I don't know what definition of carbon nanotube includes something with a long chain of carbon, but in general carbon nanotubes refers to tiny tube structures composed of aromatic carbon( carbon connected to 3 other carbons). Vehicle exhaust and other combustion processes can produce a bunch of weird structures consisting of aromatic carbon, some of which are tube shaped:

http://www4.ncsu.edu/~apgriesh/pubs/lagally_ast_2012.pdf