PurpleAir sensors. neighborhoods. One day of Cambridge, traced hour by hour through the particulate matter we cannot see.
Particulate matter smaller than 2.5 micrometers, PM2.5, slips past the body's defenses and lodges in the lungs and bloodstream. The World Health Organization recommends an annual average below 5 µg/m³. Cambridge sits at about µg/m³ across the days behind this map. Comfortable. But comfortable averages hide the shape of the day. The real picture lives in the hours.
Before the first commuter, Cambridge sits at a citywide median of µg/m³, near the World Health Organization's annual PM2.5 guideline of 5. This is what good urban air looks like. The map is almost monochrome.
The curve tilts upward through the morning. The hottest neighborhood, , sits at µg/m³. Not a rush-hour spike, just a gentle drift as the city wakes. The textbook two-peak diurnal pattern is barely there.
By noon the citywide average is µg/m³. Neighborhoods cluster tightly together. The spread between cleanest and most-loaded is barely µg/m³, a remarkably even reading across the thirteen districts.
Something unexpected starts here. As the sun warms the surface and the Charles river breeze picks up, air mixes vertically and particulates disperse upward. The citywide average falls to µg/m³, a % drop from the morning crest.
Urban-air textbooks predict a second peak from the evening commute. Cambridge does the opposite. The cleanest hour of the typical day is right now. reads µg/m³, the day's minimum. The river breeze keeps doing its work into the evening.
With the daytime mixing layer gone, concentrations begin a slow climb toward the early-morning baseline. The citywide average is back up to µg/m³ and rising. The cycle resets.
The choropleth above is now looping the entire day in seconds. Not the dramatic two-peak shape the textbooks promise. A quieter, gentler curve that ventilates itself by evening. The data has its own story to tell.