Trees can save Dhaka from heat and air pollution

(Clockwise) : Congested concrete structures conserve heat to the inconvenience of urban dwellers. The more an urban area is interspersed by greenery the better it is for reducing its heat and air pollution. Treelined streets not only add to serenity but provide cooling effect to urban surrounding.

Parallel to the fastest growth of urbanisation, its adverse impact on the population and environment is also amplified these days thanks to the increasing number of vehicles, haphazard establishment of industries and congested residential units.

Besides experiencing a degrading living standard in the city areas due to air pollution and other anomalies, people are also facing unfavourable weather condition in the dry season. The extent of air pollution and emission of toxic gases into the air, including the greenhouse gases like carbon dioxide, carbon monoxide and sulphur dioxide, is higher in dry weather compared to the rainy season.

Trees, shrubs and vines play the pivotal role by absorbing much of the heat and pollutants in the air, and giving people respite from damaging agonies.

The adversaries of summer are so high in cities that it clearly draws a differential line between the air and surface temperatures of urban areas and that of rural areas. And thus the extent of air pollution, heat-related illness and mortality are high while energy demand and air conditioning cost are on the rise.

In the city areas, heat is formed because of the absence of natural land cover and excess of built areas like metalled roads, pavements, buildings and other infrastructure — the main elements of growing urbanisation. All these — paradoxically, for a better living — lead to the minimisation of necessary greenery or their displacement.

As a result, the overall environment and weather conditions change significantly — unfavourably. In this situation with decrease or absence of the natural cooling effects of shading and evaporation of water from soil and leaves, tall buildings and narrow streets can heat air that is trapped between them and reduce wind flow while released heat from vehicles, factories, and air conditioners may add to that, further increasing temperatures.

Hot weather may prevail during day and night compared to rural areas which cool off faster at night than cities. The stored heat in city roads and buildings is the reason behind the slow change in temperature in the evening.

Even though there are provisions in construction rules that open spaces must be left there for greenery and proper penetration of water into the ground, they are hardly maintained by the initiators of commercial or industrial and even residential units. At present, however, some newly built residential areas have adopted some plantation for relief.

In addition, more and more age-old and broadleaf trees are felled everyday for construction of structures, while the rate of plantation is very low and insignificant. Thus, it can be seen that trees are fading away from the fastest-growing urban areas where both heat and air pollution is prominent.

Unless there were public parks and some roads lined with hundreds of large trees giving shade and contributing in cooling the air of the cities, the situation would have been simply unbearable.

Because of an increase in urban temperatures, people suffer from different complications including heat stroke, physiological disruption, organ damage, and even death. The elderly and children are highly vulnerable in such case.

Moreover, it has some financial drawbacks too. As the heat mounts, people in the urban areas undergo more spending for the summertime cooling which also triggers emission of harmful pollutants.

Studies say higher temperatures also accelerate the chemical reaction that produces ground-level ozone, or smog.

Greenery — a universal solution
To create a better living place amid up-and-coming urbanisation, the culture of plantation of trees and vegetation has been widespread all over the world. But the scenario is grave in our urban areas where people prefer utilising every single inch of their land by erecting structures.

Moreover, urban design and layout must be modified, and efficient heating and cooling systems chosen to ensure a congenial environment. Vegetation also ensures better soil structure.

Apart from applying this in residential facilities, cool paving materials should be used for roads, sidewalks, and parking lots as heat mitigation measures so that people may walk in a relatively cooler space and vehicles parked do not make much of evaporative emissions.

Tree canopies also affect wind speed, its relative humidity and turbulence, as well as surface roughness.

However, although trees usually contribute to cooling summer air temperatures, their presence can increase air temperatures in some instances. In areas with scattered tree canopies, radiation can reach and heat ground surfaces; at the same time, the canopy may reduce atmospheric mixing such that cooler air is prevented from reaching the area, studies say.

While planting trees, its placement should be considered so that shades are found to be beneficial and vegetation blocks the sun’s ray, minimising heat transfer inside buildings thus reducing the need for air conditioning.

In our cities, trees should shade the east, and especially west, walls to maximise savings on cooling. Planting trees directly to the south may provide little shade in the summertime and block desired sun in the wintertime.

Experts suggest that large, healthy and broadleaf trees work well as they balance energy requirements over the course of a year. In summer, foliage cools buildings by blocking solar radiation. In winter, after the leaves have fallen, the sun’s energy passes through the trees and helps to warm buildings.

Locale for outdoor plantation is also crucial because of the concrete pavements, bituminous roads and vehicular pollution. To be benefited in terms of air quality, long-life trees should be planted in polluted areas or heavily populated areas, when the pollutant sensitive species should be avoided.

Meanwhile, trees emitting Volatile Organic Compounds (VOCs) should be averted as they contribute to the formation of ozone and carbon monoxide.

Trees reduce gaseous air pollution primarily by uptake via leaf stomata, and some gases are removed by the plant surface. They also remove pollution by intercepting airborne particles. Some particles can be absorbed into the tree. The intercepted particle often is re-suspended to the atmosphere, washed off by rain, or dropped to the ground with leaf and twig fall.

As the significance of greenery will never decrease, it is the duty of all to ensure presence of trees around us, conserve and help them grow and help the present and future generations live in a congenial atmosphere keeping pollution and temperature at a tolerable level.


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