Adsorption and hydrolysis of alcohols and carbonyls on ice at temperatures of the upper troposphere.

The uptake of gaseous ethanol, 1,1,1-trifluoroethanol, acetone, chloral (CCl(3)CHO), and fluoral (CF(3)CHO) on ice films has been investigated using a coated-wall flow tube at temperatures 208-228 K corresponding to the upper troposphere (UT), with a mass spectrometric measurement of gas concentration. The uptake was largely reversible and followed Langmuir-type kinetic behavior, i.e., surface coverage increased with the trace gas concentration approaching a maximum surface coverage at a gas phase concentration of N(max) ∼ (2-4) × 10(14) molecules cm(-3), corresponding to a surface coverage of ∼30% of a monolayer (ML). The equilibrium partition coefficients, K(LinC), were obtained from the experimental data by analysis using the simple Langmuir model for specific conditions of temperature and concentration. The analysis showed that the K(LinC) depend only weakly on surface coverages. The following expressions described the temperature dependence of the partition coefficients (K(LinC)) in centimeters, at low coverage for ethanol, trifluoroethanol, acetone, chloral, and fluoral: K(LinC) = 1.36 × 10(-11) exp(5573.5/T), K(LinC) = 3.74 × 10(-12) exp(6427/T), K(LinC) = 3.04 × 10(-9) exp(4625/T), K(LinC) = 7.52 × 10(-4) exp(2069/T), and K(LinC) = 1.06 × 10(-2) exp(904/T). For acetone and ethanol the enthalpies and entropies of adsorption derived from all available data showed systematic temperature dependence, which is attributed to temperature dependent surface modifications, e.g., QLL formation. For chloral and fluoral, there was an irreversible component of uptake, which was attributed to hydrate formation on the surface. Rate constants for these surface reactions derived using a Langmuir-Hinshelwood mechanism are reported.

Studies of single aerosol particles containing malonic acid, glutaric acid, and their mixtures with sodium chloride. II. Liquid-state vapor pressures of the acids.

The vapor pressures of two dicarboxylic acids, malonic acid and glutaric acid, are determined by the measurement of the evaporation rate of the dicarboxylic acids from single levitated particles. Two laboratory methods were used to isolate single particles, an electrodynamic balance and optical tweezers (glutaric acid only). The declining sizes of individual aerosol particles over time were followed using elastic Mie scattering or cavity enhanced Raman scattering. Experiments were conducted over the temperature range of 280-304 K and a range of relative humidities. The subcooled liquid vapor pressures of malonic and glutaric acid at 298.15 K were found to be 6.7(-1.2)(+2.6) x 10(-4) and 11.2(-4.7)(+9.6) x 10(-4) Pa, respectively, and the standard enthalpies of vaporization were respectively 141.9 ± 19.9 and 100.8 ± 23.9 kJ mol(-1). The vapor pressures of both glutaric acid and malonic acid in single particles composed of mixed inorganic/organic composition were found to be independent of salt concentration within the uncertainty of the measurements. Results are compared with previous laboratory determinations and theoretical predictions.

Studies of single aerosol particles containing malonic acid, glutaric acid, and their mixtures with sodium chloride. I. Hygroscopic growth.

We describe a newly constructed electrodynamic balance with which to measure the relative mass of single aerosol particles at varying relative humidity. Measurements of changing mass with respect to the relative humidity allow mass (m) growth factors (m(aqueous)/m(dry)) and diameter (d) growth factors (d(aqueous)/d(dry)) of the aerosol to be determined. Four aerosol types were investigated: malonic acid, glutaric acid, mixtures of malonic acid and sodium chloride, and mixtures of glutaric acid and sodium chloride. The mass growth factors of the malonic acid and glutaric acid aqueous phase aerosols, at 85% relative humidity, were 2.11 +/- 0.08 and 1.73 +/- 0.19, respectively. The mass growth factors of the mixed organic/inorganic aerosols are dependent upon the molar fraction of the individual components. Results are compared with previous laboratory determinations and theoretical predictions.

Uptake of gaseous hydrogen peroxide by submicrometer titanium dioxide aerosol as a function of relative humidity.

Hydrogen peroxide (H(2)O(2)) is an important atmospheric oxidant that can serve as a sensitive indicator for HO(x) (OH + HO(2)) chemistry. We report the first direct experimental determination of the uptake coefficient for the heterogeneous reaction of gas-phase hydrogen peroxide (H(2)O(2)) with titanium dioxide (TiO(2)), an important component of atmospheric mineral dust aerosol particles. The kinetics of H(2)O(2) uptake on TiO(2) surfaces were investigated using an entrained aerosol flow tube (AFT) coupled with a chemical ionization mass spectrometer (CIMS). Uptake coefficients (gamma(H(2)O(2))) were measured as a function of relative humidity (RH) and ranged from 1.53 x 10(-3) at 15% RH to 5.04 x 10(-4) at 70% RH. The observed negative correlation of RH with gamma(H(2)O(2)) suggests that gaseous water competes with gaseous H(2)O(2) for adsorption sites on the TiO(2) surface. These results imply that water vapor plays a major role in the heterogeneous loss of H(2)O(2) to submicrometer TiO(2) aerosol. The results are compared with related experimental observations and assessed in terms of their potential impact on atmospheric modeling studies of mineral dust and its effect on the heterogeneous chemistry in the atmosphere.

