4 ms·

One reason to battle the NOx at the road surface is that the concentrations there are higher, near where vehicles emit it. Thus you don't have to scrub as larg

One reason to battle the NOx at the road surface is that the concentrations there are higher, near where vehicles emit it. Thus you don't have to scrub as large a volume of air in order to get the same effect. One HN-oriented analogy is conditioning heat out of a data center. If you wait till the heat coming off your servers mixes uniformly throughout the entire room, you have to process a lot of air. If on the other hand you catch the air just as it comes off the servers, you have to process less air in order to remove the same energy. Pushing the analogy further-- at normal temperatures (for data centers and for the ultimate place they reject heat to, the outdoors), processing a small amount of air that starts at a high temperature can be done more cheaply than a large amount of air that starts very close to the temperature you want to maintain. By analogy, it's probably true that scrubbing a fixed amount of NOx is easier when you start at a high concentration, than when you have a greater volume at lower concentration.

revorad 16y ago
Your data center analogy would make sense if we were talking about treating the exhaust at the tailpipe. Once the NOx is out in the atmosphere, it's already too late. NOx disperses very rapidly. Further, think about the ratio of boundary layer (or even at a lower height) air volume to the surface area of roads. Plus, there is no suction happening here. It's literally just the air touching the surface of the road. I would love to see some numbers on the efficiency of this process, but my gut tells me it's pretty poor.
- dmlorenzetti 16y ago
  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.)

revorad 16y ago

Your data center analogy would make sense if we were talking about treating the exhaust at the tailpipe. Once the NOx is out in the atmosphere, it's already too late. NOx disperses very rapidly. Further, think about the ratio of boundary layer (or even at a lower height) air volume to the surface area of roads. Plus, there is no suction happening here. It's literally just the air touching the surface of the road. I would love to see some numbers on the efficiency of this process, but my gut tells me it's pretty poor.

dmlorenzetti 16y ago
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.)