Mexico City officials, desperate to cut back on the air pollution that chokes the city, are encouraging citizens to cover their roofs with lightweight soil and plant gardens as a way to help control smog particles. The city has also given approval to a private company to install thousands of three-foot-tall air filter canisters on busy street corners, which the company hopes to pay for by selling advertising space on the filters. Some enviros are skeptical that either of these schemes will have much effect. Luis Guerra, who runs his own project to monitor the city’s air, says bigger solutions are needed, such as buying cleaner city buses. Big-ticket projects to help deal with air pollution, such as building an elevated train, have been stalled by a lack of funds and local opposition.