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Peak Daily One-Hour Ozone Averages for January 2026

The table below lists the peak daily one-hour ozone concentrations measured in January 2026 in each community where the TCEQ measures ozone. Concentrations are color-highlighted based on the EPA-defined Air Quality Index colors. (See Interpreting the AQI.) All ozone measurements are in parts per billion, and are truncated to an integer number.

The values that go into this table are calculated at 3:30 am each morning. Therefore, the current day will almost always have very low hourly averages that are not truly representative of local conditions for the day. Because of this, the report does not display the current day's data.

Use the selection boxes below to customize this report. You may select a different month, a different year, a different report format, and whether or not to include time on the report. Click on the Generate Report button once you have made your selections.

Month:	Year:	Report Format:	Include time:
JanuaryFebruaryMarchAprilMayJuneJulyAugustSeptemberOctoberNovemberDecember	1996199719981999200020012002200320042005200620072008200920102011201220132014201520162017201820192020202120222023202420252026	Tabular (Web Formatted)Comma-Delimited	Include time
Highlight using AQI colors

Area	Monitoring Site	POC	January 2026																													POC	Monitoring Site
			1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29
Dallas-Fort Worth
	Ft. Worth Northwest C13/AH302	2	41	41	40	38	42	41	44	38	23	41	39	42	40	40	37	44	41	45	NA	NA	NA	23	24	28	42	49	34	52	58	2	Ft. Worth Northwest C13/AH302
	Keller C17	2	39	42	35	38	40	38	45	37	23	41	38	40	42	39	35	43	41	46	40	34	44	33	27	29	42	49	35	51	55	2	Keller C17
	Frisco C31/C680	1	41	47	36	36	41	40	49	38	27	42	40	35	40	39	35	45	41	44	42	35	44	35	32	32	40	47	37	62	61	1	Frisco C31/C680
	Denton Airport South C56/A163/X157	1	42	47	35	32	45	38	37	38	25	42	39	42	41	39	35	44	41	45	40	30	44	30	27	29	43	50	40	70	61	1	Denton Airport South C56/A163/X157
	Arlington Municipal Airport C61	1	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	44	40	39	38	43	40	44	37	33	39	31	29	28	39	46	35	43	54	1	Arlington Municipal Airport C61
	Dallas North No.2 C63/C679	1	38	42	33	35	37	36	44	37	19	39	37	37	39	38	34	43	40	41	37	33	43	33	30	29	40	45	37	51	58	1	Dallas North No.2 C63/C679
	Rockwall Heath C69	1	38	38	34	32	35	33	45	33	21	36	34	33	35	34	29	40	37	37	35	31	38	29	28	28	35	41	33	47	56	1	Rockwall Heath C69
	Grapevine Fairway C70/A301/X182	1	42	NA	NA	NA	31	40	38	38	25	40	40	40	40	39	37	43	40	43	39	36	41	34	29	28	38	45	35	56	58	1	Grapevine Fairway C70/A301/X182
	Kaufman C71/A304/X071	1	46	39	39	36	41	38	49	36	24	42	41	42	42	41	37	47	42	43	41	38	35	26	30	30	39	45	39	51	60	1	Kaufman C71/A304/X071
	Granbury C73/C681	1	41	34	41	39	42	45	48	38	36	39	39	43	40	39	36	43	41	45	37	34	40	35	25	28	39	44	31	53	54	1	Granbury C73/C681
	Eagle Mountain Lake C75	1	42	45	37	41	46	40	43	41	35	42	40	44	46	42	40	46	43	48	41	35	47	29	29	30	45	50	40	59	61	1	Eagle Mountain Lake C75
