ABSTRACT
The Amazon rainforest plays a crucial role in the global climate system, acting as a major carbon sink and influencing regional and global weather patterns. Understanding the chemical composition of rainwater is essential for assessing the impact of anthropogenic activities, such as deforestation and industrial emissions, on atmospheric chemistry and hydrology. This work aimed to characterize the chemical composition of rainwater in a biological reserve of primary forest in the central Brazilian Amazon at 60 km of a large urban center. Rainwater samples were collected from March 2008 to March 2010 and were analyzed by ion chromatography, ICP-MS, and TOC-V. This is the only and longest rainfall monitoring carried out in this reserve. The results showed that the rainwater is rich in organic carbon (TOC), representing 77% of total carbon. The most abundant ions were NH4 + and Cl-. Few elements were detected, with emphasis on Al and Fe. In the dry season, most species were enriched. The lower amount of precipitation, biomass burning and the lower capacity to remove pollutants from the atmosphere are the main reasons for this seasonal difference. Only 7% had characteristics of acid rain (pH < 4.5), with acidity dominated by NO3 -. A positive matrix factorization indicated contribution of sources: crustal (48%), secondary aerosol (26%), biogenic (22%), and industrial emissions (4%). Although the forest has primary characteristics, the proximity to the urban center indicates some anthropogenic influence on the chemical composition of rainwater.
KEYWORDS:
ions; metals; soluble carbon; wet deposition; positive matrix factorization
RESUMO
A floresta amazônica desempenha um papel crucial no sistema climático global, atuando como um importante sumidouro de carbono e influenciando os padrões climáticos regionais e globais. Compreender a composição química da água da chuva é essencial para avaliar o impacto das atividades antrópicas, como o desmatamento e emissões industriais, na química atmosférica e hidrologia. Este trabalho objetivou caracterizar a composição química da água da chuva em uma reserva biológica composta por floresta primária na Amazônia central brasileira, distante 60 km de um grande centro urbano. Amostras de água de chuva foram coletadas de março 2008 a março 2010 e analisadas por cromatografia iônica, ICP-MS e TOC-V. Este é o único e mais longo monitoramento pluviométrico realizado nessa reserva. Os resultados mostraram que a água da chuva é rica em carbono orgânico (COT), representando 77% do carbono total. Os íons mais abundantes foram NH4 + e Cl-. Poucos elementos foram detectados, com destaque para Al e Fe. Na estação seca, a maioria das espécies foi enriquecida. A menor quantidade de precipitação, a queima de biomassa e a menor capacidade de remoção de poluentes da atmosfera são os principais motivos desta diferença sazonal. Apenas 7% apresentaram características de chuva ácida (pH < 4,5), com acidez dominada pelo NO3 -. Uma fatoração de matriz positiva indicou contribuição de fontes: crustal (48%), aerossol secundário (26%), biogênico (22%) e emissões industriais (4%). Embora a floresta possua características primárias, a proximidade com o centro urbano indica alguma influência antrópica na composição química das águas de chuva.
PALAVRAS-CHAVE:
íons; metais; carbono solúvel; deposição úmida; fatoração de matriz positiva
INTRODUCTION
Climate change has affected several ecosystems around the world, including the Amazon region (Flores et al. 2024Flores, B.M.; Montoya, E.; Sakschewski, B.; Nascimento, N.; Staal, A.; Betts, R.A.; et al. 2024. Critical transitions in the Amazon forest system. Nature 626: 555-564.). The region has been experiencing an intensified variation of atmospheric composition in the last decades, mainly due to the presence of particulate matter (PM) (Artaxo et al. 2006Artaxo, P.; Oliveira, P.H.; Lara, L.L.; Pauliquevis, T.M.; Rizzo, L. V.; Junior, C.P.; et al. 2006. Efeitos climáticos de partículas de aerossóis biogênicos e emitidos em queimadas na Amazônia. Revista Brasileira de Meteorologia 21: 168-189.). PM influences the world’s energy balance, atmospheric circulation, and the hydrological cycle, and plays an important role in the rain cycle, affecting the mechanism of cloud formation during the rainy and dry season (Tavares 2012Tavares, J.P.N. 2012. Interaction between vegetation and the atmosphere in cloud and rain formation in the Amazon: A review. Estudos Avançados 26: 219-227.).
