Façade-Level Monitoring of CO2 Variability under Urban Heat Island Conditions using Low-Cost Sensor Data Loggers

Summary

A simple, effective environmental monitoring protocol to measure temperature, humidity, wind speed, and carbon dioxide variations across urban surfaces with differing sunlight exposure influenced by the urban heat island effect.

Abstract

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.

Introduction

The urban heat island effect (UHIE) is a persistent climatic phenomenon in which urban regions experience elevated temperatures compared with their rural surroundings, primarily due to anthropogenic heat emissions, dense surface materials, and diminished vegetation cover^1,2,3. Elevated urban temperatures alter localized microclimates, influencing wind flow, humidity, and the retention of atmospheric CO₂^1,4. In densely built environments, thermal stratification reduces natural air mixing, thereby intensifyin....

Protocol

1. Sensor Calibration and Setup

Figure 1: AC-powered sensor module housing. A 230 V AC-powered sensor unit containing CO₂, temperature, and humidity sensors, used for continuous microclimatic monitoring. Please click he....

Results

The recorded data reveal a consistent cross-city pattern: external CO₂ concentrations are significantly higher in areas with high concretization compared to adjacent green zones.

In Chennai (Figure 9), a difference of 78 ppm was observed between the concrete-dominant east façade (490 ppm) and the shaded green north façade (412 ppm). This contrast was intensified by the presence of outdoor HVAC condenser units on the east façade, which released waste heat into the .......

Discussion

A critical step in this protocol is the baseline calibration and synchronization of CO₂, temperature, and humidity sensors. Accurate calibration in shaded, open-air settings is essential to ensure inter-sensor consistency and minimize drift during deployment¹. Incorrect placement or uneven solar exposure can introduce false gradients; therefore, maintaining a mounting height of 1-2 m and ensuring adequate distance from localized heat sources such as HVAC condenser units are important conside.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Arduino IDE	Arduino	v2.x	For programming and debugging ESP32/Arduino boards
CO2 Sensor, NDIR (Non-Dispersive Infrared)	Sensirion	SCD40 / SCD41	Used for real-time ambient CO2 monitoring
Data Logging Microcontroller (ESP32)	Espressif	ESP32 DevKitC	For logging sensor data; supports Wi-Fi and SD card
Humidity and Temperature Sensor	Sensirion	SHT21	For ambient temperature and relative humidity measurement
MicroSD Card (32 GB or higher)	SanDisk / Kingston	Various	Stores time-series CO2, temperature, and humidity data
MicroSD Card Module	Generic Supplier	N/A	Interface between ESP32 and SD card for data logging
Power Adapter (AC to 5V DC)	Generic Supplier	N/A	Converts 230 V AC to stable 5 V DC power
Python Software Environment	Python.org	v3.10 (pandas 1.5, matplotlib 3.7)	For advanced data analysis, visualization, and automation
Rainproof Sensor Housing	Polypropylene/3D Printed	N/A	Perforated casing to protect electronics from rain and dust
Rope or Mounting Harness	Generic	N/A	Used for secure façade mounting at required height
SD Card Reader/Writer	Any USB Interface	N/A	For transferring data from SD card to PC
Excel	Microsoft	N/A	For post-processing time-series data
Calc	LibreOffice	N/A	Spreadsheet Software, For post-processing time-series data
USB Cable (Micro/Type-C)	Generic	N/A	Used for programming and powering ESP32 module