~ $ cd projects/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 (Poblenou) · background, urban · mean NO2 in µg/m³ by hour of the day and month of the year · every day of the week, 1991–2026, 2026 to 13 October

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
01	47	54	54	43	39	36	31	30	43	52	49	49
02	43	49	52	43	41	37	33	32	42	48	45	44
03	39	44	48	42	41	37	33	33	40	43	40	41
04	36	41	44	40	40	38	34	32	38	39	37	38
05	34	38	41	38	38	37	35	31	35	36	35	35
06	34	39	42	40	40	40	38	33	36	36	36	36
07	39	43	46	45	45	46	45	38	42	40	40	40
08	46	51	53	53	52	52	52	43	49	47	48	46
09	52	56	57	57	55	56	56	45	53	52	53	50
10	52	55	56	55	53	53	52	43	52	53	54	51
11	51	54	51	49	45	43	42	36	48	53	52	50
12	51	51	42	40	37	34	31	28	40	49	52	52
13	49	45	35	33	31	29	25	22	32	42	49	51
14	46	38	30	28	26	24	21	18	25	35	46	49
15	42	34	28	24	23	21	18	15	21	30	41	46
16	42	34	27	23	22	20	18	15	20	30	42	45
17	45	38	30	24	24	21	19	15	21	32	46	48
18	53	45	34	26	26	23	21	16	23	37	56	57
19	60	54	40	29	29	26	23	18	26	42	63	62
20	62	59	46	33	32	28	26	21	30	47	66	64
21	62	62	52	38	36	31	28	23	34	52	66	65
22	60	63	55	41	39	33	30	26	36	54	64	63
23	57	62	56	43	39	34	31	26	38	55	60	59
24	52	58	56	42	39	35	30	27	40	55	54	53

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, and 2026 so far, to 13 October, last added to on 13 October 2026.

Each cell is the mean, in µg/m³, of every measurement taken at that hour in that month over the years selected. The colour says whether it is good or bad before the number is read: green is clean, red is the European annual limit of 40, and purple going to black is beyond it. Press a bar to see one year alone.

What is in it

At Barcelona's Poblenou, a station away from the big roads, over its whole record:

Two ridges. The morning one peaks at hour 09 and is there all year. The evening one, around hours 20 to 22, is the higher of the two from November to February, and by July it is less than half of what it was in January.
The afternoon. Between the ridges is a valley, deepest at hours 16 and 17 in August, at 15 µg/m³: a quarter of a January evening.
August. Its mean is 28 µg/m³; no other month is below 33. 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.
The years. 57.4 µg/m³ in 2003, 36.7 in 2019, 29.0 in 2020, 22.0 in 2025. It has not been over the European limit of 40 since 2017.

Choose the Eixample, which measures traffic, and the same shape is there in purple: 70 µg/m³ in 1999, 49.5 in 2019, 35.2 in 2020. The year of the lockdowns was its first under the limit; 2022 went back over it, and 2025 closed at 29.1. The same shape, higher or lower, is at every urban station here.

Height

Choose the Observatori Fabra. It is in Barcelona, 415 m up the hill the city climbs, and its table is green: 7.4 µg/m³ in 2025, when the Eixample below it closed at 29.1 and Poblenou at 22.0. It has no rush hours either. Its highest cells are late morning in summer and late afternoon in winter, and none of them reaches 20; its lowest are the early morning, just when the streets below are filling. Same city, same traffic: little of it is measured up there.

The other green table is La Castanya, in the Montseny, where the yearly mean has stayed under 4 µg/m³ since 2013: that is what air with no road under it measures. Those two are the only stations of these eleven 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 for a year once it has ended — and for the year still running once a day, which is drawn in grey: a mean of part of a year, with the winter still to come.

