Hcfyk

I'm writing my dissertation and that involves typing a lot of $mathrmCO_2$. I'd like to make a macro for it that's a lot easier to type, but if I put newcommandco2mathrmCO_2, for example, it gives me the error that mathrm can only be used in math mode. But using newcommandco2$mathrmCO_2$ doesn't work either.

Is there a way for me to make a macro that makes typing CO_2 easier, even if it doesn't use mathrm?

I'm not sure if this is a question about how to use co2 as a macro name or just a question about how to subscript in text mode. If you use coo, the macro

newcommandcooensuremathmathrmCO_2

works fine in both text and math mode. If you're only planning to use the macro in text mode, COtextsubscript2 works.

documentclassarticle

newcommandcooensuremathmathrmCO_2
newcommandcoooCOtextsubscript2

begindocument

With ensuremath: Text coo and math $coo$ both look fine.

With textsubscript: Text cooo is fine, but math $cooo$ isn't.

enddocument

Don't forget the trainling and space after the call in text mode.

You want easy?

documentclassarticle 
usepackagechemformula
begindocument
chCO2
enddocument

I would use the mhchem package. It makes a lot of chemistry things easier, including reactions as well as formulas.

Put the line

usepackagemhchem

in the header section of your document, and then CO2 is just ceCO2. You can use that in math mode or text mode, it works either way. Here are some examples of other formulae from the package documentation, to give an idea of what you can do. (Note from the very first example how easy it is to write a simple reaction.)

(Note also that, as mentioned in John Kormylo's answer, there is also a package chemformula. It seems to be very similar, and allows you to do the same things in very similar ways. It is a more recent package, but I have no experience with it.)

Using isotope package with the macro cotwo you can write easily and quickly CO_2:

documentclassarticle
usepackageisotope
newcommandcotwo$isotopeCO_2$

begindocument

cotwo

enddocument