Heterogeneous Chemistry of HONO on Liquid Sulfuric Acid: A New Mechanism of Chlorine Activation on Stratospheric Sulfate Aerosols

HONO Measurement by Catalytic Conversion to NO on Nafion Surfaces

A selective catalytic converter has been developed to quantify nitrous acid (HONO), a photochemical precursor to NO and OH radicals that drives the formation of ozone and other pollutants in the troposphere. The converter is made from a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer (Nafion) that was found to convert HONO to NO with unity yield under specific conditions. When coupled to a commercially available NO_x (=NO + NO₂) chemiluminescence (CL) analyzer, the system measures HONO with a limit of detection as low as 64 parts-per-trillion (ppt) (1 min average) in addition to NO_x. The converter is selective for HONO when tested against other common gas-phase reactive nitrogen species, although loss of O₃ on Nafion is a potential interference. The sensitivity and selectivity of this method allow for accurate measurement of atmospherically relevant concentrations of HONO. This was demonstrated by good agreement between HONO measurements made with the Nafion-CL method and those made with chemical ionization mass spectrometry in a simulation chamber and in indoor air. The observed reactivity of HONO on Nafion also has significant implications for the accuracy of CL NO_x analyzers that use Nafion to remove water from sampling lines.

The effect of (H2O)n (n = 1-3) clusters on the reaction of HONO with HCl: a mechanistic and kinetic study

The reaction between HONO and HCl is a possible pathway for the generation of ClNO, which is prone to photolyze, produce chlorine radicals, and accelerate the oxidation of tropospheric VOCs. Current experimental and theoretical studies have significant differences in rate constants under similar conditions. This study aims to examine the reasons for this difference. In this study, the effects of a single water molecule, water dimer, water trimer, excess HCl and excess HONO on the reaction mechanism of HONO + HCl were studied at the CCSD(T)/aug-cc-pVTZ//M06-2X/6-311+G(2df,2p) level and the rate constants of each reaction channel were calculated. Our results showed that the reaction potential barrier of HONO with HCl was the lowest only when the water dimer was present, and the reaction rate constants were close to the experimental results, and both the cis-HONO⋯(H₂O)₂ + HCl and the trans-HONO⋯(H₂O)₂ + HCl reaction paths are likely to occur. We think that the reason for the inconsistency between experimental and theoretical results is that the water dimer is involved in the reaction in experiments.

Persistent sulfate formation from London Fog to Chinese haze

Sulfate aerosols exert profound impacts on human and ecosystem health, weather, and climate, but their formation mechanism remains uncertain. Atmospheric models consistently underpredict sulfate levels under diverse environmental conditions. From atmospheric measurements in two Chinese megacities and complementary laboratory experiments, we show that the aqueous oxidation of SO₂ by NO₂ is key to efficient sulfate formation but is only feasible under two atmospheric conditions: on fine aerosols with high relative humidity and NH₃ neutralization or under cloud conditions. Under polluted environments, this SO₂ oxidation process leads to large sulfate production rates and promotes formation of nitrate and organic matter on aqueous particles, exacerbating severe haze development. Effective haze mitigation is achievable by intervening in the sulfate formation process with enforced NH₃ and NO₂ control measures. In addition to explaining the polluted episodes currently occurring in China and during the 1952 London Fog, this sulfate production mechanism is widespread, and our results suggest a way to tackle this growing problem in China and much of the developing world.

Heterogeneous chemistry of glyoxal on acidic solutions. An oligomerization pathway for secondary organic aerosol formation

The heterogeneous chemistry of glyoxal on sulfuric acid surfaces has been investigated at various acid concentrations and temperatures, utilizing a low-pressure fast flow laminar reactor coupled to an ion drift-chemical ionization mass spectrometer (ID-CIMS). The uptake coefficient (γ) of glyoxal ranges from (1.2 ± 0.06) × 10(-2) to (2.5 ± 0.01) × 10(-3) for 60-93 wt % H2SO4 at 253-273 K. The effective Henry's Law constant (H*) ranges from (98.9 ± 4.9) × 10(5) to (1.6 ± 0.1) × 10(5) M atm(-1) for 60-93 wt % at 263-273 K. Both the uptake coefficient and Henry's Law constant increase with decreasing acid concentration and temperature. Our results reveal a reaction mechanism of hydration followed by oligomerization for glyoxal on acidic media, indicating an efficient aqueous reaction of glyoxal on hygroscopic particles leading to secondary organic aerosol formation.

