Novel idea: Cleaning the Air with Roof Tiles

Recently WUWT carried a story about “solar roads” which had a ridiculous premise with a big batch of “feel good” Eco-egineering that is technically and economically implausible. This idea for catalytic reduction of smog looks far more plausible and less expensive.

Students develop titanium dioxide roof tile coating that removes up to 97 percent of smog-causing nitrogen oxides

RIVERSIDE, Calif. (www.ucr.edu) — A team of University of California, Riverside’s Bourns College of Engineering students created a roof tile coating that when applied to an average-sized residential roof breaks down the same amount of smog-causing nitrogen oxides per year [produced by] as a car driven 11,000 miles.

They calculated 21 tons of nitrogen oxides would be eliminated daily if tiles on one million roofs were coated with their titanium dioxide mixture. They also calculated it would cost only about $5 for enough titanium dioxide to coat an average-sized residential roof.

Tiles with and without titanium dioxide

That would have a significant impact in Southern California, where 500 tons of nitrogen oxides are emitted daily in the South Coast Air Quality Management District coverage area, which includes all of Orange County and the urban portions of Los Angeles, Riverside and San Bernardino counties.

Last month, the research by the UC Riverside team – Carlos Espinoza, Louis Lancaster, Chun-Yu “Jimmy” Liang, Kelly McCoy, Jessica Moncayo and Edwin Rodriguez – received an honorable mention award for phase two of an Environmental Protection Agency student design competition.

A UC Riverside student team who worked on the project last year received $15,000 as a phase one winner of EPA’s P3 (People, Prosperity and the Planet) competition. That team consisted of William Lichtenberg, Duc Nguyen, Calvin Cao, Vincent Chen and Espinoza (an undergraduate then who is now a graduate student at UC Riverside).

Both teams were advised by David Cocker, a professor of chemical and environmental engineering, and Kawai Tam, a lecturer at the Bourns College of Engineering.

Nitrogen oxides are formed when certain fuels are burned at high temperatures. Nitrogen oxides then react with volatile organic compounds in the presence of sunlight to create smog.

Currently, there are other roofing tiles on the market that help reduce pollution from nitrogen oxides. However, there is little data about claims that they reduce smog.

Mini atmosphere chamber

The students set out to change that. They coated two identical off-the-shelf clay tiles with different amounts of titanium dioxide, a common compound found in everything from paint to food to cosmetics. The tiles were then placed inside a miniature atmospheric chamber that the students built out of wood, Teflon and PVC piping.

The chamber was connected to a source of nitrogen oxides and a device that reads concentrations of nitrogen oxides. They used ultraviolet light to simulate sunlight, which activates the titanium dioxide and allows it to break down the nitrogen oxides.

They found the titanium dioxide coated tiles removed between 88 percent and 97 percent of the nitrogen oxides. They also found there wasn’t much of a difference in nitrogen oxide removal when different amounts of the coating were applied, despite one having about 12 times as much titanium dioxide coating. There wasn’t much of a difference because surface area, not the amount of coating, is the important factor.

The current team of students, all of whom are set to graduate in June, are hopeful a new team of students will continue with this project and test other variables.

For example, they want to see what happens when they add their titanium dioxide to exterior paint. They are also considering looking at applying the coating to concrete, walls or dividers along freeways. Other questions include how long the coating will last when applied and what impact changing the color of coating, which is currently white, would have.

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59 thoughts on “Novel idea: Cleaning the Air with Roof Tiles

  1. The titanium dioxide coated tiles break down nitrogen oxides using catalytic reduction. But what does it break down the nitrogen oxides into, oxygen and nitrogen gasses? And how long do the tiles continue to work before they must be repainted?

  2. Louis says:
    June 4, 2014 at 11:50 pm

    ….But what does it break down the nitrogen oxides into, oxygen and nitrogen gasses? And how long do the tiles continue to work before they must be repainted?

    Read a but slower. The last part of your query is answered at the end of the article:

    are hopeful a new team of students will continue with this project and test other variables.

    For example, they want to see what happens when they add their titanium dioxide to exterior paint. They are also considering looking at applying the coating to concrete, walls or dividers along freeways. Other questions include how long the coating will last when applied and what impact changing the color of coating, which is currently white, would have.

