~ $ 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, 1991–2025

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
01	48	55	55	44	39	37	31	30	43	52	49	49
02	43	50	52	44	42	38	33	32	43	48	45	44
03	40	45	49	42	42	37	34	33	40	44	40	41
04	36	41	44	41	40	38	35	33	38	39	37	38
05	34	38	42	38	39	38	35	31	36	36	35	35
06	35	40	42	40	40	40	39	33	37	37	36	36
07	40	44	47	46	46	46	46	38	42	41	40	40
08	47	52	54	53	52	53	53	44	49	48	48	46
09	53	57	58	57	56	57	57	46	53	52	53	50
10	52	56	57	56	53	53	53	44	53	53	54	51
11	51	55	52	50	46	44	43	37	49	53	52	50
12	52	52	43	41	37	34	31	29	41	49	52	52
13	50	46	36	34	31	29	26	23	32	42	49	51
14	47	39	31	29	26	25	22	18	25	36	46	49
15	43	35	28	24	23	21	18	15	21	31	41	46
16	42	35	28	24	23	21	19	15	20	30	42	45
17	46	38	31	24	24	21	20	15	22	32	46	48
18	54	46	35	26	27	23	21	16	24	37	56	57
19	61	55	41	29	29	26	23	18	27	42	63	62
20	63	60	47	33	33	29	26	21	30	48	66	64
21	63	64	53	39	37	32	28	24	35	52	66	65
22	61	65	56	42	39	34	30	26	37	55	64	63
23	58	63	57	43	40	34	31	27	38	56	60	59
24	53	59	57	43	39	35	30	28	41	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, last added to on 21 September 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 only when a year has ended.

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
01	48	55	55	44	39	37	31	30	43	52	49	49
02	43	50	52	44	42	38	33	32	43	48	45	44
03	40	45	49	42	42	37	34	33	40	44	40	41
04	36	41	44	41	40	38	35	33	38	39	37	38
05	34	38	42	38	39	38	35	31	36	36	35	35
06	35	40	42	40	40	40	39	33	37	37	36	36
07	40	44	47	46	46	46	46	38	42	41	40	40
08	47	52	54	53	52	53	53	44	49	48	48	46
09	53	57	58	57	56	57	57	46	53	52	53	50
10	52	56	57	56	53	53	53	44	53	53	54	51
11	51	55	52	50	46	44	43	37	49	53	52	50
12	52	52	43	41	37	34	31	29	41	49	52	52
13	50	46	36	34	31	29	26	23	32	42	49	51
14	47	39	31	29	26	25	22	18	25	36	46	49
15	43	35	28	24	23	21	18	15	21	31	41	46
16	42	35	28	24	23	21	19	15	20	30	42	45
17	46	38	31	24	24	21	20	15	22	32	46	48
18	54	46	35	26	27	23	21	16	24	37	56	57
19	61	55	41	29	29	26	23	18	27	42	63	62
20	63	60	47	33	33	29	26	21	30	48	66	64
21	63	64	53	39	37	32	28	24	35	52	66	65
22	61	65	56	42	39	34	30	26	37	55	64	63
23	58	63	57	43	40	34	31	27	38	56	60	59
24	53	59	57	43	39	35	30	28	41	55	54	53

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
01	48	55	55	44	39	37	31	30	43	52	49	49
02	43	50	52	44	42	38	33	32	43	48	45	44
03	40	45	49	42	42	37	34	33	40	44	40	41
04	36	41	44	41	40	38	35	33	38	39	37	38
05	34	38	42	38	39	38	35	31	36	36	35	35
06	35	40	42	40	40	40	39	33	37	37	36	36
07	40	44	47	46	46	46	46	38	42	41	40	40
08	47	52	54	53	52	53	53	44	49	48	48	46
09	53	57	58	57	56	57	57	46	53	52	53	50
10	52	56	57	56	53	53	53	44	53	53	54	51
11	51	55	52	50	46	44	43	37	49	53	52	50
12	52	52	43	41	37	34	31	29	41	49	52	52
13	50	46	36	34	31	29	26	23	32	42	49	51
14	47	39	31	29	26	25	22	18	25	36	46	49
15	43	35	28	24	23	21	18	15	21	31	41	46
16	42	35	28	24	23	21	19	15	20	30	42	45
17	46	38	31	24	24	21	20	15	22	32	46	48
18	54	46	35	26	27	23	21	16	24	37	56	57
19	61	55	41	29	29	26	23	18	27	42	63	62
20	63	60	47	33	33	29	26	21	30	48	66	64
21	63	64	53	39	37	32	28	24	35	52	66	65
22	61	65	56	42	39	34	30	26	37	55	64	63
23	58	63	57	43	40	34	31	27	38	56	60	59
24	53	59	57	43	39	35	30	28	41	55	54	53

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
01	48	55	55	44	39	37	31	30	43	52	49	49
02	43	50	52	44	42	38	33	32	43	48	45	44
03	40	45	49	42	42	37	34	33	40	44	40	41
04	36	41	44	41	40	38	35	33	38	39	37	38
05	34	38	42	38	39	38	35	31	36	36	35	35
06	35	40	42	40	40	40	39	33	37	37	36	36
07	40	44	47	46	46	46	46	38	42	41	40	40
08	47	52	54	53	52	53	53	44	49	48	48	46
09	53	57	58	57	56	57	57	46	53	52	53	50
10	52	56	57	56	53	53	53	44	53	53	54	51
11	51	55	52	50	46	44	43	37	49	53	52	50
12	52	52	43	41	37	34	31	29	41	49	52	52
13	50	46	36	34	31	29	26	23	32	42	49	51
14	47	39	31	29	26	25	22	18	25	36	46	49
15	43	35	28	24	23	21	18	15	21	31	41	46
16	42	35	28	24	23	21	19	15	20	30	42	45
17	46	38	31	24	24	21	20	15	22	32	46	48
18	54	46	35	26	27	23	21	16	24	37	56	57
19	61	55	41	29	29	26	23	18	27	42	63	62
20	63	60	47	33	33	29	26	21	30	48	66	64
21	63	64	53	39	37	32	28	24	35	52	66	65
22	61	65	56	42	39	34	30	26	37	55	64	63
23	58	63	57	43	40	34	31	27	38	56	60	59
24	53	59	57	43	39	35	30	28	41	55	54	53