The Amazon rainforest is an important source of natural trace gases, PM, and water vapor (Ramsay et al. 2020Ramsay, R.; Di Marco, C.F.; Sörgel, M.; Heal, M.R.; Carbone, S.; Artaxo, P.; et al. 2020. Concentrations and biosphere-atmosphere fluxes of inorganic trace gases and associated ionic aerosol counterparts over the Amazon rainforest. Atmospheric Chemistry and Physics 20: 15551-15584.). However, there is also a large contribution from biomass burning during the dry season, resulting in unusual PM concentrations in remote sites (Herbert et al. 2021Herbert, R.; Stier, P.; Dagan, G. 2021. Isolating large-scale smoke impacts on cloud and precipitation processes over the Amazon with convection permitting resolution. Journal of Geophysical Research: Atmospheres 126: e2021JD034615.). In addition, there is an annual contribution of 28 million tons of African dust to the Amazon basin (Rizzolo et al. 2016).
The emissions of PM strongly impact the atmospheric dynamics since it can disperse and absorb solar radiation, interfere with the process of cloud formation, and, consequently, with rainfall (Fiore et al. 2015Fiore, A.M.; Naik, V.; Leibensperger, E.M.; Fiore, A.M.; Naik, V.; Leibensperger, E.M. 2015. Air quality and climate connections. Journal of the Air & Waste Management Association 65: 645-685.). In general, PM has a predominant chemical composition of sulfate, nitrate, ammonium, sea salt, mineral dust, organic compounds, and black or elemental carbon (Moran-Zuloaga et al. 2018Moran-Zuloaga, D.; Ditas, F.; Walter, D.; Saturno, J.; Brito, J.; Carbone, S.; et al. 2018. Long-term study on coarse mode aerosols in the Amazon rain forest with the frequent intrusion of Saharan dust plumes. Atmospheric Chemistry and Physics 18: 10055-10088.). Hydrophilic species present in PM contribute to cloud formation (Wang et al. 2023Wang, Y.; Okochi, H.; Tani, Y.; Hayami, H.; Minami, Y.; Katsumi, N.; et al. 2023. Airborne hydrophilic microplastics in cloud water at high altitudes and their role in cloud formation. Environmental Chemistry Letters 21: 3055-3062.).
There are many studies about gases and aerosol in the Amazon region, but few about chemical composition of rainwater (Williams et al. 1997Williams, M.R.; Fisher, T.R.; Melack, J.M. 1997. Chemical composition and deposition of rain in the central Amazon, Brazil. Atmospheric Environment 31: 207-217.; Artaxo et al. 2006Artaxo, P.; Oliveira, P.H.; Lara, L.L.; Pauliquevis, T.M.; Rizzo, L. V.; Junior, C.P.; et al. 2006. Efeitos climáticos de partículas de aerossóis biogênicos e emitidos em queimadas na Amazônia. Revista Brasileira de Meteorologia 21: 168-189.; Honório et al. 2010Honório, B. a D.; Horbe, a. M.C.; Seyler, P. 2010. Chemical composition of rainwater in western Amazonia - Brazil. Atmospheric Research 98: 416-425.; Pauliquevis et al. 2012Pauliquevis, T.; Lara, L.L.; Antunes, M.L.; Artaxo, P. 2012. Aerosol and precipitation chemistry measurements in a remote site in Central Amazonia: the role of biogenic contribution. Atmospheric Chemistry and Physics 12: 4987-5015.). Here we analyze the data of monitoring of the chemical composition of rainwater in the central Amazon over two years of weekly measurements (from 2008 to 2010) in an area of primary forest relatively close to a large urban center. This monitoring was part of the EUCAARI project (European Integrated Project on Aerosol Cloud Climate Interactions) (Kulmala et al. 2009Kulmala, M.; Asmi, A.; Lappalainen, H.K.; Carslaw, K.S.; Pöschl, U.; Baltensperger, U.; et al. 2009. Introduction: European Integrated Project on Aerosol Cloud Climate and Air Quality interactions (EUCAARI) - integrating aerosol research from nano to global scales. Atmospheric Chemistry and Physics 9: 2825-2841.). Although the data are over a decade old, they still represent the longest monitoring of rainwater composition carried out to date in the central Amazon, allowing to establish a baseline to which compare changes over time. This is essential to assess the impacts of human activities, such as deforestation and biomass burning, on the characteristics of rainfall in the Amazon region.
