Search

A laboratory chilled mirror hygrometer (CMH), exposed to an airstream containing water vapor (H2O) and nitric acid (HNO3), has been used to demonstrate the existence of a persistent water-nitric acid condensate that has a saturation H2O vapor pressure greater than that of hexagonal ice (Ih). The condensate was routinely formed on the mirror by removing HNO3 from the airstream following the formation of an initial condensate on the mirror that resembled nitric acid trihydrate (NAT). Typical conditions for the formation of the persistent condensate were a H2O mixing ratio greater than 18 ppm, pressure of 128 hPa, and mirror temperature between 202 and 216 K. In steady-state operation, a CMH maintains a condensate of constant optical diffusivity on a mirror through control of only the mirror temperature. Maintaining the persistent condensate on the mirror required that the mirror temperature be below the H2O saturation temperature with respect to Ih by as much as 3 K, corresponding to up to 63% H2O supersaturation with respect to Ih. The condensate was observed to persist in steady state for up to 16 h. Compositional analysis of the condensate confirmed the co-condensation of H2O and HNO3 and thereby strongly supports the conclusion that the Ih supersaturation is due to residual HNO3 in the condensate. Although the exact structure or stoichiometry of the condensate could not be determined, other known stable phases of HNO3 and H2O are excluded as possible condensates. This persistent condensate, if it also forms in the upper tropical troposphere, might explain some of the high Ih supersaturations in cirrus and contrails that have been reported in the tropical tropopause region.

Global-scale atmospheric measurements are used to investigate the effectiveness of recent adjustments to production and consumption controls on hydrochlorofluorocarbons (HCFCs) under the Montreal Protocol on Substances that Deplete the Ozone Layer (Montreal Protocol) and to assess recent projections of large increases in hydrofluorocarbon (HFC) production and emission. The results show that aggregate global HCFC emissions did not increase appreciably during 2007-2012 and suggest that the 2007 Adjustments to the Montreal Protocol played a role in limiting HCFC emissions well in advance of the 2013 cap on global production. HCFC emissions varied between 27 and 29 kt CFC-11-equivalent (eq)/y or 0.76 and 0.79 GtCO2-eq/y during this period. Despite slower than projected increases in aggregate HCFC emissions since 2007, total emissions of HFCs used as substitutes for HCFCs and chlorofluorocarbons (CFCs) have not increased more rapidly than rates projected [Velders, G. J. M.; Fahey, D. W.; Daniel, J. S.; McFarland, M.; Andersen, S. O. The Large Contribution of Projected HFC Emissions to Future Climate Forcing. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 10949-10954] for 2007-2012. HFC global emission magnitudes related to this substitution totaled 0.51 (-0.03, +0.04) GtCO2-eq/y in 2012, a magnitude about two times larger than emissions reported to the United Nations Framework Convention on Climate Change (UNFCCC) for these HFCs. Assuming accurate reporting to the UNFCCC, the results imply that developing countries (non-Annex I Parties) not reporting to the UNFCCC now account for nearly 50% of global HFC emissions used as substitutes for ozone-depleting substances (ODSs). Global HFC emissions (as CO2-eq) from ODS substitution can be attributed approximately equally to mobile air conditioning, commercial refrigeration, and the sum of all other applications.

The UV photolysis of Cl(2)O(2) (dichlorine peroxide) is a key step in the catalytic destruction of polar stratospheric ozone. In this study, the gas-phase UV absorption spectrum of Cl(2)O(2) was measured using diode array spectroscopy and absolute cross sections, sigma, are reported for the wavelength range 200-420 nm. Pulsed laser photolysis of Cl(2)O at 248 nm or Cl(2)/Cl(2)O mixtures at 351 nm at low temperature (200-228 K) and high pressure (approximately 700 Torr, He) was used to produce ClO radicals and subsequently Cl(2)O(2) via the termolecular ClO self-reaction. The Cl(2)O(2) spectrum was obtained from spectra recorded following the completion of the gas-phase ClO radical chemistry. The spectral analysis used observed isosbestic points at 271, 312.9, and 408.5 nm combined with reaction stoichiometry and chlorine mass balance to determine the Cl(2)O(2) spectrum. The Cl(2)O(2) UV absorption spectrum peaks at 244.5 nm with a cross section of 7.6(-0.5)(+0.8) x 10(-18) cm(2) molecule(-1) where the quoted error limits are 2sigma and include estimated systematic errors. The Cl(2)O(2) absorption cross sections obtained for wavelengths in the range 300-420 nm are in good agreement with the Cl(2)O(2) spectrum reported previously by Burkholder et al. (J. Phys. Chem. A 1990, 94, 687) and significantly higher than the values reported by Pope et al. (J. Phys. Chem. A 2007, 111, 4322). A possible explanation for the discrepancy in the Cl(2)O(2) cross section values with the Pope et al. study is discussed. Representative, atmospheric photolysis rate coefficients are calculated and a range of uncertainty estimated based on the determination of sigma(Cl(2)O(2))(lambda) in this work. Although improvements in our fundamental understanding of the photochemistry of Cl(2)O(2) are still desired, this work indicates that major revisions in current atmospheric chemical mechanisms are not required to simulate observed polar ozone depletion.

In this work, potential atmospheric loss processes for SO2F2, a commercially used biocide (fumigant), have been studied and its global warming potential calculated. Rate coefficients for the gas-phase reactions OH + SO2F2 --> products, k1, and Cl + SO2F2 --> products, k4, were determined using a relative rate technique to be k1 < 1 x 10(-16) cm3 molecule-1 s-1 at 296 and 333 K and k4(296 K) < 5 x 10(-17) cm3 molecule(-1) s(-1). UV absorption cross sections of SO2F2 were measured at 184.9, 193, and 213.9 nm, and its photolysis quantum yield at 193 nm was determined to be <0.02. The atmospheric lifetime of SO2F2 with respect to loss by OH, Cl, and O(1D) reaction and UV photodissociation is estimated to be >300, >10000, 700, and >4700 years, respectively. The stratospheric lifetime of SO2F2 is calculated using a two-dimensional model to be 630 years. The global warming potential (GWP) for SO2F2 was calculated to be 4780 for the 100 year time horizon using infrared absorption cross sections measured in this work and a SO2F2 globally averaged atmospheric lifetime of 36 years, which is determined primarily by ocean uptake, reported by Mühle et al. (Mühle, J.; Huang, J.; Weiss, R. F.; Prinn, R. G.; Miller, B. R.; Salameh, P. K.; Harth, C. M.; Fraser, P. J.; Porter, L. W.; Greally, B. R.; O'Doherty, S.; Simonds, P. G. J. Geophys. Res., submitted for publication, 2008). Reaction channels and the possible formation of stable adducts in reactions 1 and 4 were evaluated using ab initio, CCSD(T), and density functional theory, B3P86, quantum mechanical electronic structure calculations. The most likely reaction product channels were found to be highly endothermic, consistent with the upper limits of the rate coefficients measured in this work.