Heterogeneous reactions of surface-adsorbed catechol with nitrogen dioxide: substrate effects for tropospheric aerosol surrogates

Surface-adsorbed organics can alter the chemistry of tropospheric aerosols thereby impacting photochemical cycles and altering aerosol properties. The nature of the surface can also influence the chemistry of the surface-adsorbed organic. We employed diffuse reflectance infrared spectroscopy (DRIFTS) to monitor the adsorption of gaseous catechol on several tropospheric aerosol surrogates and to investigate the subsequent reactivity of adsorbed catechol with nitrogen dioxide. The dark heterogeneous reaction of NO(2) with NaCl-adsorbed catechol produced 4-nitrocatechol, 1,2-benzoquinone, and the ring-cleaved product muconic acid, with product yields of 88%, 8%, and 4% at relative humidity (RH) < 2%, respectively. The reaction was first-order with respect to both catechol and NO(2). The reactive uptake coefficient for NO(2) + NaCl-adsorbed catechol increased from 3 x 10(-6) at <2% RH to 7 x 10(-6) at 30% RH. These reactions were more than two orders of magnitude more reactive than NaCl without adsorbed catechol. The 4-nitrocatechol product yield was enhanced on NaF, while NaBr-adsorbed catechol produced considerably more 1,2-benzoquinone and muconic acid. This substrate effect is discussed in terms of each substrate's ability to polarize the phenol group and hinder hydrogen atom abstraction from intermediate o-semiquinone radicals. These dark heterogeneous reactions may alter the UV-visible absorbing properties of tropospheric aerosols and may also contribute as a dark source of NO(2)(-)/HONO. These results contrast prior observations which found pure catechol thin films unreactive with NO(2), highlighting the need to specifically consider substrate and matrix effects in laboratory systems.

Heterogeneous oxidation of squalene film by ozone under various indoor conditions

Abstract The effects of indoor conditions (ozone concentration, temperature, relative humidity (RH), and the presence of NO(x)) on heterogeneous squalene oxidation were studied with Attenuated Total Reflectance-Fourier Transform Infrared spectroscopy. The heterogeneous kinetics of squalene-ozone reaction revealed a pseudo-first-order reaction rate constant of 1.22 x 10(-5)/s at [O(3)] = 40 ppb. Oxidation kinetics were insensitive to temperature over the range of 24-58 +/- 2 degrees C as well as to RH and presence of NO(x). Products, however, were affected by the environmental parameters. As temperature was increased, fewer surface products and more low molecular weight gaseous products were observed. Lower air exchange rates also enhanced gas phase reactions, allowing for formation of secondary gas phase products. As RH increased, there was a shift in product distribution from ketones to aldehydes, and the presence of NO(x) during squalene ozonolysis resulted in the formation of nitrated oxidation products. Identified surface products included 6-methyl-5-hepten-2-one, geranyl acetone, and long chain ketones and aldehydes, while gas phase products included formaldehyde, acetone, 4-oxopentanal (4-OPA), glyoxal, and pyruvic acid. Practical Implications Heterogeneous oxidation of squalene resulted in surface products including long chain aldehydes and ketones, and gas phase products including formaldehyde, a known human carcinogen (IARC 2006), and bicarbonyl compounds like: 4-oxopentanal (4-OPA), glyoxal, and pyruvic acid that are characterized as asthma triggers and sensitizers (Anderson et al., 2007; Jarvis et al., 2005). In addition, ozonolysis experiments in the presence of NO(x) showed the formation of nitrated surface oxidation products. Such nitrated products may have higher mutagenicity, carcinogenicity, or allergenic potential than their nitrate free counterparts (Franze et al., 2005; Pitts, 1983). Kinetic studies determined that at moderate ozone levels of 40 ppb (Uhde and Salthammer, 2007), and an estimated skin surface density of 4 x 10(15) molecules/cm(2), surface reaction would lead to a minimum product formation flux of 4 x 10(10) molecules cm(2)/s. As squalene is naturally occurring and continually produced by the human body, its concentration in the indoor environment cannot be controlled. However, this study highlights the importance of regulating air exchange rate, temperature, and ozone level in the indoor environment on the formation of potentially harmful or irritating squalene oxidation products.

Multiphase chemistry of ozone on fulvic acids solutions

By means of a wetted-wall flow tube, we studied the multiphase chemistry of ozone on aqueous solutions containing fulvic acids (FA), taken as proxies for atmospheric "humic like substances", so-called HULIS. In these experiments, the loss of gaseous O3 was monitored by UV-visible absorption spectroscopy at the reactor outlet (i.e., after contact between the gaseous and liquid phases). Measurements are reported in terms of dimensionless uptake coefficients (gamma) in the range from 1.6 x 10(-7) to 1.3 x 10(-5) depending on ozone gas phase concentration (in the range from 6.6 to 34.4 x 10(11) molecules cm(-3)) and fulvic acid aqueous concentration (in the range from 0.25 to 2.5 mg L(-1)) and pH (in the range from 2.5 to 9.2). The measured kinetics were observed to follow a Langmuir-Hinshelwood type mechanism, in which O3 first adsorbs on the liquid surface and then reacts with the Fulvic Acid molecules. The reported uptake coefficients are greatly increased over those measured on pure water, demonstrating that the presence in solution of fulvic acids does greatly enhance the uptake kinetics. Accordingly, the chemical interactions of fulvic acids (or HULIS) may be a driving force for the uptake of ozone on liquid organic aerosols and can also represent an important mechanism for the O3 deposition to the rivers and lakes.

