Mexicali's particle pollution far worse than cities to the north

Mexicali's fine-particle pollution was far worse than in California cities to the north in 2017. Annual average concentrations of fine particles known as PM2.5, measured in micrograms per cubic meter:

SOURCE California Air Resources Board. NOTE Not all sites reported every day. MITCHELL THORSON, PAMELA REN LARSON, IAN JAMES, FRANK POMPA/USA TODAY NETWORK