

Air-Purifying Road Surface Eats 45% of NOx Pollution - MikeCapone
http://www.sciencedaily.com/releases/2010/07/100706082058.htm

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cullenking
There are two problems with this that I can see right off the bat.

1) There is already a large problem with excess nitrogen runoff due to
fertilizers, which is causing massive algae blooms in oceans, destroying
aquatic life. This would have a similar effect from my understanding

2) Titanium Dioxide is kinda nasty to manufacture, in that it uses large
quantities of chlorine. Also, it has been classified as a potential
carcinogen.

With those being said, as long as the net effect is a reduction in harmful
pollutants or carcinogens, rather than an increase, it's a win-win. It's just
a bit difficult to understand that without probably rolling it out widespread.

~~~
hapless
Titanium dioxide is only suspected to be a carcinogen when inhaled in
considerable concentration.

As it stands today, I wouldn't want to breathe concrete or tarmac dust in high
concentration. Adding or refusing to add titanium dioxide will not alter my
feelings on that. :)

~~~
pasbesoin
I hadn't heard of this. Aren't many paints' pigmenting these days based on
TiO2? Would sanding painted surfaces pose a risk?

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mcritz
How nice would it be to have the negative aspects of vehicle use directly
affected drivers?

The roads in the communities where they drive become toxic. Maybe we can
create a diaper on exhaust ports of vehicles that is then scrubbed clean in
their own drinking water.

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defdac
Take the runoff water from the road with high nitrates and collect it in
ditches with algae or Hydrilla absorbing CO2. Convert the algae/aquatic plants
into bio diesel or dry it and make soil for plant nurseries.

Edit: Taken from Wikipedias Hydrilla article, "This abundant source of biomas
is a known hyperaccumulator of Mercury, Cadmium, Chromium and Lead, and as
such can be used in phytoremediation."

<http://en.wikipedia.org/wiki/Phytoremediation>

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sliverstorm
Can someone tell me, definitively, why we are trying to reduce NOx?

Yes, it comes out of tailpipes, and the stuff that comes out of tailpipes has
always been deadly.

Yes, in high concentrations, it probably plays a part in LA's smogscape of the
late 1900's.

Yes, my first car which made excessive NOx had a distinct smell and made me
feel happy...

But did you know that NOx uses sunlight to produce free oxygen in a catalytic
cycle? This free oxygen then reacts to produce ozone. Aren't we kind of having
an ozone shortage right now?

~~~
dmlorenzetti
Ozone near the earth's surface (which this road surface attacks) is a health
risk, and contributes to smog. Ozone higher up in the atmosphere (of the
famous "hole") blocks UV radiation.

<http://www.policyalmanac.org/environment/archive/ozone.shtml>

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sliverstorm
"...converts them with the aid of sunlight into harmless nitrate"

Genius! As soon as I read the article title, I realized the potential... Just
think of all the energy the black road surfaces around the globe absorb!

~~~
dmlorenzetti
Unfortunately, that energy is pretty low-grade and not very usable. Thus that
heat absorption mainly just contributes to the "heat island" effect, which
among other things increases air-conditioning costs. Better is to turn roads
and roofs into reflecting surfaces, which generally improves the quality of
urban areas.

<http://en.wikipedia.org/wiki/Urban_heat_island>

<http://www.epa.gov/heatisld/>

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revorad
Scrubbing atmospheric pollution off with paint on the road is one of the
stupidest ideas ever. Do they really think they can clean up such huge volumes
of air like this? Some local authorities in London have already wasted a load
of money trying to do this. Now the Dutch are at it.

~~~
dmlorenzetti
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
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
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...](http://www.atmos-chem-
phys.net/10/2745/2010/acp-10-2745-2010.html)

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.)

