What is the additive in natural gas that makes it smell?

Hello willb4220 and thank you for your question.

There are several additives used in natural gas to make it smell but
Mercaptan is the most common.

"Because methane--and therefore, natural gas--does not have any odor,
the gas company adds a warning "rotten-egg" smell (mercaptan or a
similar sulfur-based compound) that can be easily detected by most
people.
http://www.nidcd.nih.gov/health/smelltaste/gasdtctr.asp

"Natural gas odorization with Mercaptan is mandated for public safety
by the Department of Transportation (DOT) at the point of residential
gas use and in larger service lines, especially when these lines pass
through or are in proximity to residential areas."
http://www.flowcontrolnetwork.com/PastIssues/jan2004/4.asp

Thank you for your question and if you need any clarification of my
answer, do not hesitate to ask.

Very best regards

THX1138

Search strategy included:
"natural gas"Mercaptan  site:.gov
://www.google.com/search?hl=en&lr=&as_qdr=all&q=+%22natural+gas%22Mercaptan++++++++++site%3A.gov

Also see:
http://www.onelook.com/?w=Mercaptan+&ls=a

The mercaptan group of alkaloids really are the most evil smelling
chemicals there are. As thx1138-ga has pointed out, methane, the
chemical of natural gas, is odourless and in fact Transco, who ?ship?
the gas could make it smell of strawberries if they wanted to but
instead use a stomach turning compound to alert customers to gas
leaks, even very small ones.

In the old days when gas was made from coal by the Gas Board, a
chemical called ?tetra hydro thiothin? was the alerting agent. For
those chemically minded this is a five membered ring with four carbons
and a sulphur. I don?t know however that the change over took place
when gas was first pumped from the North Sea or at a later date.

Norman

In the above comment, read 'aliphatics' for 'alkaloids'. I think it is
what Shakespeare referd to as the 'heat opressed brain'.

Norman