CO₂ concentrations and air change rates in Chinese city-type residences

Carbon dioxide concentration (CO₂) is an important index for estimating the air exchange rate (AER). To clarify the actual conditions of the AERs in city-type residences in China, CO₂ concentrations were measured at 5-minute intervals during a period of one to two weeks in six cities (Harbin, Dalian, Beijing, Shanghai, Wuhan, and Changsha) in summer and winter. The AERs were estimated by the reciprocal average concentration method and the CO₂ fitting technique, both of which rely on measured CO₂ concentrations. The ranges of CO₂ concentrations in winter and summer were 400-9794 ppm and 400-4519 ppm, respectively. The difference of the minimum concentrations was not remarkable between winter and summer among the six cities because indoor CO₂ concentrations are strongly influenced by outdoor CO₂ concentrations. On the other hand, for the maximum concentrations, winter showed CO₂ concentrations higher than those in summer. This finding indicates that the residents do not frequently open their windows in winter, and so indoor CO₂ concentrations trend to a high value. As estimated by the CO₂ fitting technique, it became clear that when windows were closed, AERs were 0.5 h^-1 or less. In the city-type residences in China, the CO₂ concentrations at the time of sleeping with closed windows reached a maximum of 10,000 ppm around daybreak, since airtightness was also higher. However, since windows are customarily opened in the daytime, the lowest concentrations in the daytime were comparable with the outdoor air concentrations in both summer and winter.