Objective To investigate the distribution of negative air ion (NAI) concentration in Jiulong Mountain which locates in suburb of Beijing, and understand the role of forest vegetation in improving air quality.

Method KEC-900 air ion detector was used to measure and record the NAI concentration at 11 plots of P. orientalis stand in Jiulong Mountain of Beijing according to day, month, altitude and aspect by the method of fixed observation.

Result The diurnal variation of NAI concentration in Jiulong Mountain differed among seasons. The curves of diurnal variation of NAI concentration in spring and summer followed a single-peak type, and the spring and summer peak values appeared at 12: 00 and 14: 00. The curves of diurnal variation of NAI concentration in autumn and winter followed a double-peak type, and the peak values appeared at 8: 00 and 18: 00. The monthly average concentration of NAI was characterized by two peaks in May and October, with values of 691 ions per cm³ and 920 ions per cm³. The concentration of NAI rose at first and then decreased with the increase of altitude. When the altitude is higher than 450 m, the NAI concentration in the P. orientalis forest was greater than in the forest edge. In the P. orientalis forest locates in Sijiangou with the altitude of 140m, the NAI concentration on the sunny slope was slightly lower than that on the shaded slope (P > 0.05) in April, and the NAI concentration on the sunny slope was significantly higher than that on the shaded slope (P < 0.05) in August.

Conclusion The NAI concentration in the P. orientalis forest of Jiulong Mountain have obvious spatial and temporal distribution characteristics, that is, the concentration of NAI shows "single peak type" or "double peak type" with time, and shows "single peak type" with altitude, and is affected by both the aspect and the time.