Barbara J. Finlayson-Pitts

Unexpectedly high concentrations of molecular chlorine in coastal air

The fate of many atmospheric trace species, including pollutants such as nitrogen oxides and some volatile organic compounds, is controlled by oxidation reactions. In the daytime troposphere, these reactions are dominated by photochemically produced OH radicals; at night and in polluted environments, NO3 radicals are an important oxidant. Ozone can contribute to the oxidation of atmospheric species during both day and night. In recent years, laboratory investigations, modelling studies, measured Cl deficits in marine aerosols and species-nonspecific observations of gaseous inorganic chlorine compounds other than HCl have suggested that reactive halogen species may contribute significantly to—or even locally dominate—the oxidative capacity of the lower marine troposphere. Here we report night-time observations of molecular chlorine concentrations at a North American coastal site during onshore wind flow conditions that cannot be explained using known chlorine chemistry. The measured Cl2 mixing ratios range from <10 to 150 parts per 1012 (p.p.t.), exceeding those predicted for marine air by more than an order of magnitude. Using the observed chlorine concentrations and a simple photochemical box model, we estimate that a hitherto unrecognized chlorine source must exist that produces up to 330 p.p.t. Cl2 per day. The model also indicates that early-morning photolysis of molecular chlorine can yield sufficiently high concentrations of chlorine atoms to render the oxidation of common gaseous compounds by this species 100 times faster than the analogous oxidation reactions involving the OH radical, thus emphasizing the locally significant effect of chlorine atoms on the concentrations and lifetimes of atmospheric trace species in both the remote marine boundary layer and coastal urban areas.

The heterogeneous hydrolysis of NO2 in laboratory systems and in outdoor and indoor atmospheres: An integrated mechanism

The heterogeneous reaction of NO2 with water on the surface of laboratory systems has been known for decades to generate HONO, a major source of OH that drives the formation of ozone and other air pollutants in urban areas and possibly in snowpacks. Previous studies have shown that the reaction is first order in NO2 and in water vapor, and the formation of a complex between NO2 and water at the air–water interface has been hypothesized as being the key step in the mechanism. We report data from long path FTIR studies in borosilicate glass reaction chambers of the loss of gaseous NO2 and the formation of the products HONO, NO and N2O. Further FTIR studies were carried out to measure species generated on the surface during the reaction, including HNO3, N2O4 and NO2+. We propose a new reaction mechanism in which we hypothesize that the symmetric form of the NO2 dimer, N2O4, is taken up on the surface and isomerizes to the asymmetric form, ONONO2. The latter autoionizes to NO+NO3−, and it is this intermediate that reacts with water to generate HONO and surface-adsorbed HNO3. Nitric oxide is then generated by secondary reactions of HONO on the highly acidic surface. This new mechanism is discussed in the context of our experimental data and those of previous studies, as well as the chemistry of such intermediates as NO+ and NO2+ that is known to occur in solution. Implications for the formation of HONO both outdoors and indoors in real and simulated polluted atmospheres, as well as on airborne particles and in snowpacks, are discussed. A key aspect of this chemistry is that in the atmospheric boundary layer where human exposure occurs and many measurements of HONO and related atmospheric constituents such as ozone are made, a major substrate for this heterogeneous chemistry is the surface of buildings, roads, soils, vegetation and other materials. This area of reactions in thin films on surfaces (SURFACE = Surfaces, Urban and Remote: Films As a Chemical Environment) has received relatively little attention compared to reactions in the gas and liquid phases, but in fact may be quite important in the chemistry of the boundary layer in urban areas.

Nonequilibrium atmospheric secondary organic aerosol formation and growth

Airborne particles play critical roles in air quality, health effects, visibility, and climate. Secondary organic aerosols (SOA) formed from oxidation of organic gases such as α-pinene account for a significant portion of total airborne particle mass. Current atmospheric models typically incorporate the assumption that SOA mass is a liquid into which semivolatile organic compounds undergo instantaneous equilibrium partitioning to grow the particles into the size range important for light scattering and cloud condensation nuclei activity. We report studies of particles from the oxidation of α-pinene by ozone and NO3 radicals at room temperature. SOA is primarily formed from low-volatility ozonolysis products, with a small contribution from higher volatility organic nitrates from the NO3 reaction. Contrary to expectations, the particulate nitrate concentration is not consistent with equilibrium partitioning between the gas phase and a liquid particle. Rather the fraction of organic nitrates in the particles is only explained by irreversible, kinetically determined uptake of the nitrates on existing particles, with an uptake coefficient that is 1.6% of that for the ozonolysis products. If the nonequilibrium particle formation and growth observed in this atmospherically important system is a general phenomenon in the atmosphere, aerosol models may need to be reformulated. The reformulation of aerosol models could impact the predicted evolution of SOA in the atmosphere both outdoors and indoors, its role in heterogeneous chemistry, its projected impacts on air quality, visibility, and climate, and hence the development of reliable control strategies.