	Parker County C76	1	38	37	35	38	44	40	47	41	39	41	38	41	41	39	38	42	40	45	42	38	39	35	27	29	41	47	34	55	59	1	Parker County C76
	Cleburne Airport C77/C682	1	39	34	41	38	36	49	51	36	31	39	39	43	39	40	36	45	42	46	35	35	45	31	27	28	41	47	32	51	54	1	Cleburne Airport C77/C682
	Dallas Executive Airport C402	1	40	38	36	38	38	39	53	37	29	39	39	43	41	40	37	45	41	45	39	35	40	33	30	31	40	47	38	44	58	1	Dallas Executive Airport C402
	Greenville C1006/A198	1	42	41	39	34	39	39	48	36	27	41	38	36	41	38	35	44	40	39	39	38	42	33	32	33	40	46	39	60	61	1	Greenville C1006/A198
	Pilot Point C1032	1	44	46	36	34	44	40	42	39	32	43	39	41	42	38	35	45	42	45	42	32	47	33	30	30	41	48	38	70	63	1	Pilot Point C1032
	Italy C1044/A323	1	43	35	43	37	43	44	55	37	33	39	40	42	42	40	39	45	42	45	41	35	34	28	28	29	NA	NA	NA	NA	NA	1	Italy C1044/A323
	Corsicana Airport C1051	1	47	38	43	42	45	42	55	35	28	39	41	45	38	41	46	38	41	43	46	35	36	31	27	27	37	44	38	47	55	1	Corsicana Airport C1051
Area	Monitoring Site	POC	January 2026																													POC	Monitoring Site
			1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29
Tyler-Longview-Marshall
	Longview C19/A127/C644	2	51	44	35	36	41	44	55	36	42	41	45	42	47	44	53	48	43	47	48	45	42	39	30	28	36	48	44	49	62	2	Longview C19/A127/C644
	Tyler Airport Relocated C82	1	49	41	34	29	40	41	50	31	32	40	41	41	41	41	43	46	41	44	49	51	45	35	29	28	38	44	40	47	60	1	Tyler Airport Relocated C82
	Karnack C85/AFHP303	1	48	40	21	26	37	46	52	34	34	37	45	39	46	40	43	45	39	42	48	41	31	34	31	28	34	43	41	46	56	1	Karnack C85/AFHP303
Area	Monitoring Site	POC	January 2026																													POC	Monitoring Site
			1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29
El Paso-Juarez
	Ascarate Park SE C37/A332/A172/X159	1	30	32	40	32	29	29	29	38	43	37	43	39	33	40	43	38	41	36	37	36	30	30	14	14	37	NA	16	28	36	1	Ascarate Park SE C37/A332/A172/X159
	Chamizal C41/AH126	1	32	36	35	33	33	34	28	42	43	39	41	38	38	31	47	29	41	34	38	32	30	40	23	16	32	42	33	34	45	1	Chamizal C41/AH126
	Socorro Hueco C49/F312	1	34	35	45	38	34	33	26	42	42	39	43	41	36	41	44	42	42	38	38	39	42	37	21	19	33	39	37	39	43	1	Socorro Hueco C49/F312
	Skyline Park C72	1	NA	NA	NA	NA	36	37	31	45	40	39	44	42	33	42	45	41	41	39	42	39	40	41	20	18	32	40	35	37	42	1	Skyline Park C72
	Ivanhoe C414/F514	1	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	17	16	26	36	36	34	41	1	Ivanhoe C414/F514
	Ojo De Agua C1021/FG121/FG221	1	31	33	35	33	29	35	30	39	40	38	41	38	37	38	44	34	38	37	37	32	34	38	31	23	30	38	37	31	39	1	Ojo De Agua C1021/FG121/FG221
Area	Monitoring Site	POC	January 2026																													POC	Monitoring Site
			1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29
Waco
	Waco Mazanec C1037	1	47	39	46	40	45	41	48	35	27	40	38	32	41	43	39	45	43	44	41	38	37	25	26	28	40	42	38	42	52	1	Waco Mazanec C1037
	Temple Georgia C1045	1	45	38	47	39	42	42	44	35	40	41	43	44	41	44	41	47	46	47	49	42	41	28	23	27	40	46	35	48	51	1	Temple Georgia C1045
	Killeen Skylark Field C1047	1	44	40	47	41	43	44	49	39	39	40	43	41	41	43	40	47	44	45	49	40	40	33	19	27	39	45	36	46	50	1	Killeen Skylark Field C1047
Area	Monitoring Site	POC	January 2026																													POC	Monitoring Site
