Column optical aerosol properties measured by ground-based remote sensing at

AOD measurements started at the Birkenes Observatory in spring 2009, utilizing an automatic sun and sky radiometer (CIMEL type CE-318, instrument #513). The retrieval method is that of the AERONET version 2 direct sun algorithm (for details: http://aeronet.gsfc.nasa.gov).

Quality assured (Level 2) data are available for the eight years of operation, 2009 - 2016. The Cimel instrument was scheduled for calibration in spring 2016 at Valladolid (Spain), with the plan to restart measurements in July 2016 at the latest. Due to various circumstances, such as problems with Spanish customs and technical failure during the calibration, the instrument was not back in operation until end of October 2016. Shortly afterwards, further technical problems with both the instrument and the communication system at the observatory

occurred and prohibited usable measurements until April 2017. As a consequence, only a very limited data set is available for 2016, mostly before 1 May. The few measurements from November 2016 lack a second calibration afterwards (which is planned for November 2017) and are only available as level 1.5 data.

Figure 53: 2009 - 2016 time series of aerosol optical depth (AOD) at 500 nm wavelength in the atmospheric column above Birkenes (upper panel) and Ångström coefficient describing the AOD wavelength dependence (lower panel). Mean values and standard deviations are given.

The AOD and Ångström coefficient time series and seasonal variations are shown Figure 53 and Figure 54. The 2016 monthly mean and mean values for all years are shown in Table 7.

Data for all years are given in Appendix I. There are no obvious trends visible in the eight years of observations. Missing data in the summer half-year were caused by a combination of various circumstances as described above. In general, AOD measured at Birkenes are

relatively low, compared to central European observations. E.g., AERONET climatological values for Cabauw, the Netherlands, vary between 1.2 ± 0.06 in December and 3.1 ±0.19 in April). In 2016, AOD values at Birkenes were very low, compared to previous years at this

Figure 54: Monthly mean aerosol optical depth (AOD) at 500 nm wavelength in the atmospheric column above Birkenes (upper panel) and Ångström coefficient describing the AOD wavelength dependence (lower panel). Mean values and standard deviations are given. Values marked in grey are the mean and standard deviations for the time period 2009-2016.

station; none of the monthly mean AOT data were above average over the whole

measurement series. The highest monthly average AOD of 0.05 was observed in April, but this is not significant as data from the summer months with generally higher AOT values are missing. There were no episodes with significantly higher AOT values during winter and spring.

Ångström exponent monthly means are close to the long-term means observed at Birkenes and also follow the seasonal variation of previous years, with monthly means close to 1.0 in the first months of the year and in November. All 2016 monthly means are within the standard deviations of all years at the station.

Key findings column-integrated optical aerosol properties Birkenes: There are no trend in the eight years of AOD observations at Birkenes. In 2016, AOD values at Birkenes were very low, compared to previous years at this station; none of the monthly mean AOT data were above average over the whole measurement series.

Table 7: Monthly mean values for 2016 and mean for the time period 2009-2016, plus standard deviations, for aerosol optical depth (AOD) and Ångström coefficient observed in Birkenes. In addition, the number of days with cloud free and quality assured observations are given.

Month/Year Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov* Number of days with cloud-free and quality assured observations (AERONET level 2)

2016 2 11 12 9 2*

Total

09-16 9 13 61 63 93 114 127 107 68 52 13

*not yet approved as level-2 data

3.4.2 Column optical aerosol properties measured by ground-based remote sensing at Ny-Ålesund

In 2002, Physikalisch-Meteorologisches Observatorium Davos/World Radiation Center (PMOD/WRC) in collaboration with NILU, started AOD observations in Ny-Ålesund (at the Sverdrup station, 46 m a.s.l.) as part of the global AOD network on behalf of the WMO GAW program. A precision filter radiometer (PFR) measures the extinction in four narrow spectral bands at 368 nm, 415 nm, 500 nm and 862 nm. Data quality control includes instrumental control like detector temperature and solar pointing control as well as objective cloud screening. Ångström coefficients are derived for each set of measurements using all four PFR channels. Calibration is performed annually at PMOD/WRC. Quality assured data are available at the World Data Centre of Aerosols (WDCA), hosted at NILU (see https://ebas.nilu.no) In Ny-Ålesund, the solar elevation is less than 5° before 4 March and after 10 October, limiting the period with suitable sun-photometer observations to the spring-summer-early autumn seasons (NILU contributes to a Lunar Arctic initiative to fill the gap in the wintertime AOD climatology by using Lunar photometer, see Appendix II). In 2016, sun-photometer observations started on 15 March and lasted until 12 September; reliable AOD values are available on 64 days. The AOD and Ångström coefficients time series of monthly means and standard deviation are shown in Figure 55, while the 2016 values on the background of the average data and their standard deviation from the whole 15-year period are shown in Figure 56. The 2016 monthly mean values and standard deviations for all years are given in Table 8. Data for all years are given in Appendix I.

Figure 55: 2002 - 2016 time series of aerosol optical depth (AOD) at 500.5 nm wavelength in the atmospheric column above Ny-Ålesund (upper panel) and Ångström coefficient (lower panel). Monthly mean values and standard deviations are given.

0.0 0.1 0.1 0.2 0.2 0.3 0.3 0.4

Jan-02 Jan-03 Jan-04 Jan-05 Jan-06 Jan-07 Jan-08 Jan-09 Jan-10 Jan-11 Jan-12 Jan-13 Jan-14 Jan-15 Jan-16 Jan-17

AOD

Month-Year

In all months monthly mean AOD values were equal to or smaller than the long-term mean; in April the value was at the lower end of the standard deviation of all years. None of the daily average AOD values exceeded 0.2 and only 6 exceeded 0.1. Ångström coefficient monthly means relatively very stable throughout almost the whole measurement period close to a value of 1.5. Generally, in Ny-Ålesund seasonal variation of the Ångström coefficient is very weak, and the monthly mean Ångström coefficiens in 2016 lie within the standard deviation range of the long-term monthly means, but in April and May at its upper end.

Figure 56: Seasonal variation of the aerosol optical depth (AOD) (upper panel) and Ångström coefficient (lower panel) observed in Ny-Ålesund. Values marked in grey are the mean and standard deviations for the time period 2009-2016; the 2016 monthly mean and standard deviations are shown in green.

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

3 4 5 6 7 8 9

AOD

Month of year

0.0 0.5 1.0 1.5 2.0 2.5

3 4 5 6 7 8 9

Ångström coefficient

Month of year

Table 8: Monthly mean values for 2016 and mean for the time period 2002-2016 for March to September, plus standard deviations, for aerosol optical depth (AOD) and Ångström coefficient observed in Ny-Ålesund. In addition, the number of days with cloud free and quality assured observations are given.

Month/Year Mar Apr May Jun Jul Aug Sep Number of days with cloud-free and quality assured observations

2016 6 14 8 7 12 10 7

Total

2002-2016 67 174 161 152 179 135 118

Key findings column-integrated optical aerosol properties Ny-Ålesund: In 2016, Ny-Ålesund monthly mean AOD values in all months were equal to or smaller than the long-term mean.