A thick haze is hanging over the Valley of Mexico, which is home to the Mexican capital and the suburb of Ecatepec where Pope Francis is set to hold a huge outdoor Mass.
There's a notable smell of smoke in the air and skyscrapers in a central business district were obscured by the haze.
The Mexico City government says via Twitter that there was a grassland fire in San Salvador Atenco about 6 miles (10 kilometers) from the site of the papal Mass.
The government is reporting an overall air quality of "bad" and recommends that people suffering from asthma or other respiratory problems restrict their outdoor activities.
That's not unusual for the famously smoggy capital and falls short of its "very bad" and "extremely bad" ratings.
Pope Francis has arrived at the sprawling, gritty suburb of Ecatapec where he's due to say Mass for more than 300,000 people on a vast field at the edge of the Mexico City metropolis.
During the motorcade through Mexico City to catch his helicopter, the pontiff stopped his popemobile briefly to greet and hug a group of nuns.