  3. Why is someone wearing slippers in the lab? Tops of feet exposed around chemicals and heavy objects? OSHA would not approve.

  4. For example, they want to see what happens when they add their titanium dioxide to exterior paint. They are also considering looking at applying the coating to concrete, walls or dividers along freeways. Other questions include how long the coating will last when applied and what impact changing the color of coating, which is currently white, would have.

    http://www2.dupont.com/Titanium_Technologies/en_US/uses_apps/coatings/

    (…) In the areas of residential, commercial and industrial construction and remodel, you will find TiO2 white pigment in architectural paint, where it provides the ability of paint manufacturers to offer the wide variety of colors that are available today.

    (…) Automotive paint has never been more durable, with so many color variations available. Titanium dioxide white pigment is a key component of this paint. (…)

    Pleasure boats, cruise ships, shipping vessels, rail cars and airplanes – all of these modes of transportation utilize TiO2 white pigment. Similar to its uses in the automotive industry, TiO2 is used in various paint and coatings for these vehicles as well as the many higher value plastic materials that you will find.

    http://pubs.acs.org/doi/pdf/10.1021/ie50362a017

    Chalking of Titanium Dioxide Pigmentated Exterior Finishes
    R. E. Troutman , W. G. Vannoy
    Ind. Eng. Chem., 1940, 32 (2), pp 232–237
    DOI: 10.1021/ie50362a017
    Publication Date: February 1940
    Note: In lieu of an abstract, this is the article’s first page.

    Oh look, I can buy 48 hours of access to this 74-year old article for only $35!

    I think these intrepid researchers can find previous work to help them out.

  5. Titanium is quite a dangerous metal. If there are any atoms not oxydised it can explode into flames as it used to do for me when cleaning an electron evaporation machine for which I was responsible. Once oxydised, as in paint, you would probably need to paint the planet white as recommended by oblarny’s idiots.

  6. If TiO2 is such a good eliminator of Nitrogen oxides, shouldn’t it be used at source (eg, in auto catalytic converters) – or can’t it be formulated to handle the necessary temperatures? It is, I am told, slightly cheaper than platinum…

  7. Important Question:

    (…) They coated two identical off-the-shelf clay tiles with different amounts of titanium dioxide, a common compound found in everything from paint to food to cosmetics. (…)

    They found the titanium dioxide coated tiles removed between 88 percent and 97 percent of the nitrogen oxides. They also found there wasn’t much of a difference in nitrogen oxide removal when different amounts of the coating were applied, despite one having about 12 times as much titanium dioxide coating. There wasn’t much of a difference because surface area, not the amount of coating, is the important factor.

    Where Are The Control Samples?

    If how much is used doesn’t matter, then did they try it with none at all?

    You would test untreated tiles, straight from the box.

    If there was surface prep before applying the titanium dioxide, then you test prepped but bare tiles.

    I doubt they simply dusted the tiles and got the precise quantity and quality of coating they wanted. So the substance was in a binder to make it stick to the tiles. Then you test tiles with the just the binder applied, onto tiles that are or are not prepped as indicated.

    If a medical researcher said it didn’t matter how much of the active ingredient was ingested, the same beneficial effect was observed, I’d seriously wonder if the active ingredient did anything at all.

    They say this is a catalyst effect, so only the amount of surface area matters.

    But titanium dioxide is an additive in almost practically all colorized paints and coatings, and colored plastics, etc.

    If this effect is real, then won’t any exterior wall using titanium dioxide-containing paint, when exposed to sunlight, also be breaking down smog-causing nitrogen oxides?

    If the effect is real, why would any modern city have smog problems at all?

    I want to see their work. I want to see what were the controls.

  8. johnmarshall
    Paints already contain TiO2.

    Very true – glossy white paint contains a high percentage of Titanium Oxide. Titanium Oxide was famously the first ever brilliant white paint pigment – until its discovery, artists were forced to use slightly off white pigments such as zinc white.

    http://www.webexhibits.org/pigments/indiv/history/tiwhite.html

    In addition, most roofs are rapidly covered in a thin deposit of dust or even moss – it only takes a thin layer to prevent the air from reaching the roof tile. This also applies to paint on house walls – which is probably why painting walls white has no noticeable impact on pollution.

