AMS Environmental Science Seminar Series

AMS Environmental Science Seminar Series

Contribution of Black Carbon and Atmospheric Brown Clouds to Climate Warming: Impacts and Opportunities
Black carbon (BC) in soot is the dominant absorber of visible solar radiation in the atmosphere. Anthropogenic sources of black carbon, although distributed globally, are most concentrated in the tropics where solar irradiance is highest. Black carbon is often transported over long distances, mixing with other aerosols along the way. The aerosol mix can form transcontinental plumes of atmospheric brown clouds (ABCs), with vertical extents of 1.8 to 3.1 miles. Because of the combination of high absorption, a regional distribution roughly aligned with solar irradiance, and the capacity to form widespread atmospheric brown clouds in a mixture with other aerosols, emissions of black carbon are the second strongest contribution to current global warming, after carbon dioxide emissions. In the Himalayan region, solar heating from black carbon at high elevations may be just as important as carbon dioxide (CO2) in the melting of snowpacks and glaciers. The interception of solar radiation by atmospheric brown clouds leads to dimming at the Earth’s surface with important implications for the hydrological cycle, and the deposition of black carbon darkens snow and ice surfaces, which can contribute to melting, in particular of Arctic sea ice. Presently, populations on the order of 3 billion people are living under the influence of regional ABC hotspots.
Black carbon (BC) is an important part of the combustion product commonly referred to as soot. BC in indoor environments is largely due to cooking with biofuels such as wood, dung and crop residue. Outdoors, it is due to fossil fuel combustion (diesel and coal), open biomass burning (associated with deforestation and crop residue burning), and cooking with biofuels.
Soot aerosols absorb and scatter solar radiation. BC refers to the absorbing components of soot. Dust, which also absorbs solar radiation, is not included in the definition of BC. Globally, the annual emissions of BC are (for the year 1996) roughly 8.8 tons per year, with about 20% from biofuels, 40% from fossil fuels and 40% from open biomass burning. The uncertainty in the published estimates for BC emissions is a factor of two to five on regional scales and at least ±50% on global scales. High BC emissions occur in both the northern and the Southern Hemisphere, resulting largely from fossil fuel combustion and open burning, respectively.
Atmospheric brown clouds are composed of numerous submicrometer aerosols, including BC, but also sulphates, nitrates, fly ash and others. BC is also internally mixed with other aerosol species such as sulphates, nitrates, organics, dust and sea salt. BC is removed from the atmosphere by rain and snowfall. Removal by precipitation, as well as direct deposition to the surface, limits the atmospheric lifetime of BC to about one (±1) week.
Given that BC has a significant contribution to global climate warming, and a much shorter lifetime compared with CO2 (which has a lifetime of 100 years or more), a major focus on decreasing BC emissions offers an opportunity to mitigate the effects of global warming trends in the short term. Reductions in BC are also warranted from considerations of regional climate change and human health.
Causal Link between Carbon Dioxide and Air Pollution Mortality
Recent research suggests that carbon dioxide, through its increase in temperatures and water vapor, increases U.S. air pollution deaths. This effect is greatest in locations where air pollution is already high. The causes of the increased death rate are increased respiratory illness, cardiovascular diseases, and complications from asthma due to increases in ozone and particulate matter. Ozone increases with more carbon dioxide because, in urban areas, higher temperatures and water vapor independently increase ozone through enhanced chemical reactions. These effects are not so important in rural areas. However, in rural areas, higher temperatures increase organic gas emissions from vegetation, increasing ozone slightly. Particles increase with more carbon dioxide because carbon dioxide increases air temperatures more than ground temperatures, reducing vertical and horizontal dispersion of pollutants. Furthermore, water vapor from carbon dioxide’s warming increases humidity, causing particles to swell and absorb more pollutant gases. Finally, more organic gases from vegetation result in more sticky gases that convert to particles.
It now appears that an estimated 1000 (350-1800) additional people die per year in the U.S. per 1 degree Celsius of temperature rise due to carbon dioxide. To date, global temperatures have increased by 0.8 degrees Celsius due to carbon dioxide and other greenhouse gases and absorbing particles. The same effect should hold for other greenhouse gases. This incremental death rate compares with a current air pollution death rate of 50,000-100,000 per year in the U.S. California is especially hard hit because it has 6 of the 10 top polluted cities in the U.S. California was found to suffer 30% or more of the additional fatalities due to carbon dioxide although it has only 12% of the U.S. population.
Recently, the U.S. EPA denied California’s request to permit the state to regulate carbon dioxide on its own, in part, on the premise that California did not have a special circumstance relative to other states; no studies had isolated carbon dioxide’s effect (as opposed to all greenhouse gases) on air pollution; and no studies had quantified the health impacts of carbon dioxide. The research described herein may well present an opportunity for EPA to revisit its original ruling in light of these more recent scientific findings.