A form of harmful air pollution that often can't be plainly observed is now being tracked with a groundbreaking new tool developed by scientists that allows residents to see where it's coming from.

The late-afternoon air near Crissy Field in San Francisco was enjoyed recently by locals who squeezed in the last minutes of sunlight for a game of fetch with their dogs, windsurfing, or simply building a sandcastle. Among those hands deep in the sand was resident Vanessa Braunstein and her son Nicholas.

"I grew up in Los Angeles, so I feel the air in San Francisco is almost so much better," remarked Braunstein

"And it feels cool," added her 5-year-old child.

But when it comes to air quality, there's a lot more than meets the eye. There's growing concern over a tiny airborne threat - a pollu

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