Numerical Evaluation of the Effects of Green Belt Development on Local Meteorology and Air Quality over Tehran Metropolis

Urban green spaces are essential parts of the city's configuration which can be expanded in the urban areas in the forms of parks or gardens, and green urban structures such as green roofs or green walls. The green areas around the city are usually constructed in the form of Green Belts and forest parks. In new decades, land use/ land cover alterations have remarkably reduced the urban green spaces which cause environmental concerns such as biological pollutions, increase in surface runoffs and greenhouse gases, and the phenomenon of dust and the urban heat island. Studies on the role of urban greening on air quality in different regions have shown that the structure, physical characteristics and vegetation types can be the decreasing factors of air pollution, but in special circumstances, they may show adverse negative impacts. Indeed, on one hand, they can affect the wind and temperature fields, as well as the atmospheric emission patterns and the transmission of pollutants and natural ventilation over the city, and on the other hand, by releasing organic volatile organic compounds (VOCs), they may accelerate the rate of ozone production. Therefore, proper selection of plant species and management of urban green space design are substantial and complicate issues in urban management programs. One of the proposed strategies to control the excessive expansion of cities, prevention of marginalization and population growth in cities is the development of the Green Belt. According to the definitions, the Green Belt is a green striped borderline in the margins of the city that has numerous environmental impacts, such as beautification of the urban landscape, stabilization of the soil and preventing the entrance of the aerosols and dust to the city center. According to related studies, the area and density of the Green Belt play major roles in changing the concentration of atmospheric pollutants due to sedimentation and deposition processes. The history of the Green Belt Development Project of Tehran backs to about 40 years ago and its completion is one of the priorities of the urban management programs. It is predicted that the area of Tehran Green Belt reaches more than 40,000 hectares so it is expected to show significant changes on the micro-climate of the city. Therefore, in this study, the role of the Green Belt Development of Tehran on weather parameters such as wind and temperature fields, relative humidity, surface heat flux and planetary boundary layer height (PBLH), as well as the atmospheric concentration of atmospheric gas pollutants (SO2,NO2,CO and O3) are studied, using coupled Meteorological-Chemical numerical model (WRF-Chem). WRF-Chem numerical model (version 8.3) coupled with the Noah Land Surface Model and the UCM urban canopy model is used for numerical simulations. The National Centers for Environmental Prediction (NCEP) global forecast system (GFS) reanalysis data on a 1˚×1˚ grid are used for the atmosphere and soil initial and boundary conditions. MOZART (Model for Ozone and Related chemical Tracers) data are applied for boundary conditions of domain 1 and initial conditions for 4 domains in the chemistry section. The 2005 version of carbon bond chemical mechanism (CB05) and the MADE (Modal Aerosol Dynamics Model for Europe) aerosol scheme are used for simulations. Model of Emissions of Gases and Aerosols from Nature (MEGAN) is used for biochemistry. Anthropogenic emission inventory is produced by PREP-CHEM-SOURCES preprocessor from RETRO (REanalysis of TROpospheric chemical composition) global data. Tehran updated emission inventory is mapped over the global anthropogenic data in the fourth domain and emissions are introduced to the model through two separate times (wrfchemi_{00z,12z}_d04). The control case is representative of the current state of land use in the city of Tehran. Four early summer days (18-21 June 2016) with the cloudless condition, calm wind and without precipitation are selected for the simulations. In the next stage, the green belt scenario is performed to reproduce the meteorological and chemistry parameters and the results were compared with the control case. Findings show that green belt development on the eastern and southeast boundaries of the city has reduced the temperature (about 2.5°C) and increased relative humidity within the city (about 7% at nighttime and 4% at daytime) which confirms the positive role of this green area in improving environmental comfort level. Also, the cooling effect of this green space in the borders of the city is noticeable. Furthermore, this urban green approach reduces the wind speed and changes the wind direction over the region. This factor, as well as the observed decrease in PBLH (nightly changes in the center of the city are -200 meters and in the northwest and southeastern margins are -300 and -500 meters, respectively), have caused the accumulation of polluted air in the city center. Particularly during the night, the green band of the northern margin of the city has reduced the influence of local mountainous winds to the city and a significant reduction in wind speed in the city center, which has a negative impact on the status of air pollution and heat island in this area. Changes in the wind direction in the western border of the city prevents the outflow of polluted air and entrance of the clean air which has a negative effect on the concentration of industrial pollutants in this part of the city. Further studies in this area are needed in other seasons, especially in winter, with significant levels of pollution. Based on the results obtained in this study, low-density plants with limit height which have less unwanted impacts on wind flow and natural ventilation of the city, are proposed to be used in Green Belt project of Tehran Metropolis.