  9. The tiO2 in paint is non-catalytic rutile; the tiles are coated in anatase, which is catalytic.

    This idea is at least 25 years old.

  10. EPA’s P3 Program: People, Prosperity and the Planet, they sell you any crap. Just ignore their BS.

  11. Is interesting to compare the way the scientific community reacts
    to non-climate related discoveries;

    ” the announcements should be made after submission to journals and vetting by expert referees. If there must be a press conference, hopefully the scientific community and the media will demand that it is accompanied by a complete set of documents, including details of the systematic analysis and sufficient data to enable objective verification.”

    http://www.nature.com/news/big-bang-blunder-bursts-the-multiverse-bubble-1.15346

  12. (…) The tiles were then placed inside a miniature atmospheric chamber that the students built out of wood, Teflon and PVC piping.

    The chamber was connected to a source of nitrogen oxides and a device that reads concentrations of nitrogen oxides. They used ultraviolet light to simulate sunlight, which activates the titanium dioxide and allows it to break down the nitrogen oxides.

    Seriously, if you’re going to study a substance, then study just the substance.

    Grab large test tube and a two hole stopper. Use long glass tube that almost reaches the bottom, coat outside with the substance. That’s the inlet. Other hole gets a short tube, that’s the outlet.

    Purge the tube with nitrogen, then pump in the nitrogen oxides and any other atmospheric gases you think are important. Illuminate tube with ultraviolet light, measure intensity to check replication of test conditions. Test concentrations at outlet.

    Simple.

    The project, as done, seems more like something high school chem students could set up and perform, with help from the vo-tech students learning construction trades. And this is college work, that scored some EPA funds?

  13. Curious. The title says “roof tiles”, and “Garfy” is posting links to French sites featuring building materials made from hemp.

    Somewhere someone’s spam bots are suffering from a severe lack of proper context and content detection.

    This is funny. He’s advising the US readers as well to grow hemp, which currently isn’t legal by federal law.

    Let’s see what else gets posted.

    • when I say “hemp” I mean a different variety (you cannot smoke it) – and also obtain linen like with flax – and spread it under the tiles

  14. Not such a novel idea actually. It’s been around since at least 2007 when Italcementi patented a TiO2 cement that converted nitrogen oxide to soluble nitrate. When used in concrete it is called a photocatalytic or self cleaning surface.

  15. They should have removed a few tiles from roofs and tested them, to see if dust and sunlight had degraded their cleansing ability.

  16. Sounds interesting.
    Some questions.
    1. How well do these things work in stagnant air once the NOx near the surface has been eshausted? If there were much wind, there probably wouldn’t be a smog problem/
    2. I realize that there are a lot of tile roofs in coastal California, but are they really a good idea in earthquake country?
    3. Like Louis says, what are the reaction products? N2 and O2?
    4. Is there any chance of these things catalyzing unwanted reactions as well as the desired reactions?

  17. Garfy said on June 5, 2014 at 2:53 am:

    when I say “hemp” I mean a different variety (you cannot smoke it) – and also obtain linen like with flax – and spread it under the tiles

    You need background info:

    Feds make first step towards hemp legalization
    Published time: February 04, 2014 20:23
    Edited time: February 06, 2014 14:19

    The United States federal government may not be ready to sanction marijuana use, but a new agriculture bill is set to legalize preliminary stages of hemp production in states that allow the practice.

    A new farm bill, passed by Congress on Tuesday, would allow universities and state agriculture departments to establish industrial hemp growing programs. If these research programs go well, they could pave the way for commercial hemp farming to become a reality.

    Hemp is a plant in the same family as marijuana, though it lacks its cousin’s high levels of THC and is therefore much less potent. It’s used to create numerous products, including cooking oil, clothing, paper, and rope. According to the Associated Press, the US imported about $11.5 million in hemp products in 2011.

    Although hemp used to be grown in the United States, its cultivation was outlawed under the 1970’s Controlled Substances Act. Supporters of the decision’s review believe, however, that allowing states to move forward with industrial hemp will help the country gain a slice of a market that’s currently controlled by China.

