Time for the third instalment of Niall's video diary that he recorded in the field in Colorado last summer. This time he almost talks about science. Time to calibrate the eddy flux correlation time lag! It goes like this...By knowing the direction of the wind and the concentration of something, we can work out the "flux", that is, how much of the something is being emitted from the forest into the atmosphere (if the something is greater when the wind is blowing up the way) or how much of something is sinking from the atmosphere into the forest (if something is greater when the wind is, you guessed it, blowing down). The problem is, for the calculations to work we have to do this superfast - about 20 times every second.We used a "sonic anemometer" which measures wind direction by detecting changes is the speed of sound between some little nodules: if there is a gust of wind, then the sound takes longer or shorter to travel over the same distance. At the same time, we are sampling air from right next to the sonic anemometer by sucking it down a tube. This goes to an instrument in the box at the bottom which makes measurements of the particles that are suspended in the air. The problem is that, because the air takes a certain amount of time to travel down the pipe before it is measured, there is a "lag time" between the wind measurement and the particle measurements. If we know this lag time then we can just shift the measurements so they match.That's where the balloons, cigarettes and a sharp knife come in. Cigarette smoke is made up of lots of particles. The idea is that we fill the balloon with smoke and pop it right next to the sonic anemometer and the particle tube. The sonic detects the pressure wave from the balloon pop (effectively a very small fast gust of wind) and the instantaneous release of particles is measured (after they've travelled down the tube) by the instrument at the bottom. We just need to match up the pop on the sonic with the particles detected after the tube and, hey presto!, there's the lag time.
Archive for February 2012
After an accidentally long winter break, we're back this time with a podcast about the ozone layer, the hole in it, and those pesky CFCs that cause it. Find out how the ozone layer is created and destroyed, the difference between good and bad ozone, and listen to Nicky learn to pronounce "cafetiere". No interview or news this week due to some dodgy recording, but we have just ordered a very posh mixer and mics, so (hopefully) look forward to some better recording quality next time.