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In terms of day temperatures, January this year is in line to be the coldest in a decade and third coldest since 1947.
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NEW DELHI: Days are becoming colder in the month of January in the capital in recent decades and pollution could be playing a role in forcing the trend, warn experts.
In terms of day temperatures, January this year is in line to be the coldest in a decade and third coldest since 1947. It will also be the fifth consecutive year when Delhi's average maximum temperature in January has remained below the normal of 20.8 degrees Celsius.
That's not all. January month's average maximum temperature has been below 19 degrees C only four times since Independence — and all these instances (including this year) have come since 1998. In fact, the period from 1998 to 2014 has seen the six coldest months of January in the capital since 1947.
Experts say there appears to be a link between higher frequency of 'cold day' conditions and increasing levels of air pollutants. Pollutants hang lower, and so accumulate more, in winter months. This leads to haze, smog and fog. Particles in the upper layer of atmosphere absorb sunlight, leading to what's called a dimming effect. All this contributes to less sunlight reaching the surface and, consequently, low day temperatures.
"There is a consensus emerging on the link between rising pollution levels and colder days," said Gufran Beig, chief project scientist at System of Air Quality Weather Forecasting and Research (SAFAR). "In cold temperatures, the (atmospheric) boundary layer is low which leads to accumulation of pollutants. From our study, we know that pollution from human sources such as transport has increased which also plays a role in bringing down the boundary layer."
R K Jenamani, director of IGI Airport Met office, wrote a paper in Current Science in 2007 showing a strong correlation between low maximum temperatures in Delhi's winter, foggy days and high pollution levels.
"Under similar meteorological conditions, we often find Delhi has more fog than cities such as Amritsar, Varanasi and Lucknow. This suggests that increased levels of pollution in the capital could be playing a role," Jenamani said.
Statistics too point to this link. Monthly averages of Delhi's maximum temperatures in January show a drop of almost 2 degrees C since 1947, with the fall being sharper since the late 1980s. At the same time, the average number fog hours in January in the capital (visibility below 1,000m) has more than doubled since 1981, increasing from just below 5 hours per day to more than 11 hours in 2014.
Then there's air pollution, which has been steadily rising in Delhi. Data from January 14 to 16 this year shows PM10 levels at numerous locations were more than 600 micrograms per cubic metre — that's more than six times the safe limits. PM2.5 levels were similarly high, making Delhi's air worse than the much-maligned Beijing.
It continues to get worse. A recently concluded study by post doctoral fellows at Indian Institute of Tropical Meteorology ( IITM), Pune, found a steady increase in pollution and black carbon emissions in Delhi from 2010 to 2013.
According to the study, PM2.5 (fine, respirable particles) emissions were 94.26 gigagram per year (Gg/year) in 2010 increased to 107.5 Gg/year. This increasing trend was particularly seen over Rajiv Chowk, Sansad Bhawan, India Gate, IGI airport, Okhla industrial area, Pragati Maidan, IP estate and Janakpuri.
The study also found that the contribution of PM2.5 emissions from the transport sector was the highest — and growing — followed by burning of biomass. In the past three years, over 32% of the PM2.5 emissions were from transport. Say Neha Parkhi and Saroj Sahu, the fellows who analysed the data, "Open biomass burning has increased PM2.5 pollution significantly in winter. Construction work and infrastructure projects also have role in increasing PM 10 emissions which have shown an increase of about 5% in the past three years.
Experts say while pollutants could be increasing the chances of fog, the reverse could also be true. Said Jenamani, "Once a fog layer sets in, it contributes to making the air still. This, is turn, leads to more accumulation of pollutants in the air."