Reactive uptake of NO3, N2O5, NO2, HNO3, and O3 on three types of polycyclic aromatic hydrocarbon surfaces

We investigated the reactive uptake of NO3, N2O5, NO2, HNO3, and O3 on three types of solid polycyclic aromatic hydrocarbons (PAHs) using a coated wall flow tube reactor coupled to a chemical ionization mass spectrometer. The PAH surfaces studied were the 4-ring systems pyrene, benz[a]anthracene, and fluoranthene. Reaction of NO3 radicals with all three PAHs was observed to be very fast with the reactive uptake coefficient, gamma, ranging from 0.059 (+0.11/-0.049) for benz[a]anthracene at 273 K to 0.79 (+0.21/-0.67) for pyrene at room temperature. In contrast to the NO3 reactions, reactions of the different PAHs with the other gas-phase species (N2O5, NO2, HNO3, and O3) were at or below the detection limit (gamma <or= 6.6 x 10(-5)) in all cases, illustrating that these reactions are at best slow. For NO3 we also investigated the time dependence of the reactive uptake to determine if the surface-bound PAH molecules were active participants in the reaction (i.e., reactants). Reaction of NO3 on all three PAH surfaces slowed down at 263 K after long NO3 exposure times, suggesting that the PAH molecules were reactants. Additionally, NO2 and HNO3 were identified as major gas-phase products. Our results show that under certain atmospheric conditions, NO3 radicals can be a more important sink for PAHs than NO2, HNO3, N2O5, or O3 and impact tropospheric lifetimes of surface-bound PAHs.

Aging of organic aerosol: bridging the gap between laboratory and field studies

The oxidation of organics in aerosol particles affects the physical properties of aerosols through a process known as aging. Atmospheric particles compose a huge set of specific organic compounds, most of which have not been identified in field measurements. Laboratory experiments inevitably address model systems of reduced complexity to isolate critical chemical phenomena, but growing evidence suggests that composition effects may play a central role in the atmospheric aging of organic particles. In this review we seek to address the connections between recent laboratory studies and recent field campaigns addressing the aging of organic aerosols. We review laboratory studies on the uptake of oxidants, the evolution of particle-water interactions, and the evolution of particle density with aging. Finally, we review field data addressing condensed-phase lifetimes of organic tracers. These data suggest that although matrix effects identified in the laboratory have taken a step toward reconciling laboratory-field disagreements, further work is needed to understand the actual aging rates of organics in ambient particles.

The Oxidation of Oleate in Submicron Aqueous Salt Aerosols: Evidence of a Surface Process

We have studied the oxidation of submicron aqueous aerosols consisting of internal mixtures of sodium oleate (oleic acid proxy), sodium dodecyl sulfate, and inorganic salts by O3, NO3/N2O5, and OH. Experiments were performed using an aerosol flow tube and a continuous flow photochemical reaction chamber coupled to a chemical ionization mass spectrometer (CIMS). The CIMS was fitted with a heated inlet for volatilization and detection of organics in the particle phase simultaneously with the gas phase. A differential mobility analyzer/condensation particle counter was used for determining aerosol size distributions. The oxidation of oleate by O3 follows Langmuir-Hinshelwood kinetics, with gammaO3 approximately 10(-5) calculated from the observed loss rate of oleate in the particle phase. The best fit Langmuir-Hinshelwood parameters are kImax=0.05+/-0.01 s-1 and KO3=4(+/-3)x10(-14) cm3molec-1. These parameters showed no dependence on the ionic composition of the aerosols or on the presence of alkyl surfactants. Several ozone oxidation products were observed to be particle-bound at ambient temperature, including nonanoic acid. We observed efficient processing of oleate by OH (0.1<or=gammaOH<or=1), and we suggest an upper bound of gammaNuOmicron3<10(-3). We conclude that for the aerosol compositions studied, oxidation occurs near the gas-aerosol interface and that the 1 e-fold lifetime of unsaturated organics at the aerosol surface is approximately 10 min due to O3 oxidation under atmospheric conditions. In the context of a Langmuir-Hinshelwood mechanism, different underlying aerosol compositions may extend the lifetime of oleic acid at the surface by reducing KO3.

