Three major airports serve the district, though none are within the city's borders. Two of these major airports are located in suburban Northern Virginia and one in suburban Maryland. The closest is Ronald Reagan Washington National Airport, which is located in Arlington County, Virginia, just across the Potomac River about from downtown Washington, D.C. This airport provides primarily domestic flights and has the lowest number of passengers of the three airports in the region. The busiest by number of total passengers is Baltimore/Washington International Airport (BWI), located in Anne Arundel County, Maryland about northeast of the city. The busiest by international flights and the largest by land size and amount of facilities is Washington Dulles International Airport, located in Dulles, Virginia, about west of the city. Dulles has the most international passenger traffic of any airport in the Mid-Atlantic outside the New York metropolitan area, including approximately 90% of the international passenger traffic in the Washington-Baltimore region. Each of these three airports also serves as a hub for a major American airline: Reagan National Airport is a hub for American Airlines, Dulles is a major hub for United Airlines and Star Alliance partners, and BWI is an operating base for Southwest Airlines. In 2018, the Washington, D.C. area was the 18th-busiest airport system in the world by passenger traffic, accumulating over 74 million passengers between its three main commercial airports; by 2022 it had climbed to 13th-busiest for passenger traffic, even though passenger numbers decreased to less than 69 million.