August 6, 2019


Black Carbon has substantial effects on the regional climate through local pollution and the formation of Atmospheric Brown Cloud. Black carbon (BC) is the most strongly light-absorbing component of particulate matter (PM). It has a strong warming effect through its impact on ice and snow. It reduces the reflectivity and accelerates melting of snow in the Himalayan region. Black Carbon is a product of incomplete combustion. It is the solid, mostly pure carbon component of soot that is capable of absorbing light at all wave lengths; hence it is black in appearance.

BC gets mixed into the atmosphere in the form of fine particles (PM2.5). It is the most effective form of PM, by mass, at absorbing solar energy per unit of mass in the atmosphere. BC can absorb about million times more energy than carbon dioxide. BC is a major component of “soot”, a complex light-absorbing mixture that also contains some organic carbon.

Impact of Black Carbon in Climate Change

BC affects climate by directly absorbing light radiation which is responsible for increase in temperature. The Himalayas and Arctic are vulnerable to the warming and melting effects of BC as it reduces the reflectivity (albedo) of snow and ice.

Black carbon has strong light absorbing property which affects the climate in multiple ways. First, black carbon in the atmosphere absorbs sunlight and infrared radiation, warming the nearby air. This is called the direct effect. Due to presence of larger numbers of aerosols, the same amount of water in clouds condenses into a larger number of smaller cloud droplets which suppresses the formation of drops that are large enough to rain out (Black Carbon: Impacts and Mitigation in the Hindu Kush Himalayas, ICIMOD).

Black Carbon particles presence within cloud drops makes the individual drop more light absorbing, and thus it is more likely to evaporate. These effects vary greatly in time and space, and are a great source of uncertainty in climate prediction. In areas with snow and ice, black carbon is also responsible for the snow albedo effect. When black carbon is deposited onto white surfaces it darkens the surfaces, allowing them to absorb more sunlight and warm up, leading to more rapid melting. Black carbon accelerates snow melting exposing darker surfaces underneath to sunlight (Black Carbon: Impacts and Mitigation in the Hindu Kush Himalayas, ICIMOD).

During winter and spring, haze layers extending more than three kilometers above the earth’s surface are formed. Black Carbon in the upper parts of the layer absorbs sunlight and warms the air and reduces the sunlight reaching the surface underneath.  It results cooling of valleys and low land areas contributing to the increased build-up of winter fog and reduction in winter climate. The reduced sunlight also affects crop and other biomass yields. So people light more fires to stay warm and they contribute to more emission of ABCs. There is rapid increase of temperature above the haze layers i.e. at higher altitude, contributing to additional melting of snow. The direct effect of BC could be double as previous estimates of IPCC (American journal of geophysical research)

Impact of BC in Health

The most significant health impact of outdoor air pollution has been associated with particulate matter (Cohen et. al., 2003; Holgate et. al., 1999 and World Bank 2002).  In most cities in developing countries, BC component of particulate matter is a major concern because their concentration in the air is often very high. This is true for Kathmandu as well.

There is currently insufficient information to differentiate the health effects of BC and other PM. Over the past decade, the scientific community has focused increasingly on trying to identify the health impacts of particular PM2.5 constituents, such as BC.

BC contributes to the adverse impacts on human health, ecosystems, and visibility associated with ambient fine particles. Short-term and long-term exposures to BC are associated with a broad range of human health impacts. BC exposure may affect respiratory and cardiovascular system as well as may lead to premature death. BC particle may pose a greater risk on human health then other component of PM2.5 (EPA, United States Environment Protection Agency).

As the most common route for pollutants to enter the human body is by inhalation, the most common effect of BC is to damage the respiratory system. Exposure to BC can overload or break down natural defense mechanisms in the body, causing or contributing to respiratory diseases such as lung cancer, asthma, chronic bronchitis and emphysema. BC can also have adverse impacts on other important systems such as cardiovascular system and central nervous system.

Chronic Obstructive Pulmonary Disease (COPD) which is an irreversible damage of the lungs is one of the main health effects of BC. Several studies have shown that children, elderly and people with lung and heart diseases are more vulnerable to the health effects. Health effects of BC can be estimated by conducting epidemiological studies or experiments on animals and humans. Although no long-term epidemiological studies have been conducted in Kathmandu, a few studies have been done by conducting preliminary medical examination of a group of exposed population or by using dose response relationships that have been developed elsewhere. These indicate that the health impacts of BC can be quite severe. Analysis of hospital records from major hospitals in Kathmandu indicates that the number of COPD patients admitted to hospitals, as well as the percent of COPD patients as a percentage of total medical patients has increased
significantly. Hospital records also indicate that the number of COPD patients is highest in the dry winter months, which is also when air pollution in Kathmandu is at its peak.

WHO has recognized that fine particles are the most dangerous air pollutants as they can enter deep into the human body and they are often coated with toxic substances. WHO says there is not safe limit for the concentration of PM10 (particle matter less than 10 microns) or PM2.5 (particle matter less than 2.5 microns) in the air as even at low levels they can cause harm to human health. The main source of fine particles is combustion of fossil fuel, such as vehicle emission

BC Emission Inventory for Kathmandu Valley

The main sources of black carbon in contest of Kathmandu Valley are transport sector, residential sector, industrial sector and commercial sector. Use of diesel and gasoline in transport sector is responsible for BC emission. Wood, LPG and kerosene in the residential sector and use of wood and coal in the industrial sector and diesel in the commercial sector are highly responsible for BC emission in Kathmandu Valley. The study done by World Bank estimates that 221 tons of BC was emitted from diesel generators in FY2012/13 Kathmandu Valley. In this work at the same time BC emission in commercial sector is found to be 222 tons. from May 2009 to April 2010 BC was measured using anaethalo meter (AE-31) at Pulchowk Campus, an urban location of Kathmandu valley for the last two months (March April, 2010). BC concentration shows significant temporal variation with monthly mean concentration varying between 14.9 mg/m3 and 3.0 mg/m3.