Table 5 Description of the main variables and attributes of the NetCDF files.

time

- units: seconds since 1970-01-01 00:00:00;

int32

- valid_min: minimum value;

- valid_max: maximum value;

sensornumber

Corresponds to the sensing element history. If the sensor has never been replaced it’ll take a value of “1”

int16

- valid_min: minimum value (equal to “1”);

- valid_max: maximum value (e.g. if it has been replaced just once it’ll take the value of 2, and so on);

- date_first_measurement: start date of measurements for a specific sensing element;

- date_last_measurement: end date of measurements for a specific sensing element;

calibration

It describes the data product.

int16

- flag_values: indication of the calibrated product. It can take the values of “1”, “2” or “3”;

- flag_meanings:

out-of-box (for flag_values = 1): data product produced by default by the manufacturer;

cal1 (for flag_values = 2), first corrected/calibrated data product;

cal2 (for flag_values = 3), second corrected/calibrated product;

- date_calibration_applied: the date on which a new data product was released;

Note 1: NO, NO₂, O₃, PM_2.5 and PM₁₀ are the only variables for which suppliers generated different data products.

Note 2: a special case is the PurpleAir sensors, which can only take the flag_values “1” (flag_meanings = atmospheric) and “2” (flag_meanings = indoor) which are the default data products from that company. See²⁷ for more details.

*var (i.e., air_pressure, relative_humidity, air_temperature, pm1, pm10, pm2p5, no2, no, o3, co2, co)

Measurement values.

float64

- units for meteorological variables:

○ Air_pressure: hectopascal (hPa);

○ relative_humidity: percentage (%);

○ air_temperature: Kelvin (K);

- units for the air pollutants:

○ pm1, pm10 and pm2p5: ug/m3;

○ no2, no, o3, co2 and co: ppb;

- FillValue: missing or invalid data is shown as −9999.

- valid_min: minimum valid value;

- valid_max: minimum valid value;

- coordinates: co-location sites coordinates (reference system WGS84):

○ LAQS: 51.449694 N, −0.037389E;

○ YoFi: 53.951917 N, −1.075861E;

○ MAQS: 53.444222 N, −2.214417E;

- Ancilliary_variables: qc_flag_*var and qc_flag_reason_*var (see Table 6 for more details).