I always assumed it was mostly from the soot of diesel rigs. I know they have gotten cleaner in recent years but diesel is still pretty dirty. I remember an apa

I always assumed it was mostly from the soot of diesel rigs. I know they have gotten cleaner in recent years but diesel is still pretty dirty. I remember an apartment I had in SF near some of the muni lines that still ran diesel and it was absolutely disgusting outside the window.