Jonathan P. D. Abbatt

Heterogeneous oxidation of atmospheric aerosol particles by gas-phase radicals

Atmospheric aerosol particles play pivotal roles in climate and air quality. Just as chemically reduced gases experience oxidation in the atmosphere, it is now apparent that solid and liquid atmospheric particulates are also subject to similar oxidative processes. The most reactive atmospheric gas-phase radicals, in particular the hydroxyl radical, readily promote such chemistry through surficial interactions. This Review looks at progress made in this field, discussing the radical-initiated heterogeneous oxidation of organic and inorganic constituents of atmospheric aerosols. We focus on the kinetics and reaction mechanisms of such processes as well as how they can affect the physico-chemical properties of particles, such as their composition, size, density and hygroscopicity. Potential impacts on the atmosphere include the release of chemically reactive gases such as halogens, aldehydes and organic acids, reactive loss of particle-borne molecular tracer and toxic species, and enhanced hygroscopic properties of aerosols that may improve their ability to form cloud droplets.

Resurgence in Ice Nuclei Measurement Research

Understanding cloud and precipitation responses to variations in atmospheric aerosols remains an important research topic for improving the prediction of climate. Knowledge is most uncertain, and the potential impact on climate is largest with regard to how aerosols impact ice formation in clouds. In this paper, we show that research on atmospheric ice nucleation, including the development of new measurement systems, is occurring at a renewed and historically unparalleled level. A historical perspective is provided on the methods and challenges of measuring ice nuclei, and the various factors that led to a lull in research efforts during a nearly 20-yr period centered about 30 yr ago. Workshops played a major role in defining critical needs for improving measurements at that time and helped to guide renewed efforts. Workshops were recently revived for evaluating present research progress. We argue that encouraging progress has been made in the consistency of measurements using the present generation of ic...

The role of long-lived reactive oxygen intermediates in the reaction of ozone with aerosol particles

The heterogeneous reactions of O₃ with aerosol particles are of central importance to air quality. They are studied extensively, but the molecular mechanisms and kinetics remain unresolved. Based on new experimental data and calculations, we show that long-lived reactive oxygen intermediates (ROIs) are formed. The chemical lifetime of these intermediates exceeds 100 seconds, which is much longer than the surface residence time of molecular O₃ (~10⁻⁹ s). The ROIs explain and resolve apparent discrepancies between earlier quantum mechanical calculations and kinetic experiments. They play a key role in the chemical transformation and adverse health effects of toxic and allergenic air-particulate matter, such as soot, polycyclic aromatic hydrocarbons and proteins. ROIs may also be involved in the decomposition of O₃ on mineral dust and in the formation and growth of secondary organic aerosols. Moreover, ROIs may contribute to the coupling of atmospheric and biospheric multiphase processes.

N2O5 hydrolysis on sub-micron organic aerosols: the effect of relative humidity, particle phase, and particle size

Measurements of the reactive uptake coefficient for N2O5 hydrolysis, γm, on sub-micron organic aerosols were performed in an entrained aerosol flow tube as a function of relative humidity (RH), aerosol phase, N2O5 partial pressure, and mean aerosol size. Aerosol phase and relative humidity were determined simultaneously, and chemical ionization mass spectrometry was used to detect the decay rate of N2O5 in the presence of malonic acid or azelaic acid aerosol. The γm on solid malonic acid was determined to be less than 0.001 (RH = 10–50%), and on solid azelaic acid, γm was 0.0005 ± 0.0003. Aqueous malonic acid aerosol yielded γm = 0.0020 ± 0.0005 at 10% RH and increased with RH to ∼0.03 at RH = 50–70%. We report the first evidence of an inverse dependence of the γm on the initial partial pressure of N2O5 in the flow reactor, and a dependence on particle size for aerosol with surface area-weighted radii less than ∼100 nm at 50% RH. We find that the super-saturated malonic acid aerosol results are consistent with N2O5 hydrolysis being both aerosol volume-limited where, for RH < 50%, water is the limiting reagent, and also with a surface-specific process.

