Initial Ozone 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 Ozone Concentration of 70 ppb

Monitor AQS ID	Site Name	Location (Lat, Long)	Ozone Value	AQI Category
060290232	Oildale - 3311 Manor Street	35.44, -119.02	73.0	Sensitive Group
060712002	Fontana - Arrow Highway	34.10, -117.49	73.0	Sensitive Group
060250005	Calexico - Ethel Street	32.68, -115.48	73.0	Sensitive Group

Air Quality Analysis

Monitor AQS ID

Air Quality Analysis

060290232

Hourly Ozone Analysis

At Oildale - 3311 Manor Street, hourly ozone concentrations showed no significant trend on September 20 and September 21. O3 levels exceeded 70 ppb on 7 occasions, occurring at: September 20, 10:00 AM, September 20, 11:00 AM, September 20, 12:00 PM, September 20, 01:00 PM, September 20, 03:00 PM, September 20, 04:00 PM, September 20, 05:00 PM. Among these exceedances, 2 were significant pollution spikes (peaks), recorded at: September 20, 12:00 PM, September 20, 04:00 PM. Hourly ozone levels showed a distinct day-night pattern. The highest recorded ozone concentration was 80.0 ppb at September 20, 12:00 PM, while the lowest was 23.0 ppb at September 21, 05:00 AM. Unusual ozone spikes detected at: September 20, 11:00 AM, September 20, 12:00 PM.

Daily ozone Analysis of the Year

Throughout 2026, ozone levels at Oildale - 3311 Manor Street exceeded 70 ppb on 15 days, which accounts for 4.1% of the year. On September 20, 2026, the ozone concentration reached 73.0 ppb, ranking #7 among exceedance events this year. The highest ozone exceedance this year was 79.0 ppb on September 09, 2026, which was 6.0 ppb higher than this event. The majority of exceedance events occurred in August, with 6 occurrences. The season with the highest exceedance frequency was Summer, accounting for 10 exceedance days.

Historical daily Ozone Analysis

Between 2022 and 2026, ozone levels at Oildale - 3311 Manor Street exceeded the 70 ppb daily standard most frequently during Summer, accounting for 28.2% of all exceedances. The month with the highest number of exceedances was May, making up 9.7% of the total. The exceedance recorded on September 20, 2026 at Oildale - 3311 Manor Street reached 73.0 ppb, which is 13.1 ppb higher than the average Summer ozone level of 59.9 ppb. 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.

060712002

Hourly Ozone Analysis

At Fontana - Arrow Highway, hourly ozone concentrations showed an increasing trend on September 20 and September 21. O3 levels exceeded 70 ppb on 4 occasions, occurring at: September 20, 12:00 PM, September 20, 01:00 PM, September 20, 02:00 PM, September 20, 03:00 PM. Among these exceedances, 2 were significant pollution spikes (peaks), recorded at: September 20, 01:00 PM, September 21, 01:00 PM. Hourly ozone levels showed a distinct day-night pattern. The highest recorded ozone concentration was 90.0 ppb at September 20, 01:00 PM, while the lowest was 3.0 ppb at September 20, 05:00 AM. Unusual ozone spikes detected at: September 20, 01:00 PM, September 20, 02:00 PM.

Daily ozone Analysis of the Year

Throughout 2026, ozone levels at Fontana - Arrow Highway exceeded 70 ppb on 72 days, which accounts for 19.7% of the year. On September 20, 2026, the ozone concentration reached 73.0 ppb, ranking #62 among exceedance events this year. The highest ozone exceedance this year was 106.0 ppb on March 20, 2026, which was 33.0 ppb higher than this event. The majority of exceedance events occurred in July, with 19 occurrences. The season with the highest exceedance frequency was Summer, accounting for 49 exceedance days.

Historical daily Ozone Analysis

Between 2022 and 2026, ozone levels at Fontana - Arrow Highway exceeded the 70 ppb daily standard most frequently during Summer, accounting for 27.7% of all exceedances. The month with the highest number of exceedances was July, making up 9.3% of the total. The exceedance recorded on September 20, 2026 at Fontana - Arrow Highway reached 73.0 ppb, which is -0.6 ppb higher than the average Summer ozone level of 73.6 ppb. 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.

