Air quality improvement in Chinese metropolis
di Davide Triacca
The quick deterioration of the air quality in Chines metropolis, triggered by anthropic activities, causes – according to the World Health Organization statistics – hundreds of thousands deaths per year by respiratory and cardiovascular diseases and high socioeconomic costs. An important contribution to reduce such air pollution can be obtained through a wise and strategic planning of the urban (smart) green infrastructure.
As clearly shown by studies and researches performed in many different urban areas affected by similar air quality issues all over the world, in fact, trees has a strong potential in removing air pollutants circulating in the atmosphere. Trees can reduce air pollutants in two ways1:1. by direct reduction from the air
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 surfaces2. Tree canopies can also intercept particulate matters in the air3. 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 areas4 5.
Tree direct reduction of atmospheric pollutants was estimated and measured in several world megacities providing the following results.
In 1994, trees in New York City removed an estimated 1,821 metric tons of air pollution at an estimated value to society of $9.5 million. Air pollution removal by urban forests in New York was greater than in Atlanta (1,196 t; $6.5 million) and Baltimore (499 t; $2.7 million), but pollution removal per m2 of canopy cover was fairly similar among these cities (NewYork: 13.7 g/m2/year; Baltimore: 12.2 g/m2/year; Atlanta: 10.6 g/m2/year)6. These standardized pollution removal rates differ among cities according to the amount of air pollution, length of in-leaf season, precipitation, and other meteorological variables. Large healthy trees greater than 77 cm in diameter remove approximately 70 times more air pollution annually (1.4 kg/yr) than small trees less than 8 cm in diameter (0.02 kg/yr). Air quality improvement in New York City due to pollution removal by trees during daytime of the in-leaf season averaged 0.47% for particulate matter, 0.45% for ozone, 0.43% for sulfur dioxide, 0.30% for nitrogen dioxide, and 0.002% for carbon monoxide.
