Satellite images confirm a decrease in airborne pollution in São Paulo

A drop of 33% in levels of nitrogen dioxide (NO₂) is associated with a reduction in diesel vehicle traffic and industrial production due to isolation measures taken to slow the spread of novel coronavirus.

By Maria Fernanda Ziegler  |  Agência FAPESP – Airborne pollution has improved considerably in several large Brazilian cities as a result of the social distancing measures implemented to slow the spread of COVID-19. Satellite images produced by the Royal Netherlands Meteorological Institute (KNMI) show a 33% decrease in levels of nitrogen dioxide (NO₂) in metropolitan São Paulo. NO₂ is emitted by diesel vehicles and coal- and oil-fired power plants, as well as forest fires and crop burning.

“The images show that NO₂ emissions fell more than 30% in São Paulo in March and April compared with the same two months of last year. A similar decrease can be seen to have occurred in other metropolitan areas, such as those of Curitiba, Rio de Janeiro and Vitória. This reduction was due mainly to the fall in industrial activity and motor traffic,” said Eduardo Landulfo, a researcher in the Nuclear and Energy Research Institute’s Center for Lasers and Applications (CLA-IPEN).

Although São Paulo has the largest vehicle fleet and the most intense industrial activity of all Brazilian cities, emissions have fallen drastically even there, Landulfo added, and the images clearly show where traffic and manufacturing have halted. “It’s curious that the highest levels of NO₂ in metropolitan São Paulo are now seen in the vicinity of the Marginal Tietê freeway [the main east-west artery] and the flyover at the western exit of the city proper [leading to the Castelo Branco state highway], reflecting the amount of truck traffic, which is still heavy,” he said. “The southeastern and southern part is fairly clear, including Santo Amaro [a São Paulo City neighborhood] and the ABC region [industrial cities in the metropolitan area].”

Through several projects funded by FAPESP, Landulfo has developed a LIDAR system for the detection of pollutants via active remote sensing. LIDAR (light detection and ranging) is a surveying method that illuminates a target with laser light and measures the distance traveled by the reflected light. Differences in laser return times and wavelengths are then processed to make digital 3D representations of the target.

“We collaborated with the European Space Agency, which supplied the satellite images. I handle the LIDAR remote sensing part and satellite data validation,” Landulfo said. Henk Eskes, a researcher at KNMI, compiles the image database.

The data showing a decrease in pollution in metropolitan São Paulo will be important for studies currently being conducted by researchers as part of a Thematic Project on greenhouse gas emissions supported by FAPESP.

The principal investigator for the project is Maria de Fátima Andrade, who says the data collected during the quarantine will enable scientists to measure levels of pollutants that are normally estimated.

“We plan to contribute to the greenhouse gas inventory for São Paulo City,” Andrade said. “It’s worth noting that the satellite images confirm our prior observations of a fall in pollution levels using data from CETESB [the São Paulo state environmental agency]” (read more at: agencia.fapesp.br/33086).

The level of primary airborne pollutants, especially carbon monoxide (CO) and nitrogen oxides (NOx), fell by approximately 50% on March 22-28 compared with on March 15-21, according to CETESB’s monitoring data.

In addition to the significant decrease in the levels of primary pollutants directly linked to vehicle emissions, there was also a decrease of approximately 30% in respirable particulate matter, especially PM10 (fine particles with a diameter of 10 micrometers or less) and PM2.5 (2.5 micrometers). PM10 is mostly resuspended dust, while PM2.5 comes from secondary processes that occur after fuel combustion.

The researchers are using radars in three areas of the city to measure greenhouse gas emissions. “The aim of the Thematic Project is to measure greenhouse gas emissions in São Paulo City. It focuses on carbon dioxide [CO₂] and methane, but other gases, such as CO and NO₂, are important because they result from vehicle fuel combustion. These levels help us interpret the dataset as a whole,” said Landulfo, a coprincipal investigator for the project led by Andrade.