Rural Air Chemistry Dataset

728,595

Observations

607

Series

123

Subjects

1987–2026

Period covered

What this looks like

One real series from the dataset, and real rows from the head of its published extract. Neither is a mock-up: the rows below are the same rows, unchanged, that are in the file you would download.

Nitric acid in air at Vincennes

The longest single series this dataset holds — 1,989 weekly values, one of 607. Drawn from 600 of them, evenly spaced across the record. The dataset is not this chart.

Drawn to show the shape and resolution of what the dataset contains, not to make a point about Vincennes. Gaps are breaks, never interpolated.

View the data behind this chart

240 of 462 periods, evenly spaced across the record. Every value is in the download and the API; a table of all 462 would be longer than this page and unusable with a screen reader.
Period	Vincennes (µg/m³)
1987.6666666666667	1.43
1987.8333333333333	1.72
1988	1.68
1988.1666666666667	1.70
1988.3333333333333	2.69
1988.5	3.55
1988.6666666666667	2.14
1988.8333333333333	0.979
1988.9166666666667	1.20
1989.0833333333333	1.45
1989.25	4.59
1989.4166666666667	3.73
1989.5833333333333	4.18
1989.75	1.41
1989.9166666666667	1.32
1990.0833333333333	1.33
1990.25	2.20
1990.4166666666667	4.89
1990.5833333333333	5.43
1990.75	2.85
1990.9166666666667	2.32
1991.0833333333333	2.80
1991.1666666666667	2.71
1991.3333333333333	3.13
1991.5	3.29
1991.6666666666667	2.07
1991.8333333333333	1.66
1992	0.915
1992.1666666666667	2.13
1992.3333333333333	4.32
1992.5	2.60
1992.6666666666667	2.40
1992.8333333333333	1.76
1993	1.16
1993.1666666666667	1.64
1993.3333333333333	2.58
1993.4166666666667	3.69
1993.5833333333333	3.71
1993.75	1.82
1993.9166666666667	2.05
1994.0833333333333	1.23
1994.25	1.67
1994.4166666666667	5.28
1994.5833333333333	4.12
1994.75	1.53
1994.9166666666667	2.67
1995.0833333333333	1.15
1995.25	1.74
1995.4166666666667	5.86
1995.6666666666667	2.31
1995.75	2.51
1995.9166666666667	1.49
1996.1666666666667	0.866
1996.3333333333333	3.09
1996.5	4.40
1996.6666666666667	1.41
1996.8333333333333	0.957
1997	0.984
1997.1666666666667	1.56
1997.3333333333333	2.25
1997.5	4.55
1997.6666666666667	1.99
1997.8333333333333	1.24
1998	0.930
1998.0833333333333	1.57
1998.25	2.29
1998.4166666666667	3.12
1998.5833333333333	3.46
1998.75	1.37
1998.9166666666667	1.72
1999.0833333333333	1.63
1999.25	2.09
1999.4166666666667	4.04
1999.5833333333333	3.61
1999.75	1.98
1999.9166666666667	1.40
2000.0833333333333	1.52
2000.25	2.05
2000.3333333333333	2.23
2000.5	3.39
2000.6666666666667	0.875
2000.8333333333333	0.630
2001	2.57
2001.1666666666667	1.36
2001.3333333333333	1.39
2001.5	1.69
2001.6666666666667	2.13
2001.8333333333333	1.21
2002	0.618
2002.1666666666667	2.36
2002.3333333333333	1.94
2002.5	4.97
2002.5833333333333	2.83
2002.75	1.27
2002.9166666666667	1.19
2003.0833333333333	0.974
2003.25	1.69
2003.4166666666667	3.78
2003.5833333333333	3.21
2003.75	1.62
