European AQI Requirements
EAQI requirements
What the European Air Quality Index is?
The European Air Quality Index uses hourly concentrations of PM2.5, PM10, NO2, O3 and SO2. Each pollutant gets a sub-index and the worst one sets the overall level (Good / Fair / Moderate / Poor / Very poor / Extremely poor).
Current index bands (hourly concentrations, µg/m³) – in force since 4 July 2025
| Pollutant | Good | Fair | Moderate | Poor | Very poor | Extremely poor |
|---|---|---|---|---|---|---|
| PM2.5 | 0–5 | 6–15 | 16–50 | 51–90 | 91–140 | > 140 |
| PM10 | 0–15 | 16–45 | 46–120 | 121–195 | 196–270 | > 270 |
| O3 | 0–60 | 61–100 | 101–120 | 121–160 | 161–180 | > 180 |
| NO2 | 0–10 | 11–25 | 26–60 | 61–100 | 101–150 | > 150 |
| SO2 | 0–20 | 21–40 | 41–125 | 126–190 | 191–275 | > 275 |
Source: ETC HE Report 2024/17, Table 5.2, and the live EEA index page.
Station classification
The stations are classified:
- by predominant source: traffic, industrial, background
- by area: urban, suburban, rural (+ rural sub-types)
A station's index is shown as complete only if it reports the minimum set for its type:
- traffic stations need NO2 + PM,
- industrial and background stations need NO2 + O3 + PM
- SO2 is used when available but is never required.
For stations which have missing pollutants, the sub-indices are still computed per available pollutant. The circle is semi-transparent: grey when the partial index is good/fair/moderate, coloured when it is poor/very poor/extremely poor.
Missing hourly values are gap-filled with downscaled CAMS model data:
- difference method for NO2, PM2.5, PM10;
- multiplicative method for O3;
- no gap-filling for SO2.
Gap-filled values are marked with an asterisk in station details.
A pollutant's index is not determined if measurements are missing on three of the four previous days.
What is E2a?
E2a is one of the lettered data flows of the EU air quality e-reporting system set up by Implementing Decision 2011/850/EU (the "IPR Decision"). Its full name is "primary up-to-date assessment data, measurements" under Article 10 of that Decision. "Up-to-date" (UTD) means provisional, not yet validated hourly data, pushed to the EEA shortly after measurement. It is the only flow that feeds the EEA Air Quality Index, the "Air quality now" viewer, the public download service's "unverified" dataset, and the Copernicus CAMS near-real-time service.
Data flows and its content
| Flow | Content |
|---|---|
| B | Zones and agglomerations |
| C | Assesment regime (which sampling points assess which pollutant) |
| D | Metadata: networks, stations, sampling points, measurement processes |
| E1a | Validated primary measurements, full calendar year |
| E2a | Modelled and objective-estimation results |
| G | Attainment of environmental objectives |
| H to K | Air quality plans and measures |
How it is delivered?
Countries push XML files to an EEA-managed sFTP account. Alternatively the EEA harvests from a national Sensor Observation Service that implements the AQD data model. Credentials come from the AQ IPR helpdesk (aqipr.helpdesk@eionet.europa.eu). This is separate from the Reportnet/CDR envelopes used for B to K.
The data are sent only by the national data provider of a member, cooperating or voluntary reporting country. A private station never talks to the EEA directly.
To have data displayed on EEA website https://airindex.eea.europa.eu/AQI/index.html#, the national competent authority (ANPM/RNMCA) needs to be contacted.
What E2a file contains?
Each E2a file is the same XML structure as E1a. It has two parts:
- aqd:AQD_ReportingHeader: who reports, the reporting period (for UTD typically just the hours covered), and the change flag.
- One om:OM_Observation per sampling point and pollutant, following the INSPIRE Observations and Measurements model. The key elements are:
-
om:phenomenonTime: start and end of the period the values cover.
-
om:resultTime: when the result became available (for UTD, the same as the phenomenon time).
-
om:parameter AssessmentType: .../assessmenttype/fixed (or indicative).
-
om:parameter SamplingPoint: an xlink to the AQD_SamplingPoint identifier registered in Dataset D.
-
om:procedure: xlink to the AQD_SamplingPointProcess in Dataset D (the measurement method, equipment, equivalence).
-
om:featureOfInterest: xlink to the AQD_Sample (the inlet) in Dataset D.
