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FROM VOLUME 5, NUMBER 11, NOVEMBER 1992
TROPOSPHERIC OH, PHOTOCHEMISTRY AND TRACE SPECIES
"Evidence for a Missing Carbon Monoxide Sink Based on Tropospheric
Measurements of 14CO," J.E. Mak (NCAR, POB 3000, Boulder CO 80307), C.A.M.
Brenninkmeijer, M.R. Manning, Geophys. Res. Lett., 19(14),
1467-1470, July 24, 1992. The first 14CO observations in the free troposphere
during different seasons from 90° S to 34° N help constrain OH
concentration, and show that another sink besides OH may exist.
"Kinetics of the OH Reaction with Methyl Chloroform and Its
Atmospheric Implications," R.K. Talukdar, A. Mellouki et al., Science,
257(5067), 227-230, July 10, 1992.
Measurements of rate coefficients from 243 to 379 K with the
pulsed-photolysis laser-induced fluorescence method show that tropospheric OH
concentrations derived from CH3CCl3 budget analysis are increased by about 15%,
requiring corresponding changes in global warming and ozone depletion potentials
of several trace gases.
Two items from Geophys. Res. Lett., 19(13), July 6, 1992:
"Relative Rate Constants for the Reactions of OH with Methane and
Methyl Chloroform," W.B. DeMore (Jet Propulsion Lab., 4800 Oak Grove Dr.,
Pasadena CA 91109), 1367-1370. The reported rate constants, measured in a
slow-flow temperature controlled photochemical reactor, were based on the
disappearance of the parent compounds as measured by FTIR spectroscopy.
"Kinetics of the Reactions of OH with Methyl Chloroform and Methane:
Implications for Global Tropospheric OH and the Methane Budget," B.J.
Finlayson-Pitts (Dept. Chem., California State Univ., Fullerton CA 92634), M.J.
Ezell et al., 1371-1374.
Rate constants were determined with fast flow discharge over a range of
temperatures and pressures, and were found to be less than that now used to
estimate global OH concentrations. The recent suggestion that methane flux may
have been overestimated in the past is supported.
Special issue: "Mauna Loa Observatory Photochemistry
Experiment," J. Geophys. Res., 97(D10), June 30, 1992.
Contains 13 papers describing measurements of selected odd nitrogen
constituents, hydrocarbons, peroxides and other species made in May 1988.
"An Intercomparison of Tropospheric OH Measurements at Fritz Peak
Observatory, Colorado," G.H. Mount (ERL, NOAA, 325 Broadway, Boulder CO
80303), F.L. Eisele, Science, 256, 1187-1190, May 22, 1992.
Two completely different techniques, chemical analysis and spectroscopic
absorption, gave consistent results, showing that ambient OH concentrations can
now be measured with sufficient sensitivity to test photochemical models.
Concentrations on all days were significantly lower than model predictions,
perhaps indicating the presence of an unknown scavenger.
"Interhemispheric Asymmetry in OH Abundance Inferred from
Measurements of Atmospheric 14CO," C.A.M. Brenninkmeijer (Phys. Sci. Div.,
DSIR, POB 31 312, Lower Hutt, New Zealand), M.R. Manning et al., Nature,
356(6364), 50-52, Mar. 5, 1992.
Because of its atmospheric lifetime under certain conditions, 14CO is a more
sensitive indicator of OH than the longer-lived trace gases commonly used. 14CO
data obtained using accelerator mass spectrometry show that, surprisingly,
concentrations in the Southern Hemisphere are about 40% lower than at comparable
Northern Hemisphere latitudes. This result suggests that OH abundance is higher
in the Southern Hemisphere, in contrast to predictions from current
"Model Calculations of Nighttime Atmospheric OH," Y. Lu (Global
Change Res. Ctr., Oregon Graduate Inst., Beaverton OR 97006), M.A.K. Khalil,
Tellus, 44B(2), 106-113, Apr. 1992.
Nighttime OH concentrations may have decreased significantly between ice-age
and preindustrial, and between preindustrial and present times, although daytime
concentrations did not vary much during these transitions.
"The Measurement of Tropospheric OH by Long Path Absorption. 1.
Instrumentation," G.H. Mount (Aeronomy Lab., ERL, NOAA, 325 Broadway,
Boulder CO 80303), J. Geophys. Res., 97(D2), 2427-2444, Feb. 20,
The method and instrumentation described succeeded in measuring OH
concentration in a clean environment at a sensitivity limit of about 0.01 pptv
with an integration time of several hours. This limit is well below predicted
noontime OH concentrations and should be low enough to provide a rigorous test
of photochemical theories of OH formation.
"Tropospheric OH: Model Calculations of Spatial, Temporal and Secular
Variations," Y. Lu (Global Change Res. Ctr., Oregon Graduate Inst.,
Beaverton OR 97006), M.A.K. Khalil, Chemosphere, 23(3), 397-444,
Found strong diurnal variations in OH, as well as clear vertical, seasonal
and latitudinal variations. OH levels decrease only slightly during different
climatic and atmospheric conditions, from the ice ages to the present, but the
calculated nighttime OH concentration in the ice ages is three times the present
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