Update from Dalhousie....

Post written by Eddy Barratt and Rob Trigwell

Well we’ve had a little practise and I (Eddy) had my first shift running the lidar solo on Monday night, though a combination of instant messaging, e-mails and phone calls meant that I had all the support that I needed. You might expect that the lidar can pretty much run itself, but there are still quite a lot of tasks that kept me busy. The most important of these is to do with aircraft safety.  We have a radar that shuts down the laser whenever an aircraft is in our vicinity. When this happens I have to go outside, look for the aircraft in question, then log it’s height and it’s direction before turning the laser back on. There are also helicopters that occasionally buzz over the city.  We have a microphone on the roof and we can hear them approaching from a fair distance, and we can shut off the lidar manually before the radar even detects them.

 The LIDAR running at night.

The data retrieval software also needs to be started every 60 minutes, and to produce the plots requires a little skill and patience. If you look carefully at the plot below (taken monday night, current plots can be found at http://aolab.phys.dal.ca/data/current/) you’ll see black bands at 5 and 5.5 km. These are where I’d set the normalisation limits. The software has to be told how much light to expect from clear, aerosol free air, and we do that by selecting a region and telling it that in that region the air is free of aerosols. Clearly if their are aerosols in that region the plots will look wrong, so selecting the right normalisation region is important. Usually you’d choose a region high up in the atmosphere but for that shift I was having to continually lower the region, to bring it under the cloud which had been steadily sinking all evening.

This plot demonstrates some interesting features. First, and most obviously, you can see the bottom edge of the cloud. It had fallen from about 9km at 2100 (GMT, 6pm local time) to 6km when I took the screenshot at 0230.. Occasionally, like between 2200 and 2300, the cloud was thin enough so that we were still getting some readings from above it, but for most of the evening the bottom of the cloud had been as far as our laser has reached. The second thing is the band of bright colours down low, in the first 700m or so. This is thin cloud or fog, it wasn’t thick enough to stop the beam, but it did provide enough reflections to show up quite significantly.

The third thing you might notice, with a keener eye, is the swirling blue tinted patterns between the two cloud layers. These are aerosols, which is exactly what we’re out here to look for. I’ll hand over to Rob who ran some further analysis, to try and find out where that aerosol came from...

Lidar profile above Halifax, and 5 days backward trajectory for air that was at 2000 m elevation at 2200 h on monday night, produced by the NOAA HYSPLIT model. Click on the figure to enlarge it.

Running the NOAA HYSPLIT model at the altitude at which the aerosols were being detected by lidar can give us an insight into source regions of these aerosols. The model uses what it knows about wind strengths and directions in the past few days to calculate what paths a given parcel of air has taken to be where it is now. As you can see from the plot, the back trajectories suggest that the aerosols could have originated somewhere in the US midwest, before making their way across Ontario and Quebec. These are not heavily industrialised regions so the aerosols are probably not anthropogenic. The actual source and type of these aerosols is a matter for further analysis. 

Back in Edinburgh, Mark Parrington is using forecast output from the NASA GEOS-5 system to predict the trajectories of biomass burning plumes, and when they pass over Halifax. The forecast CO profiles over Halifax (http://xweb.geos.ed.ac.uk/~mparring/BORTAS/Halifax/), plotted in a similar form to our lidar plots, highlight where Carbon Monoxide is strongly concentrated with the measured aerosol. CO is produced in a number of different processes, both industrial and natural, but in particular CO would be expected to coexist with aerosols from Biomass burning, so correlations between his plots and our own are very exciting.

CO forecast profiles over Halifax, plots courtesy of Mark Parrington, University of Edinburgh. Click to enlarge.

A last word from Eddy:

I should have mentioned in my last post that I’m here thanks to a grant awarded by the Nuffield Foundation, which I’m very grateful for, and very sorry not to acknowledge it last post.

First Taste of Atmospheric Research

Post written by Eddy Barratt. 

As Sarah mentioned in a previous post, the BORTAS project includes scientists from a number of different disciplines within the atmospheric science community. I can’t quite claim to be one of them, but happily the project is also offering me, an undergraduate student at Edinburgh University, the chance to take my first step into the world of atmospheric research.

Together with Rob Trigwell, who is due to start his Masters at Edinburgh in the Autumn, I arrived in Halifax yesterday to assist Tom Duck with the operation of his Lidar (see the 4th May entry) for five weeks this summer. Initially the idea was that we’d be flying out a couple of weeks ahead of the rest of the team, to familiarise ourselves with the instrument, and then during the flight campaign the information we gathered from lidar observations would continually be available for everybody else to assist analysis, flight planning, and the like. The fact that the flying campaign has been postponed hasn’t dramatically changed our particular role at all; measurements of aerosol levels in the sky above Nova Scotia can still tell us a lot about the transport of boreal fire smoke plumes. Together with the myriad of other scientific measurements being made this summer (by radiosonde, satellite, mountain top observatory; just not aircraft), Rob and I will hopefully be able to contribute some important observations that will help the rest of the team to refine their models, fine tune their science, and for us all to reach a better understanding of what goes on within a plume of smoke.

 The roof of the physics building at Dalhousie University. The white instrument is the RADAR (RAdio Detection And Ranging) instrument, and when it's running the LIDAR beam comes up through the hatch on the right.

So today we met Tom and his team, and we had our first introduction to the lidar, and our first safety briefing. At the heart of the lidar is a very powerful laser, so the introduction was as much about how to operate the machine safely as it was about gathering data from it. For example, the beam shoots directly upwards from the physics building in downtown Halifax, and could potentially cause a danger to passing aircraft; so the lidar is also equipped with a radar, and if a plane’s path brings it close enough overhead then the whole instrument is automatically shut off.

Over the course of the next week or so we will be learning the ins and outs of the machine, which Tom designed and built (and which, I should admit, seems a little Heath-Robinson to my un-accustomed eyes. The photons of light, which have reflected from aerosols in the atmosphere and have found their way down to the receiver telescopes of the lidar, are transported to photomultipliers, which count them, in optical fibres inside garden hoses). The lidar hasn’t been used much since last summer, so we’ll all be interested in what we discover over the next few weeks. We’ll keep you posted.