060250005

Hourly Ozone Analysis

At Calexico - Ethel Street, hourly ozone concentrations showed an increasing trend on September 20 and September 21. O3 levels exceeded 70 ppb on 3 occasions, occurring at: September 20, 10:00 AM, September 20, 11:00 AM, September 20, 12:00 PM. Among these exceedances, 1 were significant pollution spikes (peaks), recorded at: September 20, 11:00 AM. Hourly ozone levels showed a distinct day-night pattern. The highest recorded ozone concentration was 91.0 ppb at September 20, 11:00 AM, while the lowest was 2.0 ppb at September 20, 03:00 AM. Unusual ozone spikes detected at: September 20, 11:00 AM, September 20, 12:00 PM.

Daily ozone Analysis of the Year

Throughout 2026, ozone levels at Calexico - Ethel Street exceeded 70 ppb on 16 days, which accounts for 4.4% of the year. On September 20, 2026, the ozone concentration reached 73.0 ppb, ranking #9 among exceedance events this year. The highest ozone exceedance this year was 83.0 ppb on August 22, 2026, which was 10.0 ppb higher than this event. The majority of exceedance events occurred in April, with 3 occurrences. The season with the highest exceedance frequency was Summer, accounting for 7 exceedance days.

Historical daily Ozone Analysis

Between 2022 and 2026, ozone levels at Calexico - Ethel Street exceeded the 70 ppb daily standard most frequently during Summer, accounting for 27.1% of all exceedances. The month with the highest number of exceedances was May, making up 9.4% of the total. The exceedance recorded on September 20, 2026 at Calexico - Ethel Street reached 73.0 ppb, which is 16.6 ppb higher than the average Summer ozone level of 56.4 ppb. 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 Ozone for Days Around Event

Time series of 1-hour ozone concentrations at Oildale - 3311 Manor Street starting on Sept. 20, 2026. The number '1' indicates the highest 1-hour ozone concentration of the day, which is also the time used for HYSPLIT backward trajectory analysis.

Time series of 1-hour ozone concentrations at Fontana - Arrow Highway starting on Sept. 20, 2026. The number '1' indicates the highest 1-hour ozone concentration of the day, which is also the time used for HYSPLIT backward trajectory analysis.

Time series of 1-hour ozone concentrations at Calexico - Ethel Street starting on Sept. 20, 2026. The number '1' indicates the highest 1-hour ozone concentration of the day, which is also the time used for HYSPLIT backward trajectory analysis.

Historical Daily Maximum 8-Hour Ozone Concentration

Time series plot of daily maximum 8-Hour ozone concentrations at Oildale - 3311 Manor Street for each year between 2022 and 2026. The colored dots represent the daily maximum 8-Hour ozone concentrations on Sept. 20, 2026. The black line indicates the 5-year average daily maximum 8-Hour ozone concentration, and the grey dashed line represents the 70 ppb daily ozone standard.

Time series plot of daily maximum 8-Hour ozone concentrations at Fontana - Arrow Highway for each year between 2022 and 2026. The colored dots represent the daily maximum 8-Hour ozone concentrations on Sept. 20, 2026. The black line indicates the 5-year average daily maximum 8-Hour ozone concentration, and the grey dashed line represents the 70 ppb daily ozone standard.

Time series plot of daily maximum 8-Hour ozone concentrations at Calexico - Ethel Street for each year between 2022 and 2026. The colored dots represent the daily maximum 8-Hour ozone concentrations on Sept. 20, 2026. The black line indicates the 5-year average daily maximum 8-Hour ozone concentration, and the grey dashed line represents the 70 ppb daily ozone standard.

Prescribed Fires within 50 Miles

Monitor AQS ID	On the Day	One Day Before	Two Days Before
060290232	No fires	No fires	No fires
060712002	No fires	No fires	No fires
060250005	No fires	No fires	No fires

Wildfires within 100 Miles

Monitor AQS ID	2-Day Range to Exceedance Date
060290232	No fires
060712002	No fires
060250005	No fires

