The climate of São Paulo has changed. Summer days are getting hotter and hotter, and the winter days, drier. The average temperature of Brazil’s biggest city is 1.3ºC (degrees Celsius) higher than it was four decades ago. And, contrary to what one might imagine, the effects of urbanization, above all the process of making the ground impermeable and the excess of vehicles, are not the main culprits for the change: they account for some 30% of the alterations, whilst the 70% come from the forces of Nature, in particular, the heating up of the Atlantic Ocean in this period.
Besides explaining these alterations, the research coordinated by Pedro Leite da Silva Dias, of the Insitute of Astronomy, Geophysics and Atmospheric Sciences of the University of São Paulo (IAG-USP), shows something worse: the Metropolitan Area of São Paulo – the capital and the neighboring 38 municipalities – is an export center for pollutants. Hemmed in to the south by the Mar mountain range, which separates the coastal plain from the plateau – and to the north by the Cantareira mountain range, some 1,200 meters in height, the region occupies a quadrilateral of 200 by 150 km (kilometers), where some 17 million people live.
Its polluted air, particularly in the winter, can reach cities located up to 100 km from the capital, albeit in lower concentrations than those in the vicinity of the avenues or industrial installations where it is produced. Silva Dias estimates that, according to the time of year, from 20% to 30% of the pollution in Campinas, Tatuí and Sorocaba, for example, comes from São Paulo.
Pollution has therefore become, no longer just a local problem, but a regional one. Bad news for the inhabitants of São Paulo themselves, who, at weekends and on holidays, go to take refuge in the mountains in search of pure air, one of the attractions of the nearby hill towns. Models made on the computer attest that the air can not be as pure as one would think, due to the pollution brought surreptitiously by the winds that blow from the capital. If swayed by stronger winds, the polluted air from the metropolis can reach Bauru, almost 400 km away.
Another problem has been outlined for the neighbors of São Paulo: there are places where the concentration of ozone (O3) close to the ground is even higher than in the capital. Formed by the pollutants emitted by cars, this ozone is harmful, unlike the ozone that exists in the higher layers of the atmosphere, which protects the planet for noxious radiation. In the upper atmosphere, this form of oxygen filters the ultraviolet rays of the Sun, but close to the ground it can irritate the eyes and cause rhinitis, coughing and other respiratory problems. It is poisonous to plants as well.
In Barueri, Embu and Jundiaí, for example, the level of this kind of polluting ozone can be up to 50% higher than in the Sé square or in the Anhangabaú valley – at these points, in downtown, the hourly average of ozone, 60 ppb (parts per billion), oscillates according to the season of the year and sometimes exceeds the safety limit, which is 80 ppb. When one thinks about solutions, a complicating factor arises. Who should take responsibility for the health problems caused by pollution: the municipality that exports pollutants or the one that receives them? Not even the specialists in Environmental Law in Europe or the United States can agree on the subject.
In the capital, there are also some surprising place where ozone forms, like the Cantareira mountain range and the Jaraguá peak. Although they are considered fresh air refuges, they are high regions, and so they block the passage of the air and can have the same concentrations of ozone as in the densely urbanized areas, according to surveys by the IAG and the Institute of Energy and Nuclear Research (Ipen). The situation is worrying, because today ozone is the pollutant that most exceeds the safety limit, above all in the São Paulo districts of Ibirapuera and Mooca, as well as in Cubatão, in the Baixada Santista. The formation of ozone in places distant from the points of origin of the pollutants is a problem common to the big centers.
Silva Dias believes that there is a lot of ozone in the surroundings of Brasília and Curitiba, for example, since the phenomenon usually occurs in cities with over 400,000 inhabitants. The joint work of physicists, chemists, meteorologists and mathematicians shows why today the Land of Drizzle is no more than a recollection. This nickname for São Paulo refers to a situation that persisted until the 60s, when the fine rain was unrelenting and added itself to a colder climate: in the winter, the inhabitants of São Paulo could not do without thick overcoats, gloves and scarves. Today, there is practically no drizzle, while torrential rain is frequent, causing flooding in the hot season.
