Urban air pollution consists of tiny particles, known as particulate matter (PM), and gases such as ozone (O3), nitrogen dioxide (NO2) and sulphur dioxide (SO2). These pollutants are formed mainly as a result of vehicle and industrial emissions.
Poor air quality is a serious threat to human health, causing problems for the respiratory system and cardiovascular diseases (Pope et al., 1995; 2002). In many UK cities, including Leeds, average levels of NO2 in the air exceed the legally binding limits set by the European Union (EC, 2008).
Worldwide it is estimated that approximately 3.7 million deaths per year are caused by exposure to poor ambient air quality (WHO, 2014). At the local scale, exposure to particulate air pollution is estimated to cause 350 premature deaths annually in Leeds, and 29,000 across the whole UK (PHE, 2014).
Trees and shrubs have multiple impacts on air quality. They can improve air quality by removing both particles and gases from the air; particles stick to the surface of the leaves, and gases are taken up through pores on the leaf surface. Trees with complex, ridged or hairy leaves (such as pines) tend to capture more particles than trees with broader, smoother leaves (Beckett et al., 2000; Freer-Smith et al., 2005; Rasanen et al., 2013). However, plants also emit gases (volatile organic compounds, VOCs ; Owen et al., 2003) into the atmosphere that can result in the formation of O3 and PM under certain conditions (Chameides et al., 1988; Donovan et al., 2005; Curci et al., 2009; Sartelet et al., 2012).
In places, trees may exacerbate local pollution by reducing the ventilation of air. The presence of large trees in narrow street canyons can obstruct wind flow and limit the ability of trees to remove pollutants (Buccolieri et al., 2009; Vos et al., 2013). As a result, planting hedges (Wania et al., 2012) or adding “green walls” (Pugh et al., 2012) in polluted street canyons may be more beneficial.
Current understanding suggests that the presence of urban vegetation results in an overall reduction in air pollution (Nowak et al., 2000; 2006). For example, schools surrounded by green space have been shown to experience lower levels of traffic-related pollution in their classrooms (Dadvand et al., 2015). However, more research is required to fully understand the multiple ways in which urban vegetation can affect air quality.