Our aim was to assess the magnitude of anthropic influence on rainfall in the forest at this spatial scale and provide a basis for comparison with later data on rainfall composition in the region. We quantified ions, elemental composition, and total carbon from rainwater samples in the rainy and dry season of the year, and discuss the likely sources of the rainfall components. Positive matrix factorization (PMF) was used for the first time in the Amazon region with rainfall samples to identify sources.
MATERIAL AND METHODS
Study area and sampling procedures
The study was carried out in the Cuieiras Biological Reserve (CBR) (02o35´21.08”S, 60o06´53.63”W, 130 m.a.s.l), located about 60 km north-northwest of the city of Manaus, stae of Amazonas (Brazil), and 2000 km from the Atlantic Ocean (Figure 1). It has an area of 22,735 ha covered by relatively undisturbed primary forest. The prevailing winds blow from the northeast over vast expanses of intact tropical forest, bringing Atlantic Ocean air masses, long-range transported Saharan dust and regionally originated pollutants from biomass burning (Moran-Zuloaga et al. 2018Moran-Zuloaga, D.; Ditas, F.; Walter, D.; Saturno, J.; Brito, J.; Carbone, S.; et al. 2018. Long-term study on coarse mode aerosols in the Amazon rain forest with the frequent intrusion of Saharan dust plumes. Atmospheric Chemistry and Physics 18: 10055-10088.).
Rainwater samples were collected from March 2008 to March 2010. Samples were collected at intervals of two to four days, totaling 114 samples (100 to 1000 mL), with 47 in the dry (June to October) and 67 in the rainy (November to May) season. Samples were collected with a““wet-onl”” collector (AeroChem Metrics, Model 301 automatic sensing wet/dry) that was installed about 1 m above the ground. This collector uncovers the sample container after detecting (through an infrared optical rainfall sensor) five drops of rainfall. The cover returns to its place within 2 min after rainfall stops, to minimize dry deposition and evaporation.
Data quality of individual rainwater samples was checked by ionic balance, according to US NADP criteria, i.e., the samples were discarded when the ion difference in the samples was 15%-30% for samples having total ion concentrations > 100 µeq L-1, and 30 %-60 % for samples having total ion concentrations of 50-100 µeq L-1.
Chemical analyses
The ionic conductivity and pH were measured at the end of each sampling. Immediately after sampling, the samples were subdivided and properly preserved: a) one aliquot was preserved with Thymol in order to minimize biological activity and to determine ion concentrations by ion chromatography; b) other aliquot was preserved with mercury chloride in order to determine TOC; and c) the third aliquot was preserved with HNO3 to determine metals and other elements by ICP-MS. All samples were filtered (Whatman - 41 filters) and stored in a refrigerator at 4 oC before analyses.
Elements such as Al, Ca, Fe, Mg, K, Na, Li, Ti, V, Cr, Mn, Co, Ni, Cu, As, Zn, Se, Rb, Sr, Y, Cd, Sb, Cs, Ba, La, Ce, Nd, W, Pb, Bi were determined by the ICP-MS technique (Perkin-Elmer ELAN 6000). The concentrations of the elements were determined using In and Tl as internal standards. More details can be found in Godoy et al. (2009). The ion concentrations (Cl-, NO3 -, SO4 2-, PO4 3-, CH3COO-, HCOO-, Na+, K+, NH4 +, Ca2+, Mg2+) were determined by ion chromatograph (Dionex model ICS-2000, USA) in the same way as described by Gioda et al. 2023Gioda, A.; Mateus, V.L.; Hacon, S.S.; Ignotti, E.; Gomes, R.G.S.; Pedreira, M.F.S.; et al. 2023. Assessing over decadal biomass burning influence on particulate matter composition in subequatorial Amazon: literature review, remote sensing, chemical speciation and machine learning application. Anais da Academia Brasileira de Ciências 95: e20220932. . Total organic carbon (TOC), total carbon (TC) and inorganic carbon (IC) were determined by TOC-V CPHCPN (SHIMADZU, TOC-4200, Japan). TC - IC result in TOC concentration and calculated by subtracting inorganic carbon (IC) (Gioda et al. 2011Gioda, A.; Amaral, B.S.; Monteiro, I.L.; Saint’Pierre, T.D. 2011. Chemical composition, sources, solubility, and transport of aerosol trace elements in a tropical region. Journal of Environmental Monitoring 13: 2134-2142. ; Duarte et al. 2013Duarte, A.F.; Gioda, A.; Ziolli, R.; Duó, D. 2013. Contaminación atmosférica y deposición húmeda en la Amazonia brasileña. Revista Cubana de Salud Publica 39: 627-639.). The instrument´s detection limit was calculated as three times the standard deviation of ten replicated blank values for all techniques.
