Tree rings reveal the reduction of trace metals in the atmosphere of São Paulo, Brazil
The Metropolitan Region of São Paulo (MRSP) is ranked as the 12th largest megacity in the world with 21 million inhabitants. Similar to most megacities MRSP also suffers from air quality degradation. For example, the latest air quality report published by the São Paulo State Environmental Company (CETESB 2018), using data points measured over 16 air quality stations, in the year 2017, showed that none of them achieved the recent national air quality standard for fine particulate matter (PM2.5, annual mean). Nevertheless, this post is not about poor air quality in Latin American cities, but about an interesting biomonitoring technique to measure pollutants in the atmosphere.
Scientists at the Institute of Biosciences at the University of São Paulo (USP) are using tree rings to investigate the atmospheric composition in the MRSP. The scientists used the Tipuana tipu (Benth.) Kuntze (Fabaceae), a typical tree found in abundance in the tropics. This tree is about 15 m height and it can reach up to 1 meter diameter at the breast height. The Tipuana has (a couple of) favorable conditions to serve as a biomonitor, for instance, it loses the leaves during the dry season (seasonal behavior), which results in the formation of semi-porous rings delimited by marginal parenchyma; living tissue that serves as storage cells, which are clearly distinct tree rings that are well suited for the study.
The scientists collected samples at the University of São Paulo, which is located at the west side of the city, next to an air quality station run by CETESB. The sampling increment cores were taken using a special increment borer coupled to a motor drill. This system does not heat up the sample and produces 15 mm wide increment cores. But do not feel sorry for the trees, their injuries were treated carefully and no more than 2 samples were taken per tree to guarantee its healing.
The increment cores were taken to the laboratory and prepared to be analyzed. The rings and the chemical elements present in cores were investigated using very sophisticated instrumentation (e.g. high-resolution optical scanner, high-resolution laser ablation and ICP-MS system, see the manuscript for further information).
The scientists found a decreasing trend over the last three decades in the central region of São Paulo for most analyzed chemical elements, consistent with CETESB air quality station that also showed a similar trend for PM10. Stronger reduction was obtained for Pb, which was likely related to the implementation of political policies in the early 1980’s in Brazil when this element was banned from the gasoline.
The results allowed the scientists to conclude that the chemical elements (Pb, Z, Ni, Cd and Cu) usually measured in environmental studies are non-randomly distributed in tree rings. The scientists involved in this study explained that: “the accumulation of these elements in the vessels suggests that they have been transported from roots to shoots. Part of the elements found in the upper layers of the soil are transferred from the atmosphere by precipitation, washing off from leaves and barks, and by senescence of vegetative organs. Trees uptake these elements by the roots, transporting them along with water inside the vessels.”
This research is able to reconstruct the air quality historical time series in an urban environment, such as São Paulo. Lessons learned from this study can benefit research in other environments; for instance, they can be used to understand the quality of life in the past and relate the air quality improvements to social impacts in the future.
Learn more in: https://globoplay.globo.com/v/6942865/ (video in Portuguese only)