-
om:observedProperty: pollutant URI from the Eionet vocabulary, for example .../aq/pollutant/8 for NO2 (SO2 is 1, PM10 is 5, O3 is 7, CO is 10, PM2.5 is 6001).
-
om:result: a swe:DataArray with five fields per row: StartTime, EndTime, Verification, Validity, Value. Rows are text-encoded, comma between fields and @@ between rows:
2013-10-23T00:00:00+00:00,2013-10-23T01:00:00+00:00,3,1,15@@2013-10-23T01:00:00+00:00,2013-10-23T02:00:00+00:00,3,1,9
Here 3 is the verification flag (not verified) and 1 the validity flag (valid), then the value in µg/m³.
-
Rules that govern the content
Time: ISO 8601 extended with UTC offset, in local winter time all year (no daylight saving), for example +02:00 for Romania. Hourly is the minimum resolution; half-hour raw data must be aggregated.
Units: µg/m³ for gases and PM, mg/m³ for CO, via the Eionet unit vocabulary.
PM: reported as equivalent to the EN 12341 gravimetric reference method, meaning the correction factor is already applied.
Validity flags: 1 valid; 2 valid but below detection limit; 3 valid, below detection limit, replaced by half the limit; -1 invalid or missing; -99 invalid due to maintenance or calibration.
Verification flags: 1 verified; 2 preliminary verified; 3 not verified. UTD data normally carry 2 or 3.
Consistency: every xlink must resolve to an existing Dataset D object, and the pollutant must match that sampling point's observing capability. The EEA's QA checks E2a against the preliminary Dataset C and the latest Dataset D, so an unregistered sampling point is rejected.
Lifecycle after upload
The EEA merges E1a and E2a into one database. E2a rows are overwritten when the verified E1a series arrives, within nine months after year end. If a country never sends E1a for those points, the E2a data stay in the database with their unverified flags, and derived statistics are marked "verification incomplete". Public products show validated data; the index and UTD viewer show the latest submission.
Airbox cannot send E2a itself. It needs to hand ANPM an hourly series per pollutant that already looks like a DataArray row: start, end, verification flag, validity flag, value in the right unit, timestamped in local winter time, for a sampling point ANPM has registered in Dataset D with the matching process and inlet metadata. Everything else, the XML wrapping, the sFTP push and the QA against Datasets C and D, is done on the national side.
Station / sampling point metadata (Dataset D)
Required or expected fields include:
- European station ID (EoI-style code, e.g.
RO0071A), national station code, station name, municipality, network ID, measurement configuration ID. - Geographical coordinates (longitude, latitude, altitude).
- Classification of the station by predominant emission source (traffic / industrial / background) per pollutant, main sources, classification of the area (urban / suburban / rural).
- For traffic stations: distance to major junction, traffic volume, heavy-duty fraction, traffic speed, street canyon width, distance of inlet from nearest lane and from next building.
- For industrial stations: distance from predominant industrial source.
- Sampling point: height of air inlet above ground, measurement start/end dates.
- Measurement / sampling / analytical method, equipment, detection limit, demonstration of equivalence, sampling time and interval.
- Data quality objectives: time coverage, data capture.
Measurement requirements
Reference methods
| Pollutant | Reference method |
|---|---|
| SO2 | EN 14212:2012 – UV fluorescence |
| NO2 / NOx | EN 14211:2012 – chemiluminescence |
| PM10 | EN 12341:2023 – gravimetric |
| PM2.5 | EN 12341:2023 – gravimetric |
| O3 | EN 14625:2012 – UV photometry |
| CO | EN 14626:2012 – non-dispersive infrared |
| Benzene | EN 14662 parts 1, 2, 3 |
Any other method (e.g. beta-attenuation or optical PM monitors, electrochemical or optical gas sensors) may be used only after a demonstration of equivalence (Annex VI Section B, JRC "Guide to the demonstration of equivalence"). Automatic PM monitors are typically type-approved under EN 16450.
