Effects of Outdoor Environment on Air Exchange Rate

Indoor air quality is the chemical, physical and biological properties that indoor air must have to not cause any negative impact on occupants’ health and provide comfort: feel fresh, pleasant and stimulating. 
The indoor air quality is mainly determined by the indoor pollution sources and the outdoor air imported to the building. The indoor pollution sources are related to occupants, the building and their activities. The effect of the outdoor air in the indoor environment depends on its level of pollution and the amount entering the building through different ventilation systems and infiltrations. Ventilation and infiltrations are essential to ensure an optimal level of indoor air quality by refreshing the air. However, this can mean extra energy consumption because it can affect thermal comfort. Therefore, it is important to evaluate the efficiency of ventilation.  
One way to characterize the ventilation rate, and the amount of air entering or leaving the indoor environment, is by the air exchange rate. The air exchange rate indicates the number of times that air in a fixed space is replaced by outdoor air in one hour. This study aims to determine the changes in the air exchange rate throughout the day depending on the weather conditions in the Mechanical Engineering Department building within the university campus located in the south of Coimbra city, Portugal. In line with this purpose, the air exchange rate determined using the metabolic CO2 decay tracer gas technique, by measuring the CO2 concentration inside and outside the building during a 3-month period. Meteorological observations were obtained from a weather station set in the car park. The variations and correlations of the air exchange rate against various outdoor meteorological factors, namely temperature, dew point, barometric pressure, wind speed and direction, precipitation and solar irradiation were tested using statistical regression techniques.  
This paper concludes that the air exchange rate measured by the CO2 tracer gas technique in an unoccupied building depends on meteorological conditions. Almost 80% of the variation in the air exchange rate can be explained by meteorological variables such as temperature, dew point, humidity, and wind speed and direction. Other parameters that affect air exchange rate are wind direction and precipitation. In these cases, the effects are conditioned to the situation. For example, only the wind coming from the direction in which the building is exposed has a noticeable effect. The precipitation episodes have different effects. Generally, it rains in cloudy skies, which reduces the amount of solar radiation that reaches the surface of the earth and the building. Thus, it can increase photosynthesis and the production of environmental CO2 thanks to the extra contribution of water, an essential component in the respiration-photosynthesis process.   All these parameters indirectly affect the air exchange rate.