The urban forest in Beijing and its role in air pollution reduction

Abstract

Tree planting has been proposed by the municipal government as a measure to alleviate air pollution in Beijing, the capital of China. This study examines that proposal. It is based on the analyses of satellite images and field surveys to establish the characteristics of current urban forest in the central part of Beijing. The influence of the urban forest on air quality was studied using the Urban Forest Effects Model. The results show that there are 2.4 million trees in the central part of Beijing. The diameter distribution of the trees is skewed toward small diameters. The urban forest is dominated by a few species. The condition of trees in the central part of Beijing is not ideal; about 29% of trees were classified as being in poor condition. The trees in the central part of Beijing removed 1261.4 tons of pollutants from the air in 2002. The air pollutant that was most reduced was PM₁₀ (particulate matters with an aerodynamic diameter smaller than 10 μm), the reduction amounted to 772 tons. The carbon dioxide (CO₂) stored in biomass form by the urban forest amounted to about 0.2 million tons. Future research directions to improve our understanding of the role of individual tree species in air pollution reduction are discussed.

Introduction

Urban forests play an important role in improving the quality of the urban environment. A healthy, well-managed urban forest can provide many ecological benefits. For example, the urban forest can reduce storm water runoff (Xiao et al., 1998); alleviate the intensity of the heat island (Akbari and Konopacki, 2004), sequestrate CO₂ (McPherson, 1998a; Nowak, 1994b), and reduce air pollution (DeSanto et al., 1976; Dochinger, 1980; Nowak, 1994a; McPherson and Simpson, 1998). Among the benefits provided by urban forests, the function that is most important to Beijing, capital of China, is the ability of the urban forest to remove air pollutants from the atmosphere.

Like other metropolitan areas in developing countries, Beijing faces many environmental problems caused by poor urban planning and rapid development. Air pollution is the most outstanding of these. Beijing is among the ten cities of the world with the worst air pollution problem (World Bank, 2000). Severe air pollution causes serious health problems and loss of life. Research indicates that when the air pollution index (AQI) in Beijing rose by 10%, the daily number of deaths caused by respiratory disease increased by 3.52% (Zhang et al., 2003).

Trees can reduce air pollutants in two ways: (1) by direct reduction from the air, and (2) by indirect reduction by avoiding the emission of air pollutants. In direct reduction, trees absorb gaseous pollutants like sulfur dioxide (SO₂), nitrogen dioxide (NO₂), and ozone (O₃) through leaf stomata and also can dissolve water-soluble pollutants onto moist leaf surfaces (Nowak, 1994a). Tree canopies can also intercept particulate matters in the air (Beckett et al., 1998). Indirectly, trees can reduce the air temperature through direct shading and evapotranspiration in the summer, thus reducing the emission of air pollutants from the process of generating energy for cooling purposes. Also, reduced air temperature can lower the activity of chemical reactions, which produce secondary air pollutants in urban areas (Taha, 1996; Nowak et al., 2000). Trees can also be a source of air pollutants; they emit biogenic volatile organic compound (BVOC). BVOC can react with nitro oxides (NO_x) and form O₃ and aerosols (Benjamin and Winer, 1998). The production of pollen by trees is a source of particles that can have serious health effects on people allergic to pollen (Beckett et al., 1998).

People have known that trees can help to reduce air pollutants for a long time. The Roman senate recognized the value of orchards in villas surrounding the city of Rome for maintaining air quality and forbid their conversion to urban housing (Cowell, 1978). In more recent times scientific studies have quantified the amount of air pollutants removed by trees in cities. DeSanto et al. (1976) studied the removal of five major air pollutants, SO₂, carbon monoxide (CO), O₃, NO_x and particulate matters by street trees in St. Louis area. Dochinger (1980) conducted early studies of the interception of particulates by trees. In these studies, the air-pollutants reduction effects of the urban forest were calculated by extrapolating the field measurements of several trees to the whole urban forest. There are, however, problems with this approach. Primary among these problems is that the concentration of air pollutant varies spatially and temporally and the conditions of trees are highly variable within a city. Large samples are needed to account for this variation. Another approach to determine the air pollution reduction effect of trees in urban areas is to integrate the knowledge of meteorology and atmospheric chemistry with tree biology to model air pollution reduction in a certain area and time. This method was used by Nowak (1994a), Nowak and Dwyer (2000) and Scott et al. (1998) to study several American cities. Their studies show that urban forest can contribute significantly to air pollution reduction. There are some uncertainties and limitations in this modeling method that need to be improved. Scott et al. (1998) gave a detailed discussion of these limitations. The main limitation of this method is that the direct air pollutants reduction by trees is calculated using a “big-leaf” model. The air pollutant concentration was assumed as homogenous in whole city, and the trees were assumed as occurring in a homogeneous, connected layer. In reality, the urban building configuration, the local photochemical reaction, and the meteorology condition influences the air pollutants concentration in a city resulting in a somewhat less than homogenous concentration. Also, the trees vary with different heights and crown configurations. So the air pollutants deposition rate estimated using a “big-leaf” model is only a rough approximation of the real situation. However, the method is still the best available method and was used in this study.

Trees in urban areas also work to sequestrate CO₂ from the atmosphere and store it in the form of woody biomass. Because of its role as a greenhouse gas in global warming, the reduction of CO₂ levels in the atmosphere was defined as an important task in the Kyoto Treaty. Furthermore, urban forest helps to reduce energy use for cooling and heating, resulting in the reduction of CO₂ emission from power plants (McPherson and Simpson, 1999).

Studies on the structure of the urban forests in Beijing and its role in air pollution control are limited. Zhu and Wu (1995) studied the pattern of roadside greenspace. Zhang (2001) studied species diversity. Zhao et al. (2001) conducted investigations on street tree planting. Profous (1992) studied the influence that management practice, nursery production, public preference, historical development and land use had on the urban forest structure in Beijing. Chen et al. (1998) conducted the most comprehensive studies to date on the Beijing greenspace and its ecological benefits. They studied the distribution, structure and management of greenspace in Beijing. Also, they estimated CO₂ sequestration, absorption of SO₂, chlorine gas (Cl₂), dust interception, and the influence of evapotranspiration from vegetation on air temperature by plants in the Beijing greenspace. However, their study did not provide the fundamental characteristics of the urban forest such as the DBH (the diameter of tree at the breast height) class distribution, the species percentage, and health conditions of the trees. In their studies of CO₂ sequestration and reduction of air pollution Chen et al. (1998) extrapolated field measurements from about dozens trees of 21 species to the entire city. As mentioned before, this method neglects the spatial and temporal change of air pollution concentration. It also does not account for the variations in different species, different growth habit and differences in tree condition in different part of the city.

The study presented here was designed to provide a more detailed analysis of the Beijing urban forest and its effect on air pollution. Four objectives were addressed in the study: (1) to describe the current composition and structure of the Beijing urban forest; (2) to quantify the major air pollutants including SO₂, NO₂, PM₁₀, and O₃ that are reduced from the atmosphere by urban forest; (3) to quantify the BVOC emission from the urban forest; (4) to calculate the sequestration of CO₂.