Soot aging from OH-initiated oxidation of toluene

We have conducted laboratory experiments to investigate the impacts of secondary organic aerosol formation on soot properties from OH-initiated oxidation of toluene. Monodisperse soot particles are exposed to the oxidation products of the OH-toluene reaction in an environmental chamber, and variations in particle size, mass, organic mass faction, morphology, effective density, hygroscopicity, and optical properties are simultaneously determined by an integrated aerosol analytical system. The thickness of the organic coating, correlated to reaction time and initial reactant concentrations, is shown to largely govern the particle properties. With the development of organic coating, the soot core is changed from a highly fractal to compact form, evident from the measured effective density and dynamic shape factor. The organic coating increases the particle hygroscopicity, and further exposure of coated soot to elevated relative humidity results in a more spherical particle. The single scattering albedo and scattering and absorption cross sections are also enhanced with the organic coating. Our results suggest that the oxidation products of anthropogenic pollutants alter the composition and properties of soot particles and lead to increased particle density, hygroscopicity, and optical properties, considerably enhancing their impacts on air quality, climate forcing, and human health.

Measurements of nitrous acid in commercial aircraft exhaust at the Alternative Aviation Fuel Experiment

The Alternative Aviation Fuel Experiment (AAFEX), conducted in January of 2009 in Palmdale, California, quantified aerosol and gaseous emissions from a DC-8 aircraft equipped with CFM56-2C1 engines using both traditional and synthetic fuels. This study examines the emissions of nitrous acid (HONO) and nitrogen oxides (NO(x) = NO + NO(2)) measured 145 m behind the grounded aircraft. The fuel-based emission index (EI) for HONO increases approximately 6-fold from idle to takeoff conditions but plateaus between 65 and 100% of maximum rated engine thrust, while the EI for NO(x) increases continuously. At high engine power, NO(x) EI is greater when combusting traditional (JP-8) rather than Fischer-Tropsch fuels, while HONO exhibits the opposite trend. Additionally, hydrogen peroxide (H(2)O(2)) was identified in exhaust plumes emitted only during engine idle. Chemical reactions responsible for emissions and comparison to previous measurement studies are discussed.

Heterogeneous reaction of NO(2) on fresh and coated soot surfaces

The heterogeneous reaction of nitrogen dioxide (NO(2)) on fresh and coated soot surfaces has been investigated to assess its role in night-time formation of nitrous acid (HONO) in the atmosphere. Soot surfaces were prepared by incomplete combustion of propane and kerosene fuels under lean and rich flame conditions and then processed by heating to evaporate semivolatile species or by coating with pyrene, sulfuric acid, or glutaric acid. Uptake kinetics and HONO yield measurements were performed in a low-pressure fast-flow reactor coupled to a chemical ionization mass spectrometer (CIMS), using atmospheric-level NO(2) concentrations. The uptake coefficient and the HONO yield upon interaction of NO(2) with nascent soot depend on the type of fuel and combustion regime and are the highest for samples prepared using fuel rich flame. Heating the nascent soot samples before exposure to NO(2) removes the organic material from the soot backbone, leading to a significant increase in NO(2) uptake coefficient and HONO yield. Continuous exposure to NO(2) reduces the reactivity of soot because of irreversible deactivation of the surface sites. Our results support the oxidation-reduction mechanism involving adsorptive and reactive centers on soot surface where NO(2) is converted to HONO and other products. Coating of the soot surface by different materials to simulate atmospheric aging has a strong impact on its reactivity toward NO(2) and the resulting HONO production. Coating of pyrene has little effect on either reaction rate or HONO yield. Sulfuric acid coating does not alter the uptake coefficient, but significantly reduces the amount of HONO formed. Coating of glutaric acid significantly increases NO(2) uptake coefficient and HONO yield. The results of our study indicate that the reactivity and HONO generating capacity of internally mixed soot aerosol will depend on the chemical composition of the coating material and hence will vary considerably in different polluted environments.

