Pollution over Amazon Rainforest inhibits photosynthesis and reduces cloud formation

Article published in PNAS presents the results of research derived from the Green Ocean Amazon field campaign.

By Peter Moon  |  Agência FAPESP – Scientists have shown empirically that urban pollution produced by Manaus, the capital of Amazonas State in Brazil, directly influences the biogeochemistry of the Amazon Rainforest in potentially harmful ways. Throughout the region, the pollution plume interferes with the mechanisms that produce aerosol particles, with consequences for cloud formation processes and rain production. Interactions between the urban plume and the forest’s natural emissions produce levels of ozone that can be toxic to plants.

This information is detailed in the article “Isoprene photochemistry over the Amazon rainforest”, recently published in Proceedings of the National Academy of Sciences of the United States of America (PNAS).

The paper is one of the first to publish results from Green Ocean Amazon (GOAmazon), an international scientific collaboration involving many initiatives, including a field campaign conducted in 2014 and 2015 at various sites in and around Manaus. GOAmazon projects and programs are funded by the US Department of Energy (DoE), FAPESP, and the Amazonas State Research Foundation (FEPEAM), as well as other partners.

One such project is entitled “GOAmazon: interactions of the urban plume of Manaus with biogenic forest emissions in Amazonia”, supported by FAPESP under its Research Program on Global Climate Change (RPGCC). Among the resources used for the project thus far are two research aircraft, equipped with state-of-the-art instruments, which flew extensively over central Amazonia to collect data during 2014 (more information at: agencia.fapesp.br/20157).

The principal investigator for the project is Paulo Artaxo, Full Professor at the University of São Paulo’s Physics Institute (IF-USP). “Our research has focused on understanding the ways in which pollution from Manaus interacts with forest emissions,” said Artaxo, who is also a member of GOAmazon’s steering committee.

With some 400 scientific publications and more than 12,000 citations, Artaxo was one of four Brazilians included among the world’s most influential scientific minds by Thomson Reuters in January 2016. The ranking is based on a citation analysis that identifies the scientists who have had the most significant global impact in their field of study, as determined by fellow researchers.

According to Artaxo, the rainforest naturally emits volatile organic compounds (VOCs) as part of its metabolism. Once in the atmosphere, these compounds interact with other gases and are oxidized. This process plays a key role in cloud formation and hence precipitation in the region.

The Manaus pollution plume also affects these processes. Emissions from factory smokestacks and motor vehicle exhaust pipes form a column of pollutants in the troposphere over the city. The column forms a plume as the winds bear it steadily away from the city, usually westward, until its dispersal forms a vast atmospheric blot extending for 100, 200 or even 300 km.

The plume’s pollutant gases modify the chemical reactions triggered by the VOCs in the atmosphere, producing more ozone and more aerosol particles than would occur naturally without the presence of the pollution plume. “Ozone is a phytotoxic gas,” Artaxo said. “High concentrations of it are harmful and even lethal for plants.”

Ozone concentrations in the troposphere over Amazonia are normally very low, in the range of 10-15 parts per billion (ppb) at midday, but they quadruple to 40 ppb or 50 ppb under the influence of the Manaus pollution plume. “Higher levels of ozone inhibit photosynthesis by inducing stomatal closure,” Artaxo said. Stomata need to open for plants to absorb CO2, which they turn into sugars for food through photosynthesis. “So, the forest absorbs less carbon from the atmosphere where it’s influenced by the Manaus plume, and long-term exposure to high ozone levels leads to a reduction in its total biomass.”

This reduction has not yet been verified empirically. “It’s very hard to do that because you need to monitor the forest over a long period,” Artaxo said. “We know about the phytotoxicity of high ozone concentrations from greenhouse studies. Photosynthesis is reduced when plants are submitted to ozone concentrations in the range of 40-50 ppb. We’re currently designing new experiments to try to quantify the effects of the Manaus plume on the forest.”

Particle interactions

Another significant problem identified in the GOAmazon experiment is the large amount of particles produced by interactions between the VOCs naturally released by the forest and the nitrogen oxide emitted from motor vehicles and industrial plants.

These particles affect cloud formation mechanisms, producing smaller droplets of moisture that take longer to grow and evolve. The result is a potential decrease in precipitation from clouds formed by the interactions between pollution and natural emissions from the forest. “The effect has not yet been quantified precisely,” Artaxo said. “So far, we’ve only studied the mechanisms.”

An understanding of these effects will apply to the entire Amazon region because pollution plumes over the forest are not produced only by Manaus. They are also produced by other cities in the region, including Belém and Santarém in Pará, Porto Velho in Rondônia, and Rio Branco in Acre, albeit less intensely.

The article “Isoprene photochemistry over the Amazon rainforest” (doi: 10.1073/pnas.1524136113) by Yingjun Liu, Joel Brito, Matthew R. Dorris, Jean C. Rivera-Rios, Roger Seco, Kelvin H. Bates, Paulo Artaxo, Sergio Duvoisin Jr., Frank N. Keutsch, Saewung Kim, Allen H. Goldstein, Alex B. Guenther, Antonio O. Manzi, Rodrigo A. F. Souza, Stephen R. Springston, Thomas B. Watson, Karena A. McKinney, and Scot T. Martin was published in Proceedings of the National Academy of Sciences and can be read at pnas.org/content/early/2016/05/10/1524136113.abstract.