Reactive uptake of N2O5 by aerosols containing dicarboxylic acids. Effect of particle phase, composition, and nitrate content.

Reactive uptake coefficients for loss of N(2)O(5) to micron-size aerosols containing oxalic malonic, succinic, and glutaric acids, and mixtures with ammonium hydrogen sulfate and ammonium sulfate, are presented. The uptake measurements were made using two different systems: atmospheric pressure laminar flow tube reactor (Cambridge) and the Large Indoor Aerosol Chamber at Forschungszentrum Juelich. Generally good agreement is observed for the data recorded using the two techniques. Measured uptake coefficients lie in the range 5 x 10(-4)-3 x 10(-2), dependent on relative humidity, on particle phase, and on particle composition. Uptake to solid particles is generally slow, with observed uptake coefficients less than 1 x 10(-3), while uptake to liquid particles is around an order of magnitude more efficient. These results are rationalized using a numerical model employing explicit treatment of both transport and chemistry. Our results indicate a modest effect of the dicarboxylic acids on uptake and confirm the strong effect of particle phase, liquid water content, and particulate nitrate concentrations.

Temperature dependence of pentyl nitrate formation from the reaction of pentyl peroxy radicals with NO.

Alkyl nitrate yields from the reaction of 1-pentyl, 2-pentyl and 2-methyl-2-butyl peroxy radicals with NO have been determined over the temperature range (261-305 K) and at 1 bar pressure from the photo-oxidation of the iodoalkane precursors in air-NO mixtures. Yields were observed to increase with decreasing temperature and, contrary to previous observations, along the series primary < secondary congruent with tertiary. Our results suggests a significant temperature dependence for the formation of nitrates from the reaction of pentyl peroxy radicals with NO and represent an extension in the temperature range over which this reaction has been studied experimentally in the past.

Reactive uptake of N2O5 by aerosol particles containing mixtures of humic acid and ammonium sulfate.

The kinetics of reactive uptake of N2O5 on submicron aerosol particles containing humic acid and ammonium sulfate has been investigated as a function of relative humidity (RH) and aerosol composition using a laminar flow reactor coupled with a differential mobility analyzer (DMA) to characterize the aerosol. For single-component humic acid aerosol the uptake coefficient, gamma, was found to increase from 2 to 9 x 10(-4) over the range 25-75% RH. These values are 1-2 orders of magnitude below those typically observed for single-component sulfate aerosols (Phys. Chem. Chem. Phys. 2003, 5, 3453-3463;(1) Atmos. Environ. 2000, 34, 2131-2159(2)). For the mixed aerosols, gamma was found to decrease with increasing humic acid mass fraction and increase with increasing RH. For aerosols containing only 6% humic acid by dry mass, a decrease in reactivity of more than a factor of 2 was observed compared with the case for single-component ammonium sulfate. The concentration of liquid water in the aerosol droplets was calculated using the aerosol inorganic model (for the ammonium sulfate component) and a new combined FTIR-DMA system (for the humic acid component). Analysis of the uptake coefficients using the water concentration data shows that the change in reactivity cannot be explained by the change in water content alone. We suggest that, due to its surfactant properties, the main effect of the humic acid is to reduce the mass accommodation coefficient for N2O5 at the aerosol particle surface. This has implications for the use of particle hygroscopicity data for predictions of the rate of N2O5 hydrolysis.

Reduction of NO2 to nitrous acid on illuminated titanium dioxide aerosol surfaces: implications for photocatalysis and atmospheric chemistry.

TiO2, a component of atmospheric mineral aerosol, catalyses the reduction of NO2 to nitrous acid (HONO) when present as an aerosol and illuminated with near UV light under conditions pertinent to the troposphere.

A low cost instrument based on a solid state sensor for balloon-borne atmospheric O(3) profile sounding.

The design of an instrument based on a solid state tungsten oxide sensor for making profile measurements of atmospheric ozone from balloon platforms is described. The sensor is operated at a constant temperature, typically 530 [degree]C. The importance of a detailed consideration of the electronic design is demonstrated, with particular reference to the circuit to control the sensor heater. Calibration methods which are straightforward to implement are illustrated, and the results of a test flight alongside an electrochemical ozonesonde are shown. Quantitative agreement within 25% for most of the profile demonstrates the potential of this type of sensor for ozone sounding. Future improvements from manufacture to analysis are expected to improve on this.

The kinetics of the gas-phase decomposition of the 2-methyl-2-butoxyl and 2-methyl-2-pentoxyl radicals.

The kinetics of the title reactions have been studied by relative-rate methods as a function of temperature. Relative-rate coefficients for the two decomposition channels of 2-methyl-2-butoxyl have been measured at five different temperatures between 283 and 345 K and the observed temperature dependence is consistent with the results of some previous experimental studies. The kinetics of the two decomposition channels of 2-methyl-2-pentoxyl have also been investigated, as a function of temperature, relative to the estimated rate of isomerisation of this radical. Room-temperature rate coefficient data for the two decomposition channels of both 2-methyl-2-pentoxyl and 2-methyl-2-butxoyl (after combining the relative rate coefficient for this latter with a value for the rate coefficient of the major channel, extrapolated from the data presented by Batt et al., Int. J. Chem. Kinet., 1978, 10, 931) are shown to be consistent with a non-linear kinetic correlation, for alkoxyl radical decomposition rate data, previously presented by this laboratory (Johnson et al., Atmos. Environ., 2004, 38, 1755-1765).