Heterogeneous reaction of ozone with liquid unsaturated fatty acids: detailed kinetics and gas-phase product studies

Detailed kinetics and product yield studies have been performed for the heterogeneous reaction between gas-phase ozone and three liquid fatty acids using a coated-wall flow tube and chemical ionization mass spectrometry. Gas-surface reaction probabilities for ozone loss of (8.0 ± 1.0) × 10−4, (1.3 ± 0.1) × 10−3, and (1.8 ± 0.2) × 10−3 have been measured at room temperature (298 K) for oleic acid, linoleic acid and linolenic acid, respectively. The temperature dependence of the uptake coefficients was found to be small and positive. Comparison of these results to the kinetics of the equivalent gas-phase reactions implies that there is a definite enhancement in the rate for the heterogeneous process due to entropic factors, i.e. due to collisional trapping of ozone in the surface layers of the liquid, and a possible effect on the activation energy of the reaction. For linoleic acid, the reaction probability was found to be independent of relative humidity (up to 55%), to ±10%, at 263 K. Volatile reaction products were observed using proton-transfer-reaction mass spectrometry. Nonanal was observed with a 0.50 (±0.10) yield for the reaction with oleic acid, whereas hexanal and nonenal were observed for linoleic acid with 0.25 (±0.05) and 0.29 (±0.05) yields, respectively. These results indicate that the primary ozonide formed initially in the reaction can decompose via two equal probability pathways and that a secondary ozonide is not formed in high yield in the aldehydic channel. These reactions represent a source of oxygenates to the atmosphere and will modify the hygroscopic properties of aerosols.

Sea spray aerosol as a unique source of ice nucleating particles

Ice nucleating particles (INPs) are vital for ice initiation in, and precipitation from, mixed-phase clouds. A source of INPs from oceans within sea spray aerosol (SSA) emissions has been suggested in previous studies but remained unconfirmed. Here, we show that INPs are emitted using real wave breaking in a laboratory flume to produce SSA. The number concentrations of INPs from laboratory-generated SSA, when normalized to typical total aerosol number concentrations in the marine boundary layer, agree well with measurements from diverse regions over the oceans. Data in the present study are also in accord with previously published INP measurements made over remote ocean regions. INP number concentrations active within liquid water droplets increase exponentially in number with a decrease in temperature below 0 °C, averaging an order of magnitude increase per 5 °C interval. The plausibility of a strong increase in SSA INP emissions in association with phytoplankton blooms is also shown in laboratory simulations. Nevertheless, INP number concentrations, or active site densities approximated using “dry” geometric SSA surface areas, are a few orders of magnitude lower than corresponding concentrations or site densities in the surface boundary layer over continental regions. These findings have important implications for cloud radiative forcing and precipitation within low-level and midlevel marine clouds unaffected by continental INP sources, such as may occur over the Southern Ocean.

Kinetic limitations in gas-particle reactions arising from slow diffusion in secondary organic aerosol

The potential for aerosol physical properties, such as phase, morphology and viscosity/ diffusivity, to affect particle reactivity remains highly uncertain. We report here a study of the effect of bulk diffusivity of polycyclic aromatic hydrocarbons (PAHs) in secondary organic aerosol (SOA) on the kinetics of the heterogeneous reaction of particle-borne benzo[a]pyrene (BaP) with ozone. The experiments were performed by coating BaP-ammonium sulfate particles with multilayers of SOA formed from ozonolysis of alpha-pinene, and by subsequently investigating the kinetics of BaP loss via reaction with excess ozone using an aerosol flow tube coupled to an Aerodyne Aerosol Mass Spectrometer (AMS). All reactions exhibit pseudo-first order kinetics and are empirically well described by a Langmuir-Hinshelwood (L-H) mechanism. The results show that under dry conditions (RH < 5%) diffusion through the SOA coating can lead to significant mass transfer constraints on the kinetics, with behavior between that previously observed by our group for solid and liquid organic coats. The reactivity of BaP was enhanced at -50% relative humidity (RH) suggesting that water uptake lowers the viscosity of the SOA, hence lifting the mass transfer constraint to some degree. The kinetics for -70% RH were similar to results obtained without SOA coats, indicating that the SOA had sufficiently low viscosity and was sufficiently liquid-like that reactants could rapidly diffuse through the coat. A kinetic multi-layer model for aerosol surface and bulk chemistry was applied to simulate the kinetics, yielding estimates for the diffusion coefficients (in cm2 s(-1)) for BaP in alpha-pinene SOA of 2 x 10(-14), 8 x 10(-14) and > 1 x 10(-12) for dry (RH < 5%), 50% RH and 70% RH conditions, respectively. These results clearly indicate that slow diffusion of reactants through SOA coats under specific conditions can provide shielding from gas-phase oxidants, enabling the long-range atmospheric transport of toxic trace species, such as PAHs and persistent organic pollutants.

