What's light green, sprayed on top of asphalt, and lowers the temperature of parking lots by 30 degrees If you went to school at Robert L. Duffy school in Phoenix, you'd know the answer.

Late last month, the charter school became the testing ground for "green asphalt," a concrete-based reflective covering designed to make asphalt surfaces cooler and more durable.

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Phoenix-based Emerald Cities installed the coating at the school to demonstrate the viability of the technology to city leaders and contractors, company CEO Sheri Roese said yesterday.

As part of its efforts to be carbon neutral, the school had its parking lot resurfaced to lower the energy needed for cooling adjacent buildings and make the parking lot more comfortable for students during lunch and recess.

On very hot days, temperatures of lots can get so hot--sometimes topping 200 degrees Fahrenheit--that the asphalt will begin to soften and offgas, Roese said. The lighter "cool pavement" can reduce temperatures by 30 degrees to 40 degrees, according to Emerald Cities.

The idea of using reflective materials to lower temperatures locally and as a tool against global warming is gaining ground.

Last month, the Department of Energy installed a "cool roof" on one of its buildings, a white-colored coating that replaced a roof in need of replacing. A cooler roof can mean a 10 percent to 15 percent reduction in the cooling load, DOE under secretary Cathy Zoi said in a blog.

In aggregate, cool roofs (PDF) can make a significant difference on energy use and act as a way to reflect heat back into space, according to researchers at the Lawrence Berkeley National Laboratory. They estimated that if just over three quarters of commercial buildings were covered with cool roofs, the reduction in air conditioning load would be the equivalent of taking a millions cars off the road, or 6 million metric tons of carbon dioxide per year, Zoi said.

Lawrence Berkeley now is working with the Oak Ridge National Laboratory and he California Energy Commission on the Cool Colors Project to research and develop cool-colored roofing materials.

Nano-engineered concrete Emerald Cities' Roese said she was inspired by the cool roofs' work at Lawrence Berkeley National Labs to make a reflective material for pavements.

The company has developed a line of coatings with colors ranging from gray to a sage green that can be used to cover existing asphalt. It is also working to develop a clear, reflective coating that would address concerns over glare on roadways, Roese said

"After researching it, we realized [reflective materials] would be a wonderful way to curb CO2 and heat and prevent smog [from offgassing]. It's just that people are not focused on it. They don't realize that color holds the key to some carbon emission and heat reductions," she said.

To make the coating, reflective pigments are dissolved in a special type of concrete engineered to be much stronger than traditional concrete, Roese said. This concrete is nano engineered to create a dense silica structure of tiny crystals that makes concrete stronger.

Because it's stronger than regular concrete, a thin layer, just a fraction of an inch thick, can be applied to surfaces and last for five to eight years, Roese said. The cost is one dollar per square foot, plus labor.

The coating can be sprayed on, rolled on with painting rollers, or pressed on with a squeegee-like applicator, depending of the thickness. The installation at the school was done with sprayers similar to those used by swimming pool professionals and house painters.

For municipalities, cool pavements can be part of sustainability efforts and address the urban heat island effect, where built-up areas are hotter than rural areas. Colors can be chosen for decorative purposes and put on pavement such as crosswalks or bike paths and even walls, said Roese, a former surface designer who worked with architects.

"When I saw you could paint with this and we could stop this march toward destroying the Earth, that was it. This is a calling for me," she said.