26 research outputs found
CFCI3 (CFC-11): UV Absorption Spectrum Temperature Dependence Measurements and the Impact on Atmospheric Lifetime and Uncertainty
CFCl3 (CFC-11) is both an atmospheric ozone-depleting and potent greenhouse gas that is removed primarily via stratospheric UV photolysis. Uncertainty in the temperature dependence of its UV absorption spectrum is a significant contributing factor to the overall uncertainty in its global lifetime and, thus, model calculations of stratospheric ozone recovery and climate change. In this work, the CFC-11 UV absorption spectrum was measured over a range of wavelength (184.95 - 230 nm) and temperature (216 - 296 K). We report a spectrum temperature dependence that is less than currently recommended for use in atmospheric models. The impact on its atmospheric lifetime was quantified using a 2-D model and the spectrum parameterization developed in this work. The obtained global annually averaged lifetime was 58.1 +- 0.7 years (2 sigma uncertainty due solely to the spectrum uncertainty). The lifetime is slightly reduced and the uncertainty significantly reduced from that obtained using current spectrum recommendation
NF3: UV Absorption Spectrum Temperature Dependence and the Atmospheric and Climate Forcing Implications
Nitrogen trifluoride (NF3) is an atmospherically persistent greenhouse gas that is primarily removed by UV photolysis and reaction with O((sup 1)D) atoms. In this work, the NF3 gas-phase UV absorption spectrum, sigma(delta,T), was measured at 16 wavelengths between 184.95 and 250 nm at temperatures between 212 and 296 K. A significant spectrum temperature dependence was observed in the wavelength region most relevant to atmospheric photolysis (200-220 nm) with a decrease in sigma(210 nm,T) of approximately 45 percent between 296 and 212 K. Atmospheric photolysis rates and global annually averaged lifetimes of NF3 were calculated using the Goddard Space Flight Center 2-D model and the sigma(delta,T) parameterization developed in this work. Including the UV absorption spectrum temperature dependence increased the stratospheric photolysis lifetime from 610 to 762 years and the total global lifetime from 484 to 585 years; the NF3 global warming potentials on the 20-, 100-, and 500-year time horizons increased less than 0.3, 1.1, and 6.5 percent to 13,300, 17,700, and 19,700, respectively
1,2-Dichlorohexafluoro-Cyclobutane (1,2-c-C4F6Cl2, R-316c) a Potent Ozone Depleting Substance and Greenhouse Gas: Atmospheric Loss Processes, Lifetimes, and Ozone Depletion and Global Warming Potentials for the (E) and (Z) stereoisomers
The atmospheric processing of (E)- and (Z)-1,2-dichlorohexafluorocyclobutane (1,2-c-C4F6Cl2, R-316c) was examined in this work as the ozone depleting (ODP) and global warming (GWP) potentials of this proposed replacement compound are presently unknown. The predominant atmospheric loss processes and infrared absorption spectra of the R-316c isomers were measured to provide a basis to evaluate their atmospheric lifetimes and, thus, ODPs and GWPs. UV absorption spectra were measured between 184.95 to 230 nm at temperatures between 214 and 296 K and a parametrization for use in atmospheric modeling is presented. The Cl atom quantum yield in the 193 nm photolysis of R- 316c was measured to be 1.90 +/- 0.27. Hexafluorocyclobutene (c-C4F6) was determined to be a photolysis co-product with molar yields of 0.7 and 1.0 (+/-10%) for (E)- and (Z)-R-316c, respectively. The 296 K total rate coefficient for the O(1D) + R-316c reaction, i.e., O(1D) loss, was measured to be (1.56 +/- 0.11) 10(exp 10)cu cm/ molecule/s and the reactive rate coefficient, i.e., R-316c loss, was measured to be (1.36 +/- 0.20) 10(exp 10)cu cm/molecule/s corresponding to a approx. 88% reactive yield. Rate coefficient upper-limits for the OH and O3 reaction with R-316c were determined to be <2.3 10(exp 17) and <2.0 10(exp 22)cu cm/molecule/s, respectively, at 296 K. The quoted uncertainty limits are 2(sigma) and include estimated systematic errors. Local and global annually averaged lifetimes for the (E)- and (Z)-R-316c isomers were calculated using a 2-D atmospheric model to be 74.6 +/- 3 and 114.1 +/-10 years, respectively, where the estimated uncertainties are due solely to the uncertainty in the UV absorption spectra. Stratospheric photolysis is the predominant atmospheric loss process for both isomers with the O(1D) reaction making a minor, approx. 2% for the (E) isomer and 7% for the (Z) isomer, contribution to the total atmospheric loss. Ozone depletion potentials for (E)- and (Z)-R-316c were calculated using the 2-D model to be 0.46 and 0.54, respectively. Infrared absorption spectra for (E)- and (Z)-R-316c were measured at 296 K and used to estimate their radiative efficiencies (REs) and GWPs; 100-year time-horizon GWPs of 4160 and 5400 were obtained for (E)- and (Z)-R-316c, respectively. Both isomers of R-316c are shown in this work to be long-lived ozone depleting substances and potent greenhouse gases
Ozonolysis can produce long-lived greenhouse gases from commercial refrigerants
