TSA, which was created after the 9/11 attacks, has been chronicling the plunge in air traffic, posting numbers on how many people its officers screen each day. On March 1, it was nearly 2.3 million — almost the same as a year earlier. The one-way roller coaster ride — a sheer downward scream — began in the second week of March and slowed only in the last several days, when there wasn’t much more room to drop.