Tuesday, 24 January 2012

First BELATMOS winter aerosol measurements

As mentioned in the blogpost before, the TEOM-FDMS and the aethalometer collected data during last winter, at least from end of February to mid of April. This is something very interesting because the two instruments collected data when there was really no emission of aerosols for energy generation (only wind power and batteries) or from station operation. This makes them valuable data with information on real background concentrations. In addition, these data are special because they were sampled without any direct maintenance/attendance of personnel. Measurements in Antarctica with these kinds of instruments are done normally at stations with overwintering personnel. The data I am reporting here are nevertheless preliminary and several quality assurance steps have to be applied. This is in particular important because the TEOM-FDMS and the aethalometer are measuring near their detection limits.

The average of the total aerosol mass concentration measured by the TEOM-FDMS was 0.85 microgram per cubic meter with a standard deviation of 1.03. For the moment slight negative values are still accepted because of the general low aerosol concentration and because of the detection limit of the TEOM-FDMS, which is around 1-2 microgram per cubic meter. I have to make additional checks to justify if negative values are skipped or not. For the winter 2011, 12% of the data are negative, but almost all just between 0 and –1.0 microgram per cubic meter. That the standard deviation is higher than the average means that the average total aerosol mass concentration was not – from a statistical point of view – different from zero, neither definitely positive of negative. To be able to determine this further, a longer time series will be necessary. Another possibility is that the integration time for determining a TEOM-FDMS sample would have to be longer (meaning more mass sampled on the filter before the data value is taken). However, I set the integration time already to 2 hours.

Another possibility is to cross check the TEOM-FDMS data with data from the other aerosol instruments. However, the aethalometer measures the strong absorbing particles, amounting only to a small part of the total aerosol mass. The arrival of the new instruments this season will provide more insight. In particular the Laser Aerosol Spectrometer measuring the size distribution could prove valuable to check the aerosol mass concentration measurements. By integrating over the number concentrations of all measured size classes, and assuming a certain mass density of the particles and a certain shape, it can be cross-checked if either data is reasonable. The winter aethalometer data shows an average of 5.2 nanogram per cubic meter with a standard deviation of 5.4. This means that we are really measuring near the detection limit and a longer time series is necessary to be able to make statistically sound statements how much absorbing particles (or black carbon) there are in Antarctica. However, concentrations between 0 and 10 nanogram per cubic meter would be the expected level.

The images above show the state of the shelter for the aerosol instruments after the winter (no power, heating for 7 months), after cleaning and letting it dry, and an image of the interior of the aethalometer. It shows the filter tape on which the samples are collected. On the right one can see some darker spots. These are the old samples, and the tape is drawn further when a certain absorbance is reached. The snow and ice inside the shelter freightened us a bit if there was damage to the instruments. But it appeared that this was not the case, luckily.

The departure for this season's expedition comes nearer. I will fly this Friday from Brussels via London to Cape Town, South Africa, and the flight from Cape Town to Antarctica is scheduled for Monday evening. So, the next news will come from Antarctica (or from Cape Town, if the weather conditions delay the flight to Antarctica).