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Pop Citi CEO email into a search engine and within the first few hits you'll find email addresses to many of their execs, CEO included.
Either is fine. I'm a huge fan of the Saturation Technique, which is getting your letter into as many different hands as possible. Contacting the CEO just gets your letter to the Executive Office. Some have reported more favorable results through the EO with things like GW adjustments, others find front line CSRs easier to get something out of. The more avenues you try, the better your odds IMO.