~ $ cd open-data/no2 && cat README.md

NO2 by the hour and the month

Nitrogen dioxide in a city is mostly traffic. The Generalitat de Catalunya measures it every hour at fixed points and publishes every reading since 1991. Looked at a day at a time it is noise. Averaged by hour of the day and month of the year, it is a picture:

Barcelona (Eixample) · traffic, urban · mean NO2 in µg/m³ by hour of the day and month of the year · every day of the week, 1996–2025

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
01	54	62	61	53	53	49	45	42	51	60	56	56
02	48	55	56	49	50	44	42	40	47	55	49	50
03	42	49	51	45	48	42	39	37	43	50	44	44
04	39	45	47	43	46	41	38	36	41	46	41	40
05	36	42	44	43	44	40	37	35	38	42	38	37
06	37	44	46	45	45	44	40	36	40	43	40	38
07	45	52	54	52	53	52	48	43	48	51	47	45
08	59	68	70	67	65	64	61	54	63	65	62	56
09	67	75	77	73	71	70	66	57	69	74	69	64
10	66	71	70	69	68	69	65	53	66	73	66	64
11	59	62	60	61	63	63	61	49	58	66	60	59
12	54	57	55	56	59	59	59	48	56	61	56	54
13	51	54	52	54	57	56	56	46	54	59	54	52
14	51	53	51	50	54	53	52	42	53	58	55	52
15	50	50	46	45	49	47	46	36	47	54	54	52
16	49	50	45	42	46	44	43	34	44	51	54	50
17	55	55	49	44	49	47	45	34	46	55	60	55
18	63	62	55	48	54	51	48	37	51	61	69	63
19	69	70	63	52	58	54	52	41	57	66	74	68
20	72	74	70	58	63	58	56	45	63	71	76	71
21	73	76	73	63	67	61	59	49	65	72	76	72
22	70	75	71	62	64	60	57	49	61	70	74	71
23	64	70	67	57	58	54	51	45	55	66	68	65
24	59	66	64	54	55	50	47	42	52	63	61	60

The mean of each year, every day of the week

Source: Generalitat de Catalunya, Xarxa de Vigilància i Previsió de la Contaminació Atmosfèrica. Dades obertes. The dataset, at its source. This site keeps sums of the finished years 1991 to 2025, last added to on 20 September 2026.

Each cell is the mean, in µg/m³, of every measurement taken at that hour in that month over the years selected. Press a bar to see one year alone.

What is in it

At Barcelona's Eixample, over its whole record:

Two ridges. The morning one peaks at hour 09 and is there all year. The evening one, around hours 20 and 21, is as high as the morning's from October to February, and lower from spring to the end of summer.
August. Its mean is 43 µg/m³; no other month is below 51. The city leaves, and it shows at every hour.
The week. From Monday to Friday the mean is about a quarter higher than on Saturdays and Sundays. Choose the weekends and the morning ridge is gone, while the small hours are higher than on a working day: the night before was a Friday or a Saturday.
The years. 70 µg/m³ in 1999, 49.5 in 2019, 35.2 in 2020. The year of the lockdowns was the first under the European limit of 40; 2022 went back over it, and 2025 closed at 29.1.

The same shape, lower, is at every urban station here. At La Castanya, in the Montseny, the yearly mean has stayed under 4 µg/m³ since 2013: that is what air with no road under it measures. It is also the only station of these nine that ended 2025 under the 10 µg/m³ the WHO has recommended since 2021.

What it is not

The cells are means, and a mean hides the bad days. Public holidays are counted as whatever day of the week they fell on. The dataset numbers its hours 01 to 24 and does not say by which clock. A year drawn as an outline had less than three quarters of its hours measured; Eixample has no 2010 at all. These are open data added up, not validated science: for anything that matters, go to the source.

Where it comes from

I first drew this table in 2021, for myself, in a small app that asked the portal for everything on every visit. This page is that analysis again, with the asking moved out of the reader's way: the site keeps the sums, a finished year at a time, and asks the portal only when a year has ended.