The nature of water on surfaces of laboratory systems and implications for heterogeneous chemistry in the troposphere

A number of heterogeneous reactions of atmospheric importance occur in thin water films on surfaces in the earths boundary layer. It is therefore important to understand the interaction of water with various materials, both those used to study heterogeneous chemistry in laboratory systems, as well as those found in the atmosphere. We report here studies at 22 °C to characterize the interaction of water with such materials as a function of relative humidity from 0–100%. The surfaces studied include borosilicate glass, both untreated and after cleaning by three different methods (water, hydrogen peroxide and an argon plasma discharge), quartz, FEP Teflon film, a self assembled monolayer of n-octyltrichlorosilane (C8 SAM) on glass, halocarbon wax coatings prepared by two different methods, and several different types of Teflon coatings on solid substrates. Four types of measurements covering the range from the macroscopic level to the molecular scale were made: (1) contact angle measurements of water droplets on these surfaces to obtain macroscopic scale data on the water-surface interaction, (2) atomic force microscopy measurements to provide micron to sub-micron level data on the surface topography, (3) transmission FTIR of the surfaces in the presence of increasing water vapor concentrations to probe the interaction with the surface at a molecular level, and (4) X-ray photoelectron spectroscopy measurements of the elemental surface composition of the glass and quartz samples. Both borosilicate glass and the halocarbon wax coatings adsorbed significantly more water than the FEP Teflon film, which can be explained by a combination of the chemical nature of the surfaces and their physical topography. The C8 SAM, which is both hydrophobic and has a low surface roughness, takes up little water. The implications for the formation of thin water films on various surfaces in contact with the atmosphere, including building materials, soil, and vegetation, are discussed.

Atmospheric Chemistry of Tropospheric Ozone Formation: Scientific and Regulatory Implications

Tropospheric ozone continues to be of significant concern worldwide due to its effect on human health, forest ecosystems and agricultural crops. Its levels have also increased globally over the last century. Photolysis of ozone (O3) is the major source of the hydroxyl (OH) radical in the remote troposphere, so that an increase in O3 will produce more OH, resulting in decreased lifetimes of many trace species, such as methane and the hydrochlorofluorocarbons (HGFG), which are of stratospheric importance. A brief overview of the chemistry of volatile organic compounds (VOC) and oxides of nitrogen (NOX) which lead to ozone formation is included. The question of whether controlling VOC or NOX or both is most effective in urban/suburban and downwind/rural regions is discussed. The impact of uncertainties in emissions inventories is briefly discussed. Finally, the relative contributions of individual VOC to ozone formation and how this information forms the technical basis for California’s new regulations for V...

Bromine activation in the troposphere by the dark reaction of O3 with seawater ice

There is increasing evidence that bromine atoms play a role in tropospheric chemistry in the marine boundary layer. In addition, they are believed to lead to rapid depletion of surface level ozone in the Arctic at polar sunrise. While mechanisms have been proposed for recycling bromine atoms from sea salt particles, the initial reaction(s) leading to the formation of bromine atom precursors is not known. We report here the formation of gaseous Br2 from the reaction of seawater ice with O3 in the dark. These observations suggest that this reaction is a potential source of tropospheric photolyzable bromine in high latitude coastal regions in winter. In addition, it may be the source of the photolyzable bromine gas measured recently in the Arctic by Impey et al. (1997), which is believed to be responsible for the ozone destruction at polar sunrise.

The photochemical production of HONO during the heterogeneous hydrolysis of NO2

The heterogeneous hydrolysis of NO2 in thin water films, a major source of HONO and hence OH radicals in polluted urban atmospheres, has been previously reported to be photoenhanced (H. Akimoto, H. Takagi and F. Sakamaki, Int. J. Chem. Kinet., 1987, 19, 539, ) which has important implications for OH production both in environmental chambers and in the lower atmosphere. We report here studies of the impact of 320–400 nm radiation on HONO formation during the heterogeneous NO2 hydrolysis at 296 K. The experiments were carried out in a borosilicate glass cell using long path Fourier transform infrared (FTIR) spectroscopy with three initial NO2 concentrations (20, 46, and 54 ppm) at relative humidities of 33, 39, and 57%, respectively. Nitrous acid was first allowed to accumulate from NO2 hydrolysis in the dark, and then the mixture of reactants and products was irradiated. The measured concentration–time profiles of the gases were compared to the predictions of a kinetics model developed for this system. The initial loss of HONO upon irradiation was consistent with its photolysis and known secondary gas phase chemistry without any photoenhancement. While the fundamental NO2 heterogeneous hydrolysis is not itself photoenhanced, there is clear evidence in these experiments for the generation of gas phase HONO by photolysis of adsorbed HNO3 formed during the heterogeneous hydrolysis. The mechanisms and atmospheric implications of HONO as well as NO2 formation by the photolysis of surface-adsorbed HNO3 are discussed.

