Laboratory and field evaluation of miniaturized CO₂ and NO_x sensors for bird-borne air quality monitoring: The ARIA project

The growing availability of miniaturized low-cost gas sensors has opened new possibilities for high-resolution monitoring of atmospheric pollutants, including their use on mobile platforms such as free-flying birds. Their reliability under rapidly changing environmental conditions must be assessed through laboratory and field testing.

Within the framework of the ARIA project (Animal-boRne systems for envIronmentAl monitoring), this study evaluates miniaturized CO₂ sensors based on photoacoustic and non-dispersive infrared (NDIR) principles, and a metal oxide NO_x sensor, integrated into an air sampling device. Sensors were first characterized in an exposure chamber against certified reference instruments, assessing linearity, response time, and sensitivity to temperature, relative humidity, and chemical interferents such as ozone. Their performance was subsequently evaluated at a semi-rural location (Liberti observatory) and during homing pigeon flights.

All sensors showed a generally linear response within the investigated concentration ranges. The NDIR CO₂ sensor exhibited the best overall performance, with good accuracy and limited sensitivity to environmental variables, whereas the photoacoustic sensor required correction for humidity and temperature and was greatly affected by turbulence, making it unsuitable for bird-borne measurements. The NO_x sensor, despite its high linearity, proved sensitive to humidity, temperature, and ozone interference, requiring a multivariate analysis to characterize and compensate for its response under real atmospheric conditions.

The spatial resolution achieved during pigeon flights allowed the detection of local pollutant variability beyond the reach of fixed monitoring stations. Seasonal tropospheric NO₂ vertical column densities (VCDs) retrieved from the TROPOspheric Monitoring Instrument (TROPOMI) were used to provide regional-scale perspective on pollution patterns during the flight. Together, these observations highlighted the added value of this mobile biological platform for air quality monitoring. These results suggest that bird-borne sampling may provide useful information on the spatial variability of air pollutants, although careful calibration and correction of environmental effects remain essential.