{"id":"dry-deposition","slug":"dry-deposition","name":"Dry Atmospheric Deposition Dataset","description":"Weekly dry deposition of sulphate, sulphur dioxide, nitrate, nitric acid and ammonium onto United States CASTNET sites since 1987, in kilograms per hectare. This is the acidifying and nutrient load actually reaching the ground without rain — the step between air chemistry and catchment chemistry, and the only part of that chain Hydrionis holds.","methodology":"Collected from the EPA CASTNET open data directory and stored as published. The flux is not measured: the publisher computes it as the measured weekly air concentration multiplied by a deposition velocity modelled from meteorology, land cover and surface properties, under the inferential method. Every value is therefore stored with basis modelled. The publisher reports what share of each week’s deposition velocity had to be substituted for missing inputs, and that share is not uniform through the record.","domains":["atmospheric"],"metricIds":["sulphate-deposition-dry","sulphur-dioxide-deposition-dry","nitrate-deposition-dry","nitric-acid-deposition-dry","ammonium-deposition-dry"],"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 modelled dry deposition fluxes at United States rural sites, from 1987."]}],"limitations":["Modelled, not measured. The concentration was observed; the deposition velocity was not. The flux is their product and inherits every assumption in the velocity model.","The deposition velocity is substituted where the inputs to model it are missing. Across the record 38% of weeks are fully substituted, and in 2017 to 2021 that reaches 85 to 87% as on-site meteorology was reduced — so the series carries a change in how it was produced partway through, and a trend read across that change is partly a trend in method.","Dry deposition only. Wet deposition — what rain brings down, and often the larger share — is measured by a different programme and is not held, so nothing here is total deposition and total cannot be computed from it.","A flux at a site is not additive across sites. Adding two weekly fluxes from different places produces a number with no area behind it.","Deposition onto a monitoring site is not deposition onto a catchment. It is a point estimate for one surface, and real catchments vary in land cover, roughness and elevation, all of which change the velocity.","United States only."],"context":{"hydrionisClass":"water-driver","whyHeld":"Deposition is the actual pathway from air to water: the mechanism by which industrial emissions acidified upland streams and by which nitrogen still enriches them. Holding the flux rather than only the concentration is what makes this a driver of water chemistry instead of general environmental background. It remains a modelled quantity and it measures what reached a monitoring site, not what reached a river.","waterRelevance":["The acidifying load arriving on catchments without rain","Nitrogen input driving both acidification and nutrient enrichment","The measured record of recovery after emission controls","The step that connects air chemistry to water chemistry"],"suitableUses":["Tracking dry deposition of one species at one site over decades, within a consistent period","Comparing sulphur and nitrogen deposition trajectories at the same site","Providing a deposition context for long water chemistry records, with the modelled basis stated"],"unsuitableUses":["Reading these as measurements","Reading a trend across the change in deposition velocity substitution without accounting for it","Treating this as total deposition, or adding it to a wet deposition figure from elsewhere without checking the two are commensurable","Attributing an observed change in water chemistry to deposition because the two moved together; soil buffering, geology, land use and catchment residence time all intervene, and the recovery of an acidified stream lags its deposition by years to decades","Applying a site flux to a catchment, a region or a country"],"joinDimensions":[{"kind":"measured-proximity","on":"Publisher-stated site coordinates","limitation":"Proximity is not catchment membership. The flux is modelled for the surface at one monitoring site, which may sit outside the catchment of any gauge near it, and nearness establishes neither that the same load fell on that catchment nor which way the wind blew."},{"kind":"time-only","on":"Weekly periods, against any other Hydrionis series","limitation":"Deposition acts on catchments over years. A same-week alignment with a water chemistry reading compares an input with a state that reflects decades of previous inputs."}],"relationshipConfidence":"contextual","eligibility":{"api":true,"download":true,"overlay":true,"publicPage":false}},"observationCount":698245,"seriesCount":525,"subjectCount":105,"from":1987,"to":2026,"sourceIds":["us-epa-castnet"],"units":["kg/ha"],"redistributable":true,"commerciallyReusable":true,"citation":"Hydrionis (2026), Dry Atmospheric Deposition Dataset, version 1.0.0. Retrieved from https://hydrionis.com/data/datasets/dry-deposition/","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/dry-deposition.json","lastFetched":"2026-08-28","fields":[{"id":"sulphate-deposition-dry","name":"Dry sulphate deposition","definition":"Modelled dry deposition of sulphate over one week, in kilograms per hectare.","unit":"kg/ha","nativeGeography":["monitoring-station"],"comparabilityNotes":"A modelled quantity, not a measurement. The flux is the measured air concentration multiplied by a deposition velocity the publisher models from meteorology, land cover and surface properties. The concentration is observed; the velocity is not, and the product inherits every assumption in it. The deposition velocity is substituted where the inputs to model it are missing, and the publisher reports how much of each week was substituted. Across the record 38% of weeks are fully substituted, and in 2017 to 2021 that reaches 85 to 87% — so the series carries a change in how it was produced partway through, and a trend read across that change is partly a trend in method. Dry deposition only. What rain brings down is measured by a separate network and is not held, so nothing here is a total deposition figure and total cannot be computed from it. 