Atmospheric Aerosols: Composition, Transformation, Climate and Health Effects

Aerosols are of central importance for atmospheric chemistry and physics, the biosphere, climate, and public health. The airborne solid and liquid particles in the nanometer to micrometer size range influence the energy balance of the Earth, the hydrological cycle, atmospheric circulation, and the abundance of greenhouse and reactive trace gases. Moreover, they play important roles in the reproduction of biological organisms and can cause or enhance diseases. The primary parameters that determine the environmental and health effects of aerosol particles are their concentration, size, structure, and chemical composition. These parameters, however, are spatially and temporally highly variable. The quantification and identification of biological particles and carbonaceous components of fine particulate matter in the air (organic compounds and black or elemental carbon, respectively) represent demanding analytical challenges. This Review outlines the current state of knowledge, major open questions, and research perspectives on the properties and interactions of atmospheric aerosols and their effects on climate and human health.

Kinetics of ClONO2 Reactive Uptake on Ice Surfaces at Temperatures of the Upper Troposphere

The reactive uptake kinetics of ClONO(2) on pure and doped water-ice surfaces have been studied using a coated wall flow tube reactor coupled to an electron impact mass spectrometer. Experiments have been conducted on frozen film ice surfaces in the temperature range 208-228 K with P((ClONO)(2)) < or = 10(-6) Torr. The uptake coefficient (gamma) of ClONO(2) on pure ice was time dependent with a maximum value of gamma(max) approximately 0.1. On HNO(3)-doped ice at 218 K the gamma(max) was 0.02. HOCl formation was detected in both experiments. On HCl-doped ice, uptake was gas-phase diffusion limited (gamma > 0.1) and gas-phase Cl(2) was formed. The uptake of HCl on ice continuously doped with HNO(3) was reversible such that there was no net uptake of HCl once the equilibrium surface coverage was established. The data were well described by a single site 2-species competitive Langmuir adsorption isotherm. The surface coverage of HCl on HNO(3)-doped ice was an order of magnitude lower than on bare ice for a given temperature and P(HCl). ClONO(2) uptake on this HCl/HNO(3)-doped ice was studied as a function of P(HCl). gamma(max) was no longer gas-phase diffusion limited and was found to be linearly dependent on the surface concentration of HCl. Under conditions of low HCl surface concentration, hydrolysis of ClONO(2) and reaction with HCl were competing such that both Cl(2) and HOCl were formed. A numerical model was used to simulate the experimental results and to aid in the parametrization of ClONO(2) reactivity on cirrus ice clouds in the upper troposphere.

Model of Uptake of OH Radicals on Nonreactive Solids

A model of adsorption and recombination of OH radicals was developed for nonreactive solid surfaces of atmospheric interest. A parametrization of this heterogeneous mechanism was carried out to determine the role of the catalytic properties of these solid surfaces, taking into account the adsorption energy, defects, surface diffusion, and chemical reactions in the gas-solid interface. The uptake process was simulated for diffusion-controlled chemical reactions on the surface on the basis of Langmuir-Hinshelwood and Eley-Rideal mechanisms. Using an analytical approach and the Monte Carlo technique, we show the dependencies of the uptake probability of the heterogeneous reactions on the OH concentration and adsorption energy. The model is employed in the analysis of the empirically derived uptake coefficient for water ice, Al(2)O(3), NaCl, NH(4)NO(3), NH(4)HSO(4), and (NH(4))(2)SO(4). We found the following values for the free energy of adsorption of OH radicals: E(ice) = 7.3-7.6 kcal/mol, E(Al)(2)(O)(3) = 11-11.7 kcal/mol, E(NH)(4)(NO)(3) = 10.2 kcal/mol, E(NaCl) = 10.2 kcal/mol, E(NH)(4)(HSO)(4) = 9.8 kcal/mol, and E((NH)(4))(2)(SO)(4) = 9.8 kcal/mol. The atmospheric implications of the catalytic reactions of OH with adsorbed reactive molecules are discussed. The results of the modeling of the uptake process showed that the heterogeneous decay rate can exceed the corresponding gas-phase reaction rate under atmospheric conditions.

Atmospheric Pressure Coated-Wall Flow-Tube Study of Acetone Adsorption on Ice

An atmospheric pressure variant of the coated-wall flow-tube technique in combination with a Monte Carlo simulation is presented. In a performance test of simple first-order wall loss, the Monte Carlo simulation, which uses a simplified model of transport in laminar flow, reproduced results of an analytical solution of the transport equations in a flow tube. This technique was then used to investigate the reversible adsorption of acetone on ice films between 203 and 223 K and a surface coverage of below 5% of a formal monolayer. Simulation of the experimental uptake traces allowed retrieving an adsorption enthalpy of -46 +/- 3 kJ mol(-1) for acetone on ice, which is in good agreement with other static and flow-tube methods. For the experimental conditions adopted here, the transport of acetone molecules along the ice film is governed by equilibrium thermodynamics. Therefore, the surface accommodation coefficient, S(0), and the preexponential factor, tau(0), for the activated desorption cannot be independently determined. These two main microphysical parameters describing partitioning can rather be estimated through their relation to the adsorption entropy. A first estimate for S(0) of acetone on ice in the range of 0.004-0.043 is given.