The most important criterion is research experience. I will often take students with weaker grades if they have demonstrated talent for research. GRE scores (for EPS applicants) are a very minor consideration. Although the Harvard application form allows you to be ambivalent about which professor you wish to work with, in practice I get so many applications that I only look seriously at the ones that express a clear intent to work with me (your statement of purpose is important for this). I also look for students with clear interests in atmospheric chemistry modeling - if you think you want to do experimental work or climate dynamics then you shouldn't apply with me. I strongly encourage you to email me before your application to express interest, so that I can give you feedback on your preparation and also look out for your application. I am committed to gender and racial diversity in the group and specifically encourage applications from under-represented minorities.