It's interesting, because the acronym doesn't actually work any more. Rather than explaining what the acronym is, it's a consortium of groups, government and non-government organizations in Canada and the United States that have worked on providing policy advice on air quality issues.
NARSTO is the North American Research Strategy for Tropospheric Ozone, but they've expanded, and the “S” is in there for something else. They've expanded beyond ozone now.