Cities Are Spraying Asphalt With This Chemical to Cool Urban Heat Islands

As record high temperatures scorch much of the country, some southern cities are treating asphalt with a chemical that its creator says will cool down hot pavement and reduce urban heat island effects.

Pavement Technologies, Inc. is piloting a titanium dioxide-based treatment for hot, aging asphalt across cities like Raleigh and Charlotte, North Carolina, Orlando, Florida, and Greenville and Charleston, South Carolina. 

Billed as a “road rejuvenator,” the substance acts as a seal: It’s sprayed atop an existing layer of asphalt. The company says it penetrates deeply into asphalt to repair blemishes, reduce road temperatures, and dissolve airborne vehicle pollutant particles that surround roadways. 

The company is collaborating with researchers at Texas A&M University to collect data on the spray’s efficacy: It’s sending road cores (literal segments of asphalt from strips of treated streets) and air quality data to the university to determine whether the chemical does what the company expects it to. 

If the spray works as Pavement Technology claims, it could have the effect of cooling down roads and reducing urban heat island effects. Dark roads tend to absorb energy from the sun and turn it into heat, which is re-emitted and often trapped in the atmosphere, especially in regions with tall, densely-packed buildings where wind flow is minimal. Without much greenery to offer shade and absorb some of this heat, cities tend to be, on average, one to seven degrees warmer than the rural and suburban areas that surround them. 

Rooting out the beginning of this cycle at the source—roads—could make cities cooler, Pavement Technologies wagers. When hit by photons from the sun, titanium dioxide acts as a barrier, reflecting, scattering and absorbing UV rays and reducing the amount of heat roads trap. (Titanium dioxide is also a popular ingredient in sunscreens for the same reason.)

“That’s really going to shorten that time period of that high effect of the heat island,” said Ken Holton, technical consultant for Pavement Technologies.

This process also creates a chemical reaction called photocatalysis (one sped up by light)  that generates energized electrons that oxidize and break down pollutants from cars and trucks. This creates a “smog-eating” effect, which serves as a sink for pollutants the same way trees do (one mile of the stuff has the same impact as 20 acres of trees, the company claims).    

Though the spray is still in pilot phases, Holton says it’s so far been found to reduce levels of Nitrogen Oxides (NOx) by 30 to 40 percent. These pollutants, emitted from tailpipes, contribute to levels of smog and particulate matter, the latter of which can penetrate deeply into the lungs and cause lung irritation or long-term respiratory illness.

Alongside other southern cities, Charleston County rolled out A.R.A.‑1 Ti i in April in three regions. Charleston has worked with Pavement Technologies to dispatch a similar road preservation spray, Reclamite, every three to five years for around a decade. 

According to pavement manager MacKenzie Kelley, the substance has increased the lifespan of the Charleston roads by around five years, allowing the city to reduce how often it strips and replaces them entirely. So, when the opportunity to try out a new treatment arose, it felt like a no-brainer. 

“We're already using the rejuvenator payment preservation technique—that’s our main goal, is to protect our roads,” Kelley said. “But if there's an additional benefit, then why not? Charleston’s already a hot place in the summer.” 

“Clean and cool is kind of what we’re going with,” she says. 

Asphalt is made up of two key ingredients: Minerals, like sand, rock, or crushed stone sealed together with bitumen, a thick, viscous form of petroleum made up of carbon, hydrogen, oxygen and nitrogen molecules. Over time, these materials degrade and the bitumen that holds asphalt together loses what are called maltenes: An oily compound that makes asphalt flexible. That’s why newly-repaired roads feel smooth and bouncy, while older roads become brittle and crack. 

Reclamite replaces these maltenes, allowing asphalt to stay stronger for longer. That, in and of itself, has environmental benefits, Holton says.

“That’s my whole goal, is lengthening the life cycle of the road,” he said. “That way we don't have to use as much natural resources, or heavy construction, which helps us with our carbon footprint. So it's kind of a big layered effect. The longer something lasts, the lower the CO2 emissions.”