Conservation Science

220 Chapter 9

low as pH 3.5 from a combination of sulfuric (derived from sulfur dioxide)
and nitric acid (derived from nitrogen oxides). The acidic solution will react
with the limes (calcium carbonate) according to the reactions below:

Reaction 1.

Reaction 2.

Reaction 3.

Dry deposition refers to sulfation, the direct gas/solid reaction between sulfur
dioxide and limestone. This reaction does require alteration of the sulfur dioxide
to sulfur trioxide in the atmosphere as well as the presence of a high humidity
level (80% or more) as a catalyst for the reaction. The chemical reaction
involved is:

Reaction 4.Basic Reaction

Reaction 5.Reaction Stages

For both these depositional routes, the rate of reaction is dependent upon
environmental conditions. Temperature can enhance the reaction rates and
for sulfation, water vapour is essential for the reaction to even occur. Similarly,
the effectiveness of both reactions depends upon the availability of a reactive
surface. For exposed surfaces this is not as easy as it sounds. Surfaces are likely

First Stage CaCO (s) SO (g) CaSO (s)

Second Stage CaSO (s) O

32 3

32





→

((g)→CaSO (s) 4

SO (g) CaCO (s) H O(g) CaSO .2H O(s) CO (g)

Sulfur trioxide

332 → 422





Calcium carbonate water vapour

→Calcium sulfate carbon diooxide

2(H NO )(aq) Ca CO (aq) Ca (NO ) (aq)

HO(l) CO(g)

N

3 2 32232

22





→

iitric acid Calcium carbonate Calcium nitrate water

carbon





→

dioxide

Ca CO (aq) 2H SO (aq) Ca SO (aq) CO (g) H O(l)

Calc

2  32  422 → 42 22

iium carbonatesulfuric acid→Calcium sulfate carbon dioxideewater

Ca CO (aq) H CO (aq) Ca 2HCO

Calcium carbonate c

2  32  23 2  3 



+ 


aarbonic acid
Calciumbicarbonate