-particle conversion following the oxidation of emitted vapors (BBSOA). Measurements of laboratory burns of individual or ensemble biomass fuel types have suggested that the BBSOA source could be anywhere from negligible to twice as large as the BBPOA source (8–11). On the other hand, field measurements have generally suggested little to no net change in BBOA in wildfire plumes (12–17). A leading hypothesis is that any BBSOA formation is offset by evaporation of BBPOA due to dilution-driven repartitioning of semivolatile components to the gas phase, making the magnitude of each difficult to discern (e.g., refs. 12, 14, 16–19). Recent plume modeling has investigated this behavior in detail, although the magnitudes of evaporation and BBSOA formation remain unclear due to limited observational constraints (20, 21). This hypothesis is uncertain in part because previous wildfire studies of BBSOA often did not include all potentially important precursor gases such as phenolic compounds. Laboratory studies have indicated that oxygenated aromatic compounds (i.e., phenolic compounds) are a large and often dominant source of BBSOA in BB smoke, although other sources including reduced aromatic compounds, biogenic compounds, and heterocyclic compounds (e.g., furans) can also be major BBSOA sources (11, 22–25).BB is also considered a major contributor to atmospheric brown carbon aerosol (BrC), which absorbs solar radiation and thus acts similarly to black carbon (BC) to potentially warm and stabilize the atmosphere (26–28). The lifetime of BrC and the importance of secondary BrC (sBrC) formation both remain uncertain. Phenolic compound emissions from wildfires are thought to be potentially important precursors for sBrC formation given their propensity to form light-absorbing nitroaromatics upon oxidation in the nitrogen oxide-rich fire plumes (29, 30). Phenolic compound oxidation products, including nitrophenolic compounds, have been estimated to account for up to approximately one-third to one-half of BrC light absorption for regions impacted by residential wood smoke and agricultural BB (31–34). Similar to BBSOA, field measurements of BrC absorption have generally shown little or no secondary formation in wildfire plumes, instead showing mostly decay with lifetimes from 9 h up to more than 1 d (16, 35). Understanding the controls on BrC and its secondary source can enable better predictions of its contribution and fate as wildfire smoke ages.We introduce a method for quantifying the effects of secondary formation versus dilution-driven evaporation for both total organic aerosol (OA) and BrC evolution in wildfire plumes. OA was measured using a high-resolution time-of-flight aerosol mass spectrometer (AMS), and BrC was quantified using a photoacoustic absorption spectrometer (PAS). To address the relative contributions to BBSOA formation from either primary emitted precursors (e.g., phenolic compounds) versus evaporated BBPOA, we quantified all major secondary OA (SOA) precursor gases measured using an iodide-adduct, high-resolution time-of-flight chemical ionization mass spectrometer (I⁻ CIMS) and a proton-transfer reaction time-of-flight mass spectrometer (PTR-ToF-MS) in authentic wildfire smoke plumes during the airborne Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE-CAN) field campaign. The WE-CAN project deployed a research aircraft across the western United States between 22 July and 13 September 2018 to sample wildfire smoke during the first several hours of atmospheric evolution. To investigate the contributions of particulate nitrophenolic compounds to BrC Abs₄₀₅, environmental chamber experiments were performed at the National Center for Atmospheric Research (NCAR) chamber facility in May/June 2019 as part of the Monoterpene and Oxygenated aromatic Oxidation at Night and under LIGHTs (MOONLIGHT) campaign. These experiments simulated phenolic compound oxidation chemistry in fresh wildfire plumes. Through a combined analysis of these data, we characterize the importance of phenolic compound emissions and nitrophenolic oxidation products as potential contributors to BBSOA and sBrC, compared to the sources from evaporated BBPOA vapors.     

Aqueous production of secondary organic aerosol from fossil-fuel emissions in winter Beijing haze

Secondary organic aerosol (SOA) produced by atmospheric oxidation of primary emitted precursors is a major contributor to fine particulate matter (PM_2.5) air pollution worldwide. Observations during winter haze pollution episodes in urban China show that most of this SOA originates from fossil-fuel combustion but the chemical mechanisms involved are unclear. Here we report field observations in a Beijing winter haze event that reveal fast aqueous-phase conversion of fossil-fuel primary organic aerosol (POA) to SOA at high relative humidity. Analyses of aerosol mass spectra and elemental ratios indicate that ring-breaking oxidation of POA aromatic species, leading to functionalization as carbonyls and carboxylic acids, may serve as the dominant mechanism for this SOA formation. A POA origin for SOA could explain why SOA has been decreasing over the 2013–2018 period in response to POA emission controls even as emissions of volatile organic compounds (VOCs) have remained flat.

