The rural site located in the Xibaidian (XBD) Village, Daxingzhuang town, Pinggu District, Beijing, which is in the northeastern direction of Beijing urban area. The size of the village is 835 meters from south to north, and 344 meters from west to east. The sampling site located in the north end of the village, which is away from residential areas. The sampling area consisted of yard and a row of single-story brick building with several rooms, which will be used specifically for clinical visits and instrument deployment. Around the building, there’s a small woods of peach trees on the west side, and some greenhouses on the east side. A two-lane road is about 200-300 meters north to the building. The road would be a pollution source but its traffic volume is relatively low.