The spatial-temporal pattern and influencing factors of negative air ions in urban forests

The spatial-temporal pattern and influencing factors of negative air ions in urban forests,

Shanghai, China

Hong Liang · Xiaoshuang Chen · Junguang Yin · Liangjun Da

【摘 要】Abstract：Negative air ions are natural components of the air we breathe. Forests are the main continuous natural source of negative air ions (NAI). The spatio-temporal patterns of negative air ions were explored in Shanghai, based on monthly monitoring in 15 parks from March 2009 to February 2010. In each park, sampling sites were selected in forests and open spaces. The annual variation in negative air ion concentrations (NAIC) showed peak values from June to October and minimum values from December to January. NAIC were highest in summer and autumn, intermediate in spring, and lowest in winter. During spring and summer, NAIC in open spaces were significantly higher in rural areas than those in suburban areas. However, there were no significant differences in NAIC at forest sites among seasons. For open spaces, total suspended particles (TSP) were the dominant determining factor of NAIC in summer, and air temperature and air humidity were the dominant determining factors of NAIC in spring, which were tightly correlated with Shanghai’s ongoing urbanization and its impacts on the environment. It is suggested that urbanization could induce variation in NAIC along the urban-rural gradient, but that

may not change the temporal variation pattern. Furthermore, the effects of urbanization on NAIC were limited in non-vegetated or less-vegetated sites, such as open spaces, but not in well-vegetated areas, such as urban forests. Therefore, we suggest that urban greening, especially urban forest, has significant resistance to theeffect of urbanization on NAIC.

【期刊名称】林业研究（英文版） 【年(卷),期】2014(000)004 【总页数】10

【关键词】Keywords：negative air ion concentration, spatial-temporal pattern, urbanization, urban ecosystem, urban greening ORIGINAL PAPER

Received: 2013-04-23; Accepted: 2013-07-09

? Northeast Forestry University and Springer-Verlag Berlin Heidelberg 2014

Introduction

Negative air ions (NAI) are natural components the air we breathe (Kosenko et al. 1997). When sufficient force displaces an outer electron from a molecule of common gases, such as oxygen and nitrogen, the free electron is promptly picked up by an adjacent molecule, which then becomes negatively charged. Meanwhile, the residual molecule is left with a positive charge. These molecular ions are soon attracted to water

molecules and uncharged gaseous molecules in the air, which form singly charged molecules (Krueger 1985). Superoxide (O2-) is the main negatively charged species of NAI (Goldstein et al. 1992; Kosenko et al. 1997; Wu and Lee 2004) and is more stable than other ions.

In nature, NAI can be generated by the minute shearing of water droplets, radioactive components of soil, cosmic radiation, ultraviolet rays, coronal discharge or lightning, and forests (Krueger and Reed 1976; Yates et al. 1986; Iwama 2004). Of these, forests are the main continuous natural source of NAI (Wang and Li 2009), because leaf tips have a photoelectric effect during photosynthesis, which are able to promote electrolysis and produce large quantities of NAI (Tikhonov et al. 2004). In addition, aromatic substances released by forests, such as phytoncide, can also promote air ionization and increase negative air ion concentrations (NAIC) (Shao et al. 2005; Bai and Wu 2008). Additionally, NAI can also be produced by artificial combustion sources, hot surfaces, and flames, such as motor vehicles and power lines (Fialkov 1997; Maricq 2006; Peineke and Schmidt-Ott 2008; Ling et al. 2010).

Under stable conditions, the background level of NAIC is in the magnitude of hundreds (H?rrak et al. 2003; Vana et al. 2008), usually about 300?400 ions/cm3(Ling et al. 2010). NAIC can increase to the magnitude of thousands in the presence of natural or anthropogenic ion sources, such as forests (Wu et al. 1998),waterfalls (Laakso et al.