Chlorofluoromethanes in the environment

Abstract

The molecules CF2Cl2 and CFCl3 are released into the atmosphere following their extensive use as propellants for aerosol spray cans and in refrigeration. Since they are chemically inert and have low solubility in water, these chlorofluoromethanes have very long atmospheric residence times and can be detected throughout the troposphere in amounts roughly corresponding to the integrated world industrial production to date. The most important sink for atmospheric CFCl3 and CF2Cl2 appears to be photolytic dissociation in the stratosphere by ultraviolet radiation around 2000 Å. Upon photolysis the chlorofluoromethanes release chlorine atoms, which initiate an extensive catalytic chain reaction Cl + O3 → ClO + O2 ClO + O → Cl + O2 leading to the net destruction of O3 in the stratosphere. This chain reaction can be diverted through reaction of ClO with NO, which interconnects the NOx and ClOx catalytic cycles. The Cl‐ClO chain is interrupted by the reaction of Cl with methane or other hydrogenous species to form HCl, and it is renewed by reaction of OH with HCl. One‐dimensional diffusion calculations show that present O3 depletion levels resulting from the presence of the chlorofluoromethanes are of the order of 1%. This depletion would increase up to 15 or 20% if the chlorofluoromethane injection were to continue indefinitely at present rates. Furthermore, the calculations show that the full stratospheric effect of the photodissociation of CF2Cl2 and CFCl3 is not immediately felt after their introduction at ground level because of the delay required for upward diffusion to the 25‐ to 30‐km level. If the atmospheric injection of these compounds were to terminate only after causing an observable depletion of stratospheric ozone, the depletion would intensify for sometime thereafter and would remain significant for a period of a century or more.

The Molecular Structures and Energetics of Cl2CO, ClCO, Br2CO, and BrCO

“…The carbonyl halides, CX 2 O (where X ) Cl or Br) have been found to be major products of the photolysis of chlorinated and brominated methane in the presence of oxygen. [51][52][53][54][55] They result from the reaction of trihalomethyl radicals with molecular oxygen and from the dissociation of CX 3 57 It is assumed that the C-Cl bond cleavage is the primary photolytic route. 57 The energy of the 184.9 nm photon is about 154 kcal/mol, and the present calculations suggest that the dissociation limit for breaking the C-Cl bond is 75.7 ( 1 kcal/mol; together this leads to 78.3 kcal/mol excess energy remaining in the ClCO fragment.…”

Dynamics Study of the O2(v) + HO2 Atmospheric Reaction

“…6,18,19,21,27,28,[30][31][32][33][34][35][36] However, remarkably absent from this progress list is HO 2 , which is an important natural species of the atmosphere. In fact, the HO 2 radical is a key intermediate for many chemical reactions such as those involved in atmospheric chemistry, [37][38][39][40][41][42][43][44] stratospheric formation and destruction of ozone, [45][46][47] and photochemical air pollution. [48][49][50][51][52] An investigation of the collisional properties of highly vibrationally excited O 2 with HO 2 radicals is therefore of great interest.…”

Absolute rate of the reaction of hydrogen atoms with ozone from 219–360 K

“…was first proposed as the source of the presumed oil overtone hands (I'ieinel bands) 4 in order to explain some fe atures in the ni,`,ht airglow spectrum s . Subsequent laboratory studies have tihtnvii that vil'rat itntal lt' and rotationally excited 011( 2 ") up to v' _ 9 is produced in reaction (1). 'There have h been man\ spect t'ost'op i c tit i,d i is of tilt , reFU l t t lli`, overt one lands Reaction (1) as the source of y il'laltonally ilot Oil has also been used for ki tie t is studies of t e ll (v' V) reac t i Nits Subsequent airglow studies contirm diet reaction (1) is occurring in the mesospliere where 'T -200 K. From model calculations which include the rate constant for reaction (1), the mesospheric 'h y drogen atom density can be estinuitedgl).q, Reaction (1) and the resu 1 t i ng hot hvd roxy 1 radicals have been additionally studied by the "arrested relaxat..on" technique 10 in order to NSL'ertaIII the energy part it which occurs III react ion.…”

“…', l l, ,111d molocular spec it's With ozolle. 1 1 11t, Sltt'h mt't:oNpllt't'lt' r('act loll i.1-Z it i-lt ,l oil + t '1) (1) Reaction (1), which is 77 I.calttiole exothermic. was first proposed as the source of the presumed oil overtone hands (I'ieinel bands) 4 in order to explain some fe atures in the ni,`,ht airglow spectrum s .…”