An MIT analysis of how best to reduce fine particulate matter in the atmosphere has brought some surprising results. Due to past regulations, levels of key emissions that form those harmful particles are now lower than they were a decade ago, causing some experts to suggest that cutting them further might have little effect. Not true, concludes the MIT study. Using an atmospheric model, the researchers found that new policies to restrict the same emissions would be even more effective now than they were in the past. Further analysis elucidated the chemical processes — some unexpected — that explain their findings. Their results demonstrate the importance of tailoring air pollution policies to specific situations and of addressing a variety of emissions in a coordinated way.