Parts of Arizona, California, Idaho, New Mexico, Oregon and Texas all appear to have higher concentrations of submicron particulate matter, shown by darker red areas on the researchers' map, as below.

A map to show the use of 2,4-D pesticides in different US states. Uncredited/U.S. Geological Survey What To Know 2,4-D is used to kill broadleaf weeds in various places, from lawns to fruit and ...