Why is California's air pollution so bad

Summer smog in CaliforniaCool roofs could increase ozone levels

Black is out in California. New or renovated house roofs must have a light surface so that they reflect incident sunlight and do not get too hot. New climate protection laws in the heat-plagued US state dictate that. And the usually white roof surfaces also make a difference, as Scott Epstein, atmospheric chemist and employee of an environmental agency that monitors air quality in the greater Los Angeles area, says:

"Cool roofs have several advantages. They soften the heat in the city and thus reduce thermal stress for the residents. This is a positive health effect. It also reduces the energy required for cooling the building. This leads to lower electricity bills."

Concentration of ozone could increase

But Epstein and some colleagues have now discovered: The cooling roof tiles and panels also have an undesirable side effect that was previously unknown. Not only do they reflect visible sunlight as desired. Most of them also reflect a certain amount of UV radiation, which is also contained in the solar spectrum. And that's bad. Because this could exacerbate another environmental problem: pollution with air pollutants.

"If you increasingly use materials that reflect UV light, the concentration of ground-level ozone could increase. That is a reason for us to be concerned."

Ozone is a gas that irritates the respiratory tract and is an integral part of summer smog. It is created photochemically. This requires the sun's UV radiation. It encounters nitrogen oxides in the air, breaks them down, and ozone is then formed in a subsequent reaction with atmospheric oxygen. A process that the white roofs also stimulate when they also reflect UV light. Chris Field, professor of environmental studies at Stanford University. He reviewed the new study from Southern California:

"When a roof like this reflects the sun's UV radiation, the light passes through the atmosphere a second time and produces more ozone. That's why I think the new study is important: It warns us of errors in the development of technologies for white roofs. "

Only a small increase in the values ​​is still a cause for concern

Scott Epstein and his colleagues anticipated the development in a computer model: If all future cooling roofs in the greater Los Angeles area reflect UV light, then the ozone concentration will increase everywhere. The increase in the peak values ​​is quite small. In the simulation it was a maximum of one to two percent. But summer smog is often stronger in southern California than anywhere else in the USA, and the limit values ​​for ozone are to be tightened further:

"In terms of health, the increase may be small. But it is becoming more and more difficult for us to adhere to the maximum ozone levels that will still be permitted in the future. Every little bit counts! We are already spending a lot of money today to add the smallest amounts of ozone from the atmosphere Obtain."

The solution: harmless, light-colored roof coatings

More cool roofs, more summer smog - but that doesn't necessarily have to happen. Because that is also stated in the study: There are light-colored roof paints and coatings that do not reflect UV radiation. That emerges from laboratory analyzes that are already available in isolated cases today. Chris Field therefore advises only allowing such uncritical materials:

"I think the study is so nice because it also offers a solution: We have types of cool roofs available that do not aggravate the ozone problem."