The cocktail of pollutants blanketing South Asia has far-reaching consequences. Longer-lived pollutants can drift higher up into the atmosphere and diffuse globally. Moreover the pollutants have the potential to weaken the South Asia monsoon.
Air pollution has risen in Asia in the last few decades, especially in areas of high population density, says Vanisa Surapipith, atmospheric modeller at the Kathmandu-based International Centre for Integrated Mountain Development (ICIMOD).
“Data from the Aura satellite and modelling studies have provided strong evidence that water vapour and other chemical constituents in the upper troposphere and lower stratosphere are transported by strong convection over northeast India and the Himalaya foothills regions, and disperse globally,” William Lau, professor at the earth system science inter-disciplinary centre at the University of Maryland, explained tothethirdpole.net.
Surapipith says that besides hurting the health of local residents and tourism-based economies, the impacts of the pollutants could be regional and global. Strong convection currents can drive some of the pollutants from South Asia to the upper troposphere from where they can spread across the globe. And pollutants can also impact on the monsoon system.
In general, water vapour concentrations in the upper troposphere and the lower stratosphere (UTLS) are lower than water vapour concentrations in the lower troposphere. But recent research from the Tibetan Plateau, in which scientists from China used balloon-borne instruments to study water vapour and ozone profiles over Naqu and Lhasa and nearby Tengchong region, shows that ozone concentrations are lower than expected, while water vapour concentrations are higher. The team, led by Yan Xialou from the Chinese Academy of Meteorological Sciences, Beijing, reported the findings in Science China.
Water vapour traps heat, and thus adds to global warming.
Yan’s findings concur with the findings of a team led by Laura Pan from the National Centre for Atmospheric Research (NCAR), Boulder, Colorado, US. These findings were published in the Journal of Geophysical Research, in 2014. Pan’s study was conducted over Alajuela, Costa Rica, in July and August 2007, in the transition zone between the troposphere and stratosphere. Pan’s team also studied the transition zone in Kunming, China, a part of Asia that experiences the monsoon.
Warmer planet, higher clouds
The changes in water vapour and ozone levels “can only happen during the vertical transport of water vapour due to rise of clouds,” Surapipith says. “This is likely due to the convection clouds over the plateau, particularly during the summer monsoon.”
Earlier, in 2010, an NCAR team led by William Randel had reported findings from satellite observations on hydrogen cyanide, an air pollutant produced by biomass burning. Randal’s team identified transport of air masses from the surface, through the Asian monsoon, and deep into the stratosphere. “The monsoon circulation provides an effective pathway for pollution from Asia, India, and Indonesia to enter the global stratosphere,” the report in Science says.
“This air will have enhanced black and organic carbon, sulphur dioxide, reactive nitrogen species, and possibly short-lived halogen compounds from Asian industrial emissions,” it adds. “The monsoon influence on the stratosphere is expected to become increasingly important given the ongoing growth of Asian emissions.”
However, Sarath Guttikonda, founder of from Urban Emissions.Info — an independent organisation working on air pollution — cautions that the findings on ozone and water vapour are not applicable to all pollutants, especially those that are heavier and/or have a shorter life than ozone and water vapour.
“The long transport characteristics are different for different pollutants,” Guttikonda explained to thethirdpole.net. “Ozone and carbon monoxide have the tendency to get lifted past tropopause (the transition zone) and travel to higher altitudes and distances. The same is not possible for heavy pollutants, which end up depositing on various surfaces during travel.”
The monsoon connection
Other scientists are studying whether, and how, rising pollution, especially of larger particles (aerosols), is affecting the summer monsoon in South Asia. Data from 1948 to 2014 shows a decreasing trend in the East and South Asian monsoon and monsoon-induced rainfall anomalies in the pre-monsoon and monsoon season, says Surapipith.
The studies show that increases in black carbon and other aerosols are changing the monsoon. Air pollution is decreasing rainfall in the lower plains but increasing rainfall in mountains. It is also changing the monsoon onset date.
“There is no doubt that aerosols are impacting monsoon, but to what extent and in which direction (weakening or strengthening), is still a debated topic,” says Sagnik Dey, assistant professor at the Centre for Atmospheric Sciences (CAS), Indian Institute of Technology, Delhi. His team has studied the impacts of aerosols on the dynamics of the summer monsoon.
Dey says there are two theories on the impact of aerosols on monsoon. One group of scientists, for example Chul Eddy Chung and V. Ramanathan from Scripps Institute of Oceanography; and Massimo Bollasina and Yi Ming from National Oceanic and Atmospheric Administration of the US say that aerosols transported over the Indian Ocean induce changes in sea surface temperature, which weaken the monsoon.
Dey’s colleague Dilip Ganguly, who was earlier with the Pacific Northwest National Laboratory, had reported that aerosols impact in two ways. One is a fast response for a few days during which the aerosols scatter and absorb the incoming sun’s rays, leading to ‘dimming’ of light which cools the Earth’s surface. The second is long-term effect of cooling down sea surface temperatures, which impacts the circulation of air over land and sea, and ultimately the monsoon system.
A second group of scientists believe that the light-absorbing aerosols over the Indo-Gangetic Plains and Himalayan foothills act as an elevated “heat pump” that draws in heat from the sea surface in summer, which advances the onset of the monsoon.
“Pollution can produce aerosols that absorb solar radiation and heat the atmosphere,” says Lau. “Atmospheric heating by aerosols can induce rainfall changes in the Indian summer monsoon through interaction with monsoon circulation. Such changes can have impacts not only on the Asian monsoon, but also other regions of the globe through changes in large scale circulation induced by aerosols over India and regions of East Asia.”
Aerosols block sun light from reaching the ground, cool the land, and weaken the monsoon overall. However, absorbing aerosols (dust and black carbon) can enhance rainfall over northern India and the Himalayan foothills during the early part of the monsoon. Aerosols can also change the processes of cloud formation and thus impact rainfall.
Dey however cautions that aerosols are not the only factor influencing the monsoon. “The monsoon is a large-scale phenomenon, and aerosols are not the only factor.”
“Our interpretation of such complex processes (as the monsoon) also depends on the climate model used,” he says, including how well the aerosols are simulated and whether the complex physical processes are represented in the model.
More studies are needed for a better understanding of this crucial phenomenon, the monsoon. It accounts for around 80 per cent of the annual rainfall in South Asia, and over two-thirds of the farmers in the region depend on it to water their crops.