Content

Ozone, UV and Aerosol studies

Results

Meteorology

Since February 2009, there are continuous data of an automatic weather station around 300 m east of PE station. A general description of the instrumentation and results can be found on the HYDRANT website.  At Utsteinen, there are two predominant meteorological regimes:  cold catabatic (air masses from Antarctic interior) and  synoptic (extratropical cyclones, frontal systems). The station and its activities are situated in the sector SSW-N, and as the main wind directions are confined to the sector NE-S, the aerosol measurement container is  most of the time exposed to non-contaminated upwind air. Any contamination can further be detected and excluded by the signals of the ultra-fine condensation particle counter and of the aethalometer. Time periods of very low wind speed (<4m/s) are in addition carefully looked at .

Ozone and UV

The Brewer ozone spectrophotometer on the northern roof of the station takes measurements of the incoming UV radiation between 286 to 363 nm. These measurements are used to derive the total amount of atmospheric ozone in the atmosphere. Ozone is a strong absorbing gas in the UV-B region. The largest amount of ozone is found in the middle stratosphere and through its interaction with UV-B radiation ozone protects us from this hazardous radiation potentially causing skin cancer. By combining ozone and UV measurements, the UV index can be calculated, a measure how fast skin is burned by the radiation. In Antarctica the so-called ozone hole appears for almost 25 years now each austral spring, destroying ozone almost completely in altitudes between 15 to 20 km, and reducing total ozone by a third or even more. This large reduction of total ozone increases the UV-B radiation at the surface. E.g., on 14 December 2011, a remainder of the ozone hole moved above Dronning Maud Land, reducing total ozone distinctly and increasing the UV index to nearly 10. This extreme value meant that unprotected skin would have been burned within minutes.

The situation was different in summer 2012/13. Total ozone recovered rather fast and the total ozone map over Antarctica showed no particular region of reduced ozone. The measurements of the Brewer spectrophotometer were in line with it. In November and during the first half of December total ozone values were rather high with values above 320 DU. Afterwards, total ozone values (direct sun observations) fluctuated around 270 to 310 DU. Overall, the calculated UV index indicated moderate levels of potentially hazardous UV-B radiation. However, due to the high albedo of the Antarctic landscape, the time span to be burned, indicated by the UV index, might be distinctly reduced.  It is therefore strongly recommended to always use sunscreen.

The permanent measurements of the pyranometer and UV-A and UV-B sensors allow in addition to follow in near-real time the evolution of the UV index also during winter. However, for the winter period, no direct simultaneous total ozone measurements are available. Climatological ozone values are used then.

 

Total ozone map for the southern hemisphere for 14 December 2011 (www.woudc.org)

UV index for 14 December 2011. Total ozone was only 230 DU around that day.

Time series of total ozone (both from direct sun and zenith sky observations) and of the calculated UV index for the summer season 2012/13 at Utsteinen.

Cimel Sunphotometer

The sunphotometer measures sun radiances (direct and sky) at eigth wavelengths (340, 380, 440, 500, 675, 870, 936, 1020 nm). One result is the aerosol optical depth (AOD) for the total atmospheric column. The AOD is a proxy for the aerosol amount in the atmosphere. The spectral dependency of the AOD gives further information on the size of the aerosol. Smaller particles scatter light more efficiently at shorter wavelengths and when small particles dominate, the AOD increases thus with decreasing wavelength. From the spectral AOD dependence, the so-called Angstrom exponent can be derived, describing the exponential curve of AOD with wavelength. If this exponent is distinctly above 1 (>1.5), this is an indication that submicron particles dominate. An Angstrom exponent below 1 indicates the presence of coarse particles. Exponents below 0.5, or even negative ones, are signs of cloud contamination or very coarse particles (e.g., during a dust storm, what is not happening in Antarctica). The sunphotometer needs an unobstructed view of the sun for its measurements, so care is taken to exclude cloudy periods. The instrument does also almucantar and principal plane sky radiances observations, and from these further parameters (e.g., single scattering albedo, size distribution) are calculated with a special algorithm (see the AERONET webpage for more information). The data shown below belong to quality level 1.5 (automatically cloud-cleared). In addition, some cloud-contaminated data not detected by the algorithm were manually excluded. The figure below shows the average AOD values from the sunphotometer for all summer seasons since February 2009 to February 2014. The average Angstrom exponent (calclulated between 440-870 nm) was 2.0 +/- 0.6, indicating a dominance of sub-micron particles.

 

total aerosol optical depth averages from February 2009 to February 2015 from the Cimel sunphotometer at Utsteinen, and derived from the Brewer ozone spectrophotometer (at 340 nm only)

TEOM-FDMS

The Tapered Element Oscillating Microbalance with Filter Dynamic Measurement System is operated with a bulk inlet on top of the aerosol shelter and an averaging time of 1 hour.  The total aerosol mass concentrations (from 24hour running means) are around  1.5 ± 0.8 μg/m3.  These values are at the detection limit of the instrument, indicating also that there are not many coarse particles in the atmosphere around Princess Elisabeth.