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
01	47	54	54	43	39	36	31	30	43	52	49	49
02	43	49	52	43	41	37	33	32	42	48	45	44
03	39	44	48	42	41	37	33	33	40	43	40	41
04	36	41	44	40	40	38	34	32	38	39	37	38
05	34	38	41	38	38	37	35	31	35	36	35	35
06	34	39	42	40	40	40	38	33	36	36	36	36
07	39	43	46	45	45	46	45	38	42	40	40	40
08	46	51	53	53	52	52	52	43	49	47	48	46
09	52	56	57	57	55	56	56	45	53	52	53	50
10	52	55	56	55	53	53	52	43	52	53	54	51
11	51	54	51	49	45	43	42	36	48	53	52	50
12	51	51	42	40	37	34	31	28	40	49	52	52
13	49	45	35	33	31	29	25	22	32	42	49	51
14	46	38	30	28	26	24	21	18	25	35	46	49
15	42	34	28	24	23	21	18	15	21	30	41	46
16	42	34	27	23	22	20	18	15	20	30	42	45
17	45	38	30	24	24	21	19	15	21	32	46	48
18	53	45	34	26	26	23	21	16	23	37	56	57
19	60	54	40	29	29	26	23	18	26	42	63	62
20	62	59	46	33	32	28	26	21	30	47	66	64
21	62	62	52	38	36	31	28	23	34	52	66	65
22	60	63	55	41	39	33	30	26	36	54	64	63
23	57	62	56	43	39	34	31	26	38	55	60	59
24	52	58	56	42	39	35	30	27	40	55	54	53

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
01	47	54	54	43	39	36	31	30	43	52	49	49
02	43	49	52	43	41	37	33	32	42	48	45	44
03	39	44	48	42	41	37	33	33	40	43	40	41
04	36	41	44	40	40	38	34	32	38	39	37	38
05	34	38	41	38	38	37	35	31	35	36	35	35
06	34	39	42	40	40	40	38	33	36	36	36	36
07	39	43	46	45	45	46	45	38	42	40	40	40
08	46	51	53	53	52	52	52	43	49	47	48	46
09	52	56	57	57	55	56	56	45	53	52	53	50
10	52	55	56	55	53	53	52	43	52	53	54	51
11	51	54	51	49	45	43	42	36	48	53	52	50
12	51	51	42	40	37	34	31	28	40	49	52	52
13	49	45	35	33	31	29	25	22	32	42	49	51
14	46	38	30	28	26	24	21	18	25	35	46	49
15	42	34	28	24	23	21	18	15	21	30	41	46
16	42	34	27	23	22	20	18	15	20	30	42	45
17	45	38	30	24	24	21	19	15	21	32	46	48
18	53	45	34	26	26	23	21	16	23	37	56	57
19	60	54	40	29	29	26	23	18	26	42	63	62
20	62	59	46	33	32	28	26	21	30	47	66	64
21	62	62	52	38	36	31	28	23	34	52	66	65
22	60	63	55	41	39	33	30	26	36	54	64	63
23	57	62	56	43	39	34	31	26	38	55	60	59
24	52	58	56	42	39	35	30	27	40	55	54	53

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
01	47	54	54	43	39	36	31	30	43	52	49	49
02	43	49	52	43	41	37	33	32	42	48	45	44
03	39	44	48	42	41	37	33	33	40	43	40	41
04	36	41	44	40	40	38	34	32	38	39	37	38
05	34	38	41	38	38	37	35	31	35	36	35	35
06	34	39	42	40	40	40	38	33	36	36	36	36
07	39	43	46	45	45	46	45	38	42	40	40	40
08	46	51	53	53	52	52	52	43	49	47	48	46
09	52	56	57	57	55	56	56	45	53	52	53	50
10	52	55	56	55	53	53	52	43	52	53	54	51
11	51	54	51	49	45	43	42	36	48	53	52	50
12	51	51	42	40	37	34	31	28	40	49	52	52
13	49	45	35	33	31	29	25	22	32	42	49	51
14	46	38	30	28	26	24	21	18	25	35	46	49
15	42	34	28	24	23	21	18	15	21	30	41	46
16	42	34	27	23	22	20	18	15	20	30	42	45
17	45	38	30	24	24	21	19	15	21	32	46	48
18	53	45	34	26	26	23	21	16	23	37	56	57
19	60	54	40	29	29	26	23	18	26	42	63	62
20	62	59	46	33	32	28	26	21	30	47	66	64
21	62	62	52	38	36	31	28	23	34	52	66	65
22	60	63	55	41	39	33	30	26	36	54	64	63
23	57	62	56	43	39	34	31	26	38	55	60	59
24	52	58	56	42	39	35	30	27	40	55	54	53