Objective Studying the variation of negative air ion concentration in three main types of stand at Chongli district and the influencing factors in order to lay basis for ecological benefit evaluation and ecotourism planning.

Method Based on continuous observation of negative air ion concentration in typical stand at Chongli, the diurnal variation of negative air ion concentration in poplar (Populus hopeiensis Hu et Chow) forest (PHF), birch (Betula platyphylla Suk.) forest (BF), pine (Pinus tabulaeformis Carr.) forest (PF).and non-forest site (NF) was studied. The effects of meteorological factors and plant photosynthetic characteristics on negative air ion concentration were analyzed. The air quality of different stands was evaluated by unipolar coefficient method and air quality coefficient method.

Result The variation trend of negative air ion concentration had two peaks. The lowest value appeared at noon but the maximum value appeared in different time. The negative air ion concentration in different stands was obviously different. Broadleaved forest was superior to coniferous forest. The negative air ion concentrations of PHF, BF, PF and NF were 1 101, 847, 755 and 344 ion·cm^-3 respectively. The negative air ion concentration in different stands was negatively correlated with temperature, but positively correlated with humidity and the photosynthetic characteristics of plants. The single grade coefficients of PHF, BF, PF and non-forest land were 0.92, 1.01, 1.25 and 1.37. The air quality evaluation index of PHF, BF, PF and non-forest land were 1.25, 0.85, 0.80 and 0.27.

Conclusion The concentration of negative air ions in different stands is different and the broadleaved forest is superior to coniferous forest. The concentration of air ions is affected by external climatic factors and the photosynthetic characteristics of plants. There are also differences in the air quality evaluation index of different stands. Reasonable allocation of forest resources is beneficial to air quality improvement.