I’ve just arrived in Moscow for a meeting — the subject will be soot.

You may hear it called black carbon or even elemental carbon. Scientists getting technical will call it the “light-absorbing part of particles suspended in the atmosphere.” Let’s just keep it simple and call it soot.* (More on black carbon.)

Soot: pollution that is prehistoric, pollution that sits

It’s almost certainly the most visible air pollutant. You have seen it in the black smoke of old diesel engines or a smoldering campfire.

It’s by no means new. We’ve been fouling the air with it for as long as we’ve had fire. Once we began to burn stuff to extract energy, we brought this form of pollution into our midst. You can find the stuff coating the ceilings of prehistoric cave sites.

It surely was a problem in the Middle Ages. The word used for it — soot — derives from the Old English sot or sooth, which dates back to the year 725, and is also apparently a cognate of the Middle English sittan, which means to sit, I suppose because the stuff settles out of the air and “sits” on stuff. And it was a problem in the 20th century. When I was a kid growing up in New York City, leaving a window open for a day would invite the nasty stuff to form a black layer on the windowsill.

And it’s still a problem in the very modern 21st century. Estimates put global emissions of soot at 7.6 million metric tons in 2000 [pdf], with today’s major sources being the burning of diesel, coal and biomass [pdf].

And yet while soot pollution is pretty ubiquitous, it’s not inevitable. It is produced by the incomplete combustion of fuels containing carbon. Make that combustion more efficient or capture it from the exhaust on filters, and you can largely eliminate this pollution.

Soot and the City

New York City, though still a work in progress, is a case in point. After the U.S. Army built the first solid waste incinerator there in 1885, NYC became home to so much incineration activity that “more than 110 million metric tons of waste — arguably more than any city in the world” — were incinerated there during the 20th century.

One result of all this burning was a huge soot problem, so much so that by the time the U.S. Clean Air Act of 1970 was passed and New York State limited soot-containing particulate pollution to comply, it was found that “emissions from municipal incinerators in NYC exceeded the new state standard by more than a factor of 6.”

Over the next several decades, between pollution controls and especially incinerator closures, the particulate pollution (including soot) from incinerators decreased considerably. (Proposals to re-introduce cleaner-burning incinerators in the city have met with major resistance because of their reputation as a major polluter.)

Both federal and state laws have reduced soot pollution from diesel trucks as well. Among the results of all this pollution control is a lot less soot on open window sills in the Big Apple — but a wipe with a tissue will tell you it’s not completely gone.

The trouble with soot

But is soot that big a deal? Given all our economic problems, can’t we just live with the stuff? The answer is not entirely.

The impacts of soot emissions are quite significant — soot poses major health problems when it gets into people’s lungs (see also here and here), and its ability to absorb light makes it a significant contributor to global warming. Lowering these emissions offers environmental and economic benefits.

A killer

As an air pollutant, soot or black carbon is implicated in hundreds of thousands of premature deaths.

According to estimates by the Environmental Protection Agency’s Susan Anenberg and co-authors, fine particulate pollution (of which soot is a component) is responsible for about 3.7 million premature deaths worldwide; halving black carbon emissions by implementing existing pollution controls and technologies could avoid 157,000 premature deaths.

A 2011 report by the UN Environmental Programme estimates that fast-tracking soot-reducing actions could avoid 2.4 million premature deaths by 2030. (Most of those avoided deaths would occur in East Asia where people are typically exposed to high burdens of indoor air pollution through the use of inefficient and outmoded cook stoves.)

A warmer

Red Square the night before the BC workshop.

Red Square the night before the BC workshop. Spectacular. Impressive carbon footprint too.

In the case of climate change, soot is a rather effective warmer of the atmosphere. But, as I’ve discussed previously, it’s different from greenhouse gases like carbon dioxide (CO2). For one, soot doesn’t warm through the greenhouse effect. It warms by absorbing sunlight and putting the energy from those rays (some of which would have been reflected back to space) into the atmosphere. That’s what is known as a direct effect.

There is also an indirect effect. When soot absorbs light and warms the atmosphere, it causes clouds to evaporate — fewer clouds mean even less radiation reflected to space and more warming.

And there’s more: when soot is emitted in polar regions and settles on ice and snow, it decreases the reflectivity of the ice and snow causing it to melt faster.

Another important distinction between soot and greenhouse gases is that it is removed relatively rapidly from the atmosphere (days to weeks as opposed to decades and centuries). And so soot is a regional pollutant and its warming effects are most strongly felt in the region where it is emitted.

This makes soot emissions in the polar regions such as Siberia especially problematic — they accelerate warming in the part of the globe that is most sensitive to warming, the polar regions. It’s estimated that the Arctic has been warming twice as fast as the rest of the globe and that “more than 15-55 percent of Arctic warming since the mid-18th century may be attributable to (soot or) black carbon.

Focus on Russia

Soot emissions from Russia, primarily from biomass burning and diesel exhaust, are not especially large — they’re about 16 percent of the Arctic’s black carbon emissions — but because they occur in or near the Arctic they are problematic from a climate point of view. And so the U.S. State Department in collaboration with EPA and the Energy and Agriculture Departments have initiated a cooperative program with Russia to understand that country’s sources of soot, its effects, and the opportunities for mitigating the sources and effects.

As one of the initiating steps in that collaboration, the State Department has asked the U.S. National Academies to form a scientific delegation to visit Moscow to participate in a three-day workshop on “Addressing the Challenges of Black Carbon in Russia” with our colleagues from the Russian National Academy of Sciences. And so I have just arrived in Moscow where I will be leading the American scientific delegation. I promise to take pictures and send a postcard — or at least a post (or Tweet) or two.

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End note

* Technically black carbon, elemental carbon and soot are not exactly the same. For our purposes this approach will suffice.