The air in Mexico City — described by the U.N. in 1992 as the world’s worst — seems to be getting cleaner. Even as the number of cars and people picks up in the city, tougher environmental rules calling for cleaner fuels, catalytic converters on cars, emissions tests, and limits on industrial pollution have caused pollution numbers this winter to drop significantly. In the 1990s, simply breathing in the city was the equivalent of smoking two packs of cigarettes a day. An air quality index in 1996 reached 394 on a scale from zero to 500, with anything over 100 viewed as unsatisfactory; on Wednesday this week the index topped out at 69. Big challenges remain, but Nobel Laureate Mario Molina, who is helping to lead a Massachusetts Institute of Technology study on the issue, expressed hope that new technologies would offer solutions.