Urban air pollution and its effect on fog occurrence: a view from Northern India

Dense fog in Delhi (Source: Hindustan Times)

Dense fog in Delhi (Source: Hindustan Times)

Many Asian countries, including China and India, are facing a major problem of urban air pollution. This is alarming because most of the population of these countries are now living in urban cities. Furthermore many inhabitants of rural areas are aiming to relocate to urban cities for better life opportunities. The rising populations in megacities are a major driver in creating air pollution. Over the last few years, the increasing levels of air pollutant concentrations, mainly as secondary aerosol, in the south Asian urban atmosphere is making a serious impact on environmental and human health. In particular, atmospheric fog is the key problem for these countries, mainly in winter season.

Taking India as the example, over the last two decades fog phenomena have significantly changed mainly in the northern part of country. This part consists of the Indo-Gangetic Plain, which has more than 500 million inhabitants and stretches from the north-west to the north-east. It is found that anthropogenic air pollution has hiked 50-70% from 1975 to 2012 in this area. Here fog is a big peril to all types of transport: air, rail and road during the winter season, causing great economic loss (Mohan and Payra, 2009) and sometimes accidental death due to low visibility (less than 1000 meters). For example, over the last three years Indian Railways (IR) has lost several thousand million pounds due to train cancellations (Mathur et al, 2012) and two hundred and seventeen air flights were diverted in 2010 in New Delhi airport (Syed et al, 2012) alone. Thus fog has been alarming to transport services due to reduction in visibility. This is directly or indirectly related to the air pollution and thus visibility reduction, causing a significant impact on the environment as well as daily human activities (Singh and Dey, 2012). In 1970, Collier observed that fog occurrence is more frequent in the city in comparison to airport and rural areas due to the greater presence of sources of air pollution inside the city. In the last five years the amount of scientific literature in this area has increased significantly, but there are still uncertainties in the field of fog formation identification with different aerosol composition. From the past literature it is clear that high atmospheric aerosol is the most important factor that can be responsible for rapidity of fog formation over urban areas (Mircea et al, 2002; Yasmeen et al, 2012). In 2009, Mohan and Payra observed that high levels of pollution in urban regions led to increased fog frequencies, and that air pollutants led to atmospheric reactions, which are responsible for secondary aerosol. In other way the mixture of atmospheric pollutant and meteorological conditions (such as temperature and the relative humidity) are responsible for fog formation, but the pollutant characteristics in terms of size, shape, concentration and composition in the lower troposphere can change the frequency and mathematics of fog formation. Fog formation in northern India is mainly due to aerosol radiative cooling (Dutta, 2010) with low wind speed and high RH (Syed et al, 2012). Because of low wind speed in this region, during the winter season transported pollutants are less responsible for fog formation than pollutants coming from power plants, vehicles and factory emissions.

Figure 1: The mean frequency of fog (%) in winter for 82 stations from 1976 to 2010 (Source: Syed et al, 2012)

Figure 1: The mean frequency of fog (%) in winter for 82 stations from 1976 to 2010 (Source: Syed et al, 2012)

The recent study of Syed et al, 2012 has clearly shown (Figure 1) that winter fog frequency over the Indo-gangetic plain has increased over the last 35 years due to increased loading of aerosols in this region (Dey and Tripathi, 2007) and this region represents a hub of coal power plants, factories and inhabitants. In the 1950s the United Kingdom was tackling the same kind of environmental problem which was responsible for fog occurrence, but after the 1956 Clean Air Act, the annual fog frequency declined (Gomez and Smith, 1984). This act was presented as a very useful tool for reduction of air pollutants and thus smog control. The effect and role of these atmospheric pollutant compositions on seasonal and annual visibility for Indian urban city during 1980-2010 were explained by Singh and Dey et al, 2012. A significant decreasing trend was observed, but found stable after the year 2000 (Figure 2). However, an air quality policy by Delhi state Government has adopted during 2001, marked by bold solid line in figure 2. They also observed that in the winter season, reduction in mass concentration of soot and water soluble particles may improve visibility as well reduction in fog formation. Fortunately, these aerosol components are anthropogenic and could be controlled by air pollution control policies (As Delhi has shown a good improvement in terms of visibility after 2001, Compressed Natural Gas adoption for vehicles).

Figure 2: Annual visibility trend over Delhi during 30 years period (1980-2010).  (Source: Singh and Dey, 2012)

Figure 2: Annual visibility trend over Delhi during 30 years period (1980-2010) (Source: Singh and Dey, 2012).

There remain big questions about the influence of air pollutants on fog formation and magnification in different seasons. There is a critical need to identify the effect that pollutant composition has on fog droplet activation and its occurrence.


Dey, S., and Tripathi, S. N. (2007), Estimation of aerosol optical properties and radiative effects in the Ganga basin, northern India, during the wintertime, Journal of Geophysical Research: Atmospheres (1984–2012), 112(D3).

Dutta, H.N. (2010), Acoustic sounding probing of fog dynamics, forecaster-users interactive workshop on fog monitoring and forecasting services 2010–2011, Dec, 2010.

Gomez, B., and Smith, C. G. (1984), Atmospheric pollution and fog frequency in Oxford, 1926–1980, Weather, 39(12), 379-384.

Mathur, V., Singh, K., and Chawhan, M. D. (2012), Zero-visibility navigation for the Indian Railways, MPGI National Multi Conference, (IJCA) ISSN: 0975 – 8887, 28-31.

Mircea, M., Facchini, M. C., Decesari, S., Fuzzi, S., and Charlson, R. J. (2002), The influence of the organic aerosol component on CCN supersaturation spectra for different aerosol types, Tellus B, 54(1), 74-81.

Mohan, M., and Payra, S. (2009), Influence of aerosol spectrum and air pollutants on fog formation in urban environment of megacity Delhi, India, Environmental monitoring and assessment, 151(1-4), 265-277.

Singh, A., and Dey, S. (2012), Influence of aerosol composition on visibility in megacity Delhi, Atmospheric Environment, 62, 367-373.

Syed, F. S., Körnich, H., and Tjernström, M. (2012), On the fog variability over south Asia, Climate dynamics, 39(12), 2993-3005.

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Dr Ajit Singh

Postdoctoral Research Fellow at the University of Birmingham. Area of Research: Atmospheric aerosol, air quality and health impact.
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