			1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29
Beaumont-Port Arthur
	Beaumont-Downtown C2/A112/C2002	2	50	41	34	39	38	34	31	32	23	28	39	38	NA	44	39	46	37	42	52	50	35	21	23	18	28	34	42	49	49	2	Beaumont-Downtown C2/A112/C2002
	West Orange C9/A141	2	51	39	32	38	39	28	35	33	25	26	41	38	36	42	43	48	41	48	53	49	37	23	20	19	28	39	40	49	54	2	West Orange C9/A141
	Port Arthur West C28/A128/A228	1	48	38	39	39	39	30	27	34	25	27	41	38	41	44	44	47	43	43	53	51	37	29	23	18	28	38	46	53	52	1	Port Arthur West C28/A128/A228
	Hamshire C64/C654	1	51	40	38	41	39	34	31	31	24	26	41	44	36	42	42	46	42	44	55	54	38	22	25	18	28	39	45	50	55	1	Hamshire C64/C654
	SETRPC Port Arthur C628	1 N	53	43	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	1	SETRPC Port Arthur C628
	SETRPC 40 Sabine Pass C640/C1654	1	49	37	41	34	38	30	29	32	23	28	41	37	38	40	46	52	47	47	58	56	44	34	26	18	32	40	49	53	59	1	SETRPC 40 Sabine Pass C640/C1654
	SETRPC Mauriceville 42 C642/C311/C665	1 N	52	44	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	1	SETRPC Mauriceville 42 C642/C311/C665
	SETRPC 43 Jefferson Co Airport C643	1	49	36	36	38	37	26	29	30	23	27	40	39	41	43	41	45	42	42	52	49	38	27	25	18	28	38	44	50	50	1	SETRPC 43 Jefferson Co Airport C643
	Nederland 17th Street C1035	1	50	40	37	39	38	33	31	32	23	28	40	38	40	44	40	47	41	41	52	50	37	23	23	19	29	39	43	46	52	1	Nederland 17th Street C1035
Area	Monitoring Site	POC	January 2026																													POC	Monitoring Site
			1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29
Austin
	Austin North Hills Drive C3/A322	2	48	41	50	46	45	44	44	35	38	41	44	41	39	43	36	48	45	45	53	41	40	32	17	25	36	46	34	45	51	2	Austin North Hills Drive C3/A322
	Audubon C38	1	41	37	45	43	42	43	44	32	36	38	41	39	39	39	39	47	42	42	48	38	38	29	13	22	34	42	37	43	47	1	Audubon C38
	CAPCOG Lake Kyle Park C1630	1 N	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	40	36	39	45	38	32	35	21	21	33	38	37	36	44	1	CAPCOG Lake Kyle Park C1630
Area	Monitoring Site	POC	January 2026																													POC	Monitoring Site
			1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29
Houston-Galveston-Brazoria
	Houston East C1/G316	2	48	41	41	47	41	36	32	33	25	27	39	36	16	36	33	48	39	41	54	44	35	21	27	19	30	35	42	34	52	2	Houston East C1/G316
	Houston Aldine C8/AF108/X150	2	50	43	NA	NA	NA	NA	NA	15	23	31	44	38	21	43	39	51	44	45	55	49	33	18	30	22	34	40	41	46	54	2	Houston Aldine C8/AF108/X150
	Channelview C15/AH115	3	50	44	45	49	37	38	34	36	26	33	45	42	20	43	37	49	44	47	50	47	37	25	31	22	32	42	40	46	53	3	Channelview C15/AH115
	Northwest Harris Co. C26/A110/X154	2	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	38	42	49	43	44	53	45	32	21	24	23	33	39	44	51	57	2	Northwest Harris Co. C26/A110/X154
	Hou.DeerPrk2 C35/235/1001/AFH139/FP239	1	51	43	48	48	42	36	33	35	25	31	44	43	28	44	38	49	45	43	56	51	38	32	32	24	31	41	42	46	55	1	Hou.DeerPrk2 C35/235/1001/AFH139/FP239
	Seabrook Friendship Park C45	1	42	38	41	43	35	29	24	30	21	30	42	38	28	39	35	43	38	42	44	43	31	23	21	19	27	36	43	45	50	1	Seabrook Friendship Park C45
