“Will It Rain?” Activities Investigating Aerosol ... - ACS Publications

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“Will It Rain?” Activities Investigating Aerosol Hygroscopicity and Deliquescence L. Caetano-Silva,*,† A. G. Allen,† M. L. A. M. Campos,‡ and A. A. Cardoso† †

Departamento de Química Analítica, Instituto de Química, Universidade Estadual Paulista, Araraquara, 14800-060, São Paulo, Brazil Departamento de Química, FFCLRP, Universidade de São Paulo, Ribeirão Preto, 14040-901, São Paulo, Brazil

‡

S Supporting Information *

ABSTRACT: Climate change and its consequences seem to be increasingly evident in our daily lives. However, is it possible for students to identify a relationship between these large-scale events and the chemistry taught in the classroom? The aim of the present work is to demonstrate that chemistry can assist in elucidating important environmental issues. Simple experiments are used to demonstrate the mechanism of cloud formation, as well as the influence of anthropogenic and natural emissions on the precipitation process. The experiments presented show the way in which particles of soluble salts commonly found in the environment can absorb water in the atmosphere and influence cloud formation.

KEYWORDS: High School/Introductory Chemistry, Environmental Chemistry, Hands-On Learning/Manipulatives, Atmospheric Chemistry

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INTRODUCTION Newspaper headlines and media reports increasingly describe events and catastrophes related to intense rainfall or extended periods of drought. The associated loss of life and livelihood can be huge: In 2012, floods accounted for around 53% of the victims of natural disasters, while droughts and storms accounted for 27 and 16%, respectively. Economic losses in 2012 were around 10% greater than the annual average for the period 2002−2011, with the total economic prejudice reaching approximately 157 billion dollars.1 An important task of science is now to try to understand the reasons for changes in precipitation patterns and predict what the future might hold for us. The importance of introducing and/or increasing environmental chemistry content in courses at colleges and universities has been recognized for more than a decade,2−4 but there have been few similar efforts at the high school level or below. In an earlier paper, Felix and Cardoso5 used very simple experiments to demonstrate to high school students the ways in which certain parameters, such as temperature, pressure, and the presence of aerosols, affected the formation of rain clouds. Brooks et al. have also shown the effect of surface tension on cloud formation.6 In the present work, we consider another parameter equally important for the formation of clouds: the chemical composition of atmospheric aerosols.

once in the atmosphere, how can these particles affect the formation of clouds and rain? The water present on the surface of the planet, in the soils, oceans, rivers, and lakes, evaporates naturally using the energy of the sun. In addition, part of the water ingested by living beings is released in the form of vapor during the process known as evapotranspiration. The molecules of water present in the vapor phase in the atmosphere subsequently condense and return to the surface, mainly in the form of rain or snow. The process as a whole is known as the hydrological cycle. Although it may not appear so, the process of condensation of water vapor that occurs in the atmosphere is highly complex. In order for it to proceed, a pre-existing surface is required that can act as a nucleus for the formation of liquid water. Aerosols, which are abundant in the atmosphere, are highly effective as nuclei for the condensation of water vapor, and in the case of clouds, these particles are known as cloud condensation nuclei (CCN).9−14 Nonetheless, not all types of aerosols are able to perform this role: they need to possess an essential property, namely, an affinity for water. What is the chemical composition of these aerosols that have great affinity for water? From where do these atmospheric aerosols originate? Atmospheric aerosols are derived from multiple sources that can be both natural and anthropogenic (resulting from human activity). Furthermore, the origin of an aerosol is directly reflected in its chemical composition. Figure 1 shows two

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ATMOSPHERIC AEROSOLS AND THEIR SOURCES Aerosols are small solid or liquid particles suspended in a gas,7 and are sometimes described as particulate matter.8 However, © XXXX American Chemical Society and Division of Chemical Education, Inc.