    It’s easier to grow marijuana, in the states that have legalized it, than it is to grow hemp, which they banned when previously banning the growing of marijuana.

  18. “They also calculated it would cost only about $5 for enough titanium dioxide to coat an average-sized residential roof.”

    Typical mistake of confusing raw material costs with total costs. Hiring a contractor to coat ANYTHING (be it water) on your roof will cost more than 2000$. It’s the same mistake that made nuclear power to be percieved ‘too cheap to meter’ in the 1960s

  19. Since the vast majority of homes and other buildings are coated in paints containing serious amounts of titanium dioxide already, there probably is a strong anti-pollution effect going on already (assuming this paper is correct), and it would be hard to add enough paint to a modern building to increase the effect by more than a few percent.

    If you can see the color of the paint, the surface probably isn’t “coated” enough from dirt and mold to reduce the supposed effect.

    Of course, there’s always other things we could paint.

    I know…

    “TITANIUM DIOXIDE FREAKIN’ ROADWAYS!”

  20. TiO2 is used in making paper and plastics in addition to paint. it’s quite a common chemical.

    http://en.wikipedia.org/wiki/Titanium_dioxide

    TiO2 is already used for NOx reduction in Selective Catalytic Reduction which uses NH3 (often from urea) to convert NOx to N2 and H2O. http://en.wikipedia.org/wiki/Selective_catalytic_reduction
    Perhaps they are trying Non-selective catalytic reduction, such as is found in automotive 3-way catalysts. (NSCR in industrial settings uses ammonia).
    The NOx in the air has to be on the surface and likely in a certain temp range and/or, as it looks in the picture, with lots of UV light, if they are not just heating the tile. You have to have a pretty good air exchange at the surface to get all this to happen.

  21. arthur4563 says:
    June 5, 2014 at 4:34 am
    “Nobody around here has tiles on their roof. We use shingles, ceramic or, more often, asphalt”

    Perhaps the government should mandate everyone have tile roofs, seems the green thing to do.
    I would also hope that hurricane prone states get a waiver as heavy roof tiles can be very destructive when torn off and flying around in 100 mph winds.

  22. Let’s see, on topic it is worth remembering that nitrogen oxides, produced naturally by lightning, are the original source of nitrates in soil, and nitrogen in the soil is the chemical raw material used by plants to make protein, and that we are protein, not cellulose. So this isn’t an idea for the rural masses, this is strictly a big-city idea as only in places like Los Angeles are automotive N+O compounds overt pollution instead of fertilizer. And yes, Titanium Dioxide has long been the basis for very white paint. This is a good thing. Titanium is extremely abundant (as is oxygen), and as far as I know is not toxic. Indeed, I recall that its oxide is extremely stable.

    The more interesting question is what it does to e.g. SO_2. Sulphates in the air are not fertilizer, they are the source of acid rain in the bad sense (unbalanced by being fertilizer). Of course, they are also aerosols, and hence supposedly lead to increased cooling. It is also interesting to compare a TiO_2 tile or painted rooftop to the pollution-reduction capacity of one single tree:

    http://urbanforestrynetwork.org/benefits/air%20quality.htm

    I don’t think people realize that a significant fraction of the increased carbon dioxide in the air comes not from human activity, but from the fact that we’ve cut down the world’s forests and turned them into farmland, houses, tables, charcoal, etc. This has created a huge alteration in the natural carbon cycle, as growing trees are a very significant carbon sink. To quote from the article above:

    If every American family planted one tree, the amount of CO2 in the atmosphere would be reduced by one billion pounds annually. This equates to almost 5 percent of the amount that human activity pumps into the atmosphere each year.

    I don’t know how seriously to take this, but if taken at face value then the cheapest way to achieve global carbon neutrality isn’t to spend a gazillion dollars on renewable energy, it is for every American family to fund the planting of 20 trees. Since planting a tree costs nothing at all — trees kindly provide us with any number of seeds and nuts for the cost of picking them up off of the ground — the real cost of this is land. But even if this is exaggerated, there is little doubt that deforestation is a major anthropogenic cause of the increase in CO_2 worldwide, and that reforestation is a few million times more efficient than solar cells or windmills as a means to combat increased CO_2 than trying to starve civilization of energy (and has numerous side benefits as well, as trees do indeed eliminate a lot of actual toxic pollution on an annual basis).

