Ozone Monitoring in Real-World Settings: Field Calibration and Indoor/Outdoor Dynamics with Monica Devices

Stratospheric ozone is proved to be vital for life on Earth, whereas tropospheric ground-level ozone represents a serious threat to both plants and animals, including humans. Assessing its impacts requires comprehensive monitoring. Only 25% of the Earth’s surface has ground-based data; leaving large spatial gaps. Exposure, however, is not purely outdoor: ozone penetrates indoors and reacts with indoor pollutants, reshaping what people actually breathe. Capturing both outdoor burdens and indoor penetration is therefore essential. The aim of this work is to measure ozone levels in the summer period in both indoor and outdoor environments in the Turin area using Monica low-cost sensors. To carry out this analysis, Monica low-cost sensors were co-located close to a reference instrument (Serinus 10) for two weeks to perform a calibration. After the calibration period, the three Monica were deployed in different environments: an outdoor unit installed on the roof of the Safety Laboratory at the Politecnico di Torino; an indoor unit in a closed, rarely occupied laboratory with minimal ventilation; and an indoor unit in a laboratory that is continuously ventilated due to a permanently open window. Calibration used multivariate linear regression in hourly data using Monica’s signal, temperature, and relative humidity. During deployment (30 July–30 September 2025) the outdoor unit exhibited typical summer diurnal cycles and frequent MDA8 exceedances: 18/63 days above 120 µg m⁻³ and 33/63 above 100 µg m⁻³. Indoors, ventilation dominated: the minimally ventilated room had low levels with weak coupling to the outdoors, while the continuously ventilated room showed higher I/O ratios and daytime peaks aligned with outdoors. A simple and transparent field calibration makes Monica sensors suitable for spatial-temporal mapping and indoor/outdoor studies.