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complete solubilization of the compounds present. This process is known as deliquescence.20−22

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HYGROSCOPICITY, DELIQUESCENCE, AND THE ROLE OF CLOUDS The atmosphere must be sufficiently humid in order to provide the necessary conditions for the growth of CCN and the formation of clouds. The presence of highly hygroscopic compounds in the aerosols can accelerate the process of formation and growth of cloud droplets, while the presence of less hygroscopic substances can extend the time required for cloud formation, diminishing the possibility of precipitation. Changes in the concentration and composition of the atmospheric aerosol, induced by human activities, can influence rainfall patterns at different spatial scales, leading to serious consequences for both human society and the natural environment.15,23−27 Modification of the hydrological cycle affects the distribution and availability of the fresh water that is essential for life, industry, domestic activities, agriculture, and electricity generation. For these reasons, changes in rainfall patterns are one of the main issues addressed by the Intergovernmental Panel on Climate Change.25 The role played by aerosols in the atmosphere is extremely complex and is the subject of multidisciplinary studies involving specialists from the areas of physics, meteorology, biology, and chemistry. To this end, chemistry has made a vital contribution in elucidating the chemical composition of aerosols in order to improve understanding of the processes of particle growth and the formation of cloud condensation nuclei. Two simple experiments illustrate the mechanisms underlying the formation of rain clouds. The first experiment concerns cloud formation, and the second experiment evaluates the influence of aerosol chemical composition on the capture of water vapor from the atmosphere.

Figure 1. Relative concentrations of soluble ions present in aerosols derived from (a) a rural agroindustrial region15 and (b) a densely populated and industrialized region.17

graphs illustrating aerosol composition in terms of the main soluble ions present. In the case of aerosols found in the atmosphere of rural agroindustrial regions in southeast Brazil, the composition is rich in K+ and Ca2+ ions,15,16 while aerosols collected in a densely urbanized and industrialized city in Japan are richer in NH4+ ions.17 In rural regions, the action of the wind on exposed soils results in the formation of resuspended particles containing species such as calcium and other soil nutrients, while the burning of plant residues is a source of potassium.15,16 Other natural sources of aerosols are pollen from plants, viruses, bacteria, condensed organic compounds, and dust from volcanic eruptions. In regions of high population density, domestic sewage represents a major source of gases derived from decomposition, including ammonia (NH3(g)), which once in the atmosphere can be incorporated into water droplets (eq 1). NH3(g) + H 2O(l) ⇌ NH4OH(aq) ⇌ NH4 +(aq) + OH−(aq) (1)

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FORMATION OF CLOUD CONDENSATION NUCLEI A commonly used illustration of the formation of aerosols in the atmosphere involves the reaction between gaseous ammonia (NH3(g)) and hydrogen chloride (HCl), which produces the solid ammonium chloride salt (NH4Cl(s)) (eq 2). NH3(g) + HCl(g) → NH4Cl(s)

SAFETY CONSIDERATIONS In contact with the skin and eyes, strong aqueous solutions of HCl and NH3 can cause burns, if not promptly removed by washing with copious quantities of water. Both substances are volatile, and if inhaled, the vapors can cause damage to the respiratory system. Potential symptoms of overexposure are irritation of nose, throat, and larynx: coughing, choking, and dermatitis. If, in spite of the precautions taken, there should be any accidental contact with the skin, the area should be washed immediately with large volumes of water. It is important that the person who handles the reagents should use safety glasses, gloves, and appropriate clothing. In cases of inhalation, individuals should be removed to an uncontaminated and well ventilated area. Meanwhile, it is important to stress that the quantities of reagents used in these experiments are very small, so the associated risks are low.28

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Recently formed crystals of NH4Cl(s) measure on the order of nanometers (10−9 m), but over time these particles increase in size due to the condensation of water vapor on their surfaces as well as particle aggregation. These processes lead to the formation of the type of particle known as a cloud condensation nucleus.5−11 In clouds, the droplets (nuclei) continue to grow until their mass is sufficiently large to enable them to be precipitated out in the form of rain. In the hypothetical situation of an atmosphere free of particles, the molecules of water vapor would not condense, hence preventing the formation of clouds. Due to the wide range of sources of material that can contribute to the formation of atmospheric aerosols, the composition of these particles is highly diverse and can include a variety of different inorganic and organic ions, together with nonionic components such as silica (SiO2), inorganic carbon (C), and organic molecules.7,18,19 The efficiency with which the aerosol captures molecules of water vapor is largely dependent on the presence of compounds that are soluble in water. The affinity of an aerosol for water is known as its hygroscopicity. In some cases, the efficient uptake of water vapor results in

