Onh22 h2o 2 nh3 co2

The neutralization of acidity by ammonia gas released into the air from livestock and from the use of fertilizers, and by carbonate ion suspended in air from the dust raised by farming activities, is the main reason why precipitation over the central United States is not particularly acidic, and similarly for regions of China. However, some acidification results from the ionization of the ammonium ion, NH4+, a weak acid, that is produced by ammonia neutralization:

NH4+^^NH3 + H+

Although the nitrate and sulfate salts initially are formed from acids in aqueous particles, evaporation of the water can result in the production of solid particles. The predominant ions in fine particles are the anions sulfate, S042~; bisulfate, HS04~; and nitrate, N03~; and the cations ammonium, NH4+; and hydrogen ion, H+. Aerosols dominated by oxidized sulfur compounds are called sulfate aerosols.

sulfate aerosol particle

On the west coast of North America, nitrate rather than sulfate is the predominant anion because more pollution results initially from nitrogen oxides than from sulfur dioxide, since coal mined in the western United States tends to be low in sulfur. In Great Britain, most of the fine particles in the winter months originate as soot from car exhaust and pollution from industry, whereas in the summer they arise from the oxidation of sulfur and nitrogen oxides.

If there is substantial ammonia gas in the air, nitric acid will react with it to form ammonium nitrate solid in the particulate phase. Recent simulations of smog formation in southern California indicate that although reductions in VOC concentrations without any change in NOx would reduce ozone formation, the production of nitrate-based particulates would actually increase because more nitrogen dioxide would then react to produce nitric acid and then nitrate ion. The simultaneous control of ozone and particulates presents regulators with a formidable challenge!

In summary, coarse particles are usually either soot or inorganic (soillike) in nature, whereas fine ones are mainly either soot, or sulfate or nitrate aerosols. Fine particles are usually acidic due to the presence of unneutralized acids, whereas coarse ones are usually basic because of their soil content.

Continue reading here: Air Quality Indices and Size Characteristics for Particulate Matter

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