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Dust and Ozone
Photo: Dust and ozone—from urban areas such as Fresno—create problems for agriculture in California's Central Valley
WALLPAPER
A cloak of smog gives Fresno, California, a hazy look. Smog, a hybrid of the words "smoke" and "fog," is caused when sunlight reacts with airborne pollution, including ash, dust, and ground-level ozone.
Photograph by Sarah Leen

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