{"id":"rural-air-chemistry","slug":"rural-air-chemistry","name":"Rural Air Chemistry Dataset","description":"Weekly concentrations of sulphate, nitrate, ammonium, sulphur dioxide and nitric acid in rural American air, measured on filter packs at CASTNET sites since 1987. These are the species that acidify and fertilise catchments when they reach the ground — but an air concentration is not a water concentration, and nothing here measures a river.","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.","domains":["atmospheric"],"metricIds":["air-particulate-sulphate","air-particulate-nitrate","air-particulate-ammonium","air-sulphur-dioxide","air-nitric-acid"],"subjectScope":"United States CASTNET rural monitoring sites with a long record","cadence":"weekly","version":"1.0.0","released":"2026-08-28","changelog":[{"version":"1.0.0","date":"2026-08-28","summary":"First release.","additions":["Five measured air species at United States rural sites, from 1987."]}],"limitations":["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."],"context":{"hydrionisClass":"related-context","whyHeld":"Sulphur and nitrogen falling out of the air acidified thousands of lakes and streams across the northern hemisphere, and their decline after emission controls is one of the few large-scale environmental repairs with a measured record. That record is a water story told in air measurements, which is why it is here — as context sitting beside water chemistry, never as a substitute for it.","waterRelevance":["The airborne source of acidifying and nutrient inputs to catchments","The long decline in sulphur, against which acidified water recovery is read","Nitrogen, the input that did not fall when sulphur did","Context for the deposition fluxes held separately"],"suitableUses":["Tracking a species in rural American air at one site over decades","Comparing the trajectories of sulphur and nitrogen species over the same period","Providing atmospheric context alongside a water chemistry record, with the pathway stated"],"unsuitableUses":["Reading an air concentration as a water concentration, or converting between them","Inferring that a water quality change was caused by air pollution because the two moved together; deposition, soil chemistry, geology, land use and residence time all sit between them and none is held here","Treating these as urban air quality or as a measure of what people breathe","Reading a weekly value as a peak concentration","Computing deposition from these; the flux is held separately and is a modelled quantity"],"joinDimensions":[{"kind":"measured-proximity","on":"Publisher-stated site coordinates","limitation":"Proximity is not catchment membership. A site has a position and nothing more: Hydrionis holds no catchment geometry, so a monitoring site near a river gauge may sit outside its catchment entirely, nearness does not establish that this air deposits onto that catchment, and prevailing wind is not held either."},{"kind":"time-only","on":"Weekly periods, against any other Hydrionis series","limitation":"Air responds within hours and catchment water chemistry within months to decades. A same-week alignment compares a fast signal with a slow one, and the lag between them is real, unmeasured here, and varies with soil and geology."}],"relationshipConfidence":"contextual","eligibility":{"api":true,"download":true,"overlay":true,"publicPage":false}},"observationCount":728595,"seriesCount":607,"subjectCount":123,"from":1987,"to":2026,"sourceIds":["us-epa-castnet"],"units":["µg/m³"],"redistributable":true,"commerciallyReusable":true,"citation":"Hydrionis (2026), Rural Air Chemistry Dataset, version 1.0.0. Retrieved from https://hydrionis.com/data/datasets/rural-air-chemistry/","downloads":[{"file":"/downloads/observations-atmospheric-us-epa-castnet.csv","format":"CSV","label":"atmospheric observations from EPA CASTNET, part 1 of 2 (1,000,000 rows)"},{"file":"/downloads/observations-atmospheric-us-epa-castnet-part2.csv","format":"CSV","label":"atmospheric observations from EPA CASTNET, part 2 of 2 (426,840 rows)"}],"apiPath":"/api/v1/datasets/rural-air-chemistry.json","lastFetched":"2026-08-28","fields":[{"id":"air-particulate-sulphate","name":"Particulate sulphate in air","definition":"Concentration of sulphate on the Teflon filter of a weekly filter-pack sample, in micrograms per cubic metre of air.","unit":"µg/m³","nativeGeography":["monitoring-station"],"comparabilityNotes":"An air concentration, 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 no value here can be read as saying what is dissolved in a river. Measured on a filter pack exposed for a week at a rural site, so it is a weekly average of the air passing one place rather than a peak or an instantaneous reading. CASTNET sites are deliberately rural and away from local sources, chosen to represent regional air rather than the air anybody breathes in a town. They are not urban air quality and should not be read as exposure. Weeks in which the substance was below the detection limit are held as censored observations: the week keeps its place, it carries no number because no exact value exists, and it carries the operator, the threshold and the fact that the limit is one of detection. A reader sees \"<0.06 µg/m³ — below the analytical detection limit\" rather than a gap or a number nobody measured. This affects 3.2% of nitrate weeks, 1.3% of sulphur dioxide weeks and under 1% of the rest."