Air Pollution, Aerosols and Clouds in the Climate System

Indoor or outdoor air may contain thousands or even millions of microorganisms and biological particles in just one cubic meter of air. These airborne particles are collectively referred to as bioaerosols. Examples of bioaerosols include viruses, bacteria, fungi, pollen, fragmented particles from microbial cells or insects, and by-products of living organisms (e.g. animal dander, insect excrement). The size of these particles generally varies between a fraction of a micrometer (µm) to approximately 30 µm. Bioaerosols may originate from numerous natural or man-made sources such as agriculture (harvesting, storage, composting etc) and industrial activities (manufacturing, food processing etc), indoor surfaces (ceiling, wall, carpets, house plants) and water treatment plants etc. Particle size is an important factor in determining risks associated with microbial contamination. In general, particles > 20 µm (fungi, algae, pollen etc) affect Region 1, Particles Modern airborne sampling o...

Aerosols are thought to have a cooling effect on the atmosphere, and therefore to have mitigated some of the expected global warming over this period. This is, however, a highly uncertain conclusion, in part because the total amount and vertical distribution of solar radiation that is absorbed by aerosol particles is imperfectly known. There was an interesting article in Nature (Vol-448, 2 August 2007) under “News and Views” by Prof. Peter Pilewskie . In the same issue, Ramanathan et al . (on page 575) report that the aerosol clouds above large regions of Asia actually cause as much warming as greenhouse gases — in contradiction, at first glance, to the notion of aerosol particles as a cooling agent. Figure 1. Smog drifts down India's populous Ganges valley and out into the Bay of Bengal. This is the source of 'atmospheric brown clouds' over the Indian Ocean, and the climatic effect of its constituent aerosol particles is investigated by Ramanathan and colleagues For more i...

Atmospheric aerosols in general and biomass burning aerosols in particular have recently attracted extensive interest owing to their ability to affect the climate on local to global scales. These climatic effects include a direct radiative effect due to the aerosols’ ability to scatter and absorb incoming sunlight, an indirect effect due to the aerosols’ ability to serve as cloud condensation nuclei (CCN), increasing the cloud’s reflectivity and lifetime, a semidirect effect which leads to reduction in cloud cover, owing to aerosols’ ability to absorb sunlight, changes in precipitation patterns, and export of pollutants and water vapor to the stratosphere. Therefore, it is important to assess human contribution to aerosol emissions, and to assign a source to both anthropogenic and natural aerosols, for understanding the respective contribution of different aerosol types to climate change. Levoglucosan (1,6-anhydro-β-D-glucopyranose) is a unique tracer for biomass burning sources in atm...

7Be and 210Pb are the natural radionuclides present in the atmosphere. Several studies have shown that useful information about transport, removal and residence time of aerosols in the atmosphere can be obtained from the measurements of 7Be and 210Pb. 7Be (half-life = 53.3 days) is produced by cosmic ray spallation of nitrogen and oxygen primarily in lower stratosphere and upper troposphere. Therefore, 7Be is used as a tracer of stratospheric or high tropospheric sources such as ozone, stratospheric bomb fallout debris, stratospherically injected volcanic components. In contrast, 210Pb (half-life = 22.3 years) is a progeny of 238U. The source of 210Pb is radioactive decay of 222Rn (half-life = 3.8 days), a noble gas, continuously emitting from soils to the atmosphere. Thus, 210Pb is a tracer of continental air as more than 99% 222Rn is emanating from the surface of continents. Half-life of both the radionuclides are long enough with respect to their residence time in the lower atmosphe...

Carbonaceous aerosols (organic and elemental carbon) in the atmosphere are produced by biomass burning and fossil fuel combustion but their relative contribution is not properly known. Radiocarbon (14C) is present in living and recently living material at an approximate concentration of one 14C atom per 10^12 ordinary carbon atoms (12C + 13C). This equilibrium amount is a result of the gain of 14C from its steady production by cosmic rays spallation reaction with nitrogen in the atmosphere (some fraction of which is taken up by the biosphere through photosynthesis in the form of 14CO2) versus the loss of 14C from its radioactive decay (5730 y half-life). However, 14C is absent from fossil fuels because of the ancient age of fossil carbon (due to radioactive decay to unmeasurably small amounts). This dichotomy is the basis of inferring the fraction of fossil carbon in an ambient aerosol sample by comparing its 14C content to that of living material, a methodology that has become increas...

People started quantifying the role of aerosol chemistry on radiative forcing. There is a nice study by Sussana Bauer from NASA about the effects of heterogeneous reactions among sulfate, nitrate and mineral dust aerosols on radiative forcing. Their modelling study (on global scale) suggest that the combined anthropogenic forcing of dust, nitrate and sulfate is -0.1 W/m2; however, excluding heterogeneous interactions leads to a 3 times larger negative forcing. Full abstract: Coating of mineral dust particles by air pollutants leads to core-mantle particles. These composite aerosols could interact differently with atmospheric radiation than the uncoated dust. In our simplified radiative calculations we assumed that a spherical dust core is covered uniformly by a liquid refractive material, such as sulfate or nitrate. Theoretical calculations of optical properties of such particles show that the single scattering albedo and the asymmetry parameter of core-mantle aerosols only differ sign...

In recent years, the role of atmospheric aerosols is being increasingly recognized both in climate system of Earth and in global biogeochemical cycle. Aerosols affect the radiative balance of the Earth directly by scattering or absorbing incoming shortwave radiation and indirectly by acting as cloud condensation nuclei, altering a temperature effect at the surface because of changes in cloud cover. In addition, aerosol particles are closely coupled to atmospheric chemistry as chemical reactions in the atmosphere are often accelerated on aerosol surfaces. Also, the chemistry of aerosols may alter their physical and optical properties such as size distribution (fine to coarse) due to interaction between acidic (sulpahte, nitrate) and alkaline (mineral dust) aerosols; single scattering albedo (because of coating of one type of aerosols over other); and surface properties (hydrophilic/hydrophobic), and thus, their direct and indirect effects on climate change. Long range transport and depo...