California · County air quality · 2025
Sonoma County, California
Median AQI 9 (Good). 100.0% of monitored days rated Good, direct from EPA Air Quality System stations.
- 9
- Median AQI
- 100.0%
- Good-air days
- 0
- Unhealthy days
- 99%
- Cleaner than US counties
Sonoma County's 2025 annual median AQI is 9 (Good), with 100.0% Good days
According to the U.S. EPA Air Quality System, Sonoma County, California posted a 2025 median AQI of 9 (Good) across 365 monitored days, with 100.0% rated Good and 0 unhealthy or worse. This is an annual comparison, not today's advisory.
Data source EPA Air Quality System (AQS)
- Cleaner than US
- Sonoma County's 100.0% Good-day share places it cleaner than 99% of US counties in 2025.
- Closest median-AQI peer
- Kenai Peninsula County (100.0% Good, median AQI 9) is the closest nationwide median-AQI peer to Sonoma County.
- 2020–2025 trend
- Sonoma County's floor-qualifying Good-day share is improving from 2020 to 2025 (79.2% → 100.0%).
- Median AQI
- Median AQI: 9
According to the 2025 EPA AQS county release on PlainAirData, Sonoma County, California is #1 of 951 by lowest good-day share (100.0%, median AQI 9) at median-AQI #7 ≠ Ponce County, Puerto Rico #1 highest median AQI (3, 91.4% Good days).
Annual release change
What changed in the 2025 EPA annual release
- Sonoma County's Good-day share rose from 95.1% to 100.0% between the 2024 and 2025 EPA annual releases, a change of +4.9 percentage points.
- Sonoma County's median AQI fell from 19 to 9 between the 2024 and 2025 EPA annual releases, a change of -10 points.
- Sonoma County's peak single-day AQI fell from 72 to 39 between the 2024 and 2025 EPA annual releases, a change of -33 points.
Data updated August 2026
- Good · 365 d · 100.0%
How did Sonoma County's air break down in 2025?
Across 365 monitored days, 365 fell in the Good band (AQI 0–50) and 0 reached Unhealthy or worse. The worst single day peaked at AQI 39 (Good).
EPA AQS →Share of monitored days at AQI 0-50 (Good band).
Share of monitored days above AQI 150 (Unhealthy or worse). Bar maxes at 10% (worst-county ceiling).
Where Sonoma County sits nationally
COUNT-derived placement among 951 US counties with at least 100 monitored days in 2025. Sonoma County's Good-day share is 100.0% - cleaner than 99% of that comparison set.
Sonoma County Good-day share vs. every US county
Share of monitored days in the Good band (AQI 0–50) - EPA AQS annual release
100.0% Cleaner than 99% higher than 99% of 951 US counties
Each bar is a band; taller bars hold more US counties. The dashed line + filled bar mark this entry. Hover or tap any bar for its full count, share, and where it sits relative to this entry.
Source U.S. Environmental Protection Agency, Air Quality System (AQS) · 2025
Pollutant Breakdown
Days where each pollutant was the primary driver of AQI readings in Sonoma County.
- PM10 · 365 days · 100.0%
2024 daily-monitor rhythm in Sonoma County
Month-by-month median AQI from EPA day-level county monitors (366 reported days in 2024). Dec posted the highest typical readings (median AQI 34, peak 72); Sep was cleanest at median 9. Summer months (Jun–Aug) averaged median AQI 16 vs 32 in Dec–Feb.
Five-year seasonal record
EPA day-level county files · 2020–20241,827 daily readings show a recurring local pattern: Dec has the highest matched-month median AQI (42), while Jul is lowest (24). Each cell is that month’s median daily AQI, not a current-air forecast.
Lower AQI is better. The colors summarize EPA AQI bands; gaps mean the county had no day-level record for that month.
Health Impact Assessment
Sonoma County's air quality is rated Excellent based on 100.0% good air days. No unhealthy air days were recorded in 2025.
Air quality was acceptable on 100.0% of monitored days in 2025, with a median AQI of 9. Unusually sensitive people can check real-time conditions at AirNow.gov during high-pollution events.
The dominant pollutant is PM10, driving AQI on 365 of 365 monitored days.
5-Year Air Quality Trend
Air quality is improving in Sonoma County. Highest Good-day share: 2025 (100.0%, 365 of 365 monitored days).
| Year | Good | Moderate | USG | Unhealthy | Median AQI | Max AQI |
|---|---|---|---|---|---|---|
| 2020 | 290 | 69 | 1 | 6 | 36 | 199 |
| 2021 | 283 | 82 | 0 | 0 | 37 | 89 |
| 2022 | 286 | 79 | 0 | 0 | 36 | 82 |
| 2023 | 323 | 40 | 2 | 0 | 33 | 117 |
| 2024 | 348 | 18 | 0 | 0 | 19 | 72 |
| 2025 | 365 | 0 | 0 | 0 | 9 | 39 |
How does Sonoma County compare?
How Sonoma County compares
Data Sources
Primary Source: U.S. Environmental Protection Agency (EPA), Air Quality System (AQS), Annual AQI by County dataset, 2020–2025 (aqs.epa.gov). For current conditions (not shown here — this page is historical annual data), see EPA AirNow (airnow.gov). See our methodology for how AQI, median, and coverage figures are computed.
Informational only, not professional advice. Data from EPA AQS. See our disclaimer. Spotted a figure that looks wrong? Report a correction.
Source: U.S. EPA Air Quality System (AQS) Annual AQI by county summary files for Sonoma County, California · 2020–2025
Download this county's annual AQI row as CSV , with the same EPA fields and denominator shown on this page.
Every figure on PlainAirData is rendered directly from EPA AQS air-quality monitoring data, no number is typed in by an editor. This page draws directly on EPA AQS air-quality monitoring data, no figure is typed in by an editor. See our editorial standards & corrections policy, the methodology behind these numbers, or report a data error.