Non-uniformity in outdoor CO₂ concentration in city of Copenhagen

The accurate data of outdoor CO2 concentration are important for the proper design of ventilation and thus for indoor air quality and energy use in buildings. Typical design practice is to assume outdoor CO2 concentration to be 400 ppm. However, the outdoor CO2 concentration may be different in different areas of cities.

This paper presents preliminary results of long-term (one year) outdoor CO2 concentration changes in four districts of Copenhagen (Denmark). The districts included downtown area and suburbs with different surroundings. Four buildings were selected for the measurements, one building in each district.

Outdoor CO2 concentration measurements were performed at two levels – ground level and top of the buildings. Special attention was paid to use accurate measuring instruments. The instruments were carefully calibrated before the measurements. The calibration of the instruments was checked periodically.

In this paper, preliminary results from summer and autumn measurements are presented. The outdoor CO2 concentration varied over the day and from day to day in the range between 340 and 450 ppm. The CO2 concentration at the ground of the buildings was usually 10 to 40 ppm higher than that at the top level in autumn.

At the buildings in the suburbs, during the working hours, the outdoor CO2 concentration measured on the top level close to the intake duct was on average 408 ppm. At the building in the downtown area, that was on average 414 ppm. However, the outdoor CO2 concentration varied depending on the building, level and time.

During the working hours, the 75 percentiles of outdoor CO2 concentration varied between 384 ppm and 442 ppm, which indicates that the required ventilation rate could be different over 10% depending on the building location site, measurement height and time. In order to ensure the required indoor limits of CO2 concentration, CO2 measurements must be performed close to the location of the outdoor air intake.