When the environ-communists first went after automobile emissions, the 'fix' was to use catalytic converters which actually produced sulphur dioxide out the tailpipe (that rotten egg smell). When mixed with humidity and moisture produced as a by-product of internal combustion, the result was....sulphuric acid and some plant life dying next to major highways.
I don't trust a single 'fix' ever proposed by these environmentalist fools.
Oh, so that is what it was. Feeling bad now for all the times I beat the kids for breaking wind in the car. (JUST KIDDING)