Millimeter wave spectroscopic measurements over the South Pole: 4. O 3 and N 2 O during 1995 and their correlations for two quasi‐annual cycles

In two separate papers we have previously reported observations of stratospheric O 3 and N 2 O over the South Pole during the 1993 annual cycle. Here we present (1) new O 3 and N 2 O observations at the South Pole in 1995 and (2) correlations between O 3 and N 2 O for two 11‐month observations during February 1993 to January 1994 and January‐December 1995. Strong similarities exist between the two quasi‐annual cycles for both O 3 and N 2 O. A double‐peaked profile again dominates O 3 vertical distribution in 1995 as in 1993. Features such as a pronounced summer‐fall decline in mid‐stratospheric O 3 followed by an early winter increase, a downward trend in the O 3 contour pattern associated with vertical transport, a transient enhancement of middle to upper stratospheric O 3 just before local sunrise, the timing of the ozone hole onset, and a dramatic increase of stratospheric O 3 during and following vortex breakup all show good consistency between the two annual cycles. N 2 O observations show a good agreement between the two 11‐month cycles in atmospheric descent rate during fall and winter, and in the timing of N 2 O recovery from diminished values during spring. We use O 3 ‐N 2 O correlations to further investigate the double‐peaked vertical distribution of O 3 . During springtime warmings the O 3 /N 2 O ratio shows a tight coupling between O 3 and N 2 O around 20 km, as transport creates the lowaltitude O 3 peak. A rapid and systematic decrease of the O 3 /N 2 O ratio during summer in the 25 to 30 km region (while N 2 O is essentially stable) supports the increasingly dominant role of photochemistry in producing the vertical profile for O 3 above ∼25 km while leaving a transport‐produced layer with a relatively large mixing ratio below ∼25 km. The resulting double‐peaked O 3 distribution, which persists for many months, can alter the normally negative correlations between O 3 and N 2 O in the lower and middle stratosphere, although in measurements of the N 2 O/O 3 ratios for polar air these perturbations have often been taken to be a hallmark of catalytic ozone depletion by chlorine. The present analysis should help to clarify the influence of the relatively unique O 3 vertical distribution of polar ozone when interpreting O 3 ‐N 2 O correlations.