Data quality objectives – maximum measurement uncertainty
Short-term (hourly / daily) means, expressed at 95 % confidence, applicable near the limit value:
| Pollutant / metric | Fixed measurement | Indicative measurement |
|---|---|---|
| PM2.5 (24 h) | 6.3 µg/m³ or 25 % | 8.8 µg/m³ or 35 % |
| PM10 (24 h) | 11.3 µg/m³ or 25 % | 22.5 µg/m³ or 50 % |
| NO2 (hourly) | 30 µg/m³ or 15 % | 50 µg/m³ or 25 % |
| NO2 (daily) | 7.5 µg/m³ or 15 % | 12.5 µg/m³ or 25 % |
| SO2 (hourly) | 52.5 µg/m³ or 15 % | 87.5 µg/m³ or 25 % |
| O3 (8 h mean) | 18 µg/m³ or 15 % | 30 µg/m³ or 25 % |
| CO (8 h) | 1.0 mg/m³ or 10 % | 2.0 mg/m³ or 20 % |
Long-term (annual) means:
| Pollutant | Fixed | Indicative |
|---|---|---|
| PM2.5 | 3.0 µg/m³ or 30 % (25 % before 2030) | 4.0 µg/m³ or 40 % |
| PM10 | 4.0 µg/m³ or 20 % | 6.0 µg/m³ or 30 % |
| SO2 / NO2 / NOx | 6.0 µg/m³ or 30 % (NO2 15 % before 2030) | 8.0 µg/m³ or 40 % |
Minimum data coverage (share of the year with valid data)
| Pollutant | Fixed, annual | Fixed, hourly/8 h/24 h | Indicative, annual | Indicative, hourly/8 h/24 h |
|---|---|---|---|---|
| SO2, NO2, NOx, CO | 85 % | 85 % | 13 % | 50 % |
| O3 (+ NO/NO2) | 85 % | 85 % | 13 % | 50 % |
| PM10, PM2.5 | 85 % | 75 % | 13 % | 50 % |
Fixed measurements of SO2, NO2, CO, O3, PM10, PM2.5 must run continuously all year. Maintenance must not be scheduled during pollution peaks.
Micro-scale siting (Annex IV, Section C)
- Unrestricted airflow around the inlet: arc ≥ 270° (≥ 180° at a building line); ≥ 1.5 m from buildings, balconies, trees, other obstacles.
- Inlet height 0.5 m to 4 m above ground (higher only at background sites, documented).
- Not in the immediate vicinity of sources; exhaust placed to avoid recirculation.
- Traffic stations: inlet ≥ 25 m from major junctions and ≤ 10 m from the kerbside.
- Ozone: well away from furnaces / incineration flues, > 10 m from the nearest road.
Comparison of the three station types
Detailed per-type documents: traffic-station-requirements.md, industrial-station-requirements.md, background-station-requirements.md.
| Aspect | Traffic | Industrial | Background |
|---|---|---|---|
Eionet definition (aq/stationclassification) |
Close proximity to a single major road | Close proximity to a single industrial source or industrial area | Neither traffic nor industrial; levels not dominated by a single source type |
Typical area class (aq/areaclassification) |
urban, suburban | urban, suburban, rural | urban, suburban, rural-nearcity (< 10 km), rural-regional (10–50 km), rural-remote (> 50 km) |
| What it represents (Dir. Annex IV B) | Street segment ≥ 100 m, highest-concentration streets, where people are exposed | Nearest residential area downwind of the source; ≥ 250 m × 250 m | Average exposure of the general population; several km² (urban), 10–100 km² (suburban O3), 100–10 000 km² (rural) |
| Minimum pollutants for full-colour EEA index (ETC HE 2024/17 §2) | NO2 + (PM2.5 or PM10) | NO2 + O3 + (PM2.5 or PM10) | NO2 + O3 + (PM2.5 or PM10) |
| O3 | Not required; siting conflict (O3 inlet > 10 m from road vs kerb ≤ 10 m) | Required; expect low values (NO titration); inlet away from furnaces/flues | Required; usually the pollutant that drives the index in summer |
| SO2 | Not required (road traffic is not an SO2 source) | Strongly recommended (combustion, refineries, metallurgy, ports); no EEA gap-filling | Optional, usually low |
| Other recommended pollutants | CO (Annex III transport point, Annex IX hourly info); benzene optional (validated data only) | CO, benzene, metals, B(a)P depending on source (only CO is in E2a; rest validated only) | PM2.5 preferred over PM10; NOx for O3 interpretation |
| Index pollutants typically dominating | NO2, PM | PM, SO2, NO2 | O3 (summer), PM (winter) |
| Time resolution / unit | Hourly means, µg/m³ (CO mg/m³), gases at 293 K / 101.3 kPa | Same | Same |
| Reference methods (Dir. Annex VI A) | EN 14211 (NO2), EN 12341 (PM), EN 14626 (CO) | EN 14211, EN 14625 (O3), EN 12341, EN 14212 (SO2) | EN 14211, EN 14625, EN 12341 |
