The AFD said 1/4", which I was skeptical of. So I immediately came here to check and Rodney lists 0.4", which I find more believable.
The years listed did not include 1996, which I remembered as a summer of rain. But I didn't check the record.
Do you have similar data for Baltimore? How about Richmond or Raleigh?
My meaning....if it was meteor dust, as noted, it would not be confined to DC alone.
Fascinating either way.