Thanks for bringing up this topic, and I would have appreciated it a few years earlier, however!
The important topic I am referring to is the apparent exclusive gas-phase formation of the molecule H2SO3, as correctly noted in Wikipedia on H2SO3, to quote:
There is no evidence that sulfurous acid exists in solution, but the molecule has been detected in the gas phase.[1] The conjugate bases of this elusive acid are, however, common anions, bisulfite (or hydrogen sulfite) and sulfite. Sulfurous acid is an intermediate species in the formation of acid rain from sulfur dioxide.[2]
The implication for acid rain formation has previously been noted, for example, in an MIT article, with cited Reactions (1) to (3) below:
$\ce{SO2 + .OH + M → .HOSO2 +M (1) }$
$\ce{ .HOSO2 +O2 → SO3 + .HO2 (2) }$
$\ce{SO3 + H2O → H2SO4 (3) }$
However, in this recent 2019 work: A New Mechanism of Acid Rain Generation from HOSO at the Air–Water Interface, some important chemistry:
The photochemistry of SO₂ at the air–water interface of water droplets leads to the formation of HOSO radicals. Using first-principles simulations, we show that HOSO displays an unforeseen strong acidity (pKₐ = −1) comparable with that of nitric acid and is fully dissociated at the air–water interface. Accordingly, this radical might play an important role in acid rain formation.
where the net photolysis of gaseous sulfurous acid (in addition to SO2) likely proceeds as follows:
$\ce{H2SO3 (g) + hv -> .OH (g) + .HOSO (g) }$
Supporting source: See Page S6,Table S2, Eq (1), Eq (2), Eq (5) and Eq (12) in this available supplement. Also, related results for the photolysis of nitric acid, to quote:
Here we present both field and laboratory results to demonstrate that HNO3 deposited on ground and vegetation surfaces may undergo effective photolysis to form HONO and NOx, 1–2 orders of magnitude faster than in the gas phase and aqueous phase. With this enhanced rate, HNO3 photolysis on surfaces may significantly impact the chemistry of the overlying atmospheric boundary layer in remote low‐NOx regions via the emission of HONO as a radical precursor and the recycling of HNO3 deposited on ground surfaces back to NOx.
Note, there is thus an implied similar possible acceleration in the rate of photolysis of gas-phase SO2 after becoming H2SO3 at the air–water interface.
The implications of the above chemistry is that in addition to the cited Reaction (1) above (which is a sink for the removal of the hydroxyl radical, that otherwise could be involved in an ozone depletion cycle), the UV photo-induced decomposition of also gaseous H2SO3 likely leads to more problematic radicals cited in the acid rain formation and even ozone depletion.
