SmogRank / United States / Colorado / Aspen

Aspen, month by month

Aspen has a measured daily mean on 776 days since 7 May 2023. Its worst month was August 2025, at 54 US AQI (10.5 µg/m³), 1,131st of 2,901 cities on that month's world ranking. Its cleanest was November 2023, at 2 US AQI (0.4 µg/m³).

Every month since 2023

202626 so far266 days measured2,691st of 2,791 in the world
Jan18#2,572Feb21#2,760Mar20#2,860Apr13#2,924May16#2,823Jun44#1,109Jul52#1,469Aug36#2,290Sep14#2,955Oct7 days
202524 for the year336 days measured2,604th of 2,681 in the world
Jan3 daysFeb17#2,607Mar20#2,723Apr19#2,857May16#2,768Jun23#2,387Jul36#2,135Aug54#1,131Sep21#2,973Oct13#2,601Nov17#2,657Dec25#2,792
2024too few days for a mean (a year needs 257)16 days measured
Jan16 daysFebnoneMarnoneAprnoneMaynoneJunnoneJulnoneAugnoneSepnoneOctnoneNovnoneDecnone
2023too few days for a mean (a year needs 256)158 days measured
JanFebMarAprMay25#1,478Jun20#1,525Jul33#1,109Aug18 daysSepnoneOct11 daysNov2#2,243Dec20 days
Each square is a month's mean PM2.5 in US AQI from Aspen's measured days, with its place on that month's world ranking under it. A month has a mean once 70% of its days are measured, the rule the monthly rankings use; under that, the square gives the days measured. Each square opens that month's ranking with Aspen marked; each year, the year's.
Good0–50
Moderate51–100
Sensitive groups101–150
Unhealthy151–200
Very unhealthy201–300
Hazardous301+

The US EPA's Air Quality Index for PM2.5 (2024 breakpoints). Show values as: µg/m³ · US AQI · India AQI · China AQI · European AQI · UK DAQI · Canada AQHI+. About the scales

Months without a mean, and why

18 months
MonthWhy
Oct 20267 of 11 days measured; a month needs 8. The others: 2 with no readings from the monitor, 2 with readings for part of the day (a day needs 18 hours).
Jan 20253 of 31 days measured; a month needs 22. The others: 1 with readings for part of the day (a day needs 18 hours), 27 with under 18 hours of readings or readings that failed our checks.
Dec 2024No day measured. The others: 31 with no readings from the monitor.
Nov 2024No day measured. The others: 30 with no readings from the monitor.
Oct 2024No day measured. The others: 31 with no readings from the monitor.
Sep 2024No day measured. The others: 30 with no readings from the monitor.
Aug 2024No day measured. The others: 31 with no readings from the monitor.
Jul 2024No day measured. The others: 31 with no readings from the monitor.
Jun 2024No day measured. The others: 30 with no readings from the monitor.
May 2024No day measured. The others: 31 with no readings from the monitor.
Apr 2024No day measured. The others: 30 with no readings from the monitor.
Mar 2024No day measured. The others: 30 with no readings from the monitor, 1 missing from OpenAQ's archive.
Feb 2024No day measured. The others: 28 with no readings from the monitor, 1 missing from OpenAQ's archive.
Jan 202416 of 31 days measured; a month needs 22. The others: 14 with no readings from the monitor, 1 with readings for part of the day (a day needs 18 hours).
Dec 202320 of 31 days measured; a month needs 22. The others: 10 with no readings from the monitor, 1 with readings for part of the day (a day needs 18 hours).
Oct 202311 of 31 days measured; a month needs 22. The others: 18 with no readings from the monitor, 2 with readings for part of the day (a day needs 18 hours).
Sep 2023No day measured. The others: 30 with no readings from the monitor.
Aug 202318 of 31 days measured; a month needs 22. The others: 7 with no readings from the monitor, 6 with readings for part of the day (a day needs 18 hours).

A day counts once 18 hours of it are measured; a month needs 70% of its days.

Aspen's worst months

MonthUS AQIDays measuredWorld rankEpisode
August 2025 5410.5 µg/m³31 of 311,131stof 2,901–
July 2026 529.6 µg/m³30 of 311,469thof 2,784–
June 2026 447.9 µg/m³24 of 301,109thof 2,497Utah and Colorado, United States, June 2026
July 2025 366.5 µg/m³30 of 312,135thof 2,889–
August 2026 366.5 µg/m³31 of 312,290thof 2,896–
July 2023 335.9 µg/m³30 of 311,109thof 1,432–

Monthly means over the days measured; finished months only. World rank: its place on that month's world ranking of every city with enough days. Each month opens that ranking.

Aspen's worst days

DayUS AQIEpisode
10 Aug 2025 15357.8 µg/m³–
2 Jul 2026 11139.7 µg/m³–
1 Jul 2026 10938.8 µg/m³–
28 Jun 2026 9432.4 µg/m³–
11 Aug 2025 8728.3 µg/m³–
24 Jun 2026 8426.9 µg/m³Utah and Colorado, United States, June 2026
9 Aug 2025 8426.7 µg/m³–
27 Jun 2026 8326.3 µg/m³–
6 Jul 2026 7723.1 µg/m³–
16 Jul 2025 7622.7 µg/m³–

Daily means, the highest since 7 May 2023; the WHO's daily guideline is 15 µg/m³. Each date opens that day's world ranking, and each episode the bad-air spell it was part of.

Use this history

SmogRank, PM2.5 monthly and daily means, Aspen, United States, 7 May 2023 – 11 Oct 2026. Monitor data from AirNow, via OpenAQ and from the publisher directly; sources listed at smogrank.com/sources. smogrank.com/us/aspen/history (accessed 12 Oct 2026).

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You're welcome to reuse a single chart, image or figure with this citation and a link. Terms. Raw monitor readings in bulk are published by OpenAQ, under each provider's licence.

Aspen now: the last 24 hours, its ranks today and the last 12 months day by day.

Updated 2026-10-12 18:07 UTC. Values are PM2.5 (fine particles), the median of each city's monitors, as the US AQI PM2.5 sub-index. Some low-cost readings: Sensor.Community, ODbL (data and licence). Some readings: Uzhydromet (Uzbekistan's Hydrometeorological Service); Department of Environment, Nepal; Vietnam's government monitors; Aurassure; Umhverfisstofnun (Environment Agency of Iceland); RMCAB (Secretaría Distrital de Ambiente, Bogotá); Love My Air (City and County of Denver), no reuse terms published (why we use them).