Mexico City officials want to install a carpet of green gardens on rooftops across the city, a plan they hope will reduce air pollution in a city with a notorious smog problem and little room for new ground-level gardens. Mayor Marcelo Ebrard hopes to add more than 500,000 square feet of green roofs by 2012, starting with city buildings. Leaders say the green roofs, which are part of the city’s larger $5.5 billion Green Plan, would reduce energy use for heating the buildings and also help absorb and filter the city’s air pollution. “It’s hard to increase green spaces in a city like this because there’s really no more green space,” said Tanya Muller, the city’s director of urban reforestation. “But almost all of the buildings in this city can support green roofs.” Mexico City hopes to match the success of Chicago, where more than 517,000 square feet of green roofs had been installed through 2007, according to the industry group Green Roofs.