2003.9166666666667	1.41
2004.0833333333333	2.22
2004.25	2.02
2004.4166666666667	1.64
2004.5833333333333	1.88
2004.75	0.800
2004.8333333333333	1.50
2005	1.38
2005.1666666666667	1.46
2005.3333333333333	2.04
2005.5833333333333	2.51
2005.75	2.38
2005.9166666666667	1.37
2006.0833333333333	0.922
2006.25	1.46
2006.4166666666667	1.62
2006.5833333333333	2.31
2006.75	1.11
2006.9166666666667	1.10
2007.0833333333333	0.992
2007.1666666666667	1.86
2007.3333333333333	1.61
2007.5	2.80
2007.6666666666667	1.62
2007.8333333333333	0.734
2008	0.851
2008.1666666666667	1.16
2008.3333333333333	1.75
2008.5	2.01
2008.6666666666667	1.01
2008.8333333333333	0.550
2009	0.763
2009.1666666666667	1.82
2009.3333333333333	1.64
2009.4166666666667	1.19
2009.5833333333333	1.60
2009.75	0.987
2009.9166666666667	0.696
2010.0833333333333	0.405
2010.25	1.84
2010.4166666666667	1.82
2010.5833333333333	1.13
2010.75	1.08
2010.9166666666667	0.893
2011.0833333333333	1.04
2011.25	0.745
2011.4166666666667	1.96
2011.5833333333333	1.57
2011.6666666666667	1.46
2011.8333333333333	0.661
2012	0.906
2012.1666666666667	1.52
2012.3333333333333	2.08
2012.5	2.14
2012.6666666666667	0.873
2012.8333333333333	0.914
2013	0.731
2013.1666666666667	0.549
2013.3333333333333	1.39
2013.5	1.86
2013.6666666666667	1.29
2013.8333333333333	0.799
2013.9166666666667	0.624
2014.0833333333333	1.72
2014.25	1.99
2014.4166666666667	1.47
2014.5833333333333	0.616
2014.75	0.945
2014.9166666666667	0.951
2015.0833333333333	0.517
2015.25	0.812
2015.4166666666667	1.44
2015.5833333333333	0.714
2015.75	1.21
2015.9166666666667	0.579
2016.0833333333333	1.22
2016.1666666666667	0.698
2016.3333333333333	0.944
2016.5	0.733
2016.6666666666667	0.968
2016.8333333333333	0.936
2017	0.361
2017.1666666666667	0.875
2017.3333333333333	1.62
2017.5	0.849
2017.6666666666667	0.492
2017.8333333333333	0.780
2018	0.370
2018.1666666666667	0.647
2018.3333333333333	1.07
2018.4166666666667	0.528
2018.5833333333333	1.18
2018.75	0.625
2018.9166666666667	0.387
2019.0833333333333	0.138
2019.25	0.988
2019.4166666666667	0.853
2019.5833333333333	0.747
2019.75	0.377
2019.9166666666667	0.677
2020.0833333333333	0.497
2020.25	0.843
2020.4166666666667	0.987
2020.5833333333333	0.908
2020.6666666666667	0.754
2020.8333333333333	0.515
2021	0.509
2021.1666666666667	1.32
2021.3333333333333	1.04
2021.5	0.844
2021.6666666666667	0.565
2021.8333333333333	1.03
2022	0.337
2022.1666666666667	0.980
2022.3333333333333	0.864
2022.5	0.541
2022.6666666666667	0.747
2022.8333333333333	0.477
2022.9166666666667	0.214
2023.0833333333333	0.610
2023.25	0.933
2023.4166666666667	1.68
2023.5833333333333	0.722
2023.75	0.275
2023.9166666666667	0.682
2024.0833333333333	1.05
2024.25	0.756
2024.4166666666667	1.19
2024.5833333333333	0.731
2024.75	0.625
2024.9166666666667	0.435
2025.0833333333333	0.386
2025.1666666666667	0.910
2025.3333333333333	1.08
2025.5	0.651
2025.6666666666667	0.425
2025.8333333333333	0.556
2026	0.279
2026.1666666666667	0.792
2026.3333333333333	0.891

