PhD. Student, University of Toronto
Biomass burning is the burning of vegetation and occurs in North America and Europe mostly in the form of wildfires. Every year, hundreds of forest fires burn throughout the northern hemisphere burning thousands of hectares of vegetation. The burning of vegetation emits a substantial amount of aerosols and various trace gas pollutants. Trace gases may have a considerable effect on climate. The emission of trace gases changes the chemistry of the atmosphere and the trace gases can act as greenhouse gases. Atmospheric circulation patterns have the ability to transport pollutants from fires in Canada, United States, Russia and Siberia to the high Arctic within several days.
The amount of each emitted trace gas is of great interest to atmospheric modelers as the amount of each species must be known in order to determine and properly model the chemistry of the atmosphere. Emission sources from fossil-fuel burning are generally located in populated areas and have relatively constant emissions throughout the year. There has been a substantial number of studies using either ground-based or satellite instruments to monitor human emissions of carbon monoxide (CO) and other fossil-fuel burning products. The measured emissions can then be extrapolated to other regions in order to constrain the budget on fossil-fuel emissions. Estimating the total emissions from biomass burning is somewhat more complicated. The number of wildfires that occur each year are highly variable and the emissions from each fire are dependent on the type of vegetation burned.
In August 2010, unseasonably high temperatures and drought led to hundreds of wildfires near Moscow, Russia. The smoke plumes from these fires were transported to the Arctic and detected at the Polar Atmospheric Research Laboratory (PEARL) located at Eureka, Nunavut. The Fourier Transform Infrared (FTIR) spectrometer at PEARL measures the solar absorption spectrum. From the measured absorption spectrum, the concentration of trace gases in the atmosphere can be determined and provides a means of monitoring pollutants in the Arctic. Transport of the smoke plume from the source of the fire to the Arctic can then be detected by enhancements above ambient levels of the trace gas of interest. Trace gas species such as CO, hydrogen cyanide (HCN) and ethane (C2H6) are abundant in biomass burning smoke plumes and have long atmospheric lifetimes, from several weeks to months.
A time series is shown above for CO measured at PEARL. The main sources of CO are fossil-fuel and biomass burning, while the main sink is due to reaction with the hydroxyl radical OH. The measurements of CO shown here illustrate the seasonal trends.CO is transported from lower-latitudes to the Arctic by the Brewer-Dobson circulation, and the largest abundances occur in the early spring as a result of its accumulation in the Arctic during the winter months. However, during the polar night, reaction with OH does not occur since sunlight is required. At the end of the polar night when sunlight returns to the Arctic, reaction with OH begins and CO abundances decline. During the summer months, biomass burning contributes to a considerable amount of CO pollution as can be seen by the large enhancements above background levels. Particularly evident are the Russian fires of July 2008 and August 2010.From the detection of these enhancements, we can confirm the source of the airmass from a trajectory model. The figure above shows the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model output. A backwards trajectory is shown illustrating the source of the airmass at different altitudes. The black box highlights the area of the August 2010 Russian fires near Moscow and confirms the source of the observed enhancement. This trajectory also gives an estimate of the travel time of the smoke plume of about 9 days.
Not only do FTIR measurements provide a means of monitoring trace gas abundances and fire pollution in the Arctic, but may also allow for the amount of each pollutant emitted at the fire source to be estimated. This is characterized by an emission factor which is the mass of a trace gas emitted per mass of burned vegetation. The abundance of each trace gas emitted during biomass burning are correlated. That is, the amount of a species like HCN or C2H6 emitted is proportional to the amount of CO emitted. The ratio of the abundance of each species to CO can be determined from our FTIR measurements to find what is known as the enhancement ratio. This enhancement ratio can then be converted to an emission factor using known emission factors for CO.
Emission factors are commonly used to estimate the amount of a trace gas emitted during a fire event which may then be represented in a chemical transport model. These models predict the evolution of a trace gas as it is affected by transport and chemistry and hence its impact on air quality and climate. Accurate knowledge of these emission factors are required to properly budget the sources of trace gases in chemical transport models. Measurements of biomass burning emissions provides a means of investigating the properties of these emissions, allowing for more accurate representation in chemical transport models and better assessing their impact on climate in the Arctic.