Despite increased recognition of the stress experienced by hospital nursing staffs and its effects on burnout

In a companion paper (Kroll, J. H.; Clarke, J. S.; Donahue, N. M.; Anderson, J. G.; Demerjian, K. L. J. Phys. Chem. A 2001, 105, 1554) we present direct measurements of hydroxyl radical (OH) yields for the gas-phase reaction of ozone with a number of symmetric alkenes. Yields are strongly pressure-dependent, contrary to the results of prior scavenger studies. Here we present a statistical-dynamical model of OH production from the reaction, utilizing RRKM/master equation calculations to determine the fate of the carbonyl oxide intermediate. This model agrees with our experimental results, in that both theory and observations indicate strongly pressure-dependent OH yields. Our calculations also suggest that ethene ozonolysis produces OH via a different channel than the substituted alkenes, though the identity of this channel is not clear. This channel may play a role in the ozonolysis of monosubstituted alkenes as well. Our time-dependent master equation calculations show that the discrepancy between OH yields measured in our direct study and those measured in prior scavenger studies may arise from differing experimental time scales; on short time scales, OH is formed only from the vibrationally excited carbonyl oxide intermediate, whereas on longer time scales OH formation from thermal dissociation may be significant. To demonstrate this we present time-dependent measurements of OH yields at 10 Torr and 100 Torr; yields begin increasing after hundreds of milliseconds, an effect which is much more pronounced at 100 Torr. These results are entirely consistent with theoretical predictions. In the atmosphere, the thermalized carbonyl oxide may be susceptible to bimolecular reactions which, if fast enough, could prevent dissociation to OH; however there is little experimental evidence that any such reactions are important. Thus we conclude that both mechanisms of OH formation (dissociation of vibrationally excited carbonyl oxide and dissociation of thermalized carbonyl oxide) are significant in the troposphere.

How strong is the case linking global release of chlorofluorocarbons to episodic disappearance of ozone from the Antarctic stratosphere each austral spring? Three lines of evidence defining a link are (i) observed containment in the vortex of ClO concentrations two orders of magnitude greater than normal levels; (ii) in situ observations obtained during ten high-altitude aircraft flights into the vortex as the ozone hole was forming that show a decrease in ozone concentrations as ClO concentrations increased; and (iii) a comparison between observed ozone loss rates and those predicted with the use of absolute concentrations of ClO and BrO, the rate-limiting radicals in an array of proposed catalytic cycles. Recent advances in our understanding of the kinetics, photochemistry, and structural details of key intermediates in these catalytic cycles as well as an improved absolute calibration for ClO and BrO concentrations at the temperatures and pressures encountered in the lower antarctic stratosphere have been essential for defining the link.

The observed presence of water vapor convectively injected deep into the stratosphere over the United States can fundamentally change the catalytic chlorine/bromine free-radical chemistry of the lower stratosphere by shifting total available inorganic chlorine into the catalytically active free-radical form, ClO. This chemical shift markedly affects total ozone loss rates and makes the catalytic system extraordinarily sensitive to convective injection into the mid-latitude lower stratosphere in summer. Were the intensity and frequency of convective injection to increase as a result of climate forcing by the continued addition of CO(2) and CH(4) to the atmosphere, increased risk of ozone loss and associated increases in ultraviolet dosage would follow.