Multiphase chemical kinetics of the nitration of aerosolized protein by ozone and nitrogen dioxide.

Proteins contained in pollen and other biological particles are nitrated by ozone and nitrogen dioxide in polluted air. The nitration can enhance the allergenic potential of proteins, which may contribute to the increasing prevalence of allergic diseases. The reactive uptake of NO(2) by aerosolized protein (bovine serum albumin) was investigated in an aerosol flow tube using the short-lived radioactive tracer (13)N. In the absence of O(3), the NO(2) uptake coefficient was below detection limit (γ(NO2) < 10(-6)), but with 20-160 ppb O(3) γ(NO2) increased from ~10(-6) to ~10(-4). Using the kinetic multilayer model of surface and bulk chemistry (KM-SUB), the observed time and concentration dependence can be well reproduced by a multiphase chemical mechanism involving ozone-generated reactive oxygen intermediates (ROIs), but not by NO(3) radicals formed in the gas phase. Product studies show the formation of protein dimers, suggesting that the ROIs are phenoxy radical derivatives of the amino acid tyrosine (tyrosyl radicals) which are also involved in physiological protein nitration processes. Our results imply that proteins on the surface of aerosol particles undergo rapid nitration in polluted air, while the rate of nitration in bulk material may be low depending on phase state and surface-to-volume ratio.

UVA/Vis-induced nitrous acid formation on polyphenolic films exposed to gaseous NO2.

Photochemical processes on ground and airborne surfaces have been suspected to lead to production of HONO in the sunlit lower troposphere, e.g. upon light activation of humic acids followed by reaction with adsorbed NO(2). Here, we used tannic and gentisic acids as proxies for atmospheric polyphenolic compounds to obtain further insights into the photoenhanced NO(2) conversion to HONO, which is a significant tropospheric hydroxyl radical (OH) source. The coated wall flow tube technique was used in combination with online detection of gas-phase HONO and NO(x) under different irradiation conditions. Photoenhanced HONO formation rates of 0.1 to 2 ppbv s(-1) were measured upon NO(2) (0-400 ppbv) uptake on tannic and gentisic acid coatings under irradiation with UV light. The data allow identification of three pathways of light-induced HONO formation: (I) photolysis of a nitroaromatic intermediate formed by a non-photochemical process in the dark, with a photolysis frequency of 10(4) s(-1) at 2 × 10(20) photons m(-2) photon flux; (II) direct photo-oxidation, presumably through electron or hydrogen transfer of the excited substrate; and (III) sensitized electron or hydrogen transfer as suggested before but also demonstrated for visible irradiation here. Aging of tannic acid under oxygen in the dark led to products which promoted light-induced HONO formation in the visible.

Uptake of ozone to deliquesced KI and mixed KI/NaCl aerosol particles.

The kinetics of uptake of ozone to deliquesced potassium iodide (KI) aerosol particles has been investigated in an aerosol flow tube at 72-75% relative humidity, room temperature, and atmospheric pressure. The observed loss of ozone was further analyzed in terms of a numeric model to explicitly track the iodide concentration in the particles. This allowed retrieving a value alpha(b) = 0.6 +/- (0.5)(0.4) for the bulk accommodation coefficient (alpha(b)). The second order rate constant in the bulk phase agreed with available literature (k(b) = (1.0 +/- 0.3) x 10(9) M(-1) s(-1)) even for the high ionic strength conditions of the present experiments. As long as iodide remained in excess, the average uptake coefficient was gamma = (1.10 +/- 0.20) x 10(-2). Different experiments were performed where the iodide to chloride ratio, the ozone concentration, and the surface to volume ratio of particles were varied. In combination, the results obtained indicate that uptake was driven by fast bulk accommodation and reaction in the bulk for all conditions investigated. The results further suggest that ozone uptake is not limited by the bulk accommodation coefficient alpha(b) under atmospheric conditions.

Uptake of NO2 to deliquesced dihydroxybenzoate aerosol particles.

The uptake of nitrogen dioxide (NO2), a major trace gas in the atmosphere, to deliquesced particles containing the sodium salts of hydroquinone (1,4-dihydroxybenzene) or gentisic (2,5-dihydroxybenzoic) acid was investigated at 40% relative humidity and 23 degrees C in an aerosol flow tube. The experiments were performed using the short-lived radioactive tracer 13N and a denuder technique. The observed uptake coefficient for NO2 was up to approximately 6 x 10(-3) for the hydroquinone disodium salt aerosol, which exceeds previously reported data in the range 10(-4) to 10(-3). The measured time dependence of NO2 uptake was fitted using a kinetic model taking into account reactant consumption in the particle phase, and keeping the bulk accommodation coefficient, alpha(b), and the rate constants for the reaction of dissolved NO2 with the deprotonated forms of the mentioned phenolic compounds as variables. We obtained alpha(b) = 0.024(-0.003)(+0.018) as a best estimate. For gentisic acid, the second-order rate constant was k2 = (2.9 +/- 0.1) x 10(8) L mol(-1) s(-1) and is reported for the first time. The data are consistent with bulk reaction limited uptake, without indications for a surface component in the kinetics.

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.

Intermediates formed by laser flash photolysis of [PtCl(6)](2-) in aqueous solutions.

The stationary photolysis of [PtCl(6)](2-) in aqueous solutions (10(-5)-10(-4) M) at the region of 313 nm leads to its photoaquation with a quantum yield of 0.19. Laser flash photolysis experiments (308 nm) provided evidence of the formation of Pt(iii) intermediates, namely [PtCl(4)(OH)(H(2)O)](2-) and [PtCl(4)](-), and Cl(2) (-) radical anions. The Pt(iii) complexes formed as a result of an intrasphere electron transfer from Cl(-) ligands to the excited Pt(iv) ion. However, the main ( approximately 90%) photolysis channel was not accompanied by the transfer of Cl atoms to the solvent bulk. The photoaquation of [PtCl(6)](2-) results from the back electron transfer in the secondary geminate pair, [PtCl(5)(H(2)O)](2-)-Cl. The relative yield of Pt(iii) intermediates, recorded after the completion of all processes in the geminate pair, was less than 10% of the number of disappearing initial [PtCl(6)](2-) complexes.