Quantifying carbon emission reduction (CO2) from natural ventilation in office buildings of Nanjing

As a near-zero-carbon cooling source, natural ventilation (NV) demonstrates significant potential for substituting air conditioning (AC) cooling. However, its carbon reduction benefits remain difficult to quantify within current carbon accounting systems. The displaced cooling electricity lacks direct metering and observable readings, representing "consumption that did not occur,” thus failing to enter urban emission reduction targets and accounting tools. This study targets ten baseline office buildings to construct a carbon accounting-oriented NV emission reduction computational framework, integrating scattered and disconnected key NV elements into a unified calculation chain that translates NV operation into accountable AC electricity savings and corresponding carbon reductions. We quantify carbon reduction potential across 75 NV scenarios, characterizing the relationship between NV utilization intensity and emission reduction magnitude, and extrapolate the methodology to Nanjing's office building stock (24,920 buildings, 162.85 million m2). The citywide cooling baseline is 5994 GWh, with annual cooling carbon emissions of 3.60 million tonnes. Results demonstrate that NV intensity is the most critical variable: as air speed increases from 0.1 to 0.8 m/s, carbon reduction rate per unit floor area rises from 13.3%–26.2% to 61.4%–95.7%, carbon reduction amount increases from 3.25–3.53 to 12.91–18.50 kg CO2/(m2·a), and citywide annual carbon reduction grows from 0.53 to 0.58 to 2.10–3.01 million tonnes. Findings reveal that most urban AC cooling energy can be substituted by NV. This methodology endows NV with quantifiable carbon reduction value, enabling comparison with technological measures within the same accounting framework, transforming it from a marginalized passive strategy into a measurable option for urban emission reduction.