## 4 Conclusions  
Mean air velocity and air turbulence downwind a 1:50 scaled model of a naturally ventilated pig building with a totally roofed outdoor exercise yard with different roof slopes were measured in a large boundary layer wind tunnel to investigate downwind airflow characteristics and the related air pollutant transport and dispersion properties. Before the downwind airflow measurements, wind profile properties were examined to ensure a fully-developed turbulent flow in the wind tunnel. Based on the results obtained, the following conclusions could be drawn:  
1. Reduced air velocities were observed right behind the building. At the vertical plane parallel to the building sidewalls with a distance 0.2 m (i.e., 1.5\(Z_{B}\), where \(Z_{B}\) is the building height) downwind the building, reverse flows occurred in the centre of the plane and were affected by the roof slope.
2. An elliptic-shaped low air velocity region along the distance to the building was found. It had a similar height as the building and reached 0.5 m (i.e., 3.8\(Z_{B}\) and corresponded to 25 m in full-scale) downwind from the building. Within this region, the mean air velocities were low and thus might result in a higher gaseous pollutant concentration. This suggested that applying additional treatment technologies to trap the high-concentrated gaseous pollutants (e.g., odours or ammonia) in this region might contribute to the mitigation of pollutant emissions to the atmosphere and alleviation of the burden to the surrounding environment. Apart from the mean air velocity, the transport of the air pollutants in this region was likely attributed to the turbulent eddies.
3. A wake zone with recirculated air was observed for all three roof slope cases. The larger the roof slope was, the lower and longer the wake zone became. It indicated that a steeper roof could result in the accumulated air pollutants dispersing to a farther distance.
4. The effect of the roof slope to the mean air velocity and the air turbulence could reach up to distances of 0.8 m (i.e., 6.2\(Z_{B}\)) and 0.5 m (i.e., 3.8\(Z_{B}\)) downwind from the building, respectively.  
To the best of our knowledge, the present study was the first to provide detailed information of the downwind airflow for a naturally ventilated pig building with totally roofed outdoor access. The airborne pollution transport and dispersion properties were predicted from the airflow characteristics. To support the findings, further research on the downwind gas concentration fields using a tracer gas will be conducted. The mean air velocity and air turbulence data provided in this paper can be used for validation of CFD models which permits more comprehensive studies on the effect of multiple factors on airflow patterns both indoors and around the building.  
Conceptualization, Q.Y., B.A. and T.A.; methodology, Q.Y., D.J. and G.Z.; validation, Q.Y., D.J. and L.T.; formal analysis, Q.Y.; investigation, Q.Y. and L.T.; writing--original draft preparation, Q.Y.; writing--review and editing, D.J., L.T., G.Z., B.A., S.H., S.N., E.H. and T.A.; supervision, G.Z. and T.A.; project administration, T.A.; funding acquisition, T.A. All authors have read and agreed to the published version of the manuscript.  
This research was an accompanying study of the research project \"Emissionsminderung Nutztierhaltung\" (EmiMin FKZ) funded by German Federal Ministry of Food and Agriculture (BMEL) and the Federal Office for Agriculture and Food (BLE).  
The authors greatly acknowledge the technicians [PERSON] and [PERSON] at ATB for the construction of the scaled model, and for their technical support in the wind tunnel experiment. We also appreciate the staff at Institute of Agricultural Process Engineering, CAU for providing all information about the prototype pig barn.  
The authors declare no conflict of interest.