Fire Emission Analysis

Monitor AQS ID	Emission Analysis
060290232	On September 20, 2026, an analysis was conducted to assess the potential impact of nearby fire activity on ozone levels at Oildale - 3311 Manor Street. No wildfires were identified within 100 miles of Oildale - 3311 Manor Street. No prescribed fires were identified within 50 miles of Oildale - 3311 Manor Street. While no prescribed fires or wildfires were recorded from CAL FIRE and the Prescribed Fire Reporting System (PFIRS), satellite imagery detected 56 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. However, satellite imagery does not show clear evidence of smoke reaching the monitoring site. This suggests that further analysis is needed to determine whether other factors contributed to the elevated ozone levels.
060712002	On September 20, 2026, an analysis was conducted to assess the potential impact of nearby fire activity on ozone levels at Fontana - Arrow Highway. 2 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 ozone levels on this date. The following wildfires were identified: Palos Fire, which burned 17.00, started on September 14, 2026, and last updated on September 17, 2026. It was located 29.0 miles away. Gustin Fire, which burned 30.00, started on September 19, 2026, and last updated on September 18, 2026. It was located 21.0 miles away. No prescribed fires were identified within 50 miles of Fontana - Arrow Highway. In addition to reported prescribed fire and wildfire records from CAL FIRE and the Prescribed Fire Reporting System (PFIRS), satellite imagery detected 135 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. However, satellite imagery does not show clear evidence of smoke reaching the monitoring site. This suggests that further analysis is needed to determine whether other factors contributed to the elevated ozone levels.
060250005	On September 20, 2026, an analysis was conducted to assess the potential impact of nearby fire activity on ozone levels at Calexico - Ethel Street. No wildfires were identified within 100 miles of Calexico - Ethel Street. No prescribed fires were identified within 50 miles of Calexico - Ethel Street. While no prescribed fires or wildfires were recorded from CAL FIRE and the Prescribed Fire Reporting System (PFIRS), satellite imagery detected 57 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. However, satellite imagery does not show clear evidence of smoke reaching the monitoring site. This suggests that further analysis is needed to determine whether other factors contributed to the elevated ozone levels.

Monitor AQS ID

Emission Analysis

060290232

On September 20, 2026, an analysis was conducted to assess the potential impact of nearby fire activity on ozone levels at Oildale - 3311 Manor Street. No wildfires were identified within 100 miles of Oildale - 3311 Manor Street. No prescribed fires were identified within 50 miles of Oildale - 3311 Manor Street.

While no prescribed fires or wildfires were recorded from CAL FIRE and the Prescribed Fire Reporting System (PFIRS), satellite imagery detected 56 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. However, satellite imagery does not show clear evidence of smoke reaching the monitoring site. This suggests that further analysis is needed to determine whether other factors contributed to the elevated ozone levels.

060712002

On September 20, 2026, an analysis was conducted to assess the potential impact of nearby fire activity on ozone levels at Fontana - Arrow Highway. 2 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 ozone levels on this date. The following wildfires were identified: Palos Fire, which burned 17.00, started on September 14, 2026, and last updated on September 17, 2026. It was located 29.0 miles away. Gustin Fire, which burned 30.00, started on September 19, 2026, and last updated on September 18, 2026. It was located 21.0 miles away. No prescribed fires were identified within 50 miles of Fontana - Arrow Highway.

In addition to reported prescribed fire and wildfire records from CAL FIRE and the Prescribed Fire Reporting System (PFIRS), satellite imagery detected 135 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. However, satellite imagery does not show clear evidence of smoke reaching the monitoring site. This suggests that further analysis is needed to determine whether other factors contributed to the elevated ozone levels.

060250005

On September 20, 2026, an analysis was conducted to assess the potential impact of nearby fire activity on ozone levels at Calexico - Ethel Street. No wildfires were identified within 100 miles of Calexico - Ethel Street. No prescribed fires were identified within 50 miles of Calexico - Ethel Street.

While no prescribed fires or wildfires were recorded from CAL FIRE and the Prescribed Fire Reporting System (PFIRS), satellite imagery detected 57 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. However, satellite imagery does not show clear evidence of smoke reaching the monitoring site. This suggests that further analysis is needed to determine whether other factors contributed to the elevated ozone levels.

High-Split Trajectory for Ozone Exceedance Monitor Site

HYSPLIT 24-hour Backward Trajectories from the Oildale - 3311 Manor Street site (060290232) at 100, 150, and 200 meters above ground level, initiated at the highest hourly concentration on Sept. 20, 2026.

HYSPLIT 24-hour Backward Trajectories from the Fontana - Arrow Highway site (060712002) at 100, 150, and 200 meters above ground level, initiated at the highest hourly concentration on Sept. 20, 2026.

HYSPLIT 24-hour Backward Trajectories from the Calexico - Ethel Street site (060250005) at 100, 150, and 200 meters above ground level, initiated at the highest hourly concentration on Sept. 20, 2026.