	Houston Bayland Park C53/A146	1	48	42	52	51	45	40	27	35	27	33	44	40	28	43	38	48	44	45	52	50	38	29	NA	NA	NA	NA	38	43	50	1	Houston Bayland Park C53/A146
	Conroe Relocated C78/A321	1	48	46	36	38	47	37	38	33	29	31	39	41	18	42	40	46	41	42	45	54	38	24	18	19	30	39	41	46	57	1	Conroe Relocated C78/A321
	Manvel Croix Park C84	1	49	45	56	49	43	35	31	35	27	33	44	42	27	42	36	49	42	44	48	48	30	29	27	23	31	40	49	44	53	1	Manvel Croix Park C84
	Clinton C403/C304/AH113	3	42	30	44	42	33	24	20	26	20	26	40	40	12	35	31	36	40	40	45	39	30	28	24	18	27	36	34	36	40	3	Clinton C403/C304/AH113
	Houston North Wayside C405/C1033	1	42	33	40	44	35	29	26	24	17	28	38	38	10	35	32	36	39	36	44	37	23	14	25	17	23	34	31	35	44	1	Houston North Wayside C405/C1033
	Houston Monroe C406	1	45	41	46	44	37	32	29	32	23	27	37	35	23	38	30	48	39	42	46	45	37	29	29	22	27	34	43	34	49	1	Houston Monroe C406
	Lang C408	2	45	36	42	50	42	37	30	31	23	30	40	35	18	38	36	44	37	41	45	38	36	17	23	18	31	36	39	42	53	2	Lang C408
	Houston Croquet C409	2	48	43	53	49	46	37	33	36	26	34	44	37	24	43	36	48	44	44	51	48	36	31	29	23	31	39	46	42	51	2	Houston Croquet C409
	Houston Westhollow C410/C3003	1	45	38	49	50	40	39	30	32	24	34	43	37	27	43	40	45	44	44	52	47	34	28	30	22	33	40	45	50	52	1	Houston Westhollow C410/C3003
	Park Place C416	1	44	38	47	43	40	33	30	33	24	29	41	35	23	37	30	47	41	46	49	45	38	30	30	21	29	38	42	35	51	1	Park Place C416
	Houston Harvard Street C417	1	48	41	48	50	44	39	34	33	24	30	42	36	23	40	38	45	43	45	53	46	38	23	29	22	32	37	43	42	52	1	Houston Harvard Street C417
	Sheldon C551	1 N	46	38	39	45	34	32	28	29	11	NA	NA	49	16	42	37	45	43	43	48	46	24	12	15	14	28	38	36	41	47	1	Sheldon C551
	Baytown Wetlands Center C552	1 N	49	40	40	44	36	23	21	23	12	28	38	31	17	41	41	44	44	44	53	49	34	26	27	21	18	NA	NA	NA	NA	1	Baytown Wetlands Center C552
	Crosby Library C553	1 N	47	37	34	43	32	33	29	29	17	29	39	51	18	39	36	44	40	42	51	45	32	16	24	17	18	NA	39	45	53	1	Crosby Library C553
	West Houston C554	1 N	46	36	48	55	39	39	32	33	25	33	43	38	24	43	38	48	43	43	53	45	36	26	28	22	33	40	44	49	53	1	West Houston C554
	La Porte Sylvan Beach C556	1 N	46	38	41	45	31	30	24	30	21	35	43	36	22	40	37	45	43	43	46	41	NA	23	28	20	30	39	44	47	41	1	La Porte Sylvan Beach C556
	Tom Bass C558	1 N	49	45	52	50	42	35	32	36	28	34	45	41	28	45	37	53	45	46	54	37	NA	32	32	24	32	41	48	46	54	1	Tom Bass C558
	Katy Park C559	1 N	44	35	44	49	38	35	28	31	25	33	41	38	22	36	40	40	40	37	42	42	29	24	27	19	31	37	40	42	46	1	Katy Park C559
	Atascocita C560	1 N	45	34	35	47	37	32	26	25	16	29	39	37	14	38	33	41	40	40	50	49	31	14	22	16	30	36	31	41	49	1	Atascocita C560
	Meyer Park C561	1 N	46	37	36	55	42	37	31	31	24	29	38	39	17	39	39	45	40	42	49	46	25	15	19	18	30	36	39	50	53	1	Meyer Park C561
	Bunker Hill Village C562	1 N	44	36	45	47	40	36	27	30	21	31	40	34	19	39	35	44	42	42	51	45	35	23	27	20	33	37	41	44	51	1	Bunker Hill Village C562
	Huffman Wolf Road C563	1 N	52	45	34	50	40	39	37	35	24	31	40	40	18	42	41	47	41	42	53	51	35	18	21	18	31	40	39	50	55	1	Huffman Wolf Road C563
	HRM-3 Haden Road C603/A114	1 N	48	43	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	1	HRM-3 Haden Road C603/A114
