Environment of Earth

September 16, 2009

CHEMICAL PROCESSES ON AEROSOLS

Filed under: Atmospheric chemistry — gargpk @ 3:19 pm
Tags: ,

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.

Molecule Probability
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,

hv

2CO + O2 —————-> 2CO2

TiO2, ZnO


hv

2N2 + 6H2O ————-> 4NH3 + 3O2

TiO2

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.

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