As primary station of the Network for the Detection of Stratospheric Change (NDSC), Ny-Ålesund (78,9°N 11,9°E) plays an important role in the ground-based monitoring of the Arctic stratosphere. A microwave radiometer belonging to the standard instrumentation of each station is operated at Ny-Ålesund by the Institute of Environmental Physics (University of Bremen) on behalf of the Alfred-Wegener-Institute for Polar and Marine Research. The principle of microwave radiometry is based on the detection of a pressure broadened emission line of trace gases in the microwave frequency range. From the shape of the detected line volume mixing ratio (VMR) profiles are retrieved.
| Table 1. Instrument parameters | Table 2. Retrieval parameters | |||
|---|---|---|---|---|
| frequency | 14.175 GHz | time coverage | all year round, nearly weather indipendent | |
| system noise temperature | 3200 K | time resolution | 1-5 profiles per hour> | |
| bandwidth | 1.65 GHz | vertical resolution | 10-15 km, altitude dependent | |
| frequency resolution | 1.17 MHz | altitude range | 12-55 km | |
| precision | 0.1 ppm at 20 km | |||
Figure 1. Potential vorticity (thick) and Temperature at the 475 K isentropic level for Ny-Ålesund. The approximate PSC formation temperature is indicated by the dashed line.
Figure 2. Microwave measurements of ozone in the lower stratosphere at Ny-Ålesund for mid February to mid May 1997. In the left part the ozone VMR between 12 and 35 km is shown. Contour lines indicate levels of constant VMR [ppm]. Heights corresponding to isentropic levels 400 K, 475 K and 675 K are indicated by thick lines.Isentropic levels were calculated using daily NMC pressure and temperature profiles. In the right part the measured ozone evolution on layers centered at 400 K, 475 K and 675 K isentropic levels is displayed. A linear regression was performed to estimate the mean ozone decrease on the three isentropic levels during spring 1997. The regression covers the period between day 53 and day 96 while Ny-Ålesund was located well inside the polar vortex as indicated by high and nearly constant PV on the 475 K isentropic level (Figure 1). Solid lines represent linear regressions. Errors (not indicated) are summarized in Table 3. We conclude that the strong ozone decrease is due to chemical ozone depletion. This is confirmed by low stratospheric temperatures necessary for PSC formation and the observation of enhanced ClO at Ny-Ålesund by the RAM [1]. Vortex dynamics including diabatic vertical movement is excluded as major cause for the ozone decrease. Only minor corrections to the amount of chemical ozone loss are expected if the diabatic descent in taken into account since radiative cooling decreases with increasing sunlit hours in spring. Also mixing at the vortex-edge is unimportant in spring 1997 due to the strong and stable vortex centered at the north pole.
| Table 3. Mean ozone decrease | ||
| daily decrease | total decrease | |
| 400 K isentropic level | 10 ± 1 ppb | 0.45 ± 0.04 ppb |
| 475 K isentropic level | 24 ± 1 ppb | 1.05 ± 0.04 ppb |
| 675 K isentropic level | 20 ± 2 ppb | 0.91 ± 0.08 ppb |
| column above 12.5 km | 1 ± 1 DU | 44 ± 4 DU |
Figure 3. Ozone columns above 12.5 km at Ny-Ålesund as calculated from microwave VMR-profiles. The VMR-profiles are transformed to number densities using NMC pressure and temperature profiles. Number density profiles are integrated above 12.5 km taking into account the characteristics of the VMR retrieval. The precision is about 5%. The thin line indicates mean ozone decrease.