An airborne research campaign exploring the "self-cleaning capacity" of the atmosphere has revealed summer monsoons in South Asia may both purify the air of some pollutants but disperse others. The cleansing mechanism monsoons offer may prevent larger impacts of pollution within the region, the authors claim. Whereas in East Asia, North America, and Europe, emissions of sulfur dioxide, nitrogen oxide and carbon dioxide have decreased since 2010, air pollutant levels continue to increase in South Asia, mainly due to the prevalence of coal-fired power plants. During the dry winter, from December to March, a large pollution haze drifts over South Asia toward the Indian Ocean. The "atmospheric brown cloud" has substantial impacts on air quality, climate, and the water cycle, yet scientists have yet to clarify its effects during the summer monsoon. Jos Lelieveld and colleagues hypothesized that a gigantic clockwise-circulating anticyclone, resulting from air rising in the monsoonal weather system, would carry air pollution emissions upward from South Asia. They tested this hypothesis by measuring a variety of air pollutants, including hydrogen peroxy radicals, volatile organic compounds, sulfur and nitrogen oxides, and aerosols, over South Asia between July and August 2015, in aircraft flown at high altitude. They found that, indeed, monsoon convection transported pollutants upward toward the upper troposphere and beyond, where they reacted with other gases and redistributed globally. Unexpectedly, Lelieveld et al. also observed that the summer monsoon simultaneously provided a cleansing mechanism, with some pollutants oxidized into less volatile and water-soluble products that exit the atmosphere through rain. As emissions in South Asia are rapidly increasing, the flux of pollutants through the anticyclone may intensify in years to come, the authors say.
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Journal
Science