The researchers analyzed the meteorological conditions – the variation in temperature and humidity, the distribution of the rainfall, the frequency of mists and winds – that determine the carrying of pollutants, and they concluded: the forces of Nature are decisive for the transformation of São Paulo of the Drizzle into a city of torrential rain. “There is a strong correlation between the changes in the climate of the capital and those that have taken place in the South Atlantic, where the average annual temperature has increased 1.4°C in 40 years”, Silva Dias explains.
Although one cannot guarantee that the heating up of the ocean is the direct cause of the heating up of the capital, the hypothesis is plausible. Measurements taken since 1933 at the IAG’s meteorological station in Água Funda, next to Jardim Zoológico, point to a drastic change in the pluviometric pattern: the increase of the intense rain in the summer and the decrease in the light rain in winter. The result of this was a change in the humidity level in the air. The drier air that came to predominate in the winter makes it difficult to disperse the pollutants generated by the 6 million automobiles, 400,000 trucks and buses and some 30,000 industrial installations in the Metropolitan Region.
Also based measurements from 1999 and 2000, which were added to routine information gathered by the Company of Technology and Environmental Sanitation of the State of São Paulo (Cetesb), the researchers came to have a better understanding, not only of the changes of climate, but also of the origins and the movements of the masses of air that disperse, remain stationery or change route when they find the mountain ranges and the corridors between buildings.
Most often it is the winds that originate at sea that carry the pollution produced in the city, in particular by vehicles, in volumes that are not to be overlooked: 1.6 million tons of carbon monoxide, 380,000 tons of hydrocarbons, and 64,000 tons of aerosols (particulate matter) per year. Besides showing that these pollutants affect the quality of life in the capital and the neighboring municipalities, the study is probably the first to measure the origin and destination of the air breathed in São Paulo.
The Metropolitan Region produces most of its pollutants: from 70% to 80%. The rest comes from in the interior or from other states: between the end of October and beginning of November, about 10% of the pollution in the metropolis is waste from fires, in particular from the burning of sugar cane, done at a distance of up to 300 km, in the regions of Piracicaba or Ribeirão Preto. Even the ashes from fires in the south of Amazonia can reach Brazil’s largest city, depending on the direction and intensity of the winds – the daily movement of the winds can be accompanied on the home page www.master.iag.usp.br, built with the results of the research.
The analysis of the movement and the quality of the air is based on an expanded metropolis with a radius of 100 km – to include part of the Baixada Santista (The Coastal plain around Santos and Cubatão), of the Paraíba valley (up to São José dos Campos) and of flatter areas, like Sorocaba and Campinas. This was the wide-ranging view that made it possible to know the points and the processes of the formation of ozone.
It was already known that there is less ozone in the center or in Congonhas, because the very pollutants of these areas – above all nitrogen oxides – eat it up. It is for the lack of these pollutants that there may be more ozone in the Ibirapuera park than in the neighboring 23 de Maio avenue. Carried by the masses of air, the ozone-forming pollutants given off by cars leave the capital and undergo reactions caused by sunlight, which take from two to three hours to be completed – sufficient time for them to reach the neighboring municipalities, or remain stationary on the foothills of the mountain ranges. In November, the situation is aggravated, when there are many sunny days and no clouds.
Detailing the process was hard work. The IAG’s vice-coordinator, Maria de Fátima Andrade, studied how the pollutants form and interact. With the values from the inventory of emissions, she studied the formation of ozone from nitrogen oxides, hydrocarbons and free radicals (fragments of molecules formed from oxygen). The program for forecasting the formation of ozone that she has used contains some 200 reactions with 90 pollutants.
