São Paulo residents certainly remember the event that occurred on August 19, 2019, when dark clouds covered the sky and day turned into night in the city. The phenomenon was not caused by local pollution or by emissions produced in the state itself, but by the enormous contribution of particulate matter from fires in the Amazon region, thousands of kilometers away.
The darkness made it clear — if it wasn’t already — that everything is interconnected and what happens at one end can have repercussions far from its place of origin.
But, regardless of extreme events such as the one mentioned above, the air quality in the largest Brazilian metropolis does not meet, at some times of the year, the standards established by Cetesb (Companhia Ambiental do Estado de São Paulo), and even less so the recommendations of the WHO. (World Health Organization), which are more stringent.
Who says this is researcher Regina Maura de Miranda, professor at EACH-USP (School of Arts, Sciences and Humanities at the University of São Paulo).
She coordinated, together with collaborators, a study that surveyed, in the period between the winter of 2019 and the winter of 2020, air pollution indicators in the Metropolitan Region of São Paulo. The results were published in the journal Atmosphere.
“In relation to particulate matter, the focus of our study, we found that the situation tends to worsen in the winter months, when fires occur in the interior of the state, in other regions of the country and even in other countries, such as Bolivia and Paraguay, and atmospheric circulation conditions are favorable for this pollution to reach the surface of the city of São Paulo”, says Miranda.
“During these periods, there is a greater contribution of particles with smaller diameters, which interact more efficiently with solar radiation, intensifying its climate effect and impacting human health.”
The research characterized fine particulate material, the so-called PM2.5, which encompasses particles with a diameter of less than 2.5 micrometers, and related chemical composition data obtained on the surface with optical parameters observed in the atmospheric column. Measurements were taken every 60 seconds and then the daily average was calculated.
According to the study, these daily averages exceeded the air quality standard recommended by the WHO by 75 days.
“Through the application of an appropriate model for analyzing environmental data, we identified four main sources of aerosols: heavy vehicles [42%]ground dust plus local sources [38,7%]Light vehicles [9,9%] and local sources [8,6%]”, informs Miranda.
The heading “local sources” includes both material originating from industrial emissions or fires carried out on the outskirts of the city and in the interior of the state, as well as material originating from distant fires.
“During the dry period, mainly between July and October, the burning of biomass in central Brazil and the interior of São Paulo releases large quantities of gases and particles. Driven by turbulent processes generated by the burning to relatively high layers of the atmosphere, these gases and particles are transported by the winds, potentially reaching the Metropolitan Region of São Paulo, as was explicitly the case in August 2019”, adds the researcher.
Among the various types of pollutants, due to the sources in the region studied, she highlights the importance of so-called black carbon (BC), made up mainly of carbon emitted by heavy diesel-powered vehicles or the burning of biomass.
In addition to the harm it can cause to the respiratory system, this material is a concern for another reason. By absorbing solar radiation in the visible and near-infrared spectrum range, BC can contribute to local warming of the atmosphere, worsening the effects of global warming.
“Our study was carried out on the EACH-USP campus, in the East Zone of São Paulo. It is an area with more than 6 million inhabitants, highly industrialized, and crossed by roads with intense vehicle traffic”, says Miranda.
“It is also home to the country’s largest airport. Soil impermeable by asphalt and other types of covering, heat islands and high concentration of pollutants are part of the context”, continues the researcher. “To characterize the types and concentrations of aerosols, their chemical composition, optical properties and seasonal variability, the study was conducted in different seasons, both in the dry and rainy periods.”
As for the chemical composition, the study showed a high concentration of elements originating from the soil, such as aluminum, silicon, calcium and iron. But it also identified several elements released into the air by human activity, such as sulfur, resulting mainly from combustion processes; bromine and calcium, used in lubricants and additives in light vehicles; copper and zinc, used as antioxidants in engine oils; potassium, from the burning of biomass; and chlorine, released by plastics burned along with household waste.
“The period studied was, in a way, atypical, because, at the height of the pandemic, there was a drastic reduction in pollutant emissions. Even so, WHO recommendations were exceeded. We want to publish, in a future article, the data collected in the following years. And advance the study, investigating secondary aerosols, generated by the chemical reactions of primary pollutants in the atmosphere”, comments the researcher.
The investigation is part of Erick Vinícius Ramos Vieira’s master’s degree work, supervised by Regina Maura de Miranda and first author of the article. And it was supported by FAPESP through Research Grant, granted to Miranda.
The article “Chemical Characterization and Optical Properties of the Aerosol in São Paulo, Brazil” can be read here (in English).