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USING THESE ACTIVITIES

Procedure for Activity 1: Cloud Formation

With the assistance of the teacher (or other appropriately trained person), two holes spaced about 1 cm apart are made in the lids of two 500 mL jars. The diameters of these holes must be sufficient to allow passage of a stick with cotton wool. The silica gel is added to one of the jars, and the other jar is filled with water to the same height as the silica gel. The jars are sealed, using plastic film under their lids, and allowed to stand for at least 2 h. The jars are then removed to a well-ventilated B

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An important point is that the presence of a large quantity of water vapor does not itself guarantee the formation of a cloud; the presence of hygroscopic aerosols is fundamental for the process to occur. In the present case, the airborne particles of the NH4Cl salt absorbed water vapor from the humid atmosphere, resulting in particle growth. This is similar to the processes that occur in the atmosphere of our planet, and the NH4Cl particles produced in this experiment can be described as cloud condensation nuclei. In this experiment, the instructor can repeat the procedure using other volatile or semivolatile acids in order to identify those that favor cloud formation. The authors suggest that, in addition to observing the experiment, the students could also make video recordings during the course of the experiment. Aerosols play a vital role in cloud formation, although other factors are also involved in the formation of rain. Regions with similar conditions of atmospheric humidity may have different patterns of formation of rain and fog. Different contributions of natural and anthropogenic aerosols may be an important factor that affects cloud formation. Other important factors include the topography. For example, a chain of mountains can act as a barrier to humid air masses resulting in an area with frequent rain and fog on one side, and a region of low rainfall on the other side. In urban areas, the replacement of natural vegetation by asphalt, concrete, and buildings results in the creation of heat islands, because these surfaces absorb the sun’s heat, hence increasing the local temperature and modifying the conditions of the atmosphere affecting the formation of clouds. The current challenge faced by science is to understand regional climatic changes observed over a time span of decades, including identification of the most important factors that act to modify the frequency of rain. Changes in the pattern of rainfall can affect quality of life, and increasing human activities that result in significant modification of the environment are suggested as an important cause of these changes. Altered emissions of aerosols due to increased (or different) human activities may be a relevant factor affecting rainfall patterns. This topic enables presentation of the concept of the volatility of acids and bases. As shown by Darr (2013),4 the study of atmospheric aerosols also provides an interesting way to introduce chemical thermodynamics concepts to higher-level students. The experimental observations of the behavior of the gases can be explained using a simple model of kinetic molecular theory. A possible question to introduce this topic is “Why does cloud formation initially occur nearer to the source of HCl?” The kinetic energies of the molecules (NH3 and HCl) in the gas phase are equal (both are at the same temperature). The kinetic energy is described by E = (1/2)mv2, where v is the speed of the molecule and m is the molecular mass of the gas molecule. This explains why the velocity of the NH3 molecule is greater than that of the HCl molecule. The question that remains is whether aerosols of different salts possess the same capacity to act as cloud condensation nuclei. In the next set of experiments, we shall explore the way in which chemical composition influences the extent to which a particle can absorb water.

location (or a fume cupboard). With the assistance of the teacher, and suitably protected using gloves, apron, and safety glasses, the student moistens the cotton of one stick with the HCl solution and the cotton of the other stick with the NH4OH solution. The plastic film covering one of the jars is then ruptured, and the sticks are introduced rapidly into the interior of the jar, taking care to avoid them touching each other. The formation of aerosol can be observed in the interior of the jar. For best results, it is important that both sticks should be kept inside the jar for the same time, but not for more than 10 s. The same procedure is then repeated using the other jar. The jars are compared in order to identify the conditions that provided the most efficient cloud formation. More details are provided in the Supporting Information. This experiment enables the student to gain insight into the importance of aerosols in cloud formation, by producing clouds with his/her own hands. The formation of aerosols will be observed, as well as the way in which the presence of water in the form of vapor leads to the formation of particles that are similar to cloud droplets in appearance. Photographs of the results obtained are shown in Figure 2. It can be seen that, in