},{"id":"air-particulate-nitrate","name":"Particulate nitrate in air","definition":"Concentration of nitrate on the Teflon filter of a weekly filter-pack sample, in micrograms per cubic metre of air.","unit":"µg/m³","nativeGeography":["monitoring-station"],"comparabilityNotes":"An air concentration, 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 no value here can be read as saying what is dissolved in a river. Measured on a filter pack exposed for a week at a rural site, so it is a weekly average of the air passing one place rather than a peak or an instantaneous reading. CASTNET sites are deliberately rural and away from local sources, chosen to represent regional air rather than the air anybody breathes in a town. They are not urban air quality and should not be read as exposure. Weeks in which the substance was below the detection limit are held as censored observations: the week keeps its place, it carries no number because no exact value exists, and it carries the operator, the threshold and the fact that the limit is one of detection. A reader sees \"<0.06 µg/m³ — below the analytical detection limit\" rather than a gap or a number nobody measured. This affects 3.2% of nitrate weeks, 1.3% of sulphur dioxide weeks and under 1% of the rest. Particulate nitrate and nitric acid interchange on the filter with temperature, so the two are reported separately and the split between them is less reliable than their sum."},{"id":"air-particulate-ammonium","name":"Particulate ammonium in air","definition":"Concentration of ammonium on the Teflon filter of a weekly filter-pack sample, in micrograms per cubic metre of air.","unit":"µg/m³","nativeGeography":["monitoring-station"],"comparabilityNotes":"An air concentration, 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 no value here can be read as saying what is dissolved in a river. Measured on a filter pack exposed for a week at a rural site, so it is a weekly average of the air passing one place rather than a peak or an instantaneous reading. CASTNET sites are deliberately rural and away from local sources, chosen to represent regional air rather than the air anybody breathes in a town. They are not urban air quality and should not be read as exposure. Weeks in which the substance was below the detection limit are held as censored observations: the week keeps its place, it carries no number because no exact value exists, and it carries the operator, the threshold and the fact that the limit is one of detection. A reader sees \"<0.06 µg/m³ — below the analytical detection limit\" rather than a gap or a number nobody measured. This affects 3.2% of nitrate weeks, 1.3% of sulphur dioxide weeks and under 1% of the rest."},{"id":"air-sulphur-dioxide","name":"Sulphur dioxide in air","definition":"Concentration of sulphur dioxide on the Whatman filter of a weekly filter-pack sample, in micrograms per cubic metre of air.","unit":"µg/m³","nativeGeography":["monitoring-station"],"comparabilityNotes":"An air concentration, 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 no value here can be read as saying what is dissolved in a river. Measured on a filter pack exposed for a week at a rural site, so it is a weekly average of the air passing one place rather than a peak or an instantaneous reading. CASTNET sites are deliberately rural and away from local sources, chosen to represent regional air rather than the air anybody breathes in a town. They are not urban air quality and should not be read as exposure. Weeks in which the substance was below the detection limit are held as censored observations: the week keeps its place, it carries no number because no exact value exists, and it carries the operator, the threshold and the fact that the limit is one of detection. A reader sees \"<0.06 µg/m³ — below the analytical detection limit\" rather than a gap or a number nobody measured. This affects 3.2% of nitrate weeks, 1.3% of sulphur dioxide weeks and under 1% of the rest. The share of weeks below detection did rise as air cleaned, from 0.3% in the 1980s to 2.3% in the 2020s, but it remains small enough that the omission cannot account for the observed decline."},{"id":"air-nitric-acid","name":"Nitric acid in air","definition":"Concentration of nitric acid on the nylon filter of a weekly filter-pack sample, in micrograms per cubic metre of air.","unit":"µg/m³","nativeGeography":["monitoring-station"],"comparabilityNotes":"An air concentration, 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 no value here can be read as saying what is dissolved in a river. Measured on a filter pack exposed for a week at a rural site, so it is a weekly average of the air passing one place rather than a peak or an instantaneous reading. CASTNET sites are deliberately rural and away from local sources, chosen to represent regional air rather than the air anybody breathes in a town. They are not urban air quality and should not be read as exposure. Weeks in which the substance was below the detection limit are held as censored observations: the week keeps its place, it carries no number because no exact value exists, and it carries the operator, the threshold and the fact that the limit is one of detection. A reader sees \"<0.06 µg/m³ — below the analytical detection limit\" rather than a gap or a number nobody measured. This affects 3.2% of nitrate weeks, 1.3% of sulphur dioxide weeks and under 1% of the rest. Interchanges with particulate nitrate on the filter with temperature, so the split between the two is less reliable than their sum."}]}