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[hauseref]

I am trying to figure out how to calculate the amount of sulfuric acid evaporating to the atmosphere for environmental permitting. The sulfuric acid tank holds about 2000 gallons and is roughly 6 feet in diameter. The vent is 2" diameter. The tank is located outside.

[mbeychok]

You have not told us the acid concentration and the storage temperature. So, as an example, I will assume 90 weight % acid and storage at 30 °C (86 °F).  At those conditions, the vapor pressure of sulfuric acid is extremely low, as given in Perry's Chemical Engineers' Handbook:

Partial pressure of H₂SO₄ = 0.21 x 10^-7 bar
Partial pressure of water vapor = 0.12 x 10^-4 bar
Total vapor pressure = sum of the above partial vapor pressures

It has been my experience, when developing HAZOP studies of facilities storing sulfuric acid, that the regulatory agencies involved exempted us from having to calculate vapor losses from sulfuric acid storage tanks because the extremely low partial vapor pressure of the H₂SO₄ meant that the vapor losses would be essentially nil.