How We Measure Air Quality
AirQuality.city combines two types of data sources:
- its own SensorBox stations
- official external monitoring stations
They do not use the same equipment, serve the same purpose, or have the same level of official validation. They should therefore be viewed as complementary sources rather than identical systems.
What do SensorBox stations measure?
SensorBox stations measure:
- PM2.5
- PM10
Separate AQI values are also available for these measurements.
SensorBox stations do not measure:
- NO₂
- O₃
- SO₂
- CO
Their main advantage is denser local coverage and the ability to monitor areas without an official reference station.
How does SensorBox measure particles?
SensorBox uses an optical particulate matter sensor.
The sensor estimates particle quantity based on light scattering inside a measurement chamber. The resulting signal is converted into estimated PM2.5 and PM10 concentrations.
Optical sensors allow frequent and local measurements, but they may be affected by:
- high relative humidity
- fog
- water droplets
- sensor placement
- airflow
- nearby local sources
- sensor aging and contamination
What do official external stations measure?
Official stations are part of Croatia's air quality monitoring system.
Depending on location and equipment, they may measure:
- PM2.5
- PM10
- NO₂
- O₃
- SO₂
- CO
- additional meteorological or environmental parameters
AirQuality.city imports available data from the AZO/ISZZ system.
Why are official stations not everywhere?
Reference stations use more complex and expensive equipment and require:
- regular calibration
- quality control
- maintenance
- data validation
- expert processing
There are therefore fewer of them, and they do not cover every local micro-location.
What is the role of the SensorBox network?
The SensorBox network provides:
- denser spatial coverage
- better insight into local differences
- faster detection of changes
- measurements in places without an official station
These measurements are useful for information and trend monitoring, but they should not automatically be treated as a replacement for a regulatory reference station.
How is AQI determined?
PM2.5 and PM10 each have a separate AQI based on the US EPA methodology.
The station's overall AQI is:
max(AQI PM2.5, AQI PM10)
The overall AQI therefore represents the less favorable of the two PM components.
NO₂, O₃, SO₂, and CO are not included in the overall AirQuality.city AQI.
Why does sensor placement matter?
Placement has a strong effect on the result.
Important factors include:
- installation height
- distance from roads
- distance from chimneys
- protection from direct rain
- exposure to direct sunlight
- airflow around the enclosure
- nearby walls, roofs, and obstacles
A sensor placed next to a local source may show values that are not representative of the wider area.
How does weather affect measurements?
Weather affects both actual particle concentration and optical sensor behavior.
Important factors include:
- wind
- rain
- fog
- relative humidity
- temperature inversions
- atmospheric mixing height
One reading should therefore not be viewed in isolation.
Are real-time data final?
Not necessarily.
SensorBox data are displayed as operational sensor measurements.
Official data may later undergo additional validation, correction, or flagging in the source system.
AirQuality.city therefore provides timely insight, while final official analyses rely on validated data from competent institutions.
What does it mean when a station has no data?
Possible reasons include:
- power failure
- internet connection failure
- maintenance or sensor malfunction
- delay in the external source
- temporary unavailability of a pollutant measurement
Missing data do not mean zero concentration.
How should the data be used?
The most useful approach is to:
- review trends across several measurements
- compare nearby stations
- check the latest update time
- consider station placement
- distinguish SensorBox from official stations
- avoid conclusions based on one isolated value