Figure 2. Photographs showing vertical and lateral views of the jars containing silica gel (left) and water (right), following formation of the cloud.

the jar containing water, the NH4Cl aerosol produced has the appearance of a cloud, while in the jar maintained in a dry state using silica gel, the aerosol is much less visible. This is because high humidity is maintained in the first jar by the evaporation of liquid water. The water vapor condenses when it makes contact with the surfaces of the aerosol particles, resulting in the formation of droplets. In the dry jar, the high affinity of silica gel for water molecules removes most of the water vapor present in the atmosphere within the jar. The process of condensation of water vapor is therefore restricted, and the particles of NH4Cl remain too small to be visible as a cloud.

Procedure for Activity 2: Influence of the Chemical Composition of Different Salts on the Capture of Water Vapor

The lids are divided into pairs, and approximately 5 mL of water is placed into one lid of each pair. The salt under test is placed into the other lid. Tweezers are then used to place a pair C

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Figure 3. Photographs of the salts studied, immediately prior to insertion in the glass jars containing water, and then at intervals up to 48 h.

combinations of ions and their effects on the ability of the salts to absorb water. An example is the difference observed between the calcium salts (CaCO3 and CaCl2). The solubility of salts in water depends on several factors, one of which is the ionic radius. The results observed in the experiment can be used subsequently for a discussion of the solubility of salts in water. From these results, it can be inferred that the formation of clouds depends on both the quantity of water vapor present in the atmosphere and the nature of the salts present in the aerosols. There are many natural sources of the salts found in the atmosphere, and the compounds tested here are often found in atmospheric aerosols. For example, the calcium ion is a common component of the soil, and the action of the wind, together with the movement of agricultural machinery and vehicles in plantations and on unsealed tracks, favors the resuspension of particles containing calcium, which can then remain in the atmosphere for prolonged periods.30 Calcium carbonate (limestone) is widely used as an amendment to neutralize soil acidity and to supply calcium for plant nutrition. The main sources of atmospheric chloride are the oceans (marine aerosols) and the combustion of fossil fuels and biomass.17 The potassium ion is present at high concentrations in plants, and is therefore emitted in the form of aerosols during the burning of forests and agricultural wastes.31 Sodium is especially abundant in oceanic and coastal aerosols,17 due to its release from seawater during mechanical processes. The main sources of atmospheric nitrate and sulfate are their formation from nitrogen oxides and sulfur dioxide emitted from combustion sources including road vehicles, power stations, industries, and biomass burning.32 The ammonium ion is formed from ammonia gas released from agricultural activities (especially intensive livestock production) and domestic sewage,33 and it is usually found in atmospheric aerosols associated with ions derived from acidic species such as sulfuric, nitric, and acetic acids.30 These acids are found in the atmosphere in the vapor phase or dissolved in droplets of water. The salts used in the present work therefore reflected the compounds commonly found in atmospheric aerosols from different regions.

of lids (one containing water, and the other containing the salt) into each jar. It is important to ensure that the lids do not touch each other at any moment. The glass jars are then sealed with plastic film under their caps. More details are provided in the Supporting Information. The procedure can be repeated for other salts typically found in ambient atmospheric aerosols. In the present work, the following salts were used: sodium chloride (NaCl), potassium chloride (KCl), ammonium chloride (NH4Cl), calcium chloride (CaCl2), sodium nitrate (NaNO3), potassium nitrate (KNO3), ammonium nitrate (NH4NO3), ammonium sulfate ((NH4)2SO4), and calcium carbonate (CaCO3). In the second experiment, the authors encourage the teacher, together with the students, to make a video or take photographs throughout the experiment. The teacher can question the students about the nature of the aerosol, in terms of deliquescence. The concentration of water vapor present within the jars of the second experiment is determined by the equilibrium established between the liquid and vapor phases. The presence of a solid hygroscopic salt causes the removal of water vapor from the atmosphere, in a process that continues until equilibrium is reestablished. It can be observed from the results of this experiment that the salts tested differed considerably in terms of their capacity to absorb water present in the form of vapor in the atmosphere of the jar. After different periods, some of the salts showed deliquescence and the formation of a liquid phase containing the salt in solution (Figure 3). It should be clarified that, after 24 h (Figure 3), the salts CaCl2 and NH4NO3 absorbed sufficient water from the atmosphere to solubilize all of the solid. This can even be observed by the appearance of the identification label attached to the lid of the container. The rate at which deliquescence occurred decreased in the order CaCl2 > NH4NO3 > NaNO3 > NaCl > (NH4)2SO4 > KCl > KNO3 > CaCO3. These results are in agreement with the literature.29 The CaCl2 salt absorbs water vapor at a low atmospheric relative humidity (RH) of 29%, while KNO3 requires RH of 92% in order to absorb water vapor. It may be interesting, at this point, to observe the importance of D