Comparison of FTIR and Particle Mass Spectrometry for the Measurement of Particulate Organic Nitrates

While multifunctional organic nitrates are formed during the atmospheric oxidation of volatile organic compounds, relatively little is known about their signatures in particle mass spectrometers. High resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS) and FTIR spectroscopy on particles impacted on ZnSe windows were applied to NH(4)NO(3), NaNO(3), and isosorbide 5-mononitrate (IMN) particles, and to secondary organic aerosol (SOA) from NO(3) radical reactions at 22 degrees C and 1 atm in air with alpha- and beta-pinene, 3-carene, limonene, and isoprene. For comparison, single particle laser ablation mass spectra (SPLAT II) were also obtained for IMN and SOA from the alpha-pinene reaction. The mass spectra of all particles exhibit significant intensity at m/z 30, and for the SOA, weak peaks corresponding to various organic fragments containing nitrogen [C(x)H(y)N(z)O(a)](+) were identified using HR-ToF-AMS. The NO(+)/NO(2)(+) ratios from HR-ToF-AMS were 10-15 for IMN and the SOA from the alpha- and beta-pinene, 3-carene, and limonene reactions, approximately 5 for the isoprene reaction, 2.4 for NH(4)NO(3) and 80 for NaNO(3). The N/H ratios from HR-ToF-AMS for the SOA were smaller by a factor of 2 to 4 than the -ONO(2)/C-H ratios measured using FTIR. FTIR has the advantage that it provides identification and quantification of functional groups. The NO(+)/NO(2)(+) ratio from HR-ToF-AMS can indicate organic nitrates if they are present at more than 15-60% of the inorganic nitrate, depending on whether the latter is NH(4)NO(3) or NaNO(3). However, unique identification of specific organic nitrates is not possible with either method.

Kinetics of reactions of chlorine atoms with a series of alkenes at 1 atm and 298 K: structure and reactivity

Chlorine atoms are important oxidants at dawn in the marine boundary layer where a variety of organics are also present, including alkenes. Using the relative rate technique, the kinetics of the gas phase reactions of atomic chlorine with a series of alkenes, relative to n-heptane as a reference, have been investigated at (298 ± 3) K and 1 atmosphere in either synthetic air or nitrogen. The rate constant for n-heptane, relative to n-butane whose rate constant was taken to be 2.18 × 10−10 cm3 molecule−1 s−1, was also measured and found to be (3.97 ± 0.27) × 10−10 cm3 molecule−1 s−1 (2s). Based on this value for the n-heptane reaction, the following absolute values for the rate constants, k (in units of 10−10 cm3 molecule−1 s−1) for the chlorine atom reactions were determined: propene, 2.64 ± 0.21; isobutene, 3.40 ± 0.28; 1-butene, 3.38 ± 0.48; cis-2-butene, 3.76 ± 0.84; trans-2-butene, 3.31 ± 0.47; 2-methyl-1-butene, 3.58 ± 0.40; 2-methyl-2-butene, 3.95 ± 0.32; 3-methyl-1-butene, 3.29 ± 0.36; 2-ethyl-1-butene, 3.89 ± 0.41; 1-pentene, 3.97 ± 0.36; 3-methyl-1-pentene, 3.85 ± 0.35; and cis-4-methyl-2-pentene, 4.11 ± 0.55 (±2s). The errors reflect those in our relative rate measurements but do not include the 10% error in the absolute value of the n-butane rate constant upon which these rate constants are ultimately based. A structure–reactivity scheme is presented that assumes that rate constants for addition of chorine atoms to the double bond, as well as that for abstraction of an allylic hydrogen atom, depend upon the degree of alkyl substitution at the double bond and allylic carbons. The surprising result is that the allylic hydrogen atoms react less rapidly with chlorine atoms than the analogous alkyl hydrogens in alkanes. The atmospheric implications for loss of alkenes in the marine boundary layer are discussed.

Simplified mechanism for new particle formation from methanesulfonic acid, amines, and water via experiments and ab initio calculations

Airborne particles affect human health and significantly influence visibility and climate. A major fraction of these particles result from the reactions of gaseous precursors to generate low-volatility products such as sulfuric acid and high-molecular weight organics that nucleate to form new particles. Ammonia and, more recently, amines, both of which are ubiquitous in the environment, have also been recognized as important contributors. However, accurately predicting new particle formation in both laboratory systems and in air has been problematic. During the oxidation of organosulfur compounds, gas-phase methanesulfonic acid is formed simultaneously with sulfuric acid, and both are found in particles in coastal regions as well as inland. We show here that: (i) Amines form particles on reaction with methanesulfonic acid, (ii) water vapor is required, and (iii) particle formation can be quantitatively reproduced by a semiempirical kinetics model supported by insights from quantum chemical calculations of likely intermediate clusters. Such an approach may be more broadly applicable in models of outdoor, indoor, and industrial settings where particles are formed, and where accurate modeling is essential for predicting their impact on health, visibility, and climate.