Krishnakant Budhavant and Prof Örjan Gustafsson at the station in the Maldives. Photo: Joakim Romson

India, Pakistan and Bangladesh are among the countries with the world’s worst air pollution, with the most polluted cities. With nearly 1.8 billion inhabitants in the region, factories, traffic, agriculture, fires and other sources release huge amounts of gases and particles that impact people’s health and the climate. Department researchers have long been analyzing the air at two strategically placed measurement stations in southern Asia to answer questions such as: what is the total amount of emissions, what are the sources, and how do they affect the climate?

Atmospheric Observatory in the Maldives

One of the stations is located on the island of Hanimaadhoo, in the Maldives, to the southwest of India. This location allows the station to capture emissions from the entire Indian subcontinent as they are carried over the Indian Ocean. The island has insignificant local emission sources that do no interfere with the measurements. The station was set up by Stockholm University in 2004 in collaboration with several research institutes and universities, in the Maldives, India and other parts of the world. The funding came from organizations such as the United Nations Environment Programme and the Swedish International Development Cooperation Agency, with support from some of the world’s leading climate researchers, including Stockholm University alumnus and Nobel laureate Paul Crutzen. Örjan Gustafsson, Professor at the Department of Environmental Science, took over the responsibility for the measurements in 2012. Since then, he and his colleagues have visited the Maldives two to three times a year, with a local researcher and two technicians on site year-round to operate the station. Researchers from around the world also use the station, which can be equipped with additional instruments for different types of measurements. Although the Maldives are often portrayed as “paradise islands” with pleasant climates and long sandy beaches, the air is significantly worse than in Stockholm due to the pollutants from the Asian continent, Gustafsson says.

Atmospheric Observatory in southern Bangladesh

The second station is located on the island of Bhola, off the coast of Bangladesh, where the instruments capture air pollutants from the heavily populated Indo-Gangetic Plain in northern India and Bangladesh. The so-called “brown cloud” passes over Bhola before spreading out over the sea. Hence air intercepted at the station provides a comprehensive view of the pollutants in the integrating outflow. There are also no major local sources of emissions on the island.

In collaboration with local researchers and through international partnerships, Stockholm University’s researchers have gradually built up these atmospheric observatories with instruments for continuous year-round studies of four climate-affecting aerosols and gases: black carbon, sulfate particles, and the greenhouse gases methane and carbon monoxide. Together, these four are estimated to have the same climate impact as carbon dioxide.

Brown clouds cover southern Asia

South Asia is covered in what appears to be a large brown cloud (the “Brown Cloud”) for much of the year as seen from satellites. During the winter, the cloud builds up from various combustion-related air pollutants and moves across India and down to Bangladesh and out over the Indian Ocean. Estimates show that hundreds of thousands of people die prematurely each year from various illnesses due to the poor air in the brown cloud. Soot particles are the biggest source of poor air quality in Asia. They come primarily from traffic, as well as from the burning of biofuels in homes and on agricultural fields. There is insufficient knowledge of which sources have the most significant impact on creating poor air quality. “If we can understand and explain the sources of emissions better, we can also provide decision-makers with information about which sources of emissions are most important to address,” says Örjan Gustafsson.

Both warming and cooling effect

The particles released into the brown cloud also interact with solar radiation in the atmosphere, resulting in either warming or cooling. The substances released into the air also contribute to warming to varying degrees and for different lengths of time. Aerosols and other particles are short-lived and may affect the atmosphere for a few years. Methane gas and primarily carbon dioxide, on the other hand, are more long-lived in the atmosphere.

Research has shown that soot particles have a significant impact on the local climate. Scientists also suspect that the particles may affect monsoon systems, which are essential for agriculture on the continent. In a newly published article in the journal PNAS, Örjan Gustafsson and his colleagues show that soot particles have a greater impact on the climate than previously thought and how they affect cloud formation during the monsoon period. The particles also remain in the atmosphere longer than previously thought.

“Fingerprints” reveal sources of emissions

Scientists not only determine the amount of soot particles in the atmosphere but also the type of carbon atomsthey contain, a sort of “fingerprint.” The focus is on carbon isotopes C14 and C13. The carbon in all living materials have a modern 14C and thus combustion of biofuels yields a modern 14C contribution. On the other hand, 14C (with a half-life of 5730 years) is extinct in soot coming from incomplete combustion of fossil fuels such as oil, coal, and natural gas. Studies by the researchers at the Department show that up to half of the soot particles in the atmosphere at measuring stations come from the burning of biofuels. The use of wood and other biofuels for heating in households, as well as burning in agriculture, are therefore significant sources of the brown cloud. By studying the composition of carbon isotopes and certain molecules (biomarkers), it is also possible to determine what has been burnt, such as agricultural residues. Hence, the group’s fingerprinting of the 14C in the soot particles over South Asia has shown that biomass combustion contributes about half of the total soot loading, equal to fossil fuel combustion. This is a larger fraction than in other regions and thus provide guidance to mitigation efforts. Carbon monoxide and methane are also studied at the measuring stations in the Maldives and Bangladesh. The common view is that two-thirds of the world’s methane emissions come from the tropics. But there is uncertainty about how these are distributed among different sources such as rice cultivation, livestock, landfill leaks, and so on. Therefore, there is a need to better map these sources of emissions in Asia to know which measures can have the greatest effect.

Sulfur dioxide can “mask” warming

Sulfur dioxide is believed to have a cooling effect on the climate and also “mask” how much the temperature would have actually increased if sulfur dioxide had not cooled it. But there has been great uncertainty about the extent. The shutdown of society during the coronavirus pandemic, however, resulted in a drastic reduction in sulfur dioxide emissions. Data from the Maldives observatory indicate a significant cooling effect over large parts of southern Asia and the northern Indian Ocean from sulfur dioxide emissions.

Basis for decision-makers

Taken together, the Stockholm University research results generated from the measuring stations in Bangladesh and the Maldives are of great importance to better understand how the climate system works in southern Asia and to see which sources of emissions are the greatest. The results can provide direct decision support for decision-makers and thus facilitate the prioritization of different measures to reduce the emissions of health- and climate-damaging air pollutants. The Maldives is perhaps the nation in the world most directly affected by climate warming. As the sea level rises due to a warmer climate, the low-lying archipelago risks disappearing below the sea surface. The Maldives government has therefore shown great interest in the work at the measuring station. Over the years, Örjan Gustafsson has met with the country’s climate and environment ministers on several occasions. He has also previously acted as an advisor to Sweden’s Minister for the Environment. This research at Stockholm University is primarily funded through the Swedish Research Council’s Distinguished Professor grant that Örjan Gustafsson holds and through the Swedish Research Council for Sustainable Development Formas.

The article was published March 20, 2023