Can I go to LA chinese embassy in person to apply for visa, after I bought air tix?

Yes, you can apply in person from the Chinese consulate in LA. Documents you are required are original passport, application forms both A and B (you can download the forms here on this site, you are suggested to finish it in advance to safe time), passport photo, your US Green Card, air tickets. If you will stay with your friends, you should also provide an invitation letter from them with their identity in China, otherwise, you need to provide hotel reservations.