    Finally, when I saw the article and Anthony’s lead in comments, I thought that he was going to suggest that we paint all the roadways with TiO_2, thereby increasing the mean albedo of what is (believe it or not, I worked this out with a pencil) a nontrivial fraction of the surface area of the US, especially around major urban centers, to substantially reduce the UHI effect. Of course, painted on roadways and rooftops, it would have an even larger effect. There is an enormous difference between the surface temperature above thick grass or trees (which use some of the incident sunlight to make sugars, evaporates water with some of it to maintain transpiration), black asphalt (which gets so hot one cannot walk on it in the summer daytimes), comparatively white concrete (which one can barely walk on) and the white lines drawn on the asphalt. I learned as a kid that when I walked in bare feet to the Skaneateles Lake swimming aspect in summer (which is not at all “hot” in upstate New York:-) that to cross the roads you either walked on the white lines marking the crossing or you burned the hell out of your feet.

    If we (as a society) simply added enough TiO_2 to concrete mix to cause it to come out bright white we would reduce the solar heating of all upward facing surfaces made with it by (I’m estimating) 40 to 50C compared to asphalt, 25 to 30C compared to ordinary concrete. Similar cooling would be obtained if all asphalt roof shingles were white. By “cooling”, I also mean “reduction in air conditioning bills”. I don’t know if people have any idea what the true cost of putting that cute (but dark) forest green shingle all over their roof is. Amortized over a 20 year lifetime, I’ll bet that it triples their cost relative to a bright white shingle in increased AC costs, at least in the south where I live. Ditto dark cars with black upholstery (I recently rode in one of those and its air conditioner couldn’t even keep up with the heat generated by the sun on the front dashboard as we drove — I quietly cooked).

    Covering your roof with solar cells can be an (unsubsidized) winner in terms of cost-benefit, especially in parts of the country where electricity is comparatively expensive. Obama and the EPA have just taken measures to ensure that this is basically “everywhere” in a future coming to you soon, so expect the long-awaited boom in solarization to start (ready or not) this year. A great time to invest in the technology, and you can buy the stock from Al Gore and the many, many people who have been quietly buying it up already while pushing for precisely this legislation to force it to have value. However, it takes over a decade to win the amortization of cost war, and quite a long time to win the energy war as well, as one has to pay back the energy cost of mining, smelting, making the cells, transporting them, and installing them before showing an energy “profit”. A bright white roof, OTOH, is an instant winner. Pigment is cheap, both dollar cheap and energy cheap. If there were some way of turning black asphalt (cheaply) white that would be a huge winner. Planting more trees is cheapest of all, and trees are themselves often economically valuable as a renewable resource for wood, non-commercial fuel, and food or just habitat for other things we find valuable.

    The thing is, I (like many skeptics) am sympathetic to a lot of the “Green” agenda. Reducing actual toxic pollution in our highly anthropogenically toxic environment is a super idea — one cannot eat fish from the Great Lakes more than once a month because of lead, mercury and PCBs dumped into the “great sewer” of the midwest industrial belt, and it will take a century or so before it “flushes” enough that one can. Giant bare asphalt parking lots lit at night with mercury vapor lamps that will inevitably release some of their mercury into the environment and that burn energy utterly pointlessly all night long while heating to broiling temperatures every sunny day are as offensive to my sight as to anyone’s. There is so much that we could do here — legislate a requirement that massive parking lots retain enough greenway to support both trees and watershed, paint or pigment the pavement of those parking lots white, and turn off the lights at night (or at least, but the lights on a motion detector so that they are on only on demand when humans are there to see or use the light). That alone would reduce the UHI effect of those parking lots on local temperatures, while shading the cars and sequestering all of that carbon and detoxifying all that pollution. The point is, it isn’t really about the carbon. Mankind’s largest anthropogenic effect on the climate might well not be the burning of carbon at all.

    rgb

  23. I’ve read the literature and researched photocatalysts for 2 decades. It’s a passion of mine. Some info

    ” They are also considering looking at applying the coating to concrete, walls or dividers along freeways”
    Ten years ago I submitted a proposal to the DC Council of Governments to do this. Didn’t go anywhere. I was focused on dividers and sound walls b/c of the wear on road surfaces AND because TiO2 can have the property of superhydrophilicity, which I didn’t think would do well for traction.