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D.; McFarquhar, G.; Novakov, T.; Ogren, J. A.; Podgorny, I. A.; Prather, K.; Priestley, K.; Prospero, J. M.; Quinn, P. K.; Rajeev, K.; Rasch, P.; Rupert, S.; Sadourny, R.; Satheesh, S. K.; Shaw, G. E.; Sheridan, P.; Valero, F. P. J. Indian Ocean Experiment: An Integrated Analysis of the Climate Forcing and Effects of the Great Indo-Asian Haze. J. Geophys. Res. 2001, 106, 28371−28398. (10) Tang, I. N.; Munkelwitz, H. R. Composition and Temperature Dependence of the Deliquescence Properties of Hygroscopic Aerosols. Atmos. Environ. 1993, 27, 467−473. (11) Tang, I. N.; Munkelwitz, H. R. Water Activities, Densities and Refractive Indices of Aqueous Sulfates and Sodium Nitrate Droplets of Atmospheric Importance. J. Geophys. Res. 1994, 99, 18801−18808. (12) Albrecht, B. Aerosols, cloud microphysics and fractional cloudiness. Science 1989, 245, 1227−1230. (13) Twomey, S. Influence of Pollution on Shortwave Albedo of Clouds. J. Atmos. Sci. 1977, 34, 1149−1152. (14) Pincus, R.; Baker, M. B. Effect of Precipitation on the Albedo Susceptibility of Clouds in the Marine Boundary Layer. Nature 1994, 372, 250−252. (15) Allen, A. G.; Cardoso, A. A.; Da Rocha, G. O. Influence of Sugar Cane Burning on Aerosol Soluble Ion Composition in Southeastern Brazil. Atmos. Environ. 2004, 38, 5025−5038. (16) Caetano-Silva, L.; Allen, A. G.; Lima-Souza, M.; Cardoso, A. A.; Campos, M. L. A. M.; Nogueira, R. F. P. An Analysis of Diurnal Cycles in the Mass of Ambient Aerosol Derived from Biomass Burning and Agro-Industry. J. Geophys. Res. 2013, 118, 8675−8687. (17) Khan, M. F.; Shirasuna, Y.; Hirano, K.; Masunaga, S. Characterization of PM2.5, PM2,5−10 and PM>10 in Ambient Air, Yokohama, Japan. Atmos. Res. 2010, 96, 159−172. (18) Hewitt, C. N.; Davison, B. M. Formation of Aerosol Particles from Biogenic Precursors. In Atmospheric Particles; Harrison, R. M., Van Grieken, R. E., Eds.; IUPAC Series on Analytical and Physical Chemistry of Environmental Systems, Vol. 5; Wiley & Sons: Chichester, 1998. (19) Seinfeld, J. H.; Pandis, S. N. Atmospheric Chemistry and Physics: From Air Pollution to Climate Change; John Wiley & Sons: New York, 1998. (20) Ansari, A. S.; Pandis, S. N. Prediction of Multicomponent Inorganic Atmospheric Aerosol Behavior. Atmos. Environ. 1999, 33, 745−757. (21) Cruz, C. N.; Pandis, S. N. Deliquescence and Hygroscopic Growth of Mixed Inorganic-Organic Atmospheric Aerosol. Environ. Sci. Technol. 2000, 34, 4313−4319. (22) Bzdek, B. R.; Johnston, M. V. New Particle Formation and Growth in the Troposphere. Anal. Chem. 2010, 82, 7871−7878. (23) Zamora, I. R.; Jacobson, M. Z. Measuring and Modeling the Hygroscopic Growth of Two Humic Substances in Mixed Aerosol Particles of Atmospheric Relevance. Atmos. Chem. Phys. 2013, 13, 8973−8989. (24) Freud, E.; Rosenfeld, D.; Andreae, M. O.; Costa, A. A.; Artaxo, P. Robust Relations between CCN and the Vertical Evolution of Cloud Drop Size Distribution in Deep Convective Clouds. Atmos. Chem. Phys. 2008, 8, 1661−1675. (25) IPCC (2007). Fourth Assessment Report “Climate Change 2007”, International Panel on Climate Change. Available at http://www.ipcc. ch/publications_and_data/ar4/wg2/en/ch3.html (accessed June 2014). (26) Da Rocha, G. O.; Allen, A. G.; Cardoso, A. A. Influence of Agricultural Biomass Burning on Aerosol Size Distribution and Dry Deposition in Southeastern Brazil. Environ. Sci. Technol. 2005, 39, 5293−5301. (27) Choi, M. Y.; Chan, C. K. The Effects of Organic Species on the Hygroscopic Behaviors of Inorganic Aerosols. Environ. Sci. Technol. 2002, 36, 2422−2428. (28) MSDS Solutions − Free Material Safety Data Sheets from 3E Company. Available at: http://www.msds.com (accessed May 2014). (29) Haynes, W. M. CRC Handbook of Chemistry and Physics, 90th ed.; CRC Press/Taylor and Francis: Boca Raton, 2009.

