Showing papers by "A. R. Ravishankara published in 2008"

N2O5 oxidizes chloride to Cl2 in acidic atmospheric aerosol.

Abstract: Molecular chlorine (Cl2) is an important yet poorly understood trace constituent of the lower atmosphere. Although a number of mechanisms have been proposed for the conversion of particle-bound chloride (Cl‐) to gas-phase Cl2, the detailed processes involved remain uncertain. Here, we show that reaction of dinitrogen pentoxide (N2O5) with aerosol-phase chloride yields Cl2 at low pH (

CF3CFCH2 and (Z)-CF3CFCHF: temperature dependent OH rate coefficients and global warming potentials

Abstract: Rate coefficients over the temperature range 206–380 K are reported for the gas-phase reaction of OH radicals with 2,3,3,3-tetrafluoropropene (CF3CFCH2), k1(T), and 1,2,3,3,3-pentafluoropropene ((Z)-CF3CFCHF), k2(T), which are major components in proposed substitutes for HFC-134a (CF3CFH2) in mobile air-conditioning units. Rate coefficients were measured under pseudo-first-order conditions in OH using pulsed-laser photolysis to produce OH and laser-induced fluorescence to detect it. Rate coefficients were found to be independent of pressure between 25 and 600 Torr (He, N2). For CF3CFCH2, the rate coefficients, within the measurement uncertainty, are given by the Arrhenius expression k1(T) = (1.26 ± 0.11) × 10−12 exp[(−35 ± 10)/T] cm3 molecule−1 s−1 where k1(296 K) = (1.12 ± 0.09) × 10−12 cm3 molecule−1 s−1. For (Z)-CF3CFCHF, the rate coefficients are given by the non-Arrhenius expression k2(T) = (1.6 ± 0.2) × 10−18T2 exp[(655 ± 50)/T] cm3 molecule−1 s−1 where k2(296 K) = (1.29 ± 0.06) × 10−12 cm3 molecule−1 s−1. Over the temperature range most relevant to the atmosphere, 200–300 K, the Arrhenius expression k2(T) = (7.30 ± 0.7) × 10−13 exp[(165 ± 20)/T] cm3 molecule−1 s−1 reproduces the measured rate coefficients very well and can be used in atmospheric model calculations. The quoted uncertainties in the rate coefficients are 2σ (95% confidence interval) and include estimated systematic errors. The global warming potentials for CF3CFCH2 and (Z)-CF3CFCHF were calculated to be <4.4 and <3.6, respectively, for the 100 year time horizon using infrared absorption cross sections measured in this work, and atmospheric lifetimes of 12 and 10 days that are based solely on OH reactive loss.

Evidence for liquid-like and nonideal behavior of a mixture of organic aerosol components.

Abstract: The condensation, evaporation, and repartitioning of semivolatile organic compounds (SVOCs) in the atmosphere depends both on the phase of condensed material and the effective condensed phase vapor pressures of the SVOCs. Although direct measurements of vapor pressures of individual SVOCs exist, there are limited measurements of how the properties of a given compound changes in mixtures of multiple components that exist in the atmosphere. Here, the evaporation behavior of mixtures of dicarboxylic acids, which are common atmospheric aerosol constituents, is investigated. These measurements demonstrate that complex mixtures of the individually solid organic compounds take on liquid-like properties. Additionally, the vapor pressures of individual components show strong, identity-dependent deviations from ideality (i.e., Raoult's Law), with the vapor pressures of the smaller, more volatile compounds decreased significantly in the mixtures. The addition of an inorganic compound (NaNO3) further influences the nonideal behavior, again in a compound-specific manner. These results suggest that nonideal behavior of particle-phase compounds influences the abundances of organic aerosol observed in the atmosphere and in the laboratory.