	Wallisville Road C617	1 N	52	46	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	1	Wallisville Road C617
	Texas City 34th St. C620	1 N	45	36	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	NA	1	Texas City 34th St. C620
	Lynchburg Ferry C1015/A165	1	52	41	37	47	27	33	34	38	28	34	40	40	16	41	39	48	40	43	49	48	33	28	29	21	29	38	NA	36	48	1	Lynchburg Ferry C1015/A165
	Lake Jackson C1016	1	45	36	57	44	38	30	28	33	27	30	40	41	31	43	42	47	46	46	50	49	43	38	34	28	29	37	44	51	48	1	Lake Jackson C1016
	Baytown Garth C1017	1	51	44	40	46	41	38	37	34	24	31	41	42	25	42	41	48	43	44	51	50	36	28	32	21	31	40	40	49	52	1	Baytown Garth C1017
	Galveston 99th St. C1034/A320/X183	1	48	40	52	42	38	31	34	33	29	36	45	41	36	44	45	49	44	45	54	52	42	34	33	26	28	40	48	57	56	1	Galveston 99th St. C1034/A320/X183
	Oyster Creek C1607	1 N	50	42	63	47	42	33	30	36	30	33	48	42	36	44	47	48	48	47	53	52	40	39	34	27	31	NA	NA	NA	NA	1	Oyster Creek C1607
Area	Monitoring Site	POC	January 2026																													POC	Monitoring Site
			1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29
San Antonio
	San Antonio Northwest C23	2	44	34	48	49	41	43	42	32	42	42	42	31	40	42	41	48	45	43	50	35	37	45	27	24	37	39	38	58	46	2	San Antonio Northwest C23
	Camp Bullis C58	1	44	39	49	50	40	43	44	33	40	41	43	39	42	44	41	48	44	43	51	38	37	46	27	24	37	40	38	57	47	1	Camp Bullis C58
	Calaveras Lake C59	1	44	34	50	48	42	44	38	36	29	40	41	37	42	40	37	51	40	42	50	40	36	36	32	22	35	39	43	48	51	1	Calaveras Lake C59
	Heritage Middle School C622	1 N	46	36	51	50	42	43	40	35	27	41	42	41	44	42	36	52	42	43	51	39	36	37	32	23	34	41	43	49	53	1	Heritage Middle School C622
Area	Monitoring Site	POC	January 2026																													POC	Monitoring Site
			1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29
Corpus Christi-Victoria
	Corpus Christi West C4	2	42	38	47	41	37	37	32	35	31	37	41	39	32	45	43	49	45	44	49	48	40	39	33	31	29	41	47	52	45	2	Corpus Christi West C4
	Corpus Christi Tuloso C21	1	41	38	46	39	37	42	33	36	31	37	47	38	32	45	42	49	45	45	49	45	38	36	33	29	29	41	44	49	45	1	Corpus Christi Tuloso C21
	Victoria C87	1	49	48	60	45	41	42	37	37	27	41	46	45	35	47	46	51	47	52	NA	48	38	35	34	29	36	44	47	55	53	1	Victoria C87
Area	Monitoring Site	POC	January 2026																													POC	Monitoring Site
			1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29
Lower Rio Grande Valley
	Mission C43/AP143	1	39	48	39	59	36	48	35	33	50	34	38	39	38	45	44	44	45	47	45	39	35	53	33	23	26	42	44	45	40	1	Mission C43/AP143
	Harlingen Teege C1023	1	40	41	37	45	36	38	34	36	41	35	39	41	39	46	39	45	45	42	49	43	35	39	35	25	25	41	43	45	40	1	Harlingen Teege C1023
Area	Monitoring Site	POC	January 2026																													POC	Monitoring Site
			1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22	23	24	25	26	27	28	29
Laredo
	Laredo College C44	1 N	41	48	46	58	47	68	45	48	42	40	40	40	40	49	50	53	49	49	55	45	42	58	38	29	32	43	47	51	54	1	Laredo College C44
NA - Data not available.
N - Data from this instrument does not meet EPA quality assurance criteria and cannot be used for regulatory purposes.

