Abstract
Abstract. The vast majority of emissions of fluorine-containing molecules are anthropogenic in nature, e.g. chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs). These molecules slowly degrade in the atmosphere, leading to the formation of HF, COF2, and COClF, which are the main fluorine-containing species in the stratosphere. Ultimately both COF2 and COClF further degrade to form HF, an almost permanent reservoir of stratospheric fluorine due to its extreme stability. Carbonyl fluoride (COF2) is the second-most abundant stratospheric "inorganic" fluorine reservoir, with main sources being the atmospheric degradation of CFC-12 (CCl2F2), HCFC-22 (CHF2Cl), and CFC-113 (CF2ClCFCl2). This work reports the first global distributions of carbonyl fluoride in the Earth's atmosphere using infrared satellite remote-sensing measurements by the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS), which has been recording atmospheric spectra since 2004, and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument, which recorded thermal emission atmospheric spectra between 2002 and 2012. The observations reveal a high degree of seasonal and latitudinal variability over the course of a year. These have been compared with the output of SLIMCAT, a state-of-the-art three-dimensional chemical transport model. In general the observations agree well with each other, although MIPAS is biased high by as much as ~30%, and compare well with SLIMCAT. Between January 2004 and September 2010 COF2 grew most rapidly at altitudes above ~25 km in the southern latitudes and at altitudes below ~25 km in the northern latitudes, whereas it declined most rapidly in the tropics. These variations are attributed to changes in stratospheric dynamics over the observation period. The overall COF2 global trend over this period is calculated as 0.85 ± 0.34 (MIPAS), 0.30 ± 0.44 (ACE), and 0.88% year−1 (SLIMCAT).
Highlights
Small quantities of fluorine-containing molecules are emitted into the atmosphere from natural sources, e.g. volcanic and hydrothermal emissions (Gribble, 2002), the vast majority of emissions are anthropogenic in nature, e.g. chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs)
For ACE spectra recorded at tangent heights that fall within the selected retrieval altitude range, the initial volume mixing ratios (VMRs) for the least-squares fit are taken from the set of VMR profiles established by the Atmospheric Trace MOlecule Spectroscopy (ATMOS) mission (Irion et al, 2002)
3.1 Infrared spectroscopy of carbonyl fluoride. Both Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS) and Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) retrievals make use of the COF2 linelist first released as part of the HIgh-resolution TRANsmission (HITRAN) 2004 database, with partition data taken from the Total Internal Partition Sums (TIPS) subroutine included in the HITRAN compilation
Summary
Small quantities of fluorine-containing molecules are emitted into the atmosphere from natural sources, e.g. volcanic and hydrothermal emissions (Gribble, 2002), the vast majority of emissions are anthropogenic in nature, e.g. chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs). The second-most abundant stratospheric inorganic fluorine reservoir is carbonyl fluoride (COF2), largely due to its slow photolysis. Recent studies indicate that its atmospheric abundance is increasing (Duchatelet et al, 2009; Brown et al, 2011). For the two most abundant source molecules, CFC-12 and HCFC-22, the atmospheric degradation proceeds by their initial breakdown into CF2Cl (Ricaud and Lefevre, 2006): CF2Cl2 + hv → CF2Cl + Cl CHF2Cl + OH → CF2Cl + H2O (R1). COF2 volume mixing ratios (VMRs) slowly increase with altitude up to the middle of the stratosphere, above which they decrease as photolysis of COF2 becomes more efficient, leading to the formation of fluorine atoms: COF2 + hv → FCO + F.
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