## 1 Introduction  
Natural ventilation systems have the advantages of low capital investment, energy saving [1], and absence of noise [2] compared with mechanical ventilation systems. Thus, they are widely implemented in livestock buildings. Naturally ventilated pig production systems equipped with outdoor exercise yards are receiving increasing interest in Europe, the United States,and South Africa [3; 4]. This housing system provides pigs larger living areas, enables the separation of lying and excretion areas, and allows pigs to exhibit natural behaviours [5]. It provides pigs access to an outdoor exercise area, enables pigs to mainly excrete outdoors, and thus results in improved indoor air quality [6], animal welfare [3], and meat quality [7] compared with a conventional housing system. The design of naturally ventilated pig buildings (NVPBs) with outdoor access is mainly based on the consideration of animal production performance and animal welfare [3; 8]. However, one important issue associated with this type of buildings is the amount of air pollutants produced in the buildings and their emission to the surrounding environment. The open-type barn structure allows a direct air exchange between the indoor and outdoor environment, and consequently leads to a wide dispersion of gaseous emissions into the atmosphere [9]. The gaseous pollutants produced from the buildings including ammonia, greenhouse gases, hydrogen sulphide, particulate matter, odours and aerosols contribute to environmental problems and causes nuisance to neighbouring residents [10]. The transport and dispersion processes of gaseous emissions are strongly affected by airflow [11; 12], which is influenced by the atmospheric boundary layer (ABL) such as temperature, humidity, wind speed, wind direction, and terrain [13]. The ABL experiences periodic evolution of daytime convective [14], transitional (occurs in the morning, late afternoon or early evening) [15], and nocturnal stable states [16] due to the solar diurnal cycle. This dynamic behaviour of the ABL may also affect the evolution of particulate matter [17]. In addition to the ABL, building configurations (e.g., roof structure and openings) also affect airflow field within and around the building [18], and thus may have an impact on the pollutant transport and dispersion processes. Therefore, a good understanding of airflow distributions including airflow pattern and mean and turbulent characteristics, particularly downwind of the buildings contributes to understanding air pollutant dispersion mechanisms. One of the most common ways to reduce nuisance of airborne pollutants in the vicinity of pig barns is by using artificial or natural windbreaks [19]. One function of windbreaks is acting as barriers that deflect the airflow upwards, increase the dilution of air pollutants [20], and consequently reduce their concentration at the ground level [21]. [PERSON] et al. [9] reported that using a solid wall, a screen, and another building as windbreaks placed upwind of a pig barn had different influences on air momentum and airflow patterns around the target building and might affect air dispersion patterns. Apart from the airflow redirection impact, natural windbreaks, for example shelterbels, also contribute to the reduction of wind speed, interception/absorption of chemical compounds, particulate matter and aerosols, and therefore can dilute and mitigate airborne pollutants [22]. It is found that the influence of natural windbreaks on the pollutant dispersion is related to the height, optical porosity and type of windbreaks [19; 20], and the distance from the pollutant source [20; 22]. Apart from windbreaks, it is essential to investigate the effect of building configurations e.g., the roof design on the dispersion process of gaseous pollutants. This information may help to provide building engineers guidelines for planning or design of new livestock buildings in order to reduce adverse impacts of the buildings on nearby environment and residents.