can marriage be annulled if I discover my husband is not a legal citizen

Short answer: "probably not."  Citizenship is not required for a marriage, so its absence is not a ground for divorce.  Whether the facts and circumstances of your marriage leave it susceptible to annulment for fraud or otherwise is a question best delved into during a consultation with a family law specialist at this or another firm.  I suggest starting by reading the information posted here: http://willicklawgroup.com/annulment/