J. Phys. Chem. 1988, 92, 3417-3421
3417
was included. A linear relationship means that a small change in SOz emission at a source results in a proportionate change in S042-a t a deposition site. In the discussion of Sampson's calculations in the acid deposition report,z4 emphasis was placed on the termination vs propagation aspects of the chemistry. It appeared that the termination reaction resulted in a significant loss of hydrogen radicals from the atmosphere, and hence a nonlinear SOz/S02- relationship. Stockwell's analysis indicated that the perturbations to the hydrogen radicals were not very great unless the SOzconcentration was relatively high as in a power plant or smelter plume. However, H202production is considerably increased. An important effect of the propagation mechanism seems to be that oxidant is directly generated; i.e., OH radicals are converted to H02. The H 0 2product is the source of atmospheric
rather than by an electron-transfer process. The SO5- can subsequently react with HS03- to produce sulfate. As discussed earlier; there does not appear to be a significant contribution from the addition mechanism in the gas-phase reaction of HOSOz with
H202
02.
HOz
+ H02
+
H202
+0 2
(12)
which oxidizes SOz to S042-via solution chemistry. Thus, the H 0 2 product itself contributes to further SO2 oxidation. Other and most hydrocarbons also atmospheric gases such as CO, 03, convert O H to H 0 2 ,but it is interesting to note that the gas-phase oxidation of SO2 not only conserves radicals but also generates
oxidant. As mentioned before, the complete picture is very complex and much more detail is given in ref 1 and 25 and other references cited therein. The mechanism for the oxidation of sulfur(1V) to sulfur(V1) by oxygen in aqueous solution provides an interesting contrast to the present gas-phase study. In solution the key intermediate analogous to HOSOZis the SO3- radical. The reaction of SO3with O2 has been shownz6 to proceed by an addition process
so
