Aerosols: what they are, and new analysis on how they affect climate

Summary: new analysis indicates that the cooling effect of aerosols has been underestimated. Earth will warm as the air clears from lower fossil fuel use, and that warming is likely to be larger than previously estimated.

Aerosols is one of those confusing words that mean one thing in popular parlance (something to do with hair spray) and something different to scientists. To the latter, aerosols are the solid and liquid particles in the air, such as ash, soot, and smoke.

Regulators pay attention to particle size, eg, PM10 (particulate matter <10 microns, one millionth of a meter, in size) and PM2.5 (particulate matter <2.5 microns). Regulators are most concerned about PM2.5 (and China now accepts the obligation to measure PM2.5 after the embarrassment of reporting light pollution on a day when the US embassy recorded PM2.5 levels over 500 microgram per cubic meters, 6 – 10 times the level of the most polluted cities in the world).

As described by NASA,

Climatologists typically use another set of labels that speak to the chemical composition. Key aerosol groups include sulfates, organic carbon, black carbon, nitrates, mineral dust, and sea salt. In practice, many of these terms are imperfect, as aerosols often clump together to form complex mixtures. It’s common, for example, for particles of black carbon from soot or smoke to mix with nitrates and sulfates, or to coat the surfaces of dust, creating hybrid particles.

The bulk of aerosols—about 90 percent by mass—have natural origins. Volcanoes, for example, eject huge columns of ash into the air, as well as sulfur dioxide and other gases, yielding sulfates. Forest fires send partially burned organic carbon aloft. Certain plants produce gases that react with other substances in the air to yield aerosols, such as the “smoke” in the Great Smoky Mountains of the United States. Likewise in the ocean, some types of microalgae produce a sulfurous gas called dimethylsulfide that can be converted into sulfates in the atmosphere.

Sea salt and dust are two of the most abundant aerosols, as sandstorms whip small pieces of mineral dust from deserts into the atmosphere and wind-driven spray from ocean waves flings sea salt aloft. Both tend to be larger particles than their human-made counterparts.

The remaining 10 percent of aerosols are considered anthropogenic, or human-made, and they come from a variety of sources. Though less abundant than natural forms, anthropogenic aerosols can dominate the air downwind of urban and industrial areas.

Fossil fuel combustion produces large amounts of sulfur dioxide, which reacts with water vapor and other gases in the atmosphere to create sulfate aerosols. Biomass burning, a common method of clearing land and consuming farm waste, yields smoke that’s comprised mainly of organic carbon and black carbon.

Air pollution still visible at night
Air pollution still visible at night

No one wants to keep air pollution aerosols at the current level. Even as smoking rates have remained constant in Beijing, lung cancer rates have gone up 60% in the last decade, presumably from increases in air pollution in the last decades of the 20th century.

What effect are aerosols now having on climate change?
Aerosols are an important part of climate feedback, both direct and indirect. An article in the November 11, 2011 Science looks at the complications (subscription needed).

Aerosol feedbacks

Aerosol feedbacks The left edge of each box shows the average estimate of the effects of the changes we’re responsible for; the bar shows the range of estimates. There is a 10% chance that the effect is outside the range shown. All numbers are negative—they provide for a net cooling, although there is some reason to think that the second category might be slightly positive, that is, increasing Earth’s temperature. Since aerosols are in the air only a short time, if we stopped adding aerosols, Beijing would be much cleaner within aerosol residence time of days to weeks, and temperatures would begin to rise simultaneously.

The net warming of Earth caused by people is around 1.6 watts per square meter, as if every square meter on Earth had a 1.6 W bulb running 24 hours a day. The actual estimate is 0.6 to 2.4 watts/m2, with a wide range of assumptions about how much variation is normal. (1.6 W/m2 is almost identical with the warming due to carbon dioxide. Other contributions, positive and negative, currently cancel out.) See more numbers at end.*

The upper bar, aerosol direct effects, is the cumulative change from changing reflection (some aerosols reflect more light, some absorb more and heat up), with a mean estimate of -0.5 W/m2.

The indirect aerosol effect on cloud albedo (reflection) is more complicated, and there is a greater range of estimates of its magnitude. Aerosols change precipitation and cloud patterns, in a variety of ways:
• reducing evaporation
• suppressing cloud formation and redistributing droplets
• increasing precipitation (eg, India monsoons)
• altering atmospheric circulation patterns (which could have led to drying in the Sahel)
• creating more small drops

The average estimate for the cloud albedo effect is around -0.7 W/m2.

The third category includes a number of other indirect effects from aerosols, on a longer time scale than days to weeks. Here is the new, and preliminary, analysis presented in the article.

Non-cloud albedo indirect effects include:
• physical changes to the land or ocean which affect the rate that greenhouse gases are taken up or released
• chemical changes, such as nutrients or toxins, that modify biogeochemical cycles.

Aerosols cool Earth, so more of the atmospheric carbon dioxide is taken up. As long as we keep aerosols in the air, carbon dioxide levels remain 1 – 14 parts per million lower. Aerosols with nitrogen (and gaseous nitrogen) fertilize the soil in some areas, so that an extra 0.24 – 0.7 gigatonnes (billion metric tons) is absorbed each year, a decrease of 0.1 – 0.3 ppm CO2/year. Over 135 years, the net effect is to cool Earth, a change of -0.12 to -0.35 W/m2. Aerosols from burning tropical forests provide a source of phosphorous to phosphorous-limited vegetation—this may explain half the carbon dioxide taken up by the Amazon forest, a change of -0.12 W/m2.

Oceans are not nitrogen or phosphorous limited, but in some areas, iron in desert dust may have led to 4 ppm carbon dioxide taken up by oceans, a change of 0 to -0.14 W/m2.

Aerosols can decrease carbon uptake as well. Acidic aerosols, such as sulfates and nitrates—acid rain—can leach nutrients out of the ecosystem. Acid rain increasing ocean acidification, and toxic aerosols, decrease the rate at which carbon is absorbed by the ocean.

The overall magnitude of the indirect effect of biogeochemical cycles appears to be about the same as the direct effects.

If we stopped putting aerosols in the air tomorrow…
Aerosols currently cool Earth directly, through changing clouds, and by enhancing CO2 uptake by both land and the ocean. As fossil fuel burning become cleaner, and as aerosol production drops from reduced use of fossil fuels, temperatures will rise.

To keep atmospheric temperature increases low, decreases in carbon dioxide and greenhouse gases need to be larger than previously estimated. This means that addressing climate change will be much more expensive than previously estimated.

* More numbers
• Atmospheric carbon dioxide has changed from a pre-industrial value of about 280 parts per million to 390 ppm today.
• 2.13 gigatonnes carbon = 7.8 GT CO2 added to the atmosphere = 1 ppm. The world adds about 30 GT/year from fossil fuel burning and cement manufacture (just under 1/5 of this comes from the US), but about half is currently absorbed by the oceans and land.
• Intergovernmental Panel on Climate Change on contributions from various forcings (changes, natural and anthropogenic, from pre-industrial times that are forcing changes in the climate). If the analysis on indirect effects other than cloud albedo hold up, IPCC’s chart will look different in the Fifth Assessment Report (4 volumes, scheduled to be released between September 2013 and October 2014).

IPCC: radiative forcings

IPCC Frequently Asked Questions: radiative forcings. You can read more at FAQ 2.1

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