I want to make a quick note on this. The pm2.5_alt field still returns Alt-CF3 data for backward compatibility. However, you can retrieve Alt-CF3.4 data by using a pipe and specifying the constant. In this case, it would be: pm2.5_alt|3.4.
We have plans to add this to the documentation, but for now, you can find an official note about it here: What is the Difference Between CF=1, ATM, and ALT?