What if you could look at the plants in your garden in order to learn if the air around you is clean or dirty?
At the National Center for Atmospheric Research in Boulder, scientists have planted a garden that does just that.
It's called the Ozone Garden, and the plants in that garden react – visibly – when ozone levels are high.
"You start to see damage on the leaves. A bunch of little black spots," said Danica Lombardozzi, a postdoctoral researcher at NCAR and one of the garden's creators.
Ozone at the surface of the earth is an air pollutant that is harmful to people and plants. It is created when certain pollutants, often from combustion, combine, and it can cause breathing problems, especially for children.
There are four types of plants in the ozone garden, each selected for their sensitivity to ozone. In the garden, green shoots of milkweed, snap bean, potato and cutleaf coneflower spring from the ground. The coneflower was collected with a special permit from Rocky Mountain National Park, which has experienced ozone levels above national standards, and seen plant damage as a result.
"Some plants are going to be more sensitive than other plants," said Lombardozzi, adding that she chose these four because of their sensitivity.
What happens when ozone levels are high? The plants breathe in the ozone just like people, and reacts in a way that causes some of the chlorophyll cells in the plant's leaves to die, turning portions of the leaves black. The plants do not typically die, but the damaged leaves fall off and new ones eventually grow back.
"It's kind of like the canary in the coal mine," Lombardozzi said.
The effect isn't instant, though – the leaf blackening depends on how long the ozone is in the air and how long the plants are exposed. At the NCAR ozone garden, the plants still look healthy, but the worst ozone pollution usually comes in later July and August.
Visualizing An Invisible Pollutant
Current exposure limits for humans are set by the U.S. Environmental Protection Agency at 75 parts per billion, although the agency is considering lowering this limit. Sensitive plants can start seeing effects at around 40 parts per billion, said Lombardozzi.
In the summer, the Front Range often has air quality that is worse than EPA limits, said Kateryna Lapina, a research associate at the University of Colorado Boulder and the other creator of the garden. Lapina, an atmospheric chemist, works on modeling ozone.
"With gardens like this the whole idea is to visualize the negative effects of an 'invisible' pollutant, ozone, on living systems, both plants and humans, so everybody can now see and understand it," Lapina said.
Ozone Can Affect Crop Growth
Over the years, researchers have also documented that high ozone levels damage a number of plants, including reducing yields of important agricultural crops like soybeans, wheat, and cotton. High levels of the pollutant also slow down the growth of important Colorado tree species like ponderosa pine and Douglas fir.
Lombardozzi is researching how ozone will alter crop productivity in the future.
"Understanding the magnitude of this decrease is important for future food security in regions with high ozone concentrations," she said.
So can the average person plant some of these species in their garden and use them to monitor air quality around their home?
Lombardozzi says not quite. There are a lot of other factors that could affect plant health and ozone sensitivity, like how much water they get.
Editor's Note: Due to a typographical error, an earlier version of this story mistakenly reported the EPA standard for ozone at 72 parts per billion. It is 75 ppb.