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. A flux is not a concentration: this says how much reached a hectare of ground in a week, not how much was in the air."},{"id":"sulphur-dioxide-deposition-dry","name":"Dry sulphur dioxide deposition","definition":"Modelled dry deposition of sulphur dioxide over one week, in kilograms per hectare.","unit":"kg/ha","nativeGeography":["monitoring-station"],"comparabilityNotes":"A modelled quantity, not a measurement. The flux is the measured air concentration multiplied by a deposition velocity the publisher models from meteorology, land cover and surface properties. The concentration is observed; the velocity is not, and the product inherits every assumption in it. The deposition velocity is substituted where the inputs to model it are missing, and the publisher reports how much of each week was substituted. Across the record 38% of weeks are fully substituted, and in 2017 to 2021 that reaches 85 to 87% — so the series carries a change in how it was produced partway through, and a trend read across that change is partly a trend in method. Dry deposition only. What rain brings down is measured by a separate network and is not held, so nothing here is a total deposition figure and total cannot be computed from it. 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. A flux is not a concentration: this says how much reached a hectare of ground in a week, not how much was in the air."},{"id":"nitrate-deposition-dry","name":"Dry particulate nitrate deposition","definition":"Modelled dry deposition of particulate nitrate over one week, in kilograms per hectare.","unit":"kg/ha","nativeGeography":["monitoring-station"],"comparabilityNotes":"A modelled quantity, not a measurement. The flux is the measured air concentration multiplied by a deposition velocity the publisher models from meteorology, land cover and surface properties. The concentration is observed; the velocity is not, and the product inherits every assumption in it. The deposition velocity is substituted where the inputs to model it are missing, and the publisher reports how much of each week was substituted. Across the record 38% of weeks are fully substituted, and in 2017 to 2021 that reaches 85 to 87% — so the series carries a change in how it was produced partway through, and a trend read across that change is partly a trend in method. Dry deposition only. What rain brings down is measured by a separate network and is not held, so nothing here is a total deposition figure and total cannot be computed from it. 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. A flux is not a concentration: this says how much reached a hectare of ground in a week, not how much was in the air."},{"id":"nitric-acid-deposition-dry","name":"Dry nitric acid deposition","definition":"Modelled dry deposition of nitric acid over one week, in kilograms per hectare.","unit":"kg/ha","nativeGeography":["monitoring-station"],"comparabilityNotes":"A modelled quantity, not a measurement. The flux is the measured air concentration multiplied by a deposition velocity the publisher models from meteorology, land cover and surface properties. The concentration is observed; the velocity is not, and the product inherits every assumption in it. The deposition velocity is substituted where the inputs to model it are missing, and the publisher reports how much of each week was substituted. Across the record 38% of weeks are fully substituted, and in 2017 to 2021 that reaches 85 to 87% — so the series carries a change in how it was produced partway through, and a trend read across that change is partly a trend in method. Dry deposition only. What rain brings down is measured by a separate network and is not held, so nothing here is a total deposition figure and total cannot be computed from it. 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. A flux is not a concentration: this says how much reached a hectare of ground in a week, not how much was in the air."},{"id":"ammonium-deposition-dry","name":"Dry ammonium deposition","definition":"Modelled dry deposition of ammonium over one week, in kilograms per hectare.","unit":"kg/ha","nativeGeography":["monitoring-station"],"comparabilityNotes":"A modelled quantity, not a measurement. The flux is the measured air concentration multiplied by a deposition velocity the publisher models from meteorology, land cover and surface properties. The concentration is observed; the velocity is not, and the product inherits every assumption in it. The deposition velocity is substituted where the inputs to model it are missing, and the publisher reports how much of each week was substituted. Across the record 38% of weeks are fully substituted, and in 2017 to 2021 that reaches 85 to 87% — so the series carries a change in how it was produced partway through, and a trend read across that change is partly a trend in method. Dry deposition only. What rain brings down is measured by a separate network and is not held, so nothing here is a total deposition figure and total cannot be computed from it. 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. A flux is not a concentration: this says how much reached a hectare of ground in a week, not how much was in the air."}]}