Organic aerosol (OA) is the dominant component of fine particulate matter (PM_2.5) in most polluted regions of the world, with implications for air-quality degradation and climate forcing (1, 2). Most of that OA is secondary (SOA), produced by atmospheric oxidation of volatile organic compounds (VOCs) emitted as gases from anthropogenic, pyrogenic, and biogenic sources (3–5). SOA can also result from oxidation of primary (i.e., directly emitted) OA in wet particles or fog/cloud droplets (5, 6). A number of studies have shown that uptake of oxidized VOCs by aqueous aerosols and fog/clouds, with subsequent aqueous-phase transformation to nonvolatile products, can be an important pathway for SOA formation (5–11). The mechanisms are poorly understood, however, hampering our ability to model responses to changes in VOC emissions and other factors.PM_2.5 is a severe air-pollution problem in China. Concentrations are particularly high during stagnant high-humidity wintertime conditions (winter haze) (12). OA accounts for about half of total PM_2.5 in winter haze, and most is SOA originating from fossil-fuel (FF) combustion (13), but the mechanisms responsible for this SOA formation are unknown. Incomplete FF combustion such as from residential burning of coal emits black carbon (BC) particles and primary OA (FF-POA). FF-POA has a complex composition including polycyclic aromatic hydrocarbons (PAHs) among other compounds (14–16). It may take up water at high relative humidity (RH) following coating with water-soluble species such as sulfate and nitrate (17–19). A recent study in Beijing found that 40–80% of FF-POA was water-soluble (20).Here we use detailed chemical observations during a Beijing haze event to show evidence for aqueous-phase chemical conversion of FF-POA to SOA (aq-SOA), and we show that this mechanism can account for 20% of the total OA observed in the haze event. This may explain why SOA concentrations in Beijing haze have decreased in recent years (21), as POA emissions have strongly decreased while VOC emissions have stayed flat (22). We further show that conversion of FF-POA to aq-SOA decreases the light absorption of OA with possible implications for radiative forcing and pollution-weather feedbacks.     

Contribution of isoprene-derived organosulfates to free tropospheric aerosol mass

Recent laboratory studies have demonstrated that isoprene oxidation products can partition to atmospheric aerosols by reacting with condensed phase sulfuric acid, forming low-volatility organosulfate compounds. We have identified organosulfate compounds in free tropospheric aerosols by single particle mass spectrometry during several airborne field campaigns. One of these organosulfates is identified as the sulfate ester of IEPOX, a second generation oxidation product of isoprene. The patterns of IEPOX sulfate ester in ambient data generally followed the aerosol acidity and NO(x) dependence established by laboratory studies. Detection of the IEPOX sulfate ester was most sensitive using reduced ionization laser power, when it was observed in up to 80% of particles in the tropical free troposphere. Based on laboratory mass calibrations, IEPOX added > 0.4% to tropospheric aerosol mass in the remote tropics and up to 20% in regions downwind of isoprene sources. In the southeastern United States, when acidic aerosol was exposed to fresh isoprene emissions, accumulation of IEPOX increased aerosol mass by up to 3%. The IEPOX sulfate ester is therefore one of the most abundant single organic compounds measured in atmospheric aerosol. Our data show that acidity-dependent IEPOX uptake is a mechanism by which anthropogenic SO(2) and marine dimethyl sulfide emissions generate secondary biogenic aerosol mass throughout the troposphere.     

Evidence for liquid-like and nonideal behavior of a mixture of organic aerosol components

The condensation, evaporation, and repartitioning of semivolatile organic compounds (SVOCs) in the atmosphere depends both on the phase of condensed material and the effective condensed phase vapor pressures of the SVOCs. Although direct measurements of vapor pressures of individual SVOCs exist, there are limited measurements of how the properties of a given compound changes in mixtures of multiple components that exist in the atmosphere. Here, the evaporation behavior of mixtures of dicarboxylic acids, which are common atmospheric aerosol constituents, is investigated. These measurements demonstrate that complex mixtures of the individually solid organic compounds take on liquid-like properties. Additionally, the vapor pressures of individual components show strong, identity-dependent deviations from ideality (i.e., Raoult's Law), with the vapor pressures of the smaller, more volatile compounds decreased significantly in the mixtures. The addition of an inorganic compound (NaNO₃) further influences the nonideal behavior, again in a compound-specific manner. These results suggest that nonideal behavior of particle-phase compounds influences the abundances of organic aerosol observed in the atmosphere and in the laboratory.     