Hydrofluoroolefins are being adopted as sustainable alternatives to long-lived fluorine- and chlorine-containing gases and are finding current or potential mass-market applications as refrigerants, among a myriad of other uses. Their olefinic bond affords relatively rapid reaction with hydroxyl radicals present in the atmosphere, leading to short lifetimes and proportionally small global warming potentials. However, this type of functionality also allows reaction with ozone, and whilst these reactions are slow, we show that the products of these reactions can be extremely long-lived. Our chamber measurements show that several industrially important hydrofluoroolefins produce CHF3 (fluoroform, HFC-23), a potent, long-lived greenhouse gas. When this process is accounted for in atmospheric chemical and transport modeling simulations, we find that the total radiative effect of certain compounds can be several times that of the direct radiative effect currently recommended by the World Meteorological Organization. Our supporting quantum chemical calculations indicate that a large range of exothermicity is exhibited in the initial stages of ozonolysis, which has a powerful influence on the CHF3 yield. Furthermore, we identify certain molecular configurations that preclude the formation of long-lived greenhouse gases. This demonstrates the importance of product quantification and ozonolysis kinetics in determining the overall environmental impact of hydrofluoroolefin emissions.Get fu
Release of Spring 2013 Spanish-Language MCAS Test Items
Trace atmospheric concentrations
of carboxylic acids have a potent
effect upon the environment, where they modulate aqueous chemistry
and perturb Earth’s radiative balance. Halogenated carboxylic
acids are produced by the tropospheric oxidation of halocarbons and
are considered persistent pollutants because of their weak tropospheric
and aqueous sinks. However, recent studies reported rapid reactions
between selected carboxylic acids and Criegee intermediates, which
may provide an efficient gas-phase removal process. Accordingly, absolute
rate coefficients of two Criegee intermediates, CH<sub>2</sub>OO and
(CH<sub>3</sub>)<sub>2</sub>COO, with a suite of carboxylic acids
(HCOOH, CH<sub>3</sub>COOH, CClF<sub>2</sub>COOH, CF<sub>3</sub>CF<sub>2</sub>COOH, and pyruvic acid) were measured with a view to develop
a structure–activity relationship (SAR). This SAR is based
upon the dipole-capture model and predicts the reactivity of many
further combinations of Criegee intermediates and carboxylic acids.
Complementary synchrotron-based photoionization mass spectrometry
measurements demonstrate that these reactions produce stable ester
adducts, with a reaction coordinate involving transfer of the acidic
hydrogen from the carboxylic acid to the terminal oxygen of the Criegee
intermediate. The adduct products are predicted to have low vapor
pressures, and coupling of this chemistry with a global atmospheric
chemistry and transport model shows significant production of secondary
organic aerosol at locations rich in biogenic alkene emissions
Gas-Phase Rate Coefficients for the OH + <i>n</i>‑, <i>i</i>‑, <i>s</i>‑, and <i>t</i>‑Butanol Reactions Measured Between 220 and 380 K: Non-Arrhenius Behavior and Site-Specific Reactivity
Butanol (C<sub>4</sub>H<sub>9</sub>OH) is a potential biofuel alternative
in fossil fuel gasoline and diesel formulations. The usage of butanol
would necessarily lead to direct emissions into the atmosphere; thus,
an understanding of its atmospheric processing and environmental impact
is desired. Reaction with the OH radical is expected to be the predominant
atmospheric removal process for the four aliphatic isomers of butanol.
In this work, rate coefficients, <i>k</i>, for the gas-phase
reaction of the <i>n-</i>, <i>i</i>-, <i>s</i>-, and <i>t</i>-butanol isomers with the OH radical
were measured under pseudo-first-order conditions in OH using pulsed
laser photolysis to produce OH radicals and laser induced fluorescence
to monitor its temporal profile. Rate coefficients were measured over
the temperature range 221–381 K at total pressures between
50 and 200 Torr (He). The reactions exhibited non-Arrhenius behavior
over this temperature range and no dependence on total pressure with <i>k</i>(296 K) values of (9.68 ± 0.75), (9.72 ± 0.72),
(8.88 ± 0.69), and (1.04 ± 0.08) (in units of 10<sup>–12</sup> cm<sup>3</sup> molecule<sup>–1</sup> s<sup>–1</sup>) for <i>n-</i>, <i>i</i>-, <i>s</i>-, and <i>t</i>-butanol, respectively. The quoted uncertainties
are at the 2σ level and include estimated systematic errors.
The observed non-Arrhenius behavior is interpreted here to result
from a competition between the available H-atom abstraction reactive
sites, which have different activation energies and pre-exponential
factors. The present results are compared with results from previous
kinetic studies, structure–activity relationships (SARs), and
theoretical calculations and the discrepancies are discussed. Results
from this work were combined with available high temperature (1200–1800
K) rate coefficient data and room temperature reaction end-product
yields, where available, to derive a self-consistent site-specific
set of reaction rate coefficients of the form <i>AT</i><sup><i>n</i></sup> expÂ(−<i>E</i>/<i>RT</i>) for use in atmospheric and combustion chemistry modeling