| Yienger, J. J., A. A. Klonecki, H. Levy II, W. J. Moxim, and G. R. Carmichael, 1999: An evaluation of chemistry's role in the winter-spring ozone maximum found in the northern midlatitude free troposphere. Journal of Geophysical Research, 104(D3), 3655-3667. |
| Abstract: We employ the Geophysical Fluid Dynamics Laboratory
global chemical transport model to investigate the contribution of photochemistry
to the winter-spring ozone maximum in the northern hemisphere (NH) midlatitude
free troposphere (770-240 mbar; 30°N-60°N). Free tropospheric ozone
mass slowly builds up in the winter and early spring, with net chemistry
and transport playing comparable roles. Winter and early spring conditions
are favorable to net ozone production for two reasons: (1) Winter conditions
(cold, low Sun angle, and dry) reduce HOx
and lower the level of NOx needed
for chemical production to exceed destruction (balance point); and (2)
throughout the winter and early spring, NOx,
because of its longer chemical lifetime, increases above normally net-destructive
levels in the remote atmosphere. Interestingly, net production in the midlatitude
NH free troposphere maximizes in early spring because relatively high NOx
and low balance point conditions are present at a time when increasing
insolation is speeding up photochemistry. Conceptually, the net ozone production
is associated with an annual atmospheric "spring cleaning" in
which high levels of NOx are removed
via OH oxidation. Further, we find that human activity has a major impact
on both the levels of tropospheric ozone and the role of chemistry in the
NH midlatitude, where anthropogenic NOx
emissions dominate. In that region, modern ozone levels have increased
by ~20% in the winter and ~45% in the spring, winter-spring chemistry has
switched from net destructive to net productive, the winter-spring balance
between transport and chemistry has switched from transport dominance in
preindustrial times to the present parity, and the preindustrial February
maximum has progressed to March-April. Estimated 2020 levels of NOx
emissions were found to lead to even greater net production and to push
the O3 spring maximum later into April-May. Figure 5 was reproduced clearly in Journal of Geophysical Research, 104(D7), 8329.
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