Aerosol

Aerosols are commonly biological contaminants, particulate contaminants or dust. This diagram shows the size distribution in micrometres of various aerosols.

Technically, an aerosol is a colloid suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can. The word aerosol derives from the fact that matter "floating" in air is a suspension (a mixture in which solid or liquid or combined solid–liquid particles are suspended in a fluid). To differentiate suspensions from true solutions, the term sol evolved—originally meant to cover dispersions of tiny (sub-microscopic) particles in a liquid. With studies of dispersions in air, the term aerosol evolved and now embraces both liquid droplets, solid particles, and combinations of these. Earth's atmosphere contains aerosols of various types and concentrations, including quantities of:

• natural inorganic materials: dust, smoke, sea salt, water droplets.
• natural organic materials: pollen, spores, bacteria
• anthropogenic products of combustion such as: smoke, ashes oder dusts

By far the most common aerosols in the atmosphere are clouds, which normally consist of suspensions of water droplets or ice particles of greater or lesser density.

Aerosols can be found in urban Ecosystems in various forms, for example:

• Dust,
• Cigarette smoke,
• Mist from aerosol spray cans,
• Soot or fumes in car exhaust.

Effect on climate

Winds disperse vast quantities of dust (red), sea salt (blue), sulphate (white) and black and organic carbon (green) around the world.

Global aerosol optical thickness

Aerosols over the Amazon each September for four burning seasons (2005 through 2008). The aerosol scale (yellow to dark reddish-brown) indicates the relative amount of particles that absorb sunlight.

Some anthropogenic aerosols, particularly sulfate aerosols from fossil fuel combustion, exert a cooling influence on the climate which partly counteracts the warming induced by greenhouse gases such as carbon dioxide. This effect is accounted for in many climate models. Recent research suggests that aerosol diffusion of light may have increased the carbon sink in the Earth's ecosystem.

Recent studies of the Sahel drought and major increases since 1967 in rainfall over the Northern Territory, Kimberley, Pilbara and around the Nullarbor Plain have led some scientists to conclude that the aerosol haze over South and East Asia has been steadily shifting tropical rainfall in both hemispheres southward. The latest studies of severe rainfall declines over southern Australia since 1997 have led climatologists there to consider the possibility that these Asian aerosols have shifted not only tropical but also midlatitude systems southward.

Oxides of nitrogen (NO_x) in the atmosphere are a form of pollution which can give rise to smog and act as a greenhouse gas. Their persistence in the atmosphere is affected by aerosol droplets of water. In 1964 long chain fatty acids, either naturally produced from marine organisms dispersed into the atmosphere by wave action or man-made, were found to coat these droplets. In 2006 there was a study of the effect of the LCFA on the persistence of NO_x, but the long term implications, although thought to be significant, have yet to be determined.

Currently, the net effect of the most common short-lived aerosol pollutants on climate change is small, as the radiative forcing of different species nearly cancels. Reduced emissions of sulphur dioxide, leading to lower concentrations of cooling sulphates, combined with projected increases in emission of black carbon aerosols (assuming no worldwide shift to a green economy) will lead to a net warming effect from these species. In the long run, regional effects resulting from emissions patterns are projected to average out, with this warming following the geographic distribution of warming from well-mixed greenhouse gases.

Effect on India's climate

The effects of aerosol on climate was pronounced over central and northern India. It has led to a reduction in the monsoon rainfall over India in the past 50 years and above.This was published online by the journal Science. It is based on a study done by Massimo Bollasina of Princeton University in the U.S., along with Yi Ming and V.Ramaswamy of the National Oceanographic and Atmospheric Administration's Geological Fluid Dynamics Laboratory (NOAA-GFDL) at Princeton, used a state-of-the-art global climate model to simulate what was happening in the Indian ocean with and without human-generated aerosols. However, Indian scientists have expressed reservations and claim that the model used in this study has certain wrong assumptions. They support their claim by mentioning that the soot particles could coat with dust particles and sulfate could envelope particles of soot, but the assumption made is that the particles remain separate. Mixed aerosols have different characteristics compared to particles that are separate.

An introduction to aerosols: their varied sources, brief lifetimes and erratic behaviour (NASA)

Aerosol-contamination in northeastern India and Bangladesh.

See also

• Aerosol science
• Nebulizer
• Aerogel
• Charged particle
• Aerosol spray, the spraying device
• Bioaerosol
• Global dimming
• Particulates, suspended particulate matter
• Pollution
• Stratospheric Aerosol and Gas Experiment (SAGE)

External links

• American Association for Aerosol Research
• An on-line aerosol calculator
• Climate change and aerosols. Dr Jim Haywood of the Hadley Centre explains how aerosols affect the world's climate.
• NASA GISS: Science Briefs: Particles Multi-Task to Change Climate. Dr Dorothy Koch of the Goddard Institute for Space Studies explains how aerosols can act to both warm and cool the climate.
• Atmospheric Aerosol Properties and Climate Impacts - US Government
• CAPCO (www.aerosolproducts.org) is a nonprofit organization dedicated to providing accurate information about aerosol products
• The Aerosol Products Division (www.aboutaerosols.com) of the Consumer Specialty Products Association also gives facts and relevant information about aerosol products