 

Aethalometer and Nephelometer

The aethalometer is operated at a nominal flow of 5.5 liter/min and an integration time of 60 min, with the bulk inlet on top of the aerosol shelter. The mean mass concentration of light-absorbing particles was 9.2 ± 6.7 ng/m3 during austral summers (4 seasons measured), 5.8 ± 4.6 ng/m3 during autumn (4 seasons), and 4.0 ± 3.9 ng/m3 during winter (2 seasons).  The mean absorption Angström exponent (370 to 880 nm; derived from calculated absorption coefficients) was 1.5 ± 1.0 during summer, 1.7 ± 1.3 during autumn and 0.8 ± 0.7 during winter. These values of the Absorption Angstrom exponent indicate that several absorbing aerosol types were present, as, e.g., pure soot has an absorption Angström exponent around 1. Some organic carbon compounds ('brown carbon') absorb stronger at shorter wavelength. However, to give more conclusive answers to this, chemical analysis of aerosol filter samples would be necessary.

The nephelometer is operated at a nominal flow of 5 liter/min and an averaging time of 5 min with the bulk inlet on top of the aerosol shelter. The single scattering albedo (SSA) could be derived from simultaneous aethalometer and nephelometer measurements  (SSA= scattering / (scattering + absorption ) ).  SSA values are around 0.9 ± 0.7. They showed high uncertainty, mainly due to low aerosol concentrations and the uncertainties of the absorption coefficient determination. For Antarctica, values of SSA around 0.98 and higher are expected, however, due to the high standard deviation of the data, we could not give more precise SSA values.

 

Laser Aerosol Spectrometer

The Laser Aerosol Spectrometer (LAS) measures the number of particles in different size classes. In our set-up, the LAS detects particles between 90 and 7000 nm.. The 100 size bins have been chosen to be in logarithmic scale.  In the figure below, a typcial mean size distribution is shown, for 10 January 2014.  From the size distribution, the count median diameter was calculated to be 122 nm. This typical size distribution shows that (i) there are hardly any particles larger than 1000 nm and (ii) that the lower detection limit of the LAS around 100 nm corresponds to a peak in number concentration. In lower latitudes or for Europe, in the accumulation size range (100 to 1000 nm) a peak is expected at around several hundreds of nm. That the Antarctic aerosol did show a peak at smaller sizes indicated that there was a lack of precursor gases, necessary for condensational growth of particles in(to) the accumulation size range.

 

normalised size distribution measured by the LAS integrated for 10 January 2014. Count median diameter was 122 nm.

Ultrafine Condensation Particle Counter UCPC

The UCPC detects all particles between a size diameter of 3 to 3000 nm. The first figure below shows the time series measured by the U-CPC at Utsteinen from November 2013 until end of August 2014. It can be seen that from November to March the mean concentration was around some hundreds of particles per cm3. The number concentration decreased during autumn and winter to reach a minimum in June, July. In August, number concentrations started to increase again. There were also several periods when the number concentrations increased distinctly. These periods have been further investigated, together with the data of the Laser Aerosol Spectrometer and the cloud condensation nuclei counter (see further below). The second image below shows the mean (blue + red column) and median (blue column part) number concentrations for November 2013 ('1') to August 2014 ('10'). During November to March there was a rather high variability, reflecting the influence of air masses transported to Princess Elisabeth station via synoptic scale events (cyclones, transporting air masses from easterly directions, including maritime origin). During the autumn and winter months, the number concentration fell to some tens of particles per cm3. During Antarctic winter, the Antarctic vortex, the atmospheric circulation, forms a barrier for air masses from lower latitude (which could transport more particles). Also during winter, there is hardly any sunlight for driving photooxidative aerosol chemistry and also there is a general lack of precursor gases for condensational growth of particles.

 

 

Figure UCPC-a: total aerosol number concentration at Utsteinen from November 2013 to end of August 2014

Figure UCPC-b: monthly mean (blue + red column) and median (blue column part) values of total particle number concentration; from November 2013 ('1') to August 2014 ('10')

Investigation of nucleation and Aitken mode size particles

The finding that the concentration of larger particles (> 3000 nm) was apparently negligible enabled us to combine the measurements of the CPC and the LAS (the CPC’s upper detection limit is 3000 nm). The combination of the CPC (3 - 3000 nm) and LAS (90 - 7000 nm) observations of the particle number enables us to derive the particle number in the size range 3 - 90 nm. This size range includes the nucleation mode (< 10 nm) and the Aitken size mode (some tens of nm). These size modes are important because increases of them indicate recent new particle formation events (nucleaton mode) or growth of recently formed particles (Aitken mode). Distinct increases mean that the measured aerosol is of rather local origin or has breen recently formed.  Aerosol that has been long-range transported over many days would be depleted of these size ranges (due to growth) and would have a dominant accumulation size range(100 to 1000 nm).

The figure below shows such an increase of particle concentration within the period 25 - 27 February 2014. During the morning of 27 February the number concentration increased from around 200  /cm3 to around 600 / cm3.  The total aerosol number is given in blue, the size range > 90 nm in green and the nucleation + Aitken mode size range in red (3 - 90 nm). It clearly can be seen that the particle number in the nucleation and Aitken mode was always the predominant size range and that it  was distintly increased over a short time on 27 February. The question is what caused that distinct increase. This is subject to further, ongoing investigations. For this purpose, we look additionally into the data of the Cloud Condensation Nuclei counter and into the meteorological conditions.

 

 

total aerosol number of particle size (blue), smaller than 90 nm (red,) and larger than 90 nm (blue) on 25 to 27 February 2014