Shouming Zhou

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.

Burial Effects of Organic Coatings on the Heterogeneous Reactivity of Particle-Borne Benzo[a]pyrene (BaP) toward Ozone

With an aerosol flow tube coupled to an Aerodyne aerosol mass spectrometer (AMS), room temperature (296 ± 3 K) kinetics studies have been performed on the reaction of gas-phase ozone with benzo[a]pyrene (BaP) adsorbed in submonolayer amounts to dry ammonium sulfate (AS) particles. Three organic substances, i.e., bis(2-ethylhexyl)sebacate (BES, liquid), phenylsiloxane oil (PSO, liquid), and eicosane (EC, solid), were used to coat BaP-AS particles to investigate the effects of such organic coatings on the heterogeneous reactivity of PAHs toward ozone. All the reactions of particle-borne BaP with excess ozone exhibit pseudo-first-order kinetics in terms of BaP loss, and reactions with a liquid organic coating proceed by the Langmuir-Hinshelwood (L-H) mechanism. Liquid organic coatings did not significantly affect the kinetics, consistent with the ability of reactants to rapidly diffuse through the organic coating. In contrast, the heterogeneous reactivity of BaP was reduced substantially by a thin (4-8 nm), solid EC coating and entirely suppressed by thick (10-80 nm) coatings, presumably because of slow diffusion through the organic layer. Although the heterogeneous reactivity of surface-bound PAHs is extremely rapid in the atmosphere, this work is the first to experimentally demonstrate a mechanism by which the lifetime of PAHs may be significantly prolonged, permitting them to undergo long-range transport to remote locations.

Evaluation of the effects of ozone oxidation on redox-cycling activity of two-stroke engine exhaust particles.

The effect of oxidation on the redox-cycling activity of engine exhaust particles is examined. Particles obtained from a two-stroke gasoline engine were oxidized in a flow tube with ozone on a one-minute time scale both in the presence and absence of substantial gas-phase exhaust components. Whereas ozone concentrations were high, the ozone exposures were approximately equivalent to 60 ppb ozone for 2-8 h. Oxidation led to substantial increases in redox-cycling of aqueous extracts of filtered particles, as measured using the dithiothreitol (DTT) assay. Increases in redox activity when the entire exhaust was oxidized were primarily driven by deposition of redox-active secondary organic aerosol (SOA), resulting in an upper-limit DTT activity of 8.6 ± 2.0 pmol DTT consumed per min per microgram of particles, compared to 0.73 ± 0.60 pmol min(-1) μg(-1) for fresh, unoxidized exhaust particles. Redox-cycling activity reached higher levels when VOC denuded exhaust was oxidized, with the highest DTT activity observed being 16.7 ± 1.6 pmol min(-1) μg(-1) with no upper limit reached for the range of ozone exposures used in this study. Our results provide laboratory support for the hypothesis that the toxicity of engine combustion particles due to redox-cycling may increase as they age in the atmosphere.

Connecting the oxidation of soot to its redox cycling abilities.

Although it is known that soot particles are emitted in large quantities to the atmosphere, our understanding of their environmental effects is limited by our knowledge of how their composition is subsequently altered through atmospheric processing. Here we present an on-line mass spectrometric study of the changing chemical composition of hydrocarbon soot particles as they are oxidized by gas-phase ozone, and we show that the surface-mediated loss rates of adsorbed polycyclic aromatic hydrocarbons in soot are directly connected to a significant increase in the particle redox cycling abilities. With redox cycling implicated as an oxidative stress mechanism that arises after inhalation of atmospheric particles, this work draws a quantitative connection between the detailed heterogeneous chemistry occurring on atmospheric particles and a potential toxic mechanism attributable to that aerosol.

Changes in Secondary Organic Aerosol Composition and Mass due to Photolysis: Relative Humidity Dependence

This study is focused on the relative humidity (RH) dependence of water-soluble secondary organic aerosol (SOA) aging by photolysis. Particles containing α-pinene SOA and ammonium sulfate, generated by atomization, were exposed to UV radiation in an environmental chamber at three RH conditions (5, 45, and 85%), and changes in chemical composition and mass were monitored using an aerosol mass spectrometer (AMS). Under all RH conditions, photolysis leads to substantial loss of SOA mass, where the rate of mass loss decreased with decreasing RH. For all RH conditions, the less oxidized components of SOA (e.g., carbonyls) exhibited the fastest photodegradation rates, which resulted in a more oxidized SOA after photolytic aging. The photolytic reactivity of SOA material exhibited a dependence on RH likely due to moisture-induced changes in SOA morphology or phase. The results suggest that the atmospheric lifetime of SOA with respect to photolysis is dependent on its RH cycle, and that photolysis may be an important sink for some SOA components occurring on an initial time scale of a few hours under ambient conditions.

Application of Direct Analysis in Real Time-Mass Spectrometry (DART-MS) to the Study of Gas–Surface Heterogeneous Reactions: Focus on Ozone and PAHs

A novel analytical method is presented whereby Direct Analysis in Real Time-Mass Spectrometry (DART-MS) is applied to the study of gas-surface heterogeneous reactions. To illustrate the capabilities of the approach, the kinetics of a well-studied reaction of surface-bound polycyclic aromatic hydrocarbons with ozone are presented. Specifically, using helium as the reagent gas and with the DART heater temperature of 500 °C, nanogram quantities of benzo[e]pyrene (BeP) deposited on the outside of glass melting point capillary tubes were analyzed in positive ion mode with a limit of detection of 40 pg. Using bis(2-ethylhexyl) sebacate as an internal standard, the kinetics of the ozone-BeP reaction were assessed by determining the surface-bound BeP decays, after oxidation in an off-line reaction cell. The reaction is demonstrated to follow the Langmuir-Hinshelwood mechanism, known to prevail for heterogeneous reactions of this type. In addition, a wide array of oxygenated, condensed-phase products has been observed. The present work demonstrates the capability of the DART-MS technique to investigate the heterogeneous chemistry taking place on a wide range of surfaces, such as those that form in both outdoor and indoor environments.

Evidence for Gas–Surface Equilibrium Control of Indoor Nitrous Acid

Nitrous acid (HONO) is an important component of indoor air as a photolabile precursor to hydroxyl radicals and has direct health effects. HONO concentrations are typically higher indoors than outdoors, although indoor concentrations have proved challenging to predict using box models. In this study, time-resolved measurements of HONO and NO2 in a residence showed that [HONO] varied relatively weakly over contiguous periods of hours, while [NO2] fluctuated in association with changes in outdoor [NO2]. Perturbation experiments were performed in which indoor HONO was depleted or elevated and were interpreted using a two-compartment box model. To reproduce the measurements, [HONO] had to be predicted using persistent source and sink processes that do not directly involve NO2, suggesting that HONO was in equilibrium with indoor surfaces. Production of gas phase HONO directly from conversion of NO2 on surfaces had a weak influence on indoor [HONO] during the time of the perturbations. Highly similar temporal responses of HONO and semivolatile carboxylic acids to ventilation of the residence along with the detection of nitrite on indoor surfaces support the concept that indoor HONO mixing ratios are controlled strongly by gas-surface equilibrium.