Air Pollution, Aerosols and Clouds in the Climate System

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...

"All models have assumed that particles are spherical and have chosen single values for the refractive index. If underlying model assumptions are inappropriate, then scattering, absorption and radiative forcing estimates will be incorrect." This is one the statement made by Tami Bond and Robert Bergstrom in the review paper titled ‘Light Absorption by Carbonaceous Particles: An Investigative Review’ published in Aerosol Science and Technology, 2006. I had chance to present this paper as class seminar in my ‘Atmos pheric Aerosol’ class teaching by Dr. Kirk Fuller during this summer. Let me tell you this, “this is one of the best written paper I ever read on carbon aerosols”. It reads very well and has almost all the information you need to know about atmospheric carbon aerosols and their optical properties. Personally, I congratulate and thanks Tami and Robert for putting together this wonderful paper. Below, I will briefly touch some important points from the paper. Motivati...

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...