Evolution and stoichiometry
of heterogeneous processing in the Antarctic stratosphere
L. Jaeglé,
C. R. Webster, R. D. May, D. C. Scott, R. M. Stimpfle, D.W. Kohn, P. O.
Wennberg, T. F. Hanisco, R. C. Cohen, M. H. Proffitt, K. K. Kelly, J. Elkins,
D. Baumgardner, J. E. Dye, J. C. Wilson, R. F. Pueschel, K. R. Chan, R.
J. Salawitch, A. F. Tuck, S. J. Hovde, Y. L. Yung
J. Geophys. Res.,
102, 13,235-13253, 1997.
ABSTRACT. Simultaneous
in situ measurements of HCl and ClO have been made for the first time in
the southern hemisphere, allowing a systematic study of the processes governing
chlorine activation between 15 and 20 km in the 1994 Antarctic winter.
Data for several other gases (O3,
NO, NOy, OH, HO2,
N2O, CH4,
CO, H2O, CFCs), particulates,
and meteorological parameters were collected from the ER-2 aircraft out
of New Zealand as part of the 1994 Airborne Southern Hemisphere Ozone Experiment/Measurements
of Atmospheric Effects of Stratospheric Aircraft (ASHOE/MAESA) campaign.
Observations from the ER-2 in the fall (April-May), prior to polar night,
show that chlorine activation begins with 60-75% of inorganic chlorine
as HCl. By midwinter (July-August), near-total removal of HCl is observed.
The wintertime loss of HCl in air recently exposed to extreme temperatures
is found to be correlated with high levels of reactive chlorine (ClO and
its dimer, Cl2O2)
in the linear fashion expected from the stoichiometry of the heterogeneous
reaction of hydrochloric acid with chlorine nitrate on polar stratospheric
clouds (PSCs): HCl + ClONO2 ->
Cl2 + HNO3.
To constrain the role of different heterogeneous reactions and PSC types,
we have used a photochemical trajectory model which includes heterogeneous
sulfate and PSC chemistry. Model calculations of the evolution of reactive
gases are compared with the in situ observations. In addition, simultaneous
measurements of OH and HO2
are used as a diagnostic for the occurrence of the heterogeneous reaction
HOCl + HCl -> Cl2
+ H2O, which contributes
to suppressed levels of HOx
inside the vortex. It is shown that the amount of chlorine activation is
not strongly dependent on the composition of PSCs. However, HOx
levels exhibit different signatures depending on the type of heterogeneous
surfaces that affected chlorine activation. Furthermore, this analysis
implies that in the edge region of the Antarctic vortex, the observed near-total
removal of HCl can result from latitudinal excursions of air parcels in
and out of sunlight during the winter, which photochemically resupply HOCl
and ClONO2 as oxidation
partners for HCl.