Typically, FluxLab mobile surveys involve driving around oil and gas developments looking for fugitive emissions with a truck full of gas analysers. When I started work in the lab in January, Dave asked me to take the same mobile survey techniques that the lab has been refining over the years, and apply them to a totally different environment. After a few weeks mulling this over, I decided to see what I could find out about methane emissions in cities!
We started off with a test run in Toronto this summer. Even though it’s a bit far from Antigonish, Toronto made sense since there has already been a lot of air quality research done there, and because Debra Wunch and Greg Evans, two researchers at the University of Toronto, agreed to collaborate with us. So in late July, Susan and I packed up the truck and headed off to the big city. After a few days driving (during which we got to explore the regional changes in methane between Antigonish and Toronto) we arrived downtown and were ready to get started.
The first thing to do was to get Wunch and Evan’s analysers set up in the truck along with our own. Debra Wunch’s group has been exploring methane emissions in the city by bike, towing their gas analyser along in a trailer. This meant that it was already wrapped in a small self-contained package, and it was no trouble to pop the wheels off the trailer and move the whole thing into the truck bed. The most time-consuming part was attaching their weather station to the roof of the truck which required drilling a few holes in the support beams we already have up there (shhhhh!). Both the Wunch analyser and our own measure methane, carbon dioxide, and water; ours can additionally detect ethane and methane isotopes and theirs measures carbon monoxide. The combination of the two analysers gave us a very useful suite of analytes that should extract distinctive source signatures from different types of emissions. To top it off, Greg Evans added some portable instruments for measuring particle size and black carbon which rode in the cab with us with their sampling tubing stuck out the window.
We planned out three different routes around the city (see map) targeting different types of known methane emitters, and we wanted to cover each of these three times to get a sense of how variable these emissions are.
Over the 9 days of the campaign, we managed to target 50 or so pieces of infrastructure in triplicate, including power plants, wastewater treatment plants, and different kinds of manufacturing facilities. We decided to first drive each of the routes at night, since the trapping of emissions under a low night time boundary layer might increase concentrations and allow us to sniff out some of the weaker emitters. This meant starting surveys at 9 pm and sometimes finishing after 4 am! These night time drives were full of interesting emissions and the overall stable concentrations made it easy to see some very high enhancements above background values. As cool as it was to explore new areas while the city was totally asleep, the daytime surveys were much more pleasant. When we saw big emissions during daytime surveys, we would stop and see if we could identify the specific source of the methane, scanning the various elements of the infrastructure using a Forward Looking InfraRed camera (see left picture).
In general, the highest methane emissions we saw came from parks built on top of old landfills. This was especially the case at night when low winds and a shallow mixed layer let concentrations really build up. A few times we saw some mystery methane: sudden high concentrations in areas where we hadn’t flagged any piece of infrastructure as a likely source. This would lead to a bit of detective work as we drove zigzag around the signal, trying to figure out where it was coming from. One of these mystery spikes was the highest of any we saw, and after a bit of searching, we wound up here:
It seems like this captured patch of sludge down on the lakeshore was a stronger emitter than any of the pieces of infrastructure that we had targeted! This was quite surprising, but we’d have to do a bit more investigating to confirm it, and there’s a lot more work to be done to put this into the context of overall emission volumes in the city.
On the whole, we saw a whole lot of really interesting emission signals from different types of sources, and now it’s on to crunching through the data so that we have a better overall picture of sources in the city as we plan out our next run!
By Alex Tevlin