What was made clear was the importance of the sea breeze – the low intensity current of air that is born in the ocean, as a result of the difference of temperature between the sea and the continent. It is this breeze, circulating 500 meters from the surface, that abates the temperature in the capital and enhances the dispersion of pollutants, above all when they join a southeasterly wind, more intense currents that also arise over the sea. The refreshing effect of these sea winds, the researchers discovered, may go as far as São Carlos or Pirassununga, 230 km from the capital. “São Paulo is lucky to be near the sea”, says the coordinator. “The climate would be worse, from the point of view of the impact on public health, without the breezes”. Hot and stuffy days are days when sea breeze does not reach the city.
The team also did a three-dimensional profile of the masses of air in the Metropolitan Region: the pollutants react among each other in the low region of the atmosphere, the limiting layer of the planet. Described in an article published in April 2001 in Atmospheric Environment, this region from 50 to 100 km around the center of São Paulo. Its height depends on the force of the winds that it shelters, but during the day it comes to 1,500 meters from the ground. At night, the limit falls to 400 meters or less, and, as the volume occupied by the night air diminishes, the concentration of pollutants increases.
The air gets worse with a typical phenomenon of the São Paulo winter: thermal inversion. With the arrival of a cold front, the temperature rises with height, contrary to what is usual: normally, the temperature falls 1ºC for each 100 meters of altitude. In 1999 and 2000, observations were made using Sodar –Sounding Detection and Ranging or an acoustic sounder, a device that emits audible signals, like the radar of a submarine, and draws a profile of the thermal variation at up to 1,500 meters from the ground. It was found that the limiting layer can fall to 200 meters, with strong thermal inversion. It works like the lid of a saucepan, and the lower it is the higher the concentration of pollutants. “This is the worst situation for the inhabitants of the city”, says Fátima.
Sodar also evidenced two phenomena that affect the quality of the air. One of them is the “night jets”, intense vertical winds that are the result of larger scale atmospheric mechanisms, like cold fronts – masses of air coming from the south of the continent. The jets break up the stability of the night limiting layer and may bring down pollutants like ozone, increasing its concentration close to the surface. In addition, the mixture of the air brought about by the jets may also contribute towards a decrease in the concentration of pollutants produced on the surface, like dust.The situation may improve with the second phenomenon, gravity waves. More intense at night, they are similar to waves in the water that crash against a barrier: going up the Cantareira range, the air causes oscillations, analogous to waves in water, which contributes towards reducing pollution.
“This was the first time that the three-dimensional profile of pollution in the Metropolitan Region had been studied”, comments Paulo Artaxo, a researcher with the Institute of Physics of USP who took part in the work. To arrive where they did, the specialists released balloons like the ones at birthday parties, which indicate the direction and the intensity of the winds. They also made use of a Bandeirantes aircraft from the National Institute of Space Research (Inpe). In four flights during the winters of 1999 and 2000, they collected samples of the air of the cities of São Paulo, Sorocaba, São José dos Campos, Campinas and Cubatão, flying 200 meters from the ground, below air traffic.
The researchers analyzed the concentration of the pollutant gases, ozone, nitrogen oxides, carbon monoxide, and sulfur dioxide. The concentration of particulate matter was analyzed by a technique that analyzes the X-rays generated by a sample in a particle accelerator. The analysis was done both of fine material, of less than 2 micra (1 micron is one thousandth of a millimeter), which enters the bloodstream and affects the alveoli of the lungs, and of coarse material, of over 2 micra, that causes rhinitis, coughs and colds.
The first conclusion: the concentration of pollutants in one place can vary abruptly. In a measurement on August 13th 1999 at the Campo de Marte airfield, there were 9,000 particles per cubic centimeter (cm3) at an altitude of 1,000 meters. At 1,500, the content of particulate matter fell to 2,000 per cm3. The difference also varies with the geographical distribution. “From the coastal areas to the city center, the concentration of particulate matter went up 20 times”, says Artaxo. And the sources of these pollutants vary over the year. In a study carried out in the winter, the distribution of fine particulate matter was as follows: vehicles, 28%; dust from the ground, 25%; sulphates from industrial sources, 23%; burning of industrial oil, 18%.
