Emissions from biomass burning cross the Himalayas

Contrary to the general assumption that the southern slopes of the Himalayas act as a barrier and effectively block the transportation of pollutants from India and other parts of South Asia, a study published a couple of days ago in the Nature Group journal Scientific Reports finds sound evidence to prove otherwise.

Aerosols have been found to rise and cross the entire range of the Himalayas. So much so that studies conducted in the northern slope of the Himalayas at an elevation of 4,276 MSL could find markers distinctive of pollution arising from India and other regions of South Asia.

Local meteorological conditions and regional atmospheric flow process have been the two major factors enabling the pollutants to cross over, notes Zhiyuan Cong, the first author of the paper from the Institute of Tibetan Research, Chinese Academy of Sciences, Beijing.

The culprit

The organic acids present in the aerosols serve as a unique fingerprint in identifying the source of pollution. In this case, the dicarboxylic acids served as a fingerprint.

Though dicarboxylic acids can be produced by biomass burning, vehicular exhausts and cooking (primary source), as well as atmospheric photooxidation (secondary source), the researchers were able to pinpoint the source as biomass burning.

Levoglucosan is a specific marker of biomass burning — it is “produced through the pyrolysis of cellulose during the combustion process,” Dr. Cong notes. Another unique marker of biomass burning is the water-soluble potassium. Both the markers showed strong positive correlation with dicarboxylic acids thereby confirming biomass burning as the source of pollution.

Though the pollutants were found to reach the northern slopes of the Himalayas during all the seasons — pre-monsoon, monsoon, post-monsoon and winter seasons — the amount of aerosol found peaked during pre-monsoon. This, according to them, is one more indicator of biomass burning as the source.

Agricultural burning and forest fires along the southern Himalayan foothills and the Indo-Gangetic Plain reach a high during the pre-monsoon period. That probably is the reason why the amount of biomass burning marker found peaked during the pre-monsoon time.

Dr. Cong attributes the local topographic relief of the Himalayas playing an important role in allowing the pollutants to cross the mountains and reach the northern slopes.

The up-valley wind during daytime, being maximum in the afternoon, helps in pushing the pollutants to higher altitude. On the northern slopes, a down-valley wind is prevalent during the same time. The combination of the up-valley wind in the southern slopes and down-valley wind in the northern slopes allows the accumulation of aerosol on the glacier surfaces.

“Acting as efficient channels of south-to-north air flow, the mountain valleys could allow the air pollutants to easily penetrate throughout the Himalayas,” the authors write.

“Regardless of where the pollutants come from, the study has provided compelling evidence that they are due to biomass burning. We must step up the global effort to drastically cut down biomass burning as much as we can,” Veerabhadran Ramanathan, an atmospheric scientist at the Scripps Institution of Oceanography in La Jolla, California, who is unconnected with the study, told Nature.

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