Volume-weighted mean and wet deposition
The volume-weighted mean (VWM) of ionic constituents and seasonal wet deposition (WD) fluxes in the rainwater samples were calculated according to equations [1] and [2], respectively.
[1]
[2]
where CVWM = VWM concentration; Ci = ionic concentration of individual element (µmol L-1); Pi = individual rainfall (mm) for each rainy event; Pt = total rainfall (mm); n = total number of rainfall events; WD = seasonal/total wet deposition flux expressed (mg m-2 yr-1) (Xing et al. 2017Xing, J.; Song, J.; Yuan, H.; Wang, Q.; Li, X.; Li, N.; et al. 2017. Atmospheric wet deposition of dissolved trace elements to Jiaozhou Bay, North China: Fluxes, sources and potential effects on aquatic environments. Chemosphere 174: 428-436.).
The weighted standard deviation (sdw) of the results was calculated based on equation [3].
[3]
where N = number of samples; wi = sample weight; xi = sample values; xw = weighted average; N’ = number of non-zero weights.
Enrichment factor
The enrichment factor (EF) was applied to identify the potential sources of elements and was calculated through the ratio of the concentration measurement of the elements in the rainwater samples to ratios for elements measured present in the seawater and Earth´s crust. For this, Ca2+ and Na+ were used as the reference for soil and marine sources, respectively, according to equations [4] and [5] (Zhang et al. 2007Zhang, M.; Wang, S.; Wu, F.; Yuan, X.; Zhang, Y. 2007. Chemical compositions of wet precipitation and anthropogenic influences at a developing urban site in southeastern China. Atmospheric Research 84: 311-322.). An EF < 10 was diluted, while an EF > 10 indicates enrichment relative to the reference source.
[4]
[5]
where, X is the concentration of the element of interest; X/Na+ of seawater is the ratio of seawater composition; X/Ca2+ of the crust is the ratio of crustal composition (Taylor and McLennan 1995Taylor, S.R.; McLennan, S.M. 1995. The geochemical evolution of the continental crust. Reviews of Geophysics 33: 241-265.).
Positive matrix factorization
For the positive matrix factorization (PMF), the PMF 5.0 model was used for source apportionment and characterization of the collected rainwater over the study period. Principles and modes of use are detailed in the user manual (Hristova et al. 2020Hristova, E.; Veleva, B.; Georgieva, E.; Branzov, H. 2020. Application of positive matrix factorization receptor model for source identification of PM10 in the city of Sofia, Bulgaria. Atmosphere 11: 890. doi:10.3390/atmos11090890
https://doi.org/10.3390/atmos11090890...
). This model uses two input files: i) measured concentrations of the species, and ii) estimated uncertainty of the concentration. Here, the PMF 5.0 was applied to datasets composed of 22 species (TC, Ca2+, NH4 +, HCOO-, Cl-, NO3 -, SO4 2-, Na+, Mg2+, Al, K+, V, Mn, Fe, Co, Cu, Zn, Rb, Sr, Cd, Ba, Pb). Uncertainties of the sample concentration were calculated using the method detection limit (MDL) in Equation [6] (EPA 2014EPA. 2014. EPA Positive Matrix Factorization (PM F) 5.0 Fundamentals and User Guide. Washington, 136p.).
[6]
The PMF was applied to the entire database, without separating rainy and dry seasons, since this type of analysis separately would cause a reduction in the number of degrees of freedom.
Statistical analysis
Differences between average concentrations of chemical measurements in the dry and wet season were determined using Student´s t-test. A value of p < 0.05 was considered significant. All analyses were performed using the CRAN R free software, version 4.2.1 (R Team Core 2019R Team Core. 2019. A Language and Environment for Statistical Computing. R Foundation for Statistical Computing. .).