Interpreting the AQI

Each NAAQS pollutant has a separate AQI scale, with an AQI rating of 100 corresponding to the concentration of the Federal Standard for that pollutant. Additional information about the AQI and how it can be used is available from the EPA's AirNow web site.

Place your mouse pointer over the scale displayed above to view information about the Air Quality Index, and each of the rating levels.

The actual index calculation is different for each parameter measured and is specified by the EPA. The following table shows the various breakpoints used in calculating the AQI.

AQI Breakpoint Definitions
AQI Range	1hr Ozone in ppm	8hr Ozone in ppm
0 - 50	Not Defined	0 - 0.054
51 - 100	Not Defined	0.055 - 0.070
101 - 150	0.125 - 0.164	0.071 - 0.085
151 - 200	0.165 - 0.204	0.086 - 0.105
201 - 300	0.205 - 0.404	0.106 - 0.200
301 - 400	0.405 - 0.504	Not Defined
401 - 500	Not Defined	Not Defined
500+	Not Defined	Not Defined

• The AQI for ozone is based on either the peak eight-hour running average since midnight OR the peak one-hour measurement since midnight.

PLEASE NOTE:  This data has not been verified by the TCEQ and may change. This is the most current data, but it is not official until it has been certified by our technical staff. Data is collected from TCEQ ambient monitoring sites and may include data collected by other outside agencies. This data is updated hourly. All times shown are in local standard time unless otherwise indicated.

Following EPA reporting guidelines, negative values may be displayed in our hourly criteria air quality data, down to the negative of the EPA listed Method Detection Limit (MDL) for the particular instrument that made the measurements. The reported concentrations can be negative due to zero drift in the electronic instrument output, data logger channel, or calibration adjustments to the data. Prior to 1/1/2013, slightly negative values were automatically set to zero.