Reductions of Antarctic ozone due to synergistic interactions of chlorine and bromine

In Situ Northern Mid-Latitude Observations of ClO, O 3 , and BrO in the Wintertime Lower Stratosphere

In order to test photochemical theories linking chlorofluorocarbon derivatives to ozone(O(3)) depletion at high latitudes in the springtime, several related atmospheric species, including O(3), chlorine monoxide(ClO), and bromine monoxide (BrO) were measured in the lower stratosphere with instruments mounted on the NASA ER-2 aircraft on 13 February 1988. The flight path from Moffett Field, California (37 degrees N, 121 degrees W), to Great Slave Lake, Canada (61 degrees N, 115 degrees W), extended to the center of the polar jet associated with but outside of the Arctic vortex, in which the abundance of O(3) was twice its mid-latitude value, whereas BrO levels were 5 parts per trillion by volume (pptv) between 18 and 21 kilometers, and 2.4 pptv below that altitude. The ClO mixing ratio was as much as 65 pptv at 60 degrees N latitude at an altitude of 20 kilometers, and was enhanced over mid-latitude values by a factor of 3 to 5 at altitudes above 18 kilometers and by as much as a factor of 40 at altitudes below 17 kilometers. Levels of ClO and O(3) were highly correlated on all measured distance scales, and both showed an abrupt change in character at 54 degrees N latitude. The enhancement of ClO abundance north of 54 degrees N was most likely caused by low nitrogen dioxide levels in the flight path.

Observations of the Nighttime Abundance of OClO in the Winter Stratosphere Above Thule, Greenland

Observations at Thule, Greenland, that made use of direct light from the moon on 2,3, 4,5, and 7 February 1988 revealed nighttime chlorine dioxide (OClO) abundances that were less than those obtained in Antarctica by about a factor of 5, but that exceeded model predictions based on homogeneous (gas-phase) photochemistry by about a factor of 10. The observed time scale for the formation of OClO after sunset strongly supports the current understanding of the diurnal chemistry of OClO. These data suggest that heterogeneous (surface) reactions due to polar stratospheric clouds can occur in the Arctic, providing a mechanism for possible Arctic ozone depletion.

Antarctic Ozone Depletion Chemistry: Reactions of N2O5 with H2O and HCl on Ice Surfaces

The reactions of dinitrogen pentoxide (N(2)O(5)) with H(2)O and hydrochloric acid (HCl) were studied on ice surfaces in a Knudsen cell flow reactor. The N(2)O(5) reacted on ice at 185 K to form condensed-phase nitric acid (HNO(3)). This reaction may provide a sink for odd nitrogen (NO(x)) during the polar winter, a requirement in nearly all models of Antarctic ozone depletion. A lower limit to the sticking coefficient, gamma, for N(2)O(5) on ice is 1 x 10(-3). Moreover, N(2)O(5) reacted on HCl-ice surfaces at 185 K, with gamma greater than 3 x 10(-3). This reaction, which produced gaseous nitryl chloride (ClNO(2)) and condensed-phase HNO(3), proceeded until all of the HCl within the ice was depleted. The ClNO(2), which did not react or condense on ice at 185 K, can be readily photolyzed in the Antarctic spring to form atomic chlorine for catalytic ozone destruction cycles. The other photolysis product, gaseous nitrogen dioxide (NO(2)), may be important in the partitioning of NO(x) between gaseous and condensed phases in the Antarctic winter.

Infrared measurements of increased CF(2)Cl(2) (CFC- 12) absorption above the South Pole

High-resolution ground-based solar spectra recorded at the Amundsen-Scott South Pole station in Dec. 1980 and Nov. 1986 have been analyzed in the region of the CF(2)Cl(2) (chlorofluorocarbon 12) nu(8) band Q branches at 1161 cm(-1). An increase in the CF(2)Cl(2) total vertical column above the South Pole of 1.24 +/- 0.15 over the 6-yr period, corresponding to an average rate of increase of 3.6 +/- 2.1%, is derived. This rate of increase is lower than indicated by in situ measurements at the South Pole over the same time period, but there is agreement when the rather error bars of the spectral measurement results are considered. Spectroscopic parameters that can successfully model CF(2)C1(2) absorption at low temperatures are needed to improve retrieval accuracies and could be applied to a number of pre-1980 atmospheric spectral data sets in the literature to obtain an improved record of early CF(2)Cl(2) concentration trends for comparison with estimates of historical release rates.

Global Trends in Total Ozone

Satellite ozone data from the Total Ozone Mapping Spectrometer from 1979 through 1986 show that recent decreases of total ozone have not been confined to the Antarctic spring season (the Antarctic ozone hole), but are global in extent. The losses are about twice the estimated uncertainty in the satellite data. The decreases are largest in middle and high latitudes and occur in all seasons of the year. The decreases for this 8-year period are comparable in magnitude to the increases observed during the 1960s. Southern Hemisphere values from 1986 are generally greater than those from 1985.

Antarctic Stratospheric Chemistry of Chlorine Nitrate, Hydrogen Chloride, and Ice: Release of Active Chlorine

The reaction rate between atmospheric hydrogen chloride (HCl) and chlorine nitrate (ClONO(2)) is greatly enhanced in the presence of ice particles; HCl dissolves readily into ice, and the collisional reaction probability for ClONO(2) on the surface of ice with HCl in the mole fraction range from approximately 0.003 to 0.010 is in the range from approximately 0.05 to 0.1 for temperatures near 200 K. Chlorine (Cl(2)) is released into the gas phase on a time scale of at most a few milliseconds, whereas nitric acid (HNO(3)), the other product, remains in the condensed phase. This reaction could play an important role in explaining the observed depletion of ozone over Antarctica; it releases photolytically active chlorine from its most abundant reservoir species, and it promotes the formation of HNO(3) and thus removes nitrogen dioxide (NO(2)) from the gas phase. Hence it establishes the necessary conditions for the efficient catalytic destruction of ozone by halogenated free radicals. In the absence of HCl, ClONO(2) also reacts irreversibly with ice with a collision efficiency of approximately 0.02 at 200 K; the product hypochlorous acid (HOCI) is released to the gas phase on a time scale of minutes.

Reaction of Chlorine Nitrate with Hydrogen Chloride and Water at Antarctic Stratospheric Temperatures

Laboratory studies of heterogeneous reactions important for ozone depletion over Antarctica are reported. The reaction of chlorine nitrate (ClONO(2)) with H(2)0 and hydrogen chloride (HCl) on surfaces that simulate polar stratospheric clouds [ice and nitric acid (HNO(3))-ice and sulfuric acid] are studied at temperatures relevant to the Antarctic stratosphere. The reaction of ClONO(2) on ice and certain mixtures of HNO(3) and ice proceeded readily. The sticking coefficient of ClONO(2) on ice of 0.009 +/- 0.002 was observed. A reaction produced gas-phase hypochlorous acid (HOCl) and condensed-phase HNO(3); HOC1 underwent a secondary reaction on ice producing dichlorine monoxide (Cl(2)O). In addition to the reaction with H(2)0, ClONO(2) reacted with HCl on ice to form gas-phase chlorine (Cl(2)) and condensed-phase HNO(3.) Essentially all of the HCl in the bulk of the ice can react with ClONO(2) on the ice surface. The gaseous products of the above reactions, HOCl, Cl(2)0, and Cl(2), could readily photolyze in the Antarctic spring to produce active chlorine for ozone depletion. Furthermore, the formation of condensed-phase HNO(3) could serve as a sink for odd nitrogen species that would otherwise scavenge the active chlorine.