2. Norwegian ozone measurements in 2014
2.3 Total ozone in Ny-Ålesund
Ny-Ålesund is located at a high northern latitude (79º N), which makes it more challenging to obtain reliable ozone measurements due to weak solar radiation, especially during spring and fall. Whereas most ozone instruments are based on UV absorption techniques, e.g. the Brewer and GUV instruments, the SAOZ instrument in Ny-Ålesund is based on radiation from the visible part of the solar spectrum. This requires a long pathway through the atmosphere.
NILU’s instrument in Ny-Ålesund is located at the observation platform of the Sverdrup
Station of the Norwegian Polar Institute. Measurements started up in 1990 and have continued until the present time with a few exceptions, one of which was repair and maintenance of the instrument during winter 2010/2011 at LATMOS/CNR. In October 2013 a temperature failure of the SAOZ instrument was discovered, caused by a broken electronic card, and the
instrument was sent to LATMOS, France, for repair. The instrument returned to Ny-Ålesund in March 2014. Thus, total ozone data from Ny-Ålesund are missing from February until mid-March 2014.
The SAOZ instrument is a zenith-sky UV-visible spectrometer where ozone is retrieved in the Chappuis bands (450-550 nm) twice a day (sun rise /sun set). Data from the instrument contribute to the Network of Detection of Atmospheric Composition Change (NDACC). An ozone inter-comparison shows that different SAOZ instruments are consistent within 3%.
In addition to SAOZ, a GUV-541 multi-filter radiometer is used for ozone measurements when the UV-radiation is getting stronger in the late spring, summer and early fall. These
measurements give important contributions to the ozone time series from Ny-Ålesund.
Comparisons between SAOZ and GUV data during overlapping measuring periods indicate that the GUV ozone data might be too high during summer. New comparisons between GUV and a high-quality Brewer spectrophotometer indicate the same. This is described in more detail in Section 2.3.1
Due to the SAOZ failure and repair in early 2014, less SAOZ measurements than normal are included in the 2014 ozone time series. Table 3 gives an overview of the different instruments and methods used for the 2014 ozone series in Ny-Ålesund. No ozone measurements were performed during the polar night period.
Table 3: Overview of instruments and methods applied for retrieval of the total ozone in Ny-Ålesund 2014.
Priority Method Total days with
observations
1 SAOZ instrument 96
2 GUV instrument 141
Missing days (~10 days in February) 13
Figure 3a shows daily ozone values from Ny-Ålesund in 2014. The black curve illustrates the daily ozone values, whereas the red curve shows the long-term monthly mean values for the years 1979-1989, calculated from TOMS (Total ozone Mapping Spectrometer) satellite data.
Total ozone values during winter (November to mid February) are not achievable due to absence of sunlight. Similar to Oslo and Andøya, the lowest ozone values in Ny-Ålesund normally occur in October and November, and the minimum ozone value in 2014 was 261 DU, measured 23 October. This is 5% below the long-term mean for October. A seasonal ozone minimum of 291 DU was reached on 1 March, which is 31% below normal March values.
Figure 3a: Daily total ozone values measured in Ny-Ålesund in 2014 by the SAOZ and GUV instruments (black curve).
The red line is the long-term monthly mean values from 1979 -1989.
Monthly mean ozone values in Ny-Ålesund 2014 are shown in Figure 3b. Comparison between the long-term mean and monthly mean ozone values 2014 shows that the ozone values were above the long-term mean from April and throughout the year. Similar to Andøya, the ozone values were low in February and March. It should be noted that the summer ozone
measurements are based on the GUV instrument, which might overestimate ozone by a few percent (see Section 2.3.1).
Figure 3b: Monthly mean total ozone values for 2014 (black curve) compared to the long-term monthly mean values for the period 1979-1989 (red curve).
2.3.1 Italian Brewer measurements 2014
As stated above, comparisons between SAOZ and GUV data during overlapping measuring periods have indicated that GUV ozone data are too high during the summer. Comparisons between GUV and satellite observations have also indicated such an overestimate of GUV total ozone.
In October 2014 CNR-IDASC, Italy, and NILU signed a scientific agreement that gives NILU access to the Italian Brewer (B50) measurements in Ålesund. B50 was installed in Ny-Ålesund in 1997, but unfortunately total ozone data for the entire period 1997-2014 is not available. However, from July 2013 and onward Brewer total ozone data are accessible and suitable for a preliminary inter-comparison to the Norwegian ozone measurements. Figure 4 shows GUV and Brewer ozone measurements in 2014.
Figure 4 shows that GUV values are above those from the Brewer instrument most of the year. On average, GUV ozone values are 15 DU (~4.2%) higher than Brewer ozone. This is in line with our expectations. It should be noted that B50 has not undergone calibration since 2009 and that the Brewer data shown in Figure 4 possibly need minor corrections. The Italian Brewer instrument will be calibrated by IOS Canada in summer 2015, and a final inter-comparison between Brewer and GUV will be performed after this calibration. Until then, GUV data and trend analyses are presented without corrections. An adjustment of GUV total ozone data will probably have a small impact on trend analyses, since most summer ozone measurements after 1998 are based on the GUV instrument and consequently all
measurements will be adjusted equally.
Figure 4: Comparison between GUV and Brewer ozone measurements in Ny-Ålesund 2014