Heterogeneous chemistry of alkylamines with sulfuric acid: implications for atmospheric formation of alkylaminium sulfates

The heterogeneous interaction of alkylamines with sulfuric acid has been investigated to assess the role of amines in aerosol growth through the formation of alkylaminium sulfates. The kinetic experiments were conducted in a low-pressure fast flow reactor coupled to an ion drift-chemical ionization mass spectrometer (ID-CIMS). The measurements of heterogeneous uptake of methylamine, dimethylamine, and trimethylamine were performed in the acidity range of 59-82 wt % H(2)SO(4) and between 243 and 283 K. Irreversible reactive uptakes were observed for all three alkylamines, with comparable uptake coefficients (gamma) in the range of 2.0 x 10(-2) to 4.4 x 10(-2). The measured gamma value was slightly higher in more concentrated sulfuric acid and at lower temperatures. The results imply that the heterogeneous reactions of alkylamines contribute effectively to the growth of atmospheric acidic particles and, hence, secondary organic aerosol formation.

Heterogeneous Reactions of HX + HONO and I2on Ice Surfaces: Kinetics and Linear Correlations

Reaction probabilities of gaseous nitrous acid, HONO, with HCl, HBr, and HI treated ice surfaces have been investigated in a fast flow-tube reactor coupled with a differentially pumped quadrupole mass spectrometer (QMS) at 191 K. The reaction probability increases with the HX surface coverage, and the rate is the highest for the HONO reaction on the HI-treated ice surface. Relative rate constants are correlated to the nucleophilic parameter, according to the linear free-energy relationship for this series of heterogeneous reactions on ice surfaces. The correlation was also extended to HOCl + HX(ad) reactions on the ice surface, and it can be used to treat other heterogeneous atmospheric and catalytic reactions. The reaction products ClNO and BrNO were determined by the QMS. INO was found to rapidly convert to I2 on surfaces, and I2 was observed from the reaction of HONO + HI. The uptake coefficient of I2 on the HI-treated ice surface is higher than that for I2 on the water-ice surface.

Protonation study of peroxynitric acid and peroxynitrous acid

The equilibrium structures and harmonic vibrational frequencies of peroxynitric acid (HOONO(2)) and seven structures of protonated peroxynitric acid, along with peroxynitrous acid (HOONO) and its 12 protonated peroxynitrous acid structures, have been investigated using several ab initio and density functional methods. The ab initio methods include second-order Moller-Plesset perturbation theory, quadratic configuration interaction, including single and double excitations theory (QCISD), and the QCISD(T) methods, which incorporate a perturbational estimate of the effects of connected triple excitation. The Becke three-parameter hybrid functional combined with Lee, Yang, and Parr correlation function is the density functional method used. The lowest energy form of protonated peroxynitric acid is a complex between H(2)O(2) and NO(+) rather than between H(2)O and NO(2) (+). For peroxynitrous acid, a complex between H(2)O(2) and NO(2) (+) is found to be the lowest energy structure. The ab initio proton affinity (PA) of HOONO and HOONO(2) is predicted to be 182.1 and 175.1 kcal mol(-1), respectively, at the QCISD(T)/6-311++G(3df,3pd) level of theory. The results are contrasted with an earlier study on nitrous acid, and is shown that peroxynitric acid and peroxynitrous acid have a smaller PA than nitrous acid.

MICROPHYSICS AND HETEROGENEOUS CHEMISTRY OF POLAR STRATOSPHERIC CLOUDS

Liquid and solid particles in polar stratospheric clouds are of central importance for the depletion of stratospheric ozone. Surface-catalyzed reactions on these particles, and diffusion-controlled processes in the bulk of the particles, convert halogens, which derive from compounds of mainly anthropogenic origin, from relatively inert reservoir species into forms that efficiently destroy ozone. The microphysics of these particles under cold stratospheric conditions is still uncertain in many respects, in particular concerning phase transitions such as freezing nucleation and deposition nucleation. Furthermore, there are indications that the rates of key heterogeneous reactions have not yet been established with sufficient accuracy to enable a reliable diagnosis of observed ozone losses by means of global models. The present paper reviews the current (late 1996) knowledge of the physico-chemistry of polar stratospheric clouds and evaluates the remaining uncertainties with respect to their ozone depletion potential.