On March 18, 2008, residents across South Texas were surprised to find their cars coated with a thick film of tan-colored mud from the recent rain. The National Weather Service told the San Antonio Express-News that the material was ash from fires in Mexico. The San Antonio TV meteorologists reported the material was from a massive dust storm across northern Mexico.

Who was right?

Here’s a report about this event that I sent to Bryan Lambeth at the Texas Commission on Environmental Quality and Peter Bella at AACOG
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March 18, 2008

Bryan and Pete,

I swabbed material from the window using an alcohol pad and transferred some to a flat microscope slide.

The material is predominantly tan to translucent tan mineral dust ranging in size from sub micron to 20 microns.

The material includes some pollen and large black carbon particles, the latter being up to 20 microns across. A spherical nigrospora was noted. Various vegetative matter was observed.

The large black carbon suggests outdoor burning or, more likely, a brush fire origin.

Some pollen (there isn’t much) closely matches Quercus virginiana (live oak) based on my copy of E. Grant Smith’s “Sampling and Identifying Allergenic Pollens and Molds.”

Looks like I need to visit the USDA solar instruments at Texas Lutheran University that I manage for Colorado State before solar noon to remove this matter.
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The TV weather people got it right.

As for the sunlight instruments that I manage for Colorado State, they were coated! The visible wavelengths were attenuated by about 10 percent before I cleaned them.