CONCLUSIONS Since industries, automobiles, agricultural machinery, biomass burning, and other anthropogenic activities are sources of ionic species present in the atmosphere, it has been shown that a link exists between the intensification of human action and changes in the ionic composition of atmospheric aerosols.15,16,24,26,27 These changes can then lead to modification of the processes affecting cloud formation and lifetime, and consequently to changes in rainfall patterns.34 Knowledge of the physicochemical properties of the salts found in aerosols, together with emission sources and the chemical reactions occurring within the atmosphere, is fundamental for understanding possible changes in rainfall patterns. It should also be pointed out that clouds not only play a role in the hydrological cycle but also participate in the energy balance of the planet.

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ASSOCIATED CONTENT

S Supporting Information *

Further details of the materials and procedures. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support provided by the São Paulo State Research Foundation (FAPESP, Process No. 2008/58073-5) and the National Council for Technological and Scientific Development (CNPq).

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REFERENCES

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(30) Allen, A. G.; Cardoso, A. A.; Wiatr, A. G.; Machado, C. M. D.; Paterlini, W. C.; Baker, J. Influence of Intensive Agriculture on Dry Deposition of Aerosol Nutrients. J. Braz. Chem. Soc. 2010, 21 (1), 87− 97. (31) Rissler, J.; Joakim, P.; Swietlicki, E.; Wierzbicka, A.; Strand, M.; Lillieblad, L.; Sanati, M.; Bohgard, M. Hygroscopic Behavior of Aerosol Particles Emitted from Biomass Fired Grate Boilers. Aerosol Sci. Technol. 2005, 39, 919−930. ́ (32) Rocha, J. C.; Rosa, A. H.; Cardoso, A. A. Introdução À Quimica Ambiental; Bookman: Porto Alegre, 2009. (33) Krupa, S. V. Effects of Atmospheric Ammonia on Terrestrial Vegetation: A Review. Environ. Pollut. 2003, 124, 179−221. (34) Bell, T. L.; Rosenfeld, D.; Kim, K. M.; Yoo, J. M.; Lee, M. I.; Hahnenberger, M. Midweek Increase in US Summer Rain and Storm Heights Suggest Air Pollution Invigorates Rainstorms. J. Geophys. Res. 2008, 113 (D2), D02209−D02231.

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