Sample rows

12 of 1,426,840 rows Taken from the start of observations-atmospheric-us-epa-castnet.csv. These are sample rows, not the dataset.

Sample rows from the Rural Air Chemistry Dataset extract
`metric_id`	`subject_id`	`subject_kind`	`value`	`censored`	`censor_operator`	`censor_threshold`	`censor_limit`	`censor_reported`	`unit`	`period_start`	`period_end`	`period_kind`	`basis`	`confidence`	`source_id`	`source_ref`	`qualifier`	`superseded`	`retrieved`
air-nitric-acid	castnet-abt147	geography	1.5911	—	—	—	—	—	µg/m³	1994-01-06	—	week	measured	high	us-epa-castnet	drychem.csv	—	—	2026-08-28
air-nitric-acid	castnet-abt147	geography	2.0533	—	—	—	—	—	µg/m³	1994-01-11	—	week	measured	high	us-epa-castnet	drychem.csv	—	—	2026-08-28
air-nitric-acid	castnet-abt147	geography	2.7979	—	—	—	—	—	µg/m³	1994-01-21	—	week	measured	high	us-epa-castnet	drychem.csv	—	—	2026-08-28
air-nitric-acid	castnet-abt147	geography	1.4102	—	—	—	—	—	µg/m³	1994-01-25	—	week	measured	high	us-epa-castnet	drychem.csv	—	—	2026-08-28
air-nitric-acid	castnet-abt147	geography	4.2669	—	—	—	—	—	µg/m³	1994-02-01	—	week	measured	high	us-epa-castnet	drychem.csv	—	—	2026-08-28
air-nitric-acid	castnet-abt147	geography	1.4562	—	—	—	—	—	µg/m³	1994-02-08	—	week	measured	high	us-epa-castnet	drychem.csv	—	—	2026-08-28
air-nitric-acid	castnet-abt147	geography	3.8997	—	—	—	—	—	µg/m³	1994-02-15	—	week	measured	high	us-epa-castnet	drychem.csv	—	—	2026-08-28
air-nitric-acid	castnet-abt147	geography	1.6216	—	—	—	—	—	µg/m³	1994-03-01	—	week	measured	high	us-epa-castnet	drychem.csv	—	—	2026-08-28
air-nitric-acid	castnet-abt147	geography	2.4045	—	—	—	—	—	µg/m³	1994-03-08	—	week	measured	high	us-epa-castnet	drychem.csv	—	—	2026-08-28
air-nitric-acid	castnet-abt147	geography	2.1742	—	—	—	—	—	µg/m³	1994-03-15	—	week	measured	high	us-epa-castnet	drychem.csv	—	—	2026-08-28
air-nitric-acid	castnet-abt147	geography	2.6266	—	—	—	—	—	µg/m³	1994-03-29	—	week	measured	high	us-epa-castnet	drychem.csv	—	—	2026-08-28
air-nitric-acid	castnet-abt147	geography	1.7103	—	—	—	—	—	µg/m³	1994-04-05	—	week	measured	high	us-epa-castnet	drychem.csv	—	—	2026-08-28

Download these rows as CSV — the same twelve, with the column header, for testing a loader before committing to the full file.

Methodology

Collected from the EPA CASTNET open data directory. Each value is a laboratory analysis of a filter exposed for one week at a rural site. The publisher flags every measurement and the flags are read rather than the file being treated as uniform: values flagged invalid, not analysed, or with failed flow carry no observation at all, while values below the detection limit are stored as censored observations carrying their threshold, because the number the publisher prints for them is the reporting limit rather than a measurement. Sites with fewer than 200 weekly samples are not held.

What this dataset cannot tell you

Read before relying on these figures

An air concentration is not a water concentration. Micrograms per cubic metre of air is a different quantity in a different medium from milligrams per litre of water, and the path between them runs through deposition, rainfall, soil and geology, none of which is held here.
Weeks below the detection limit are held as censored observations rather than as values or gaps: the point carries the operator, the threshold and the kind of limit, and has no exact value because none was measured. This affects 3.2% of nitrate weeks, 1.3% of sulphur dioxide weeks and under 1% of the rest. A statistic computed over them must declare how it treats them; Hydrionis does not substitute the limit, half the limit, or zero.
A weekly average of the air passing one place. Not a peak, not an instantaneous reading, and not what anyone breathes: CASTNET sites are deliberately rural and sited away from local sources to represent regional air.
Sulphur dioxide has a very long tail — a median under one microgram per cubic metre against a maximum of 206 — so its mean is a poor summary and its distribution is the honest one.
Particulate nitrate and nitric acid interchange on the filter with temperature, so the split between those two is less reliable than their sum.
The publisher also computes total sulphur dioxide and total nitrate by combining filters. Those composites carry no quality flag of their own and are not held.
United States only. There is no equivalent series here for any other country.

Coverage

Subjects	United States CASTNET rural monitoring sites with a long record — 123 held
Period	1987–2026
Units	µg/m³
Update cadence	Weekly
Identifier	`rural-air-chemistry` v1.0.0

Fields

Data dictionary →

Every metric this dataset carries, with the unit it is stored in and the geography level it is reported at. Full definitions are in the data dictionary.

