No it will not dissipate sitting under a airlock anymore then it will in a bottle which is very slowly over time.
This info written by Ben Rotter should help:
SO<SUB>2</SUB> can be lost from wine under a number of circumstances. Molecular SO<SUB>2</SUB> is volatile and some is lost from both juice and wine through vaporisation to the air, especially if the juice/wine is agitated. This loss is higher in wines stored in barrels. However, the quantity lost in this way is usually negligible.
SO<SUB>2</SUB> will be lost during alcoholic fermentation. This is partially through vaporisation with escaping carbon dioxide from the fermentation. At the end of fermentation it is common for a wine to possess zero to just a few milligrams per litre of total SO<SUB>2</SUB>, however significant deviations from this norm can be found.
Losses additionally occur through the oxidative protection of SO<SUB>2</SUB>. This is largely due to SO<SUB>2</SUB> reacting with hydrogen peroxide to form sulphuric acid. Interactions of SO<SUB>2</SUB> with quinones to form monosulphonates may also result in SO<SUB>2</SUB> loss [Lu Valle, 1952].
SO<SUB>2</SUB> is also lost in bottled wine [Ough, 1985]. Müller-Späth [1982] found that the total SO<SUB>2</SUB> had dropped by 20-30% after 5 years at 12°C in two bottled white wines. The rate of total SO<SUB>2</SUB> loss appears to be 2-3 times faster in reds than in whites [Ough, 1985]. Peynaud notes that SO<SUB>2</SUB> losses in bottle are a few mg/l per year [Peynaud, 1984, p.271]. The causes for SO<SUB>2</SUB> loss in bottle are numerous. SO<SUB>2</SUB> vapour may be lost through the cork, but this is not substantial under normal temperatures of storage. Oxidation of the SO<SUB>2</SUB> with oxygen in the bottle will certainly occur, but this reaction is very slow. Oxidation of the SO<SUB>2</SUB> by formerly oxidised phenols will lead to the production of sulphate and a loss in total SO<SUB>2</SUB>. SO<SUB>2</SUB> loss may also occur due to rearrangements in the reaction processes within the wine after long time periods, favouring redox/equilibrium reactions rather than kinetic reaction rates.
Given this information, it should be kept in mind that the total SO<SUB>2</SUB> is not the same as the amount of SO<SUB>2</SUB> that has been added to the must/wine, since some of the added SO<SUB>2</SUB> will be oxidised irreversibly to sulphate and some lost through volatisation.
Whenever losses occur, the equilibrium between free and bound SO<SUB>2</SUB> will re-establish, resulting in a small decrease in bound SO<SUB>2</SUB>.
Edited by: masta