Cytotoxic and proinflammatory effects of ambient and source-related particulate matter (PM) in relation to the production of reactive oxygen species (ROS) and cytokine adsorption by particles

The composition of airborne particulate matter (PM) varies widely depending on its source, and recent studies have suggested that particle-associated adverse health effects are related to particle composition. The objective of this study was to compare the biological/toxicological effects of different source-related PM. Specifically, we investigated the biological/toxicological effects of standard reference materials (SRMs): non-ferrous dust (PD-1, industrial), urban PM (UPM, SRM1648a), and diesel PM (DPM, SRM2975), and ambient PM2.5 (PM with an aerodynamic diameter <2.5 µm) collected at an urban site (Toronto, Canada). The dithiothreitol assay was used to measure the redox activity of the particles. Human alveolar epithelial cells (A549) were exposed to a range of concentrations (10–1000 µg/ml) of total PM, and the respective water-soluble and insoluble fractions, for 24 h. Biological responses were then evaluated in terms of cytotoxicity and interleukin (IL)-8 release, and compared with the PM composition and redox activity. We demonstrated that transition metal-enriched PD-1 exhibited the greatest cytotoxic effect (LD50 values of 100–400 µg/ml vs. >1000 µg/ml for the SRM1648a, SRM2975, and ambient PM2.5). Similarly, the PM-induced release of IL-8 was greatest for PD-1 (~6–9 ng/ml vs. ~1.5–3 ng/ml for others). These endpoints were more responsive to metals as compared with compared with secondary inorganic ions and organic compounds. Interestingly, we demonstrated a high degree of adsorption of IL-8 to the various SRMs and ambient PM2.5, and subsequently derived a new correction method to aid in interpretation of these data. These characteristics likely impart differential effects toward the toxic and immune effects of PM.

Ice formation via deposition nucleation on mineral dust and organics: dependence of onset relative humidity on total particulate surface area

We present ice nucleation results for Arizona test dust, kaolinite, montmorillonite, silica, silica coated with a hydrophobic octyl chain, oxalic acid dihydrate, Gascoyne leonardite (a humic material), and Aldrich humic acid (sodium salt). The focus was on deposition mode nucleation below water saturation at 233 K. Particles were deposited onto a hydrophobic cold stage by atomization of a slurry/solution and exposed to a constant partial pressure of water vapor. By lowering the temperature of the stage, the relative humidity with respect to ice (RHi )w as gradually increased until ice nucleation was observed using digital photography. Different numbers of particles were deposited onto the cold stage by varying the atomization solution concentration and deposition time. For the same total particulate surface area, mineral dust particles nucleated ice at lower supersaturations than all other materials. The most hydrophobic materials, i.e. Gascoyne leonardite and octyl silica, were the least active. For our limit of detection of one ice crystal, the ice onset RHi values were dependent on the total surface area of the particulates, indicating that no unique threshold RHi for ice nucleation prevails.

Formation of Volatile Organic Compounds in the Heterogeneous Oxidation of Condensed-Phase Organic Films by Gas-Phase OH

The yield of volatile organic compounds (VOCs) from the heterogeneous oxidation of condensed-phase organic and hydrocarbon soot films by gas-phase OH has been studied in a coated-wall flow tube at room temperature. Simultaneously, OH concentrations are measured using a chemical-ionization mass spectrometer (CIMS) operated in negative ion mode and VOCs are measured using a commercial proton-transfer-reaction mass spectrometer (PTR-MS). It is observed that a variety of aldehydes/carbonyls and carboxylic acids are formed. Specifically, detailed experiments were conducted with stearic acid, where products are observed that contain as many as 13 carbon atoms with the average carbon number of the products between 3 and 5. The yield of VOCs, relative to the loss of OH radicals, is strongly dependent on the partial pressure of O2 in the carrier gas, ranging from 0.08 +/- 0.03 in a nominally pure He carrier gas to 0.34 +/- 0.14 in 6 Torr of pure O2. Yields from other organics are somewhat lower than those from stearic acid, ranging in conditions of pure O2 from 0.10 +/- 0.04 for BES (bis(ethylhexyl)sebacate), to 0.03 +/- 0.01 for n-hexane soot, to 0.01 +/- 0.005 for pyrene. Under atmospheric conditions, OH oxidation of select organics may be an efficient source of small VOCs. In particular, formic acid is formed in significant yield from all the surfaces.