Im not a chemist, but the smell is usually attributed to Sulphur Dioxide. Same stuff as was in stink bombs when I was at school!
It is a byproduct of the CAT until CAT warms up and is working properly.
Can also be simulated by pedestrians on crossings who have had extra large quantaties of eggs for breakfast though!
The smell is from other vehicles... if you stand behind your car and waft the exhaust fumes and dont smell anything then you have no worries.. if yuo do smell eggs then try using another brand of fuel and also maybe a cat rejuvinator (a mix of Rhodium, Platinum and other metals in alchohol).
The smell is a Hydrogen Sulphide compound caused by Sulphur Dioxide reacting with water and cracked hydrocarbons in the cat.
Quite why it then does not burn off to Sulphur Dioxide and water is a mystery to me.