    I will also point out that researchers at Lamar University published results of auto exhaust breakdown after coating asphalt w/ TiO2 (c. 2007).

    TiO2 paint in paint is actually encapsulated in SiO2 in order to eliminate it’s catalytic effects. It is my hunch that TiO2 breakdown of the binder is why paint peeled so readily in the early 20th C, and why companies switched to Pb as a pigment.

  24. arthur4563 says:
    June 5, 2014 at 4:34 am

    Nobody around here has tiles on their roof. We use shingles, ceramic or, more often, asphalt.

    You can also coat your shingles with Titanium dioxide, or acyclovir. :P

    Bob Greene says:
    June 5, 2014 at 4:36 am

    TiO2 is used in making paper and plastics in addition to paint. it’s quite a common chemical.

    http://en.wikipedia.org/wiki/Titanium_dioxide

    TiO2 is already used for NOx reduction in Selective Catalytic Reduction which uses NH3 (often from urea) to convert NOx to N2 and H2O. http://en.wikipedia.org

    So, you can reactivate the catalyst by peeing on the roof. :P:P:P

  25. PhilCP says:
    June 5, 2014 at 4:20 am

    “They also calculated it would cost only about $5 for enough titanium dioxide to coat an average-sized residential roof.”

    Typical mistake of confusing raw material costs with total costs. Hiring a contractor to coat ANYTHING (be it water) on your roof will cost more than 2000$. It’s the same mistake that made nuclear power to be percieved ‘too cheap to meter’ in the 1960s
    _______________________
    Nope. Bubba would see his opportunity to have plenty of work and undercut the other guys. The first Mom and Pop that gritched to the local news “Gotcha” team about paying $2000 for a contractor who arrived, completed the spray job and left within 1 hour would smarten up the customer base.

  26. Beethoven

    Thanks for that Nature link. Another quote from that article that could easily be shifted to the climate debate:
    “The BICEP2 incident has also revealed a truth about inflationary theory. The common view is that it is a highly predictive theory. If that was the case and the detection of gravitational waves was the ‘smoking gun’ proof of inflation, one would think that non-detection means that the theory fails. Such is the nature of normal science. Yet some proponents of inflation who celebrated the BICEP2 announcement already insist that the theory is equally valid whether or not gravitational waves are detected. How is this possible?
    The answer given by proponents is alarming: the inflationary paradigm is so flexible that it is immune to experimental and observational tests.

  27. Ironically, titanium dioxide mineral is called rutile! Time to install new ruftiles.

  28. More white roofs would also reduce UHI. If I recall Obama’s first head of Dept. of Energy, Dr. Chu, wanted us to paint roofs and roads white to offset AGW.
    I do question how long long the roof coating could stay effective as an air cleaaner or as a reflector. If it requires an annual recoating, I doubt if it will be practical. But Tio2 is an interesting compound that is getting attention:
    An interesting side note is the ISS uses a TiO2 coated air filtration system utilizing UV lights to clean the air on the space station.

    http://spinoff.nasa.gov/ISSspin.html

    TiO2 coated surfaces are also used on earth to purify air and kill microbes:

    http://www.nature.com/srep/2014/140219/srep04134/full/srep04134.html

  29. TiO2 is well-known for use as a delustering agent in fibers, paints and other materials. It also is a great UV blocker. So, it is not new to use TiO2 in paints and coatings (eg., sun screens, until recently). Most of the basic chemistry is already known. One question is how to efficiently get the NOx in the air to contact the panels. You won’t be working with a closed, pumped system.

