Canada is updating their model to be the worst of PM2.5 and their traditional models of common pollutants.
Part of the issue is the existing model does an average over 3 hours, where PM2.5 can change rapidly (fire, traffic surge, &c). So they have a second metric that is averaged over an hour.
Part of the issue is the existing model does an average over 3 hours, where PM2.5 can change rapidly (fire, traffic surge, &c). So they have a second metric that is averaged over an hour.