Take A Ride On The Dark Side Of The Arctic

The sources of a climate heating pollutant in the Arctic have been deconstructed by our latest study [1], involving radiocarbon isotopes and atmospheric computer modeling. Black carbon aerosols are tiny atmospheric particles that originate from incomplete combustion of natural or human sources — e.g. wildfires or diesel car emissions — just like CO₂. Colloquially, black carbon is also referred to as soot.

Soot is a component of “fine particulate matter” (or PM). Its size is smaller than 2.5 micrometers (which is often referred to as PM_2.5) — or max. half as thick as spider silk. Having such a small size, it can easily enter deep into our lungs or even pass directly from the nose into the brain region. But soot is not only detrimental to our health [2], it also interacts with our climate. The dark particles absorb sunlight very efficiently. In scientific terms, we call this a strong positive radiative forcing, meaning that the presence of BC in the atmosphere is helping to heat the planet. “Positive” refers to the mathematical sign, not the feeling.

Soot’s radiative forcing is up there with carbon dioxide (CO₂), but the range of soot’s strength in changing the climate is much more uncertain than our robust knowledge of CO₂is [3]. Soot is most likely in the top three, next to CO₂ (undisputed no.1) and methane (CH₄aka natural gas). We can’t pin down the climate effect of soot as good as CO₂ for several reasons: soot emissions are more dependent on the amount of fuel burnt and the amount of soot emitted per unit of fuel burnt, which, in turn, depends on the fuel type (gas, liquid, solid) and other combustion conditions (technique). Unlike a gas (CO₂ and CH₄), soot (a particle) is unevenly distributed around the planet due to its much shorter atmospheric lifetime (days to weeks). This lifetime makes it a good target for prompt climate change mitigation — in theory. In practice, black carbon (or soot) is co-emitted with other gases and particles that potentially cool the planet (e.g. sulfate), depending on where they are in the atmosphere and how they interact with clouds or cloud-forming components.

In the Arctic, soot (and other light-absorbing impurities) depositing on the surface starts a vicious cycle by melting the snow or ice below and eventually uncovering the usually much darker surface (e.g. rock or open sea water), leading to more solar absorption and so on (we, scientists, call this a positive feedback, again due to maths). The Arctic — for many reasons — warms faster than the rest of our planet. A process called Arctic Amplification was discovered back in 1896 by the Swede Svante Arrhenius, better known for his works in chemistry (and nowadays as a distant relative of Greta Thunberg).

Now, to decrease soot emissions and mitigate its climate effect, we often use computer models. They are also the tool of choice when it comes to informing policy decisions. These Computer models are getting better by the day but have historically struggled to get seasonality and amplitude of soot concentrations in the Arctic right [4]. Part of the problem is, that these models get input from so-called ’emission inventories’. These inventories tell the model where, when and how much soot is emitted, kind of like instructions from a cookbook. But the different cookbooks don’t agree on the amount of soot that goes into our yearly soot cake. If we take a best estimate of global black carbon emissions, our cake has about the size (and weight; because of similar densities of limestone/granite and soot) of the great pyramid of Giza (~7000 gigagrams). But the range of estimates vary immensely (2000-29000 gigagrams; Figure 4). To correct these models and the emission inventories we rely on observational data. A process called reanalysis or nudging leads to improved model simulations.

Our study looked at several sites around the Arctic and collected fine aerosol particles during 1 consecutive year or more per site, for a total of 4 years. We analyzed the black carbon concentrations and determined the fuel-source types based on radiocarbon isotopes. Plants take up radiocarbon, naturally present in the atmosphere, by photosynthesis of atmospheric CO₂. All living organisms have thus more or less the same relative amount of radiocarbon atoms, we call it a similar ‘isotopic fingerprint’. Soot from biomass burning thereby has a contemporary radiocarbon fingerprint. When plants die, the radiocarbon atoms are left to decay. Radiocarbon’s half-life is 5730 years, which means that fossils and consequentially soot from fossil fuels is completely depleted of radiocarbon.

We found that soot sources had a strong seasonality, with really high contributions of fossil fuels to soot in the winter (75%) and moderate (60%) in the summer. Soot concentrations where roughly four times higher in winter than in summer. Concentrations of BC at the different stations were also relatively different from each other, but the sources were relatively uniform for all stations and almost in seasonal sync with each other (high fossil winter, low fossil summer). The model we used did really well in simulating soot concentrations, but a bit less well in simulating sources — better for fossil fuels than biofuels. And it showed that — in our case — 90% of BC emissions in the Arctic originated from countries north of 42°N.

This information gives even more support for concerted emission reduction for countries all around the Arctic, and beyond. It is important however to note, that our main focus on emission reduction should target (fossil-fuel) CO₂ emissions because they will affect the climate long after (several centuries) they have been emitted. And the reduction in these sources means a reduction in soot as well. Because soot is also a combustion product.

A reduction that targets soot specifically can be achieved by installation of particulate filters (retrofitting of old engines and stringent standard for new vehicles), shifting to cleaner fuels, burning techniques, or introduction and enforcement of inspection and maintenance programs to assure compliance with already-existing legislation.

References:

1. P. Winiger et al., “Source apportionment of circum-Arctic atmospheric black carbon from isotopes and modeling,” Sci. Adv., p. 10, 2019.
2. C. A. Pope III, “Lung Cancer, Cardiopulmonary Mortality, and Long-term Exposure to Fine Particulate Air Pollution,” JAMA, vol. 287, no. 9, p. 1132, Mar. 2002.
3.  T. C. Bond et al., “Bounding the role of black carbon in the climate system: A scientific assessment: BLACK CARBON IN THE CLIMATE SYSTEM,” J. Geophys. Res. Atmospheres, vol. 118, no. 11, pp. 5380–5552, Jun. 2013.
4. S. Eckhardt et al., “Current model capabilities for simulating black carbon and sulfate concentrations in the Arctic atmosphere: a multi-model evaluation using a comprehensive measurement data set,” Atmospheric Chem. Phys., vol. 15, no. 16, pp. 9413–9433, Aug. 2015.