اثر درختان خیابانی بر میکرو اقلیم و آلودگی هوا در یک شهر گرمسیری

ABSTRACT Effect of street trees on microclimate and air pollution in a tropical city

In the century of urbanization, as urban population densities continue to increase, living conditions continue to deteriorate. This impact may be especially severe in the tropics, where most developing countries are located, and where as much as 90% of the urbanization of the coming decades is predicted to take place (Emmanuel, 2005). Tropical cities will experience especially severe consequences of urbanization, with accelerated urban heat island effects, changes in microclimate and thermal discomfort, because of their location in hot areas (Burkart et al., 2011). This is exacerbated by the rapid growth in many tropical cities, with large scale conversions of open areas and green spaces to concreted and built land cover (Emmanuel, 2005; Kant et al., 2009; Nagendra and Gopal, 2010). The consequences can be wide ranging, including increased probability of heat strokes and health disorders, and the altered distribution of precipitation events (Dixon and Mote, 2003; Vinoj et al., 2004; Chang et al., 2007).

Urban tropical locations also face challenges of air pollution (Ghose et al., 2005; Kuvarega and Taru, 2008). As levels of industrialization increase in these cities, coupled with high densities of vehicular movement, the exposure of urban dwellers to air pollutants is anticipated to rise exponentially (Parrish and Zhu 2009). Many South Asian cities currently deal with problems of atmospheric pollution due to vehicular traffic, with unacceptably high levels of Suspended Particulate Matter (SPM), NO2, SO2, and other air pollutants (Khan and Abbasi, 2000; Emmanuel, 2005; Ghauri et al., 2007). These air pollutants can cause severe respiratory and health problems for urban residents (Pope et al., 2000), particularly impacting vulnerable sections of society such as children (Kuvarega and Taru, 2008).

Mechanisms for mitigating these challenges of urban microclimatic variations and air pollution, although much in need, have not been well developed in most tropical developing countries (Escobedo et al., 2011). While much of the discussion has remained focused on the need for phasing out old technologies and promoting public transport, the potential role of urban vegetation in such mitigation has been largely ignored by planners.Most research has been conducted in developed countries as well, although issues of microclimate maintenance and air pollution control are just as important, if not even more so, in rapidly expanding developing country cities (Escobedo et al., 2011).

Trees in cities can reduce air pollution levels significantly, by the removal of pollutant gases such as SO2, NOx, CO and O3 through leaf stomata, and through the dry deposition of suspended particulate matter on leaf surfaces (Smith, 1984). Urban vegetation can also contribute to the reduction of the urban heat island effect, and trees have a clear role to play in microclimate amelioration (Chang et al., 2007). Trees in cities can further contribute to reduction of air pollution by reducing the need for air conditioning, leading to concomitant decreases in fossil-fuel generated air pollution (Chang et al., 2007; Escobedo et al., 2008; Jim and Chen, 2009).

This study explores the impact of street trees on reducing daytime temperatures and mitigating air pollution levels in the south Indian city of Bangalore. Internationally known for its technologically intensive industries, Bangalore is the second fastest growing city in India, with a population over 9.5 million. Given the deficiency of information on the environmental benefits of trees in tropical developing cities such as Bangalore, such exploratory studies are important to provide insights into potential benefits, which can then provide the basis for in depth, long term research and monitoring.