RESULTS
Rainfall, pH, conductivity and rainwater composition
A total of 3,258 mm of precipitation occurred during the sampling period, with 2,345 mm (72%) in the wet and 913 mm (18%) in the dry season (Table 1). The mean and range of the conductivity values were 3.8 ± 2.4 µS cm-1 (0.72-13 µS cm-1) and 2.9 ± 2.1 µS cm-1 (0.22-12 µS cm-1) during the dry and wet season, respectively. Statistically significant differences for mean conductivity were found between both seasons. Mean and range of pH values were 5.17 ± 0.50 (4.20-6.50) and 5.22 ± 0.47 (3.90-6.80) during the dry and wet season, respectively (Table 1). Organic carbon (TOC) represented 77% of total carbon (TC), being slightly higher in the dry (80 %) than in the wet period (72%) (Table 1). The concentrations of ionic species in rainfall followed the sequence NH4 + > Na+ > Cl- > NO3 - > SO4 2- > K+ > Ca2+ > HCOO- > Mg2+. Ammonium and Cl- were the most dominant cation and anion, respectively. The overall contribution of cations and anions to the ionic strength in the rainwater was 58% and 42%, respectively.
The highest values of ions were recorded in the dry season (Table 1). There was a statistically significant difference in the concentration of NH4 + and NO3 - between seasons, while the other ions did not differ significantly between seasons. Only Ca2+ and Mg2+ showed a slight, statistically non-significant increase in concentration throughout the study period, while the other ions showed no incremental trend. K+, NH4 +, NO3 -, and SO4 2-, which originate from combustion processes, showed a similar behavior during the dry season.
Many elements were below the detection limit and were therefore not reported. Among the thirteen elements detected (Al, V, Mn, Fe, Co, Cu, Zn, Rb, Sr, Cd, Sb, Ba and Pb), nine (Al, Mn, Co, Cu, Rb, Cd, Sb, Ba and Pb) had higher concentrations during the dry period (Table 1). The elements did not show a clear trend over the years of study.
Wet deposition fluxes
The wet deposition fluxes showed acidic characteristics (H++NO3 -+SO4 2- = 473 µmol m-2) due to high concentrations of H+. The pooled concentration of the basic elements, which have nutrient properties, such as Na+, K+, Ca2+, and Mg2+ was not enough (221 µmol m-2) to neutralize the acidity. Na+ and Cl- represented an important fraction (266 µmol m-2) of ion deposition of marine influence. Nitrogen fluxes (272 µmol m-2) were high due to the high flux of ammonia (170 µmol m-2), one of the highest found in the study (Table 1). Soluble trace element concentrations appeared in the following order: Al > Fe > Zn > Mn > Cu > Sr > Rb > Ba > V > Co > Pb > Cd > Sb (Table 1).
Origin of the pollutants
To infer the possible sources of the studied species, the enrichment factor and PMF analysis were used. The average EF for marine sources ranged from 0.69 to 15.90 and from 0.69 to 13.80 for the dry and wet season, respectively (Table 2). Average EFsoil in the dry season ranged from 0.02 to 9.409, and from 0.02 to 9.881 in the rainy season. EFmarine for Cl- and Mg2+ was <1, but EFsoil for Cl- was >10 (Table 2). EF of Al, V, Fe, and Ba was <10. An EF >10 indicates enrichment (Zhang et al. 2007Zhang, M.; Wang, S.; Wu, F.; Yuan, X.; Zhang, Y. 2007. Chemical compositions of wet precipitation and anthropogenic influences at a developing urban site in southeastern China. Atmospheric Research 84: 311-322.), which was the case of Mn, Co, Cu, Zn, Rb, Sr, Cd, Sb, and Pb. Overall, the marine contribution was negligible compared to biogenic sources (Table 2).
We identified six source factors in the PMF analysis. Factor 1 was characterized by the dominant portions of NO3 -, SO4 2- and V, with the two first species together indicating a significant source of secondary origin (Table 3; Figure 2). Factor 2 was characterized by the high loading of NH4 +. Factor 3 had Ca2+ with the highest loading, followed by HCOO-, indicating that Ca2+ was the dominant neutralizing ion at CBR. Factor