In summer, though, the participation of automobiles drops to 24%, while dust from the ground (30%) and the burning of oil waste (21%) become more prominent. It became clear that the emission of pollutants goes hand in hand with the meteorological conditions, to determine the quality of the air. The problem is that a still mysterious logic governs this combination. “If we were to reduce the emission of pollutants by half, it could be that pollution would not fall by half”, says Artaxo. “Under some meteorological conditions, it could fall very little.”
More sophisticated studies at the Institute of Physics have indicated that the particulate matter affects the behavior of the lower layers of the atmosphere. It has now been discovered that dust, above all the finer one, absorbs and reflects light, besides heating the air around it – the polluted air 1 km from the ground is hotter than pure air at the same altitude. The particles also reduce visibility and hamper the dispersion of pollutants – and they provide the red sunset typical of the capital. The composition of this dust in the city is now known: there are particles of at least 13 elements, such as sulfur, chlorine, titanium, iron, nickel, zinc, bromine and lead. Amongst the fine particulate, sulfur predominates, and in the course particulate, there are elements coming from the ground, like silicon, calcium and iron. In this air soup, fungus spores and bacteria also circulate.
The only thing that is not known is where the particulate matter comes from, whether from autos or from the factories. So Fátima and the team from USP’s Institute of Chemistry coordinated by Lílian Carvalho survived two uncomfortable days measuring and taking samples in two of the city’s tunnels: the Jânio Quadros, through which only light vehicles pass, and the Maria Maluf tunnel, where trucks also pass. They are laboratories where the pollutants mix before they have reacted amongst themselves – amongst other reasons, because there is no radiation from the sun there. Over the next few months, as the group finishes the analyses, the contribution from vehicles will become better known.
The intensification of the researchput the solutions in the spotlight. Similar studies in Santiago do Chile, where dispersion is barred by the cordillera, made it possible to reduce the concentration of pollutants by half. According to Artaxo, it was simple: after the discovery that most pollution was caused by dust, the conclusion was that it was more feasible to invest in trucks to sweep the streets every night than to control the emissions of pollutants by factories and vehicles. “Air pollution can be resolved”, says Artaxo. “It just needs a plan to control it, with a good scientific basis, clear targets, and fines for those who do not comply with it.”
Solutions to hand
For him, it is not a question of creating, but of implementing measures already announced: more investment in collective urban transport, annual control of emissions by vehicles, and the replacement of diesel buses by natural gas-powered ones. “If these measures had been applied ten years ago, pollution today would be from 30% to 50% lower.” There are changes under way. On the Jaraguá peak, there is now a mobile Cetesb station at work, measuring the level of particulate matter, sulfur dioxide, carbon monoxide and ozone at 300 meters from the ground. There are another 23 fixed stations, and two mobile ones in the Metropolitan Region and six outside it: Cubatão (two), Campinas, Paulínia, Sorocaba and São José dos Campos.
Attentive to the future, researchers from USP are seeking to study pollution with satellite images with a resolution of from 1 to 5 km. The satellite Terra, launched last year by NASA, the space agency of the United States, shows that it is possible to detect at least the level of particles in the range of visible light and of carbon monoxide in the infrared range. In ten years, when, as is forecast, the Metropolitan Region will merge with Campinas and São José dos Campos, it may perhaps be difficult to administer hundreds of sensors to know how the air is that day. An immediate lesson of the study is to get an idea of the quality of the air by just looking at the sky. If there are clouds, it is a good sign, as they work like vacuum cleaners: they suck the polluted air from the lower layers of the atmosphere and expel them upwards.
Meteorology and Pollution of the Atmosphere in São Paulo (96/01403-4); Modality: Thematic project; Coordinator: Pedro Leite da Silva Dias – Astronomic and Geophysical Institute – USP; Investment: R$ 1,411,210.01