To get to your core comment, I agree that my analogy makes no claims about this scheme's efficacy. The posted article claims 25% to 45%, but who knows how idea

To get to your core comment, I agree that my analogy makes no claims about this scheme's efficacy. The posted article claims 25% to 45%, but who knows how ideal the conditions have to be to reach that level. You are also absolutely correct that treating at the tailpipe is even better (for exactly the reasons my analogy makes clear). That said, I will defend my analogy. My point was that dealing with NOx at the road surface will likely be more effective than dealing with it more generally in the atmosphere-- in part because the effective volumes aren't as huge as you might think (see the comment I was responding to), and in part because you're dealing with the NOx at higher concentrations. I'm sure you're right that NOx disperses rapidly, but I'd bet that concentrations still are demonstrably higher at the roadway surface than, say, a block away. For example, this study found a factor of 5 difference between NOx at the side of a busy intersection, versus that some distance away: http://www.atmos-chem-phys.net/10/2745/2010/acp-10-2745-2010.html http://www.atmos-chem-phys.net/10/2745/2010/acp-10-2745-2010... Second, I bet the boundary layer isn't all that thick at the road surface. After all, by definition you have cars passing along, stirring up a lot of turbulence. (Besides, even in quiescent conditions, you still get diffusive transport through the boundary layer, due to concentration differences-- and destroying NOx at the surface will set that up for you.)