Why less light in winter over Dhaka?

Air pollution is a sound environmental problem around the world. There are several reasons why it is a concern for Bangladesh: a) less light or visibility deteriorates; b) global warming, c) increase of cloud albedo; and d) increase of death burden or mortality and morbidity. Here, the discussion has been made with an exclusive interpretation of visibility deterioration.

Visibility deterioration causes enormous public suffering daily and interrupts traveling on road transports and flights. An extended transport queue observes on the highways. Domestic and international flights become delayed or suspended due to poor visibility at the surface level. The news often reports the rise of road accidents during foggy winter environments. The boats, ferries, launch, or ships often halts their services in the rivers, canals, or many other waterways during the dense Fog in winter. The channels are convenient and also prevalent transport routes in this country.So, thick Fog or less light in winter significantly affects economic or business activities.

Air pollutants and climate variables are responsible for changing light properties in the troposphere or near the ground to the Earth. The chemical compositions, aerosol, and refractory particles in the atmosphere play a central role in this phenomenon. Absorption of atmospheric light occurs due to the rise of black carbon (BC), nitrogen dioxide (NO2), carbon dioxide (CO2), methane (CH4), ozone (O3), and nitrous oxide (N2O). The concentration of those parameters increases near the ground or in the troposphere in winter over Bangladesh. This is a potential reason for poor visibility over the entire Bangladesh. Another process is light scattering or reflection of the light back to space by several other variables. Notably, sulfate, nitrate, silicon dioxide, or sulfur dioxide are responsible for the light scattering process occurring in the atmosphere.  These light scattering variables are reasonably higher during winter than in hot Indian summer. The light absorption and scattering properties of several air pollution or climate variables are the significant causes of visibility deterioration in Bangladesh. Poor visibility is also a concern in other South Asian countries such as India, Pakistan, Nepal, Sri Lanka, and Bhutan. These countries equally suffer from the slower economic activities in this region. Local meteorology and weather phenomenon is a hidden causes for less light in the South Asian region in winter.
A planetary boundary layer (PBL) is a vertical structure of the lower atmosphere, extending from the Earth�s surface to about 1 km. PBL governs the changing of local meteorology significantly. The depth of PBL and temperature profile are not steady but changeable. Say the temperature vertically decreases with the increase of altitude in the PBL and the troposphere (near the surface to about 15 km altitude). However, there is a steady increase in the temperature after 15 km until 50 km altitude. This layer�s name is the stratosphere, popularly known as the ozone layer. The vertical temperature profile reported above is a common atmospheric phenomenon kept occurring since the beginning of this planet. However, a temperature inversion in the troposphere interrupts the vertical movement of the chemical pollutants in the atmosphere. Thus, the concentration of atmospheric compositions increases near Earth�s surface, making the air thicker. The fog and mist are also trapped near the ground. How does the temperature inversion occur?


Ideally, air near the ground is warmer due to the heated Earth's surface, and the temperature decreases steadily with altitude. Air is relatively clean in this condition as air can mix vertically from warm to cold air. But sometimes, in the winter environment, the cold air is trapped near the surface. A new warm layer, known as the inversion layer, forms above the surface. The temperature is higher in the inversion layer,in which cold air is present above and below this new inversion layer. If there is relatively higher moisture near the surface, fog or dew may form due to the condensation of water vapor to water droplets and is trapped near the surface, causing a sharp deterioration of visibility. During winter, the cold wind transports from the Himalayas toward Bangladesh and other South Asian countries. This cold air triggers the formation of Fog and mist below the inversion layer, causing less visibility in this region. This unique change in local meteorology happens over winter conditions, a valley over a city, or a city surrounded by mountains. Under this circumstance, the PBL becomes shortened, enhancing the concentration of atmospheric compositions. Thus, the elevated concentration of those compositions reduces the light intensity through absorption and scattering processes.
Md Firoz Khan, Ph.D.is an associate professor and Leader of the Aerosol Lab, Department of Environmental Science and Management at North South University