Tuesday, 31 May 2011

Ozone Sonde Launches

I have mentioned before on Twitter that we have some brilliant collaborations going on which will help us understand the data we collect during BORTAS this summer. Many of these have already started with scientists from a number of universities and institutes collecting data during the BORTAS-A period last year. In earlier posts I talked about the Pico Mountain Observatory and the lidar at Dalhousie University but another useful and exciting activity that will be going on this summer, and in fact goes on all year round, is the launching of ozone sondes across Canada by the Environment Canada Experimental Studies Unit (ARQX).

The first question might be 'what is an ozone sonde?'....good question. Sonde is the French word for probe and so it makes sense that this is essentially a probe to measure ozone. They are lightweight instruments that can be carried by a helium filled balloon (not a party balloon, these ones are slightly bigger and more robust). The iodide redox reaction (shown below) is used to allow ozone to be detected.

2KI + O3 + H2O -> I2 + O2 + 2KOH

This reaction is used to produce an electrical signal which is proportional to the ozone concentration. This can be done in a number of ways each of which is explained at http://www.atmosp.physics.utoronto.ca/SPARC/SPARCReport1/1.08_O3sondes/1.08_O3sondes.html. The sonde also includes meteorological instruments to measure pressure, temperature and humidity. The balloon carrying the ozone sonde can travel upwards as far as 35 km before the balloon bursts. This happens because as pressure decreases higher up in the atmosphere the helium inside the balloon expands into the lower pressure surroundings. The balloon can not expand indefinitely and so eventually it bursts.

 On the left is the inside of an ozone sonde showing the two solutions used to create the electric current. The other two pictures show ozone sonde launches with the middle one using a special balloon which can get to higher altitudes (it looks very dramatic!).

ARQX launch ozone sondes weekly from ten locations across Canada. During BORTAS they will also launch daily sondes from Yarmouth, Sable Island, Goose Bay, Egbert and Bratt's Lake. There is also the possibility of additional launches if a plume is predicted to pass over one of the launch sites. The sites are all shown on the map below. Ozone sonde launch sites are identified by the symbol that resembles a balloon carrying an object (an ozone sonde of course). Also depicted on this map are sites that operate lidar instruments similar to that operated at Dalhousie University and sites which have Brewer spectrophotometers. These instruments measure total ozone and UV radiation, to find out more have a look at the Environment Canada pages at http://exp-studies.tor.ec.gc.ca/e/ozone/ozone.htm.

Network of ozone sonde launch sites, lidar locations and sites with Brewer spectrophotometers.

David Tarasick from ARQX kindly provided me with some plots from data collected during the BORTAS-A period last year. On numerous occasions the vertical profiles from ozone sonde flights show regions of elevated ozone. An example of this is shown below.

Left hand plot shows the vertical profiles taken by the ozone sonde at Edmonton with ozone mixing ratio in black. The area of high ozone is circled in red. The right hand plot shows where the air at Edmonton had come from overlayed on the fire counts for the previous day.

These plots show that the air sampled is likely to have been impacted by the fires north east of Edmonton and this could be the cause of the elevated ozone. Another example is shown below from the Goose Bay site. This shows not only the data from the ozone sonde but also aerosol measurements from the lidar and the carbon monoxide (CO) forecast carried out by scientists at Edinburgh University. All plots indicate something is happening between 4 and 8 km with CO and ozone mixing ratios and aerosol backscatter ratio and cross section all being elevated.This supports the suggestion that the elevated ozone is a result of biomass burning activity. 

The top left plot shows the ozone sonde profiles, top right is the predicted CO from the GEOS-5 model, bottom left is the aerosol backscatter cross section and bottom right is the aerosol backscatter ratio.

From this data we can say that elevated ozone is observed at ozone sonde stations downwind of large boreal forest fires. Back trajectories suggest that the sampled air passed over a region of burning. This elevated ozone may be the result of reactions involving nitrogen oxides, CO and hydrocarbons which are present in biomass burning plumes. The presence of aerosol layers at similar altitudes would support the suggestion that burning plumes have influenced the composition of the air mass. A possibility that can not yet be ruled out is that the elevated ozone is from air that has travelled into the lower atmosphere from the stratosphere where ozone concentrations are significantly higher. This is supported by the relative humidity profiles which show dry air at the altitudes where ozone is elevated but the coincidence of plume interception (suggested by the lidar aerosol data and trajectories) and downwards mixing of stratospheric air is unexplained. Hopefully measurements this summer will add some pieces to this puzzle and help us understand the processes going on in and around aging forest fire plumes.

Thanks to David Tarasick for pictures, data plots and information.

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