Friday, 16 November 2012

Smoke from field fires can travel up to 1,000km

Smoke from field fires can travel up to 1,000km

NEW DELHI: Plumes of smoke and pollutants rising from the burning fields of north India can spread through the air to places as much as 1,000km away, cause persistent fog over the region and could even be playing a part in depressing rice yields, various scientific studies on the phenomenon have found. 

The fires that raged through Punjab since the last week of October — deliberately caused by farmers to clear paddy stubbles and ready the fields for the rabi crop — were seen as a major cause of the 11-day spell of dense smog in the capital and surrounding areas. Biomass-burning is rampant, practiced in around 90% of Punjab's paddy fields, and has continued unabated for decades despite its obvious role in emitting greenhouse and a toxic mix of gases, and destroying soil nutrients. 

The practice has been studied by atmospheric and earth scientists for more than a decade and many of these have thrown up more grim facts. A 2009 study by KVS Badarinath of Indian Remote Sensing Centre and others reported that aerosols and trace gases from crop-burning in north India had been found over Hyderabad and Arabian Sea. 

"Particles from these fires can travel up to 1,000km," said 
Prof SN Tripathi of IIT Kanpur's chemical engineering department. "While smoke plumes usually clear out in a week's time, and particles rise higher in the atmosphere and can 'burn' clouds." 

Fires from 
biomass burning are also a major cause of winter fog in north India as suspended pollutants attract moisture. "We have found a direct link between fires and winter fog in north India," said Dr MM Sarin, senior professor of geosciences at Ahmedabad's Physical Research Laboratory. 

Sarin's team also found that biomass emissions contain a preponderance of organic carbon, a class of sticky particulates whose properties are different from soot, or black carbon. The proportion of black carbon — which absorb heat and light, and were linked to glacier melting — was earlier thought to be higher in these emissions. 

"Our studies also found another class of possible carcinogenic compounds called polycyclic aromatic hydrocarbons in the biomass emissions," Sarin said. 

In another paper published last year, Tripathi's team showed that once fog forms over north India, it gives rise to more particles — called secondary organic aerosols — which become seeds for more fog. "This becomes a vicious cycle and explains why winter fog persists for so long," he said. 

Given the scale of burning across north India — an estimated 17 million tonnes of paddy stubble is set afire in Punjab alone — one study has indirectly linked the practice to slowing growth rate of rice yields. In a paper published in the Proceedings of the National Academy of Sciences (USA), Maximilian Auffhammer of UC-Berkeley and others used statistical models to show that a joint reduction in brown haze (partly caused by biomass burning) and greenhouse gases would result in a rise in rice harvests. 

The results imply that adverse climate changes due to the winter brown haze and the rise in greenhouse gases have contributed to a slowdown in growth rate of rice yields in the past two decades.
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