  30. This is an intriguing idea, but what about the rest of the nation that does not use tiles for roofing? Can it be built into traditional shingles? What about metal roofs? Wolfman’s also got a great point, how well will it work in the actual atmosphere? The bench scale testing is done, now they need something to simulate more realistic conditions, like the same test rig in a barn filled with NOx, with a couple of box fans that cycle on and off during the “day” and “night”.

  31. i seem to recall reading somewhere, likely here, that having large numbers of white roofs results in less rainfall, which is the last thing we need here in #Failifornia…

    if it will still w*rk if it isn’t white, they may be on to something.

  32. Lighten up, folks, these are just kids! Universities SHOULD be giving their students the opportunities to work on creative solutions to real-world problems (and yes, oxides of nitrogen do constitute a real health threat, as they contribute to atmospheric ozone concentrations proven to contribute to mortality and morbidity).

    Not everything that every student team accomplishes will change the world, but that isn’t the point. I congratulate them for their creativity and hard work on this project.

  33. Are your collective memories that short? TiO2 struck with sunlight is self cleaning due to ozone formation when activated with UV. So, if all of your roof receives sunlight, it should stay quite clean and moss free. I thought this was already covered here on WUWT, but I seem to be mistaken.

    http://www.greenoptimistic.com/2013/06/14/self-cleaning-compound-increases-the-levels-of-air-pollution-instead-of-removing-it/

    https://www.peakpureair.com/how-tio2-uv-photocatalytic-oxidation-pco-works

  34. rgb:

    In theory, your suggestions seem plausible (white roof/concrete vs dark asphalt), but there is one place on earth that it is put into practice and it does not work nearly as well as one would think – the Middle East. Ask any serviceman/woman who has served in AOR and they will tell you: Simply put – everything over there is white or lightly colored and it is still HOT AS HADES IN LATE AUGUST over there.

    On paper is one thing, in practice is something else entirely.

  35. “up to 97 percent …”

    Ah, it’s that number again!

    Does it have some mystical significance in the field of climastrology?

  36. If this idea gains traction, it will be BITTERLY fought by the “Solar Industrial Complex,” who have other ideas for what people should be putting on their roofs.

  37. This is old stuff, PPG makes self-cleaning glass using the same photocatalytic TiO nanoparticle process and ultraviolet (UV) light, the dirt literaly oxidises and washes off in the next rain. If I can get sqeeking clean windows by garden hose and it burns off some nitrogen oxides along the way I’m a happy camper.

  38. One or more of the US automakers back in the 1970′s proposed to the EPA using a heat-activated coating on vehicle radiators to destroy air pollutants, probably ozone, in southern California. The EPA refused to give the automakers any emission reduction credits, so the idea was dropped.

  39. rgb

    Planting trees and painting white have little effect on global climate. 71% of earth’s surface is ocean. Of the remaining 29% land, 97% is deserts, forests and agricultural land. You can only paint on the 3% urban land. You can plant trees on forests and agricultural land but forests already have trees and you cannot just convert wheat fields to tree plantations. Limited area to plant trees.

    I heard ideas like planting algae and floating white ping pong balls in oceans. But too expensive and will harm ocean ecosystem.

  40. The irrelevant statistic award of the day goes to:

    “They also calculated it would cost only about $5 for enough titanium dioxide to coat an average-sized residential roof.”

  41. If my memory serves me correctly; Barium Oxide used to be the standard bright white high reflectance paint, until TiO2 came along. And who knew that unoxidized Titanium, simply explodes. They should tell our aircraft designers to stop using it. And who would want to take the risk of having an explosive Titanium bolt holding one’s hips together. I knew Titanium was somewhat difficult to machine, but didn’t realize it simply explodes in the lathe chuck.

    I’ll have to go and check all my fly rods to see that all the Titanium line guides are in good shape, and not about to explode. But most of them are either black Titanium Carbide coated, or gold Titanium Nitride, when visibility doesn’t matter.

    The bright white reflective plastic cavities used in larger seven segment LED numeric digital displays are all Titanium dioxide filled. But in that case, they blow right past the infamous 97% consensus barrier, and achieve a minimum of 98% diffuse reflectance for most visible wavelengths.

    Was this a 4-H club paper, that arose out of how to paint pig sties, so they don’t look grubby ??

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