Portland Air Pollution Mystery Solved With Tree Moss
![](https://media.wnyc.org/i/800/0/c/85/1/GettyImages-135791039.jpg)
A Japanese maple tree surrounded by moss in Portland, Oregon.
( Hai Huu Thanh Nguyen / Getty )
( Hai Huu Thanh Nguyen / Getty )
Produced by Cassandra Profita