Smog on Radar

While Nexrad radar is capable of spotting smog plumes, this is a particularly graphic example, since it maps a portion of Interstate Highway 5 as a line over 100 miles long.  

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Central California Nexrad, Sunday evening, Feb 16 '97
The large red spot at the southern end of the line appears to be Highway 5's merging with Highway 99 near the southern end of the long San Joaquin Valley in California. This bright spot persisted as the highway line's resolution window drifted north over time.
The short fork above center on the line is the HW 41 intersection at Kettleman City (heading north to Fresno).   The large bright white spot near that fork is south of Hanford, is persistent and of unknown origin to me.

Talking in private e-mail with (out-of-state severe weather) experts about this image there is consensus that the hard sharp white line is not smog, but is freeway traffic or freeway appurtenances such as signs and fences.   The fuzzy line fading out for several miles to the west of the freeway? The experts were not eager to comment on that directly, (none did) but one did say that if captured on 10 cm radar, it would look smeared to the downwind.   The majority of experts also doubted if 10 cm radar could capture smog.(??)

"I believe [they] have the right answer: AP off moving targets (especially semis), resulting from a strong inversion and almost-smart-enough clutter filters.   If this was return from smog (assuming _really_ big drops that the 10 cm radar could "see", which is unlikely), you would expect to see a much _different_ pattern of return, perhaps a bit more dense over the freeways (smeared downwind), definitely with "hot spots" near refineries and such, but generally space filling in the valleys.   There are _many_ smog/pollution sources in this area.   This line is much too sharp and intense to be smog in this badly polluted region."