...S&P Global Ratings has assigned its 'A' long-term rating to Miami-Dade County, Fla.'s pro forma $145 million series 2017 aviation revenue refunding bonds issued for Miami International Airport (MIA). At the same time, S&P Global Ratings affirmed its 'A' rating on the county's aviation revenue bonds issued for MIA. The outlook is stable. Citibank N.A. will purchase the series 2017 bonds directly. There is no contingent liquidity risk from the direct purchase, in our opinion, because the purchase shares the same events of defaults and remedies as the airport's parity aviation revenue bonds. Bond proceeds will refund MIA's series 2007 bonds outstanding for debt service savings without extending maturities. The rating reflects our assessment of a large connecting hub airport with a niche market dominance that has produced steady financial results despite having a high debt load, additional debt needs, and a high airline cost structure. Key credit strengths, in our opinion, include:...