Initial Exceedance Analysis for Sept. 20, 2026

No prescribed fires were recorded within 50 miles of monitors exceeding the daily standard on the day, one day before, or two days before.

Monitors Exceeding PM2.5 Concentration of 35

Monitor AQS ID	Site Name	Location (Lat, Long)	PM2.5 Value	AQI Category
060431001	Yosemite Village - Visitor Center	37.75, -119.59	44.6	Sensitive Group

Air Quality Analysis

Monitor AQS ID

Air Quality Analysis

060431001

Hourly PM2.5 Analysis

At Yosemite Village - Visitor Center, hourly PM2.5 concentrations showed a decreasing trend on September 20 and September 21. PM2.5 levels exceeded 35 µg/m³ on 12 occasions, occurring at: September 20, 01:00 AM, September 20, 02:00 AM, September 20, 05:00 AM, September 20, 06:00 AM, September 20, 07:00 AM, September 20, 08:00 AM, September 20, 09:00 AM, September 20, 10:00 AM, September 20, 11:00 AM, September 20, 12:00 PM, September 20, 08:00 PM, September 20, 09:00 PM. Among these exceedances, 3 were significant pollution spikes (peaks), recorded at: September 20, 01:00 AM, September 20, 08:00 AM, September 20, 09:00 PM. Hourly PM2.5 levels showed a distinct day-night pattern. The highest recorded PM2.5 concentration was 110.5 µg/m³ at September 20, 08:00 AM, while the lowest was 7.7 µg/m³ at September 21, 10:00 AM. Unusual PM2.5 spikes detected at: September 20, 08:00 AM, September 20, 09:00 AM.

Daily PM2.5 Analysis of the Year

Throughout 2026, PM2.5 levels at Yosemite Village - Visitor Center exceeded 35 µg/m³ on 1 days, which accounts for 0.3% of the year. On September 20, 2026, the PM2.5 concentration reached 44.6 µg/m³, ranking #1 among exceedance events this year. This event matches the highest PM2.5 exceedance recorded this year. The majority of exceedance events occurred in September, with 1 occurrences. The season with the highest exceedance frequency was Fall, accounting for 1 exceedance days.

Historical daily PM2.5 Analysis

Between 2022 and 2026, PM2.5 levels at Yosemite Village - Visitor Center exceeded the 35 µg/m³ daily standard most frequently during Summer, accounting for 29.6% of all exceedances. The month with the highest number of exceedances was May, making up 10.9% of the total. The exceedance recorded on September 20, 2026 at Yosemite Village - Visitor Center reached 44.6 µg/m³, which is 31.6 µg/m³ higher than the average Summer PM2.5 level of 13.0 µg/m³. This exceedance is notable but does not rank in the top 10 highest exceedances recorded from 2022 to 2026. These exceedances are often influenced by seasonal factors such as wildfires, prescribed burns, stagnant air, temperature inversions, or increased local emissions.

Average Hourly PM2.5 for Days Around Event

Time series of 1-hour PM2.5 concentrations at Yosemite Village - Visitor Center starting on Sept. 20, 2026. The number "1" indicates the highest 1-hour PM2.5 concentration of the day, which is also the time used for HYSPLIT backward trajectory analysis.

Historical 24-hr PM2.5 Concentration

Time series plot of 24-hour PM2.5 concentrations at Yosemite Village - Visitor Center for each year between 2022 and 2026. The colored dots represent the 24-hour PM2.5 concentrations on Sept. 20, 2026. The black line indicates the 5-year average 24-hour PM2.5 concentration, and the grey dashed line represents the 35 µg/m³ 24-hour PM2.5 standard.

Prescribed Fires within 50 Miles

Monitor AQS ID	On the Day	One Day Before	Two Days Before
060431001	No fires	No fires	No fires

Wildfires within 100 Miles

Monitor AQS ID	2-Day Range to Exceedance Date
060431001	Wildfire: Schell Fire Acres Burned: 12.00 Date Created: Sept. 17, 2026 Date extinguished: Sept. 18, 2026 Date Last Updated: Sept. 17, 2026, 9:49 p.m. Wildfire: Davis Fire Acres Burned: 10.00 Date Created: Sept. 17, 2026 Date extinguished: Sept. 18, 2026 Date Last Updated: Sept. 17, 2026, 9:50 p.m. Wildfire: Dome Fire Acres Burned: 2186.00 Date Created: Sept. 15, 2026 Date extinguished: None Date Last Updated: Sept. 18, 2026, 9:36 p.m.

