However, isn't this just for hydrocarbon emissions?
I don't know for certain. I'd guess no, that California's authority extends to carbon monoxide and NOx emissions, as they are primary constituents of smog. If you really pushed me, I'd suppose that the federal statute gives California authority to regulate either "pollutants" or "emissions."
If the word is "pollutants," it seems to me that the state might have an uphill fight demonstrating that carbon dioxide is a pollutant.
If the statute says "emissions," the state has an easier time, case CO2 is a tailpipe emission and thus within the literal language. However, it's pretty clear that Congress's intent at the time the legislation was adopted did not include giving California authority to regulate CO2. (Indeed, for the last 30 years, the goal has been to convert the carbon monoxide into CO2.) Thus, one could argue that California still lacks the required power. It would, however, be a much harder fight.
Of course, all of the above is speculation, as I haven't actually looked at the statute. It is, therefore, worth precisely what you've paid for it.