For 2 days, from 16 to 17 September 2023, 10 cities in the United States had PM2.5 at least twice their usual for the time of year. The worst reading was Aberdeen, South Dakota, United States: 41.8 µg/m³ on 16 September, about 7.6× its usual 5.5.
No cause namedWe have no fire or smoke records for these days, so we only say what the measurements show.
Cities reached10in 1 country
Days216 to 17 September 2023
Worst day17 September6 cities counted
Furthest above usual7.6×Aberdeen
Day by day
Every measured city in the area, coloured by its PM2.5 that day; a dark ring marks a city at twice its usual or more. Bars: cities counted each day, coloured by their median level. Choose a day or press play; without JavaScript, the days link to that day's world ranking.
How it spread
Every counted city, in the order the episode reached them, 3 days before to 3 after. Each square is a day, coloured by that day's PM2.5; outlined: at twice the city's usual or more.
Daily means in µg/m³, over the days each city counted in this episode. Highest: its highest of those days. Usual: the city's median for the time of year of that day (its days within 21 days of the date, every year, leaving out the week around it). Above usual: its highest day against that, with a usual under 5 µg/m³ taken as 5.
Against other episodes in the United States
Number 98 of 130 episodes in the United States since our records began, by cities reached.
Bars: cities reached; colour: the level of the episode's highest daily reading. All 130.
Use this page
SmogRank, Northern United States, 16 to 17 September 2023: 10 cities with PM2.5 at twice their usual or more, 16 to 17 September 2023, smogrank.com/episodes/2023-09-16-northern-united-states (accessed 4 October 2026).
You're welcome to reuse a single chart, image or figure with this citation and a link. Terms.
An episode is a run of days when many cities close to each other (four or more within 350 km on a day, 8 or more in all) each had a daily PM2.5 mean of at least 25 µg/m³ and at least 2 times their usual for that time of year. Found in our own measurements, not picked by hand. How we find episodes. Values are each city's daily mean of its own monitors; causes are the usual kinds for a pattern like this, not a confirmed source.