| Max. uncertainty, fixed / indicative (Dir. Annex V A, short-term) | NO2 1 h 15 % / 25 %; PM2.5 24 h 25 % / 35 %; PM10 24 h 25 % / 50 % | Same plus SO2 1 h 15 % / 25 %; O3 8 h 15 % / 25 % | Same as industrial minus SO2 |
| Min. data coverage, fixed (Dir. Annex V B) | Gases 85 %; PM 75 % (short-term) / 85 % (annual) | Same | Same; O3 coverage also checked separately for Apr–Sep and Oct–Mar |
| Micro-scale siting (Dir. Annex IV C) | Inlet 0.5–4 m; ≥ 25 m from major junction; ≤ 10 m from kerb; ≥ 1.5 m from obstacles; free airflow ≥ 270° (180° at building line) | Inlet 0.5–4 m; ≥ 1.5 m from obstacles; not in the immediate vicinity of the source; O3 inlet > 10 m from road, away from flues | Inlet 0.5–4 m, higher allowed if documented; ≥ 1.5 m from obstacles; O3 inlet > 10 m from road |
| Macro-scale siting (Dir. Annex IV B) | Busiest streets, street canyons considered, sensitive-group locations | Downwind in predominant wind direction, nearest residential area | Away from local emissions; rural background ≥ 20 km from urban/industrial areas; vegetation sites > 5 km from major roads (> 50 000 veh/day) |
| Type-specific Dataset D fields (Decision 2011/850/EU Annex II D) | Distance to major junction (29), traffic volume (30), heavy-duty fraction (31), traffic speed (32), street canyon width (33/34), inlet distance to building (20) and to nearest lane (21) | Distance from predominant industrial source or source area (24), main sources (23) | Area classification sub-type (28), representativeness evaluation (17), main sources optional (23) |
| Common Dataset D fields | EoI station code (RO0xxxA), name, coordinates (≥ 4–5 decimals, ETRS89), altitude, inlet height, classification per pollutant, method/equipment, equivalence demonstration, detection limit, start/end dates |
Same | Same |
| Data delivery | Hourly E2a via national authority (ANPM in Romania), validity + verification flags, consistent with Dataset D | Same | Same |
| EEA gap-filling (missing hours) | NO2, PM: CAMS difference method | NO2, PM: difference; O3: multiplicative; SO2: none | NO2, PM: difference; O3: multiplicative |
| Semi-transparent on the map when | NO2 or both PM fractions missing | Any of NO2, O3, PM missing | Any of NO2, O3, PM missing |
| Minimum sensor set for Airbox V3 | NO2 + PM2.5 (+ PM10, CO) | NO2 + O3 + PM2.5 + PM10 + SO2 | NO2 + O3 + PM2.5 (+ PM10) |
Rules shared by all three: index bands, worst-sub-index rule, sub-index not determined when the same hour is missing on at least three of the four previous days, hourly means only, and data reach the EEA only through the national E2a feed.
Outdoor Airbox checklist
[x] Sensors (minimum): NO2, O3, PM2.5 and/or PM10. Add SO2 only if the site has an SO2 source (industrial); add CO for Annex IX-style public information.
x Output: hourly mean per pollutant, µg/m³ (CO mg/m³), timestamps in UTC+offset ISO 8601 (YYYY-MM-DDThh:mm:ss±hh:mm), start and end of each hour.
x PM: apply and document the correction/equivalence to EN 12341 (co-location against a gravimetric or type-approved automatic reference monitor).
x Gases: calibration traceable to EN 14211 / 14625 / 14212 / 14626; document detection limits.
x Flags: attach validity (1/2/3/−1/−99) and verification (3 = not verified for live data) to every hourly value; never send invalid data as plain values.
x Data coverage: design for ≥ 85 % valid hours per year (≥ 75 % for PM if treated as fixed; ≥ 50 % if treated as indicative). Watchdog, local buffering and back-fill within the hour.
x Siting: inlet 0.5–4 m, free airflow, documented distances to roads/junctions/sources; classify the station (traffic / industrial / background; urban / suburban / rural).
x Metadata: prepare the Dataset D record (section 4.3) so the national authority can register the sampling point(s).
x Delivery: agree with ANPM (or the relevant national authority) on an hourly feed into their E2a submission. Verify arrival through the EEA Download Service (UTD dataset) and the index map.
x Own display: if Airbox shows an index itself, use the 2025 bands, the worst-sub-index rule, the semi-transparent rule for incomplete pollutant sets, and the official health messages.