Monitor AQS ID

2-Day Range to Exceedance Date

060431001

Wildfire: Schell Fire
Acres Burned: 12.00
Date Created: Sept. 17, 2026
Date extinguished: Sept. 18, 2026
Date Last Updated: Sept. 17, 2026, 9:49 p.m.
Wildfire: Davis Fire
Acres Burned: 10.00
Date Created: Sept. 17, 2026
Date extinguished: Sept. 18, 2026
Date Last Updated: Sept. 17, 2026, 9:50 p.m.
Wildfire: Dome Fire
Acres Burned: 2186.00
Date Created: Sept. 15, 2026
Date extinguished: None
Date Last Updated: Sept. 18, 2026, 9:36 p.m.

Fire Emission Analysis

Monitor AQS ID	Emission Analysis
060431001	On September 20, 2026, an analysis was conducted to assess the potential impact of nearby fire activity on PM2.5 levels at Yosemite Village - Visitor Center. 3 wildfires were detected within 100 miles of the site from two days before the event to the event day. The proximity of these fires to the site suggests they may have contributed to elevated PM2.5 levels on this date. The following wildfires were identified: Dome Fire, which burned 2186.00, started on September 15, 2026, and last updated on September 18, 2026. It was located 12.4 miles away. Davis Fire, which burned 10.00, started on September 17, 2026, and ended on September 18, 2026. It was located 88.3 miles away. Schell Fire, which burned 12.00, started on September 17, 2026, and ended on September 18, 2026. It was located 59.7 miles away. No prescribed fires were identified within 50 miles of Yosemite Village - Visitor Center. In addition to reported prescribed fire and wildfire records from CAL FIRE and the Prescribed Fire Reporting System (PFIRS), satellite imagery detected 19256 fire hotspots over the past two days (from two days before the event to the current day) within 100 miles of the site. This indicates ongoing fire activity in the region, which may have contributed to the observed air quality conditions. Satellite data confirms the presence of smoke plumes over the monitoring site, suggesting that emissions from nearby fires were transported into the area, potentially impacting PM2.5 levels.

Monitor AQS ID

Emission Analysis

060431001

On September 20, 2026, an analysis was conducted to assess the potential impact of nearby fire activity on PM2.5 levels at Yosemite Village - Visitor Center. 3 wildfires were detected within 100 miles of the site from two days before the event to the event day. The proximity of these fires to the site suggests they may have contributed to elevated PM2.5 levels on this date. The following wildfires were identified: Dome Fire, which burned 2186.00, started on September 15, 2026, and last updated on September 18, 2026. It was located 12.4 miles away. Davis Fire, which burned 10.00, started on September 17, 2026, and ended on September 18, 2026. It was located 88.3 miles away. Schell Fire, which burned 12.00, started on September 17, 2026, and ended on September 18, 2026. It was located 59.7 miles away. No prescribed fires were identified within 50 miles of Yosemite Village - Visitor Center.

In addition to reported prescribed fire and wildfire records from CAL FIRE and the Prescribed Fire Reporting System (PFIRS), satellite imagery detected 19256 fire hotspots over the past two days (from two days before the event to the current day) within 100 miles of the site. This indicates ongoing fire activity in the region, which may have contributed to the observed air quality conditions. Satellite data confirms the presence of smoke plumes over the monitoring site, suggesting that emissions from nearby fires were transported into the area, potentially impacting PM2.5 levels.

HYSPLIT Trajectory for Exceedance Monitor Site

HYSPLIT 24-hour backward trajectories from Yosemite Village - Visitor Center (060431001) at 100, 150, and 200 meters above ground level, initiated at the highest hourly concentration on Sept. 20, 2026.