The smoke detectors


24 February 2012 by Sarah Moller

Wildfires affect up to 20 million hectares of northern forests every year, and they don't just cause damage on the ground - the plumes of gases they release are a major source of air pollution. Sarah Moller and colleagues spent part of last summer flying over Canada to learn more about them.

On 13 July 2011, I was at Halifax airport in Nova Scotia, to meet the BAe-146 UK Atmospheric Research Aircraft (ARA). The ARA would fly through the plumes from Canadian forest fires, to measure and analyse the cocktail of gases and particles they release into the atmosphere.

Our project studies the polluting effects of fires in boreal forests, which means the subarctic - north of 50°N. This type of fire is often called biomass burning. Locally the consequences are obvious; fires remove vegetation, endanger life and fill the air with thick black smoke. But the wider impact is harder to see.

The smoke plumes contain important atmospheric pollutants - carbon monoxide (CO), nitrogen oxides (NOx), carbon dioxide (CO2), methane (CH4) and black carbon - which affect air quality and how much sunlight reaches the Earth's surface. Prevailing winds carry pollutants from North American fires across the Atlantic Ocean to the UK and the rest of Europe, potentially affecting air quality there too.

That's where our project comes in. Our challenge is to understand how these chemicals interact as the plumes travel, and specifically how they might affect air quality in the UK.

It's not just a case of measuring the different pollutants though. As plumes age, the chemicals they contain react with each other and the background air, so the pollutants that reach Europe will not be the same as those in the plume when it was emitted. This means we also need to know the age of the plume and understand how its chemistry changes over time, to let us predict what the air will be like by the time it reaches the UK.

A dirty layer of air

A dirty layer of air from forest fires can be seen quite clearly

We were particularly interested to see if ozone is produced or destroyed in ageing plumes. Ozone irritates our lungs and is an ingredient of the morning smog you sometimes see over cities. It is also a greenhouse gas. If chemicals from boreal forest fires produce ozone over the UK, it could combine with the UK's own emissions to push background levels above safe limits.

To work out the best places to plume-chase we used a map of fires that were currently burning and the expected emissions from these fires. We estimated the emissions we would expect from fires using studies that other people have done on biomass burning.

We then ran a model that used these expected emissions to predict how much CO we would find at a given location. We would pick the area of highest CO concentration for the next day's flying. We factored in the weather too, because rain 'washes out' some chemicals and changes what's going on in the plume.

Once in the air we know we've found a plume when we detect chemicals that couldn't have come from any other source, in particular acetonitrile, hydrogen cyanide and black carbon particles - our biomass-burning tracers. Seeing several of these chemicals together is a good sign we're in the right place. They can be hard to find though; sometimes they settle into thin layers at different altitudes which are easy to miss.

Taking to the Air

The first science flight out of Halifax was a bit of a let-down. We were all excited to see how well our instruments would work, how high the concentrations of different compounds would get in the plumes, and how the computer predictions of the plume location would compare to our measurements.

Unfortunately, we didn't yet have permission to fly below 13,000ft (around 4000m), so for most of the flight we were above the cloud and saw only minor fluctuations in the concentrations of the chemicals we were looking for; tantalising hints that there would be something exciting to measure, if only we could get low enough.

Armed with enough data we can begin to compare what we are seeing in the air with our computer model's prediction of what should be there. The model simulates interactions between different chemicals and tells us what we should expect to see after the plume has been around for a few days. So we can begin to understand how what we measure might relate to what was in the plume at the beginning.

Scientists on the aircraft

Scientists working on board the aircraft

We were quietly hopeful as we set off again the next morning, this time with permission to fly lower; sure enough things started to happen on our screens soon after take-off. First we saw rising CO concentrations, followed by some black carbon particles, and then my screen started showing a steady increase in reactive nitrogen; it looked like we were flying directly into an aged plume.

The chat over the radio increased as people shared their data and found correlations between methane, hydrogen cyanide and CO. At one point the CO shot up to a concentration that the chemistry operator, Steph, declared to be the highest he had seen in all his time on the aircraft. This flight was definitely more exciting and reassured us that we were in the right place.

That was the first of a series of flights during which we successfully measured plumes that were a few days old. What we needed next was more information about younger plumes.

Several fires had been reported in Ontario, but when we got there the previous day's rain had put many of the larger ones out and all we saw were a few smouldering remains. Nevertheless, our measurements were worthwhile; they showed different concentrations of pollutants to those we'd measured before, and even some compounds that we had not seen at all during our other flights.

One of the most interesting aspects of our work is when we see things in our data that the chemistry in the computer models can't explain, and we need to investigate to find out why.

Three of our flights - beneath the path of the NASA Aura satellite, over Dalhousie University's ground-based atmospheric measurement station, and around the launch site for balloon measurements of ozone at Goose Bay - will enable us to compare our own measurements with those taken using different types of instruments and methods; a useful check which can highlight potential problems in the data.

After three weeks enduring long hours and the rather limited food options around the airport, we had managed to collect important data from both fresh and older plumes.

Analysed alongside information from previous fieldtrips, this will be a useful step forward in our understanding of the impact of these plumes on European air quality. The excitement of the fieldwork is over but the hard work to tease out the answers in the data promises to be just as rewarding.

Dr Sarah Moller is a postdoctoral researcher at the University of York.

BORTAS - Quantifying the impact of BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellites.