Winter fog — a health and environmental hazard

[Naveed Yaseen]

By B. Ghauri
Every winter satellite images show a dense fog belt over northeastern India extended to northwestern India and eastern Pakistan. Winter fog thoroughly disrupts all human activities in the affected areas of the sub-continent. Low visibility causes multiple accidents, cancellation of air flights and delays in road and railway transport. Fog affects human health as the particulates’ load increases during fog episode due to inversion and low dispersion. Therefore, depression, respiratory and heart ailment and other problems have been observed to increase during the episode. Similarly increasing aerosol load of the atmosphere and reduced insulation can seriously harm agricultural produce and the economic loss to the region can be immense.

Although fog formation relates to meteorological conditions, it becomes denser and persists for longer duration due to high level of chemical species in fine particulates matter or aerosols such as sulphate. The fog thickness is largely determined by the altitude of the inversion boundary, layer, above which the air mass is warmer and drier.

The airborne particles burden over Asia have increased significantly as the population, industrialisation and urbanisation are continuously rising without prevalent control planning. The airborne fine particulate matter directly perturbs the livelihood of millions of inhabitants by inhalation and indirectly by disturbing the regional hydrological cycle and climate.

NOAA image of 26th Dec-2008 at 0922 PST received and processed at SUPARCO Satellite Ground Station (ADRPC), Karachi, showed a fog cover over northern India and north- eastern Pakistan covering an area of 46,281 sq.km

Haze consists of a combination of water droplets and minute particles. Water droplets in a haze are less than 0.001 mm in radius. There are two possible sources for particles in a haze. They are either generated naturally (e.g. sea salt, dust) or man-made (e.g. sulphate or soot). From a satellite, the haze appears brown when the fraction of soot or dust is large.

A winter fog study carried out by SUPARCO in Lahore from 1999 to date has shown high pollutants levels in all field samples of SO4-2 from 4 to 141 µg/m3 and NO3-1 from 3 to 74. 5 µg/m3. High trace elements’ concentration has also been observed.

Concentration of black carbon (BC) determined every 5 min in Lahore from November, 2005 to January 31, 2007 was among the highest urban values worldwide ranging from about 5 to 110 µg/m3 with a mean of 21.7 µg/m3. On a global basis black carbon contributes ~0.5 Wm-2 to radiative forcing. High concentration of soot over India and China are responsible for a trend toward increased flooding in the south and drought in the north. It is estimated that the reduced atmospheric trans boundary caused by high soot concentration over India and China decreases agriculture productivity by 10-20%. Soot deposited on plant leaves also reduces plant productivity.

The visibility impairment in the form of brown cloud, fog and haze/smog is experience in Asia whole year. Smog/fog is a mixture of various gases with water vapours and dust. A large part of the gases that form smog is produced when fuels are burnt. Indian Ocean Experiment (INDOEX) carried out an extensive aerosol measurement in South and Southeast Asia and reported that aerosol contained substantial amounts of both inorganic and organic pollutants, including black carbon. Scientists reported that due to man-made aerosols, India is about six to seven percent darker in about 60 years. Indian coal-based thermal power plants consume thousands of tons of coal daily and emit tons of sulfur dioxide, black carbon, arsenic etc. and introduce heavy regional atmospheric pollution load which plays a key role in the climate change process. Sulfur dioxide is converted to sulfate and change the nature of regional aerosol to acidic. Extensive agriculture burning in the region is a major source of black carbon in addition to industrial and vehicular emissions. Black carbon or soot carbon is the light-absorbing component of aerosols, which is formed in combustion processes. Sulfate and black carbon particles also play a major role in changing atmospheric and cloud chemistry in addition to direct health implications. They affect cloud nature, rain patterns, severity of rain and aerosol radiative forcing etc. which drastically affect human health and crop yield of this region which is dependent on agriculture.

An important characteristic of fog is a marked decrease in visibility. This occurs because of the presence of aerosols, i.e. particles and water droplets dispersed in atmosphere.

A major factor contributing to fog is the chemical composition of the aerosols (which showed a significant concentration of soot) and derivation of the radiative forcing associated with the aerosols. The presence of soot (black carbon) in fog warms the atmosphere, which is diametrically opposite to the effect of sulphate aerosols which cools the atmosphere. Aerosols particles can modulate the radiative energy balance of the atmosphere by changing the amount of solar and terrestrial radiation absorbed and scattered by the atmosphere. The size distribution of aerosols and their chemical composition control the manner in which aerosols alter the radiation budget of the atmosphere. The presence of aerosols decreases the solar radiation reaching the surface of the earth and usually increases the solar radiation reflected to space. In the infrared region, the presence of aerosols increases the atmospheric radiation reaching the surface and decreases the atmospheric radiation emitted to space. Aerosols can also act as cloud condensation nuclei. An increase in the concentration of aerosols increases the number of cloud droplets and hence also the reflectivity of clouds. The presence of soot in clouds increases the absorption of solar radiation and hence can lead to a cloud-cover change. Thus aerosols can also have an indirect impact on the radiation budget.

SUPARCO, the national space agency of Pakistan, is carrying out a research project to study the phenomenon of visibility impairment in the form of fog/smog in Punjab (Lahore city). The study is focused to study the atmospheric fog aerosols, their composition, and their impacts on human health, agriculture etc. Particulate matter levels (course and fine) have been observed many folds in mega cities of Pakistan, especially in Lahore, as compared to standards/guidelines given by USEPA/ WHO. For example observed 24 hours average PM2.5 (Particulates with an aerodynamic diameter less than 2.5 micrometer) concentrations were 2 ~ 14 times higher than the prescribed limits of USEPA i.e. 35µg m-3 in winter and spring. PM2.5 is of the utmost importance to be considered it can penetrate deep into lungs and many trace and toxic material and carcinogenic pollutants absorbed on these particles (PM2.5) may cause a serious damage to people health. The preliminary one hospital respiratory illness-related retrospective data analysis showed increasing trend of asthma patients and number of patients are also increasing with the passage of time. The number of asthma patients has also been correlated with monthly fine particles levels. PM2.5 concentration affects the respiratory and cardiac health. The fine particulate load increased many fold from October to April and play a key role in changing seasonal and monthly pattern of climate. The fine particles (increased concentration of greenhouse gases, black carbon and sulphate) accelerate the haze/smog/fog episode, reduced the sunny days and radiative forcing, increase the cloud cover and cause unexpected heavy rains and temperature anomalous etc. have bigger extent to damage the quality and quantity of cash and cereal crops such as cotton, rice and wheat etc. in addition to health and other environmental damages.

The writer is a researcher with SUPARCO

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