What gives hot tarmac that distinctive smell when it is rained upon, as on a summer day?

Answer 1

Part of it comes from the oils and grease loosened by the rain. That's also what makes highways so very slippery at the first rain after a dry spell. You're also subjecting those organic molecules to a large and rapid temperature change. It's also probably hot enough to cause chemical changes, hydrating some of those molecules. To get more detail, try asking in chemistry.