Most of the particulate material suspended in the atmosphere has very small size and so has a very large surface area per unit mass (around 1 million square meter per gram). Such large surface area offers considerable opportunity for the absorption of molecules from the gas phase. This is particularly true if these molecules have a low volatility. A substance having vapor pressure less than 10-6 Pa at ambient temperature will largely be adsorbed on the aerosol particles. Therefore, metals volatilized through volcanic or biological processes will probably end up attached to aerosols. The likelihood of surface reactions also increased by the large surface to volume ratio of aerosols. Generally, two types of reactions occur on aerosol: thermal reactions and photochemical reactions.
Thermal reactions: For describing thermal reactions on aerosol surfaces, following two surfaces have been common models of atmospheric aerosols:
(i) Sulfuric acid surface: Sulfuric acid is a liquid surface but acid covers the surface of many atmospheric aerosol particles so this is a good model. The effectiveness of sulfuric acid surfaces as sink has been investigated for a number of atmospheric trace gases. The effectiveness of surface may be measured in terms of the probability of reactions occurring on collision of the molecules of the gas with the surface. Such probabilities for some major atmospheric trace gases are given in the Table.
Table: Probabilities of reactions on collision of gas molecules with surface.
|Water vapor||2 x 10-3|
|Ammonia||>1 x 10-3|
|Hydrogen peroxide||7.8 x 10-4|
|Nitric acid||2.4 x 10-4|
For species like nitric acid or hydrogen peroxide, the absorption of the gas by sulfuric acid surfaces could be a sink of atmospheric gases as much important as the photolysis.
(ii) Graphite carbon surface: Absorption of gases by graphite carbon is well known. A gas like sulfur dioxide is readily absorbed and presumably oxidized on the surface. However, aerosol surface soon becomes saturated or poisoned. Absorption of gas molecules can not occur further unless there is some mechanism for ‘cleaning’ the surface. Thus it is difficult to visualize the mechanism of the removal of large amounts of a gas like sulfur dioxide from atmosphere by such a heterogeneous solid phase process.
Photochemical reactions: In addition to possibility of thermal reactions on particle surface subsequent to the absorption of the gas molecules, photochemical reactions are also possible. For example,
2CO + O2 —————-> 2CO2
2N2 + 6H2O ————-> 4NH3 + 3O2
The importance of these reactions in the atmosphere is not known. However, it is known that photo-assisted reactions on titanium oxide or zinc oxide desert sands lead to production of ammonia. It has been postulated that such reactions were the source of ammonia in the early atmosphere of Earth.