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
01	54	62	61	53	53	49	45	42	51	60	56	56
02	48	55	56	49	50	44	42	40	47	55	49	50
03	42	49	51	45	48	42	39	37	43	50	44	44
04	39	45	47	43	46	41	38	36	41	46	41	40
05	36	42	44	43	44	40	37	35	38	42	38	37
06	37	44	46	45	45	44	40	36	40	43	40	38
07	45	52	54	52	53	52	48	43	48	51	47	45
08	59	68	70	67	65	64	61	54	63	65	62	56
09	67	75	77	73	71	70	66	57	69	74	69	64
10	66	71	70	69	68	69	65	53	66	73	66	64
11	59	62	60	61	63	63	61	49	58	66	60	59
12	54	57	55	56	59	59	59	48	56	61	56	54
13	51	54	52	54	57	56	56	46	54	59	54	52
14	51	53	51	50	54	53	52	42	53	58	55	52
15	50	50	46	45	49	47	46	36	47	54	54	52
16	49	50	45	42	46	44	43	34	44	51	54	50
17	55	55	49	44	49	47	45	34	46	55	60	55
18	63	62	55	48	54	51	48	37	51	61	69	63
19	69	70	63	52	58	54	52	41	57	66	74	68
20	72	74	70	58	63	58	56	45	63	71	76	71
21	73	76	73	63	67	61	59	49	65	72	76	72
22	70	75	71	62	64	60	57	49	61	70	74	71
23	64	70	67	57	58	54	51	45	55	66	68	65
24	59	66	64	54	55	50	47	42	52	63	61	60

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
01	54	62	61	53	53	49	45	42	51	60	56	56
02	48	55	56	49	50	44	42	40	47	55	49	50
03	42	49	51	45	48	42	39	37	43	50	44	44
04	39	45	47	43	46	41	38	36	41	46	41	40
05	36	42	44	43	44	40	37	35	38	42	38	37
06	37	44	46	45	45	44	40	36	40	43	40	38
07	45	52	54	52	53	52	48	43	48	51	47	45
08	59	68	70	67	65	64	61	54	63	65	62	56
09	67	75	77	73	71	70	66	57	69	74	69	64
10	66	71	70	69	68	69	65	53	66	73	66	64
11	59	62	60	61	63	63	61	49	58	66	60	59
12	54	57	55	56	59	59	59	48	56	61	56	54
13	51	54	52	54	57	56	56	46	54	59	54	52
14	51	53	51	50	54	53	52	42	53	58	55	52
15	50	50	46	45	49	47	46	36	47	54	54	52
16	49	50	45	42	46	44	43	34	44	51	54	50
17	55	55	49	44	49	47	45	34	46	55	60	55
18	63	62	55	48	54	51	48	37	51	61	69	63
19	69	70	63	52	58	54	52	41	57	66	74	68
20	72	74	70	58	63	58	56	45	63	71	76	71
21	73	76	73	63	67	61	59	49	65	72	76	72
22	70	75	71	62	64	60	57	49	61	70	74	71
23	64	70	67	57	58	54	51	45	55	66	68	65
24	59	66	64	54	55	50	47	42	52	63	61	60

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
01	54	62	61	53	53	49	45	42	51	60	56	56
02	48	55	56	49	50	44	42	40	47	55	49	50
03	42	49	51	45	48	42	39	37	43	50	44	44
04	39	45	47	43	46	41	38	36	41	46	41	40
05	36	42	44	43	44	40	37	35	38	42	38	37
06	37	44	46	45	45	44	40	36	40	43	40	38
07	45	52	54	52	53	52	48	43	48	51	47	45
08	59	68	70	67	65	64	61	54	63	65	62	56
09	67	75	77	73	71	70	66	57	69	74	69	64
10	66	71	70	69	68	69	65	53	66	73	66	64
11	59	62	60	61	63	63	61	49	58	66	60	59
12	54	57	55	56	59	59	59	48	56	61	56	54
13	51	54	52	54	57	56	56	46	54	59	54	52
14	51	53	51	50	54	53	52	42	53	58	55	52
15	50	50	46	45	49	47	46	36	47	54	54	52
16	49	50	45	42	46	44	43	34	44	51	54	50
17	55	55	49	44	49	47	45	34	46	55	60	55
18	63	62	55	48	54	51	48	37	51	61	69	63
19	69	70	63	52	58	54	52	41	57	66	74	68
20	72	74	70	58	63	58	56	45	63	71	76	71
21	73	76	73	63	67	61	59	49	65	72	76	72
22	70	75	71	62	64	60	57	49	61	70	74	71
23	64	70	67	57	58	54	51	45	55	66	68	65
24	59	66	64	54	55	50	47	42	52	63	61	60