Façade-Level Monitoring of CO<sub>2</sub> Variability under Urban Heat Island Conditions using Low-Cost Sensor Data Loggers

Urbanization and rapid land-use change have intensified the urban heat island effect (UHIE), leading to elevated ambient temperatures, altered humidity, and increased accumulation of carbon dioxide (CO2) in dense urban environments. These microclimatic changes not only influence thermal comfort but also affect outdoor air quality and, consequently, indoor ventilation dynamics. Current monitoring approaches, such as satellite remote sensing and city-scale air quality stations, provide valuable insights but lack the façade-level spatial resolution necessary to inform building-level interventions. This study presents a reproducible, low-cost protocol for façade-specific monitoring of CO2, temperature, and humidity using non-dispersive infrared (NDIR)-based sensors integrated with microcontroller data loggers. Sensor modules are deployed on contrasting thermal zones-sun-exposed and shaded façades-to capture diurnal variations over a 24 h cycle. Calibration against outdoor baseline CO2 concentrations ensures measurement accuracy and comparability among sensors. The method was validated across four Indian cities with distinct urban forms. Results consistently revealed higher CO2 concentrations in densely built zones compared to adjacent green façades, with differences ranging from 19 ppm (Pune, India) to 78 ppm (Chennai, India). These gradients were further associated with localized temperature rises of up to 2 °C and reduced relative humidity, confirming the role of UHIE in exacerbating pollutant retention. The proposed protocol offers a scalable, field-friendly approach for urban microclimate assessment, HVAC optimization, and environmental health research. Beyond CO2, the methodology can be extended to capture additional air quality parameters such as PM₂.₅, PM₁₀, and VOCs, enabling a comprehensive assessment of urban air quality. By providing façade-level, real-time insights, this monitoring framework supports data-driven decision-making for sustainable building design, intelligent ventilation systems, and long-term climate-resilient urban planning.