Identifier	Field	Unit	Reported at
`air-particulate-sulphate`	Particulate sulphate in air Concentration of sulphate on the Teflon filter of a weekly filter-pack sample, in micrograms per cubic metre of air.	µg/m³	monitoring-station
`air-particulate-nitrate`	Particulate nitrate in air Concentration of nitrate on the Teflon filter of a weekly filter-pack sample, in micrograms per cubic metre of air.	µg/m³	monitoring-station
`air-particulate-ammonium`	Particulate ammonium in air Concentration of ammonium on the Teflon filter of a weekly filter-pack sample, in micrograms per cubic metre of air.	µg/m³	monitoring-station
`air-sulphur-dioxide`	Sulphur dioxide in air Concentration of sulphur dioxide on the Whatman filter of a weekly filter-pack sample, in micrograms per cubic metre of air.	µg/m³	monitoring-station
`air-nitric-acid`	Nitric acid in air Concentration of nitric acid on the nylon filter of a weekly filter-pack sample, in micrograms per cubic metre of air.	µg/m³	monitoring-station

Rights and access

Licence position

Open. Derivative use true; redistribution true; commercial reuse true.

Applies to material authored by US federal agencies. Third-party material carried within a federal product may be separately protected.

Permitted uses

Public display
Derived analysis
API delivery
Bulk download
Commercial use

Available to: Public, Developer, Research, Professional, Enterprise. These follow from the rights of the contributing publishers, not from a Hydrionis pricing decision — no access tier can widen what a publisher withheld.

Contributing sources

United States Environmental Protection Agency — Clean Air Status and Trends Network (US Government work — public domain)

Attribution and citation

Cite and reuse →

Two different obligations, deliberately kept apart. One is a condition of somebody else's licence; the other is a request Hydrionis has no standing to enforce.

Suggested citation for Hydrionis

Hydrionis (2026), Rural Air Chemistry Dataset, version 1.0.0, accessed 2026-09-02, hydrionis.com/data/datasets/rural-air-chemistry/

Cite Hydrionis where you rely on the compilation, the normalisation, the eligibility judgements or a chart. Where you use an unmodified published value, the publisher’s attribution is the one that is required; crediting Hydrionis as well is welcome and is not something Hydrionis can require of you.

Recommended, not required. Hydrionis does not claim rights over unmodified public data it relayed, and doing arithmetic on somebody else’s measurements does not make them Hydrionis’s. Cite the version and the access date: Rural Air Chemistry Dataset changes as publishers release and as methods are revised, and a citation without both cannot be checked later. Full guidance is on cite and reuse.

None of the contributing licences requires attribution. Crediting the publishers is still good practice and they are listed above.

Changelog

Version 1.0.0 — 2026-08-28

First release.

Additions

Five measured air species at United States rural sites, from 1987.

Use this data

Download atmospheric observations from EPA CASTNET, part 1 of 2 (1,000,000 rows) (CSV)
Download atmospheric observations from EPA CASTNET, part 2 of 2 (426,840 rows) (CSV)
Machine-readable JSON /api/v1/datasets/rural-air-chemistry.json
How to cite this, and who you must credit
How these figures were produced
Request this at a different scale, format or geography

Every bulk extract ships with a companion metadata file carrying this dataset’s version, its sources, their licences, the attribution you are required to give and the suggested citation — so the terms travel with the file rather than staying on this page.

About this data

The fields a professional user needs to judge whether this is fit for a particular purpose. Deliberately not a score.

About this data

Everything below is a property of the data rather than a rating of it. There is deliberately no single confidence score: how much a gap matters depends on what you are about to do with the series, and only you know that.

Source	United States Environmental Protection Agency — Clean Air Status and Trends Network · US Government work — public domain
How the values came to exist	Mixed across the series — the basis is recorded on every individual value.
Period held	1987–2026
Known gaps	An air concentration is not a water concentration. Micrograms per cubic metre of air is a different quantity in a different medium from milligrams per litre of water, and the path between them runs through deposition, rainfall, soil and geology, none of which is held here. Weeks below the detection limit are held as censored observations rather than as values or gaps: the point carries the operator, the threshold and the kind of limit, and has no exact value because none was measured. This affects 3.2% of nitrate weeks, 1.3% of sulphur dioxide weeks and under 1% of the rest. A statistic computed over them must declare how it treats them; Hydrionis does not substitute the limit, half the limit, or zero. A weekly average of the air passing one place. Not a peak, not an instantaneous reading, and not what anyone breathes: CASTNET sites are deliberately rural and sited away from local sources to represent regional air.
Continuity	Version 1.0.0, released 2026-08-28. 1 recorded version; breaking changes and corrections are listed in the changelog above.
Last read from the publisher	2026-08-28 · publisher updates weekly
Redistribution	Permitted by the upstream licences
Commercial reuse	Permitted by the upstream licences. No Hydrionis access tier can widen this.
Methodology	How this was produced