Aerosol formation

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  • For millennia, advances in human progress have been tied to our ability to protect ourselves from the harmful effects of the wastes we produce—ranging from human waste to the organic and inorganic by-products of everyday living. Across the world, cultures learned to bury their dead away from their homes and to burn their waste or make certain that it was carried away by streams and rivers flowing downstream from their homes. Those cultures that learned this most effectively thrived. When the industrial revolution took place in the nineteenth century, rivers again enabled progress.

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  • These efforts have attenuated the emissions engendered by growth, but 24-hour PM10 (particulate matter with aerodynamic diameter less than 10 m) concentrations exceeding several hundred µg/m3 are still measured at many monitoring sites. 1,2 A persistent haze blankets the city, especially during winter, and there is great concern among residents and visitors about the effects of suspended particles on health. Aerosols that contribute to this visibility degradation are usually a combination of primary and secondary particles.

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  • Secondary aerosol is produced by the oxidation of primary gases (sulphur dioxide, SO2, nitrogen oxides, NOx, and volatile organic compounds, VOCs,) to sulphuric and nitric acid, and organic vapours, followed by their gas-particle conversion [26, 77]. Finally, some of these acidic gases can be neutralised by reaction with ammonia gas or calcium carbonate (calcite) forming secondary aerosol (ammonium sulphate, ammonium nitrate, calcium sulphate, calcium nitrate).

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  • The available data does, however, suggest that present PM10 values exceed the recommended limit values in the majority of Member States. In future projections, planned actions should be taken into consideration. These include the SOx and NOx Protocols within UNECE and the Auto-Oil programme. Abatement policies for other pollutants will also have an impact.

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  • After its emission or secondary formation, the length of time airborne matter will remain suspended in the air will depend upon its density, shape and size and meteorological conditions. Suspended particles are deposited by dry deposition, either by sedimentation and gravitational settling or impaction due to atmospheric turbulence and diffusion. This latter process is characteristic of particles which undergo Brownian movement and sizes below 0.1µm.

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  • Smog is another secondary pollutant. This term was developed to describe a substance that is a hybrid of smoke and fog. The SOx aerosols are one source of smog formation. As discussed earlier, sulfuric acid droplets, or sulfuric acid absorbed on the surface of soot and fly ash particles, can attract moisture from the air to form what is often referred to as conventional or ‘classical’ smog.

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