Effects of anthropogenic emissions on aerosol formation from isoprene and monoterpenes in the southeastern United States

Secondary organic aerosol (SOA) constitutes a substantial fraction of fine particulate matter and has important impacts on climate and human health. The extent to which human activities alter SOA formation from biogenic emissions in the atmosphere is largely undetermined. Here, we present direct observational evidence on the magnitude of anthropogenic influence on biogenic SOA formation based on comprehensive ambient measurements in the southeastern United States (US). Multiple high-time-resolution mass spectrometry organic aerosol measurements were made during different seasons at various locations, including urban and rural sites in the greater Atlanta area and Centreville in rural Alabama. Our results provide a quantitative understanding of the roles of anthropogenic SO₂ and NO_x in ambient SOA formation. We show that isoprene-derived SOA is directly mediated by the abundance of sulfate, instead of the particle water content and/or particle acidity as suggested by prior laboratory studies. Anthropogenic NO_x is shown to enhance nighttime SOA formation via nitrate radical oxidation of monoterpenes, resulting in the formation of condensable organic nitrates. Together, anthropogenic sulfate and NO_x can mediate 43–70% of total measured organic aerosol (29–49% of submicron particulate matter, PM₁) in the southeastern US during summer. These measurements imply that future reduction in SO₂ and NO_x emissions can considerably reduce the SOA burden in the southeastern US. Updating current modeling frameworks with these observational constraints will also lead to more accurate treatment of aerosol formation for regions with substantial anthropogenic−biogenic interactions and consequently improve air quality and climate simulations.Organic aerosol (OA) is an important atmospheric component that influences climate, air quality, and human health (1). A large fraction of OA is secondary organic aerosol (SOA), which is formed through oxidation of volatile organic compounds (VOCs) emitted from human activities (anthropogenic) and vegetation (biogenic). In particular, biogenic VOCs (BVOCs), such as isoprene (C₅H₈) and monoterpenes (C₁₀H₁₆), are key precursors for global SOA formation owing to their larger emissions and higher reactivity with atmospheric oxidants compared with anthropogenic VOCs (1). However, the extent to which anthropogenic pollutants mediate the formation of SOA from biogenic VOCs (referred to as biogenic SOA) in the ambient environments is poorly understood and highly uncertain. For example, while radiocarbon analysis repeatedly indicated that more than half of the carbon in SOA is of modern (biogenic) origin in the southeastern United States (SE US) (2, 3), aircraft measurements in the same region showed that SOA correlates with anthropogenic tracers, such as CO (3).One possible explanation to reconcile the seemingly contradictory results from radiocarbon studies and ambient measurements is that the majority of SOA is produced from naturally emitted BVOCs, but its formation processes also involve pollutants originated from anthropogenic emissions (3, 4). Laboratory studies have recently revealed that biogenic SOA formation can be largely affected by anthropogenic pollutants such as NO_x and SO₂ (1, 5). According to 2011 US national emission inventory (www.epa.gov/ttn/chief/net/2011inventory.html), 90% of NO_x and 97% of SO₂ are anthropogenically emitted. NO_x can alter SOA formation by influencing peroxy radical chemistry in BVOCs oxidation mechanisms (5). The reaction of NO₂ with O₃ forms nitrate radicals, which can oxidize BVOCs to form condensable products that often have high SOA yields (6, 7). The effect of SO₂ was investigated but often explained in the context of particle acidity in laboratory studies (8). Despite intense laboratory investigations, only a few proposed mechanisms are consistent with ambient observations (9), and the reasons for the observed enhancement of biogenic SOA formation in certain polluted environments remain unclear (10, 11). For instance, while some laboratory studies found that particle acidity can enhance isoprene SOA formation (8, 12), only weak correlations have been observed in the atmosphere between tracers of isoprene SOA and particle acidity (13–16). Thus, a coherent understanding of the enhancement of biogenic SOA in polluted environments has not emerged, and these proposed mechanisms from laboratory studies have not been quantitatively established in ambient environments.Here, we provide direct observational evidence and quantification of anthropogenically enhanced biogenic SOA formation in the Southern Oxidant and Aerosol Study (SOAS; SI Appendix, Southern Oxidant and Aerosol Study) field campaign in June and July 2013. In addition, we also conducted ambient measurements from May 2012 to February 2013 at multiple rural and urban sites in the greater Atlanta area as part of the Southeastern Center of Air Pollution and Epidemiology study (SCAPE, EPA Clean Air Center; SI Appendix). The SE US is ideal for studying anthropogenic−biogenic interactions due to high natural emissions and the proximity to anthropogenic pollution sources. Here, we investigate the sources of OA using factor analysis of high-time-resolution mass spectrometry data coupled with a suite of comprehensive and collocated measurements (SI Appendix, Instrumentation). We have also performed complementary laboratory studies to examine possible chemical mechanisms to interpret results from ambient measurements. From these integrated ambient and laboratory studies, we show that anthropogenic SO₂ and NO_x emissions substantially mediate SOA formation from BVOCs such as isoprene and monoterpenes in the SE US.     

In situ measurements of gas/particle-phase transitions for atmospheric semivolatile organic compounds

An understanding of the gas/particle-phase partitioning of semivolatile compounds is critical in determining atmospheric aerosol formation processes and growth rates, which in turn affect global climate and human health. The Study of Organic Aerosol at Riverside 2005 campaign was performed to gain a better understanding of the factors responsible for aerosol formation and growth in Riverside, CA, a region with high concentrations of secondary organic aerosol formed through the phase transfer of low-volatility reaction products from the oxidation of precursor gases. We explore the ability of the thermal desorption aerosol gas chromatograph (TAG) to measure gas-to-particle-phase transitioning for several organic compound classes (polar and nonpolar) found in the ambient Riverside atmosphere by using in situ observations of several hundred semivolatile organic compounds. Here we compare TAG measurements to modeled partitioning of select semivolatile organic compounds. Although TAG was not designed to quantify the vapor phase of semivolatile organics, TAG measurements do distinguish when specific compounds are dominantly in the vapor phase, are dominantly in the particle phase, or have both phases present. Because the TAG data are both speciated and time-resolved, this distinction is sufficient to see the transition from vapor to particle phase as a function of carbon number and compound class. Laboratory studies typically measure the phase partitioning of semivolatile organic compounds by using pure compounds or simple mixtures, whereas hourly TAG phase partitioning measurements can be made in the complex mixture of thousands of polar/nonpolar and organic/inorganic compounds found in the atmosphere.     

Response of an aerosol mass spectrometer to organonitrates and organosulfates and implications for atmospheric chemistry

Organonitrates (ON) are important products of gas-phase oxidation of volatile organic compounds in the troposphere; some models predict, and laboratory studies show, the formation of large, multifunctional ON with vapor pressures low enough to partition to the particle phase. Organosulfates (OS) have also been recently detected in secondary organic aerosol. Despite their potential importance, ON and OS remain a nearly unexplored aspect of atmospheric chemistry because few studies have quantified particulate ON or OS in ambient air. We report the response of a high-resolution time-of-flight aerosol mass spectrometer (AMS) to aerosol ON and OS standards and mixtures. We quantify the potentially substantial underestimation of organic aerosol O/C, commonly used as a metric for aging, and N/C. Most of the ON-nitrogen appears as ions in the AMS, which are typically dominated by inorganic nitrate. Minor organonitrogen ions are observed although their identity and intensity vary between standards. We evaluate the potential for using fragment ratios, organonitrogen ions, ions, the ammonium balance of the nominally inorganic ions, and comparison to ion-chromatography instruments to constrain the concentrations of ON for ambient datasets, and apply these techniques to a field study in Riverside, CA. OS manifests as separate organic and sulfate components in the AMS with minimal organosulfur fragments and little difference in fragmentation from inorganic sulfate. The low thermal stability of ON and OS likely causes similar detection difficulties for other aerosol mass spectrometers using vaporization and/or ionization techniques with similar or larger energy, which has likely led to an underappreciation of these species.     

Carbohydrate-like composition of submicron atmospheric particles and their production from ocean bubble bursting

Oceans cover over two-thirds of the Earth’s surface, and the particles emitted to the atmosphere by waves breaking on sea surfaces provide an important contribution to the planetary albedo. During the International Chemistry Experiment in the Arctic LOwer Troposphere (ICEALOT) cruise on the R/V Knorr in March and April of 2008, organic mass accounted for 15–47% of the submicron particle mass in the air masses sampled over the North Atlantic and Arctic Oceans. A majority of this organic component (0.1 - 0.4 μ m^-3) consisted of organic hydroxyl (including polyol and other alcohol) groups characteristic of saccharides, similar to biogenic carbohydrates found in seawater. The large fraction of organic hydroxyl groups measured during ICEALOT in submicron atmospheric aerosol exceeded those measured in most previous campaigns but were similar to particles in marine air masses in the open ocean (Southeast Pacific Ocean) and coastal sites at northern Alaska (Barrow) and northeastern North America (Appledore Island and Chebogue Point). The ocean-derived organic hydroxyl mass concentration during ICEALOT correlated strongly to submicron Na concentration and wind speed. The observed submicron particle ratios of marine organic mass to Na were enriched by factors of ∼10²–∼10³ over reported sea surface organic to Na ratios, suggesting that the surface-controlled process of film bursting is influenced by the dissolved organic components present in the sea surface microlayer. Both marine organic components and Na increased with increasing number mean diameter of the accumulation mode, suggesting a possible link between organic components in the ocean surface and aerosol–cloud interactions.     

Influence of organic films on the evaporation and condensation of water in aerosol

Uncertainties in quantifying the kinetics of evaporation and condensation of water from atmospheric aerosol are a significant contributor to the uncertainty in predicting cloud droplet number and the indirect effect of aerosols on climate. The influence of aerosol particle surface composition, particularly the impact of surface active organic films, on the condensation and evaporation coefficients remains ambiguous. Here, we report measurements of the influence of organic films on the evaporation and condensation of water from aerosol particles. Significant reductions in the evaporation coefficient are shown to result when condensed films are formed by monolayers of long-chain alcohols [C_nH_(2n+1)OH], with the value decreasing from 2.4 × 10⁻³ to 1.7 × 10⁻⁵ as n increases from 12 to 17. Temperature-dependent measurements confirm that a condensed film of long-range order must be formed to suppress the evaporation coefficient below 0.05. The condensation of water on a droplet coated in a condensed film is shown to be fast, with strong coherence of the long-chain alcohol molecules leading to islanding as the water droplet grows, opening up broad areas of uncoated surface on which water can condense rapidly. We conclude that multicomponent composition of organic films on the surface of atmospheric aerosol particles is likely to preclude the formation of condensed films and that the kinetics of water condensation during the activation of aerosol to form cloud droplets is likely to remain rapid.     

From the Cover: Elucidating secondary organic aerosol from diesel and gasoline vehicles through detailed characterization of organic carbon emissions

Emissions from gasoline and diesel vehicles are predominant anthropogenic sources of reactive gas-phase organic carbon and key precursors to secondary organic aerosol (SOA) in urban areas. Their relative importance for aerosol formation is a controversial issue with implications for air quality control policy and public health. We characterize the chemical composition, mass distribution, and organic aerosol formation potential of emissions from gasoline and diesel vehicles, and find diesel exhaust is seven times more efficient at forming aerosol than gasoline exhaust. However, both sources are important for air quality; depending on a region’s fuel use, diesel is responsible for 65% to 90% of vehicular-derived SOA, with substantial contributions from aromatic and aliphatic hydrocarbons. Including these insights on source characterization and SOA formation will improve regional pollution control policies, fuel regulations, and methodologies for future measurement, laboratory, and modeling studies.     

On Structural Rearrangements during the Vitrification of Molten Copper

We utilise displacement analysis of Cu-atoms between the chemical bond-centred Voronoi polyhedrons to reveal structural changes at the glass transition. We confirm that the disordered congruent bond lattice of Cu loses its rigidity above the glass transition temperature (T_g) in line with Kantor–Webman theorem due to percolation via configurons (broken Cu-Cu chemical bonds). We reveal that the amorphous Cu has the T_g = 794 ± 10 K at the cooling rate q = 1 × 10¹³ K/s and that the determination of T_g based on analysis of first sharp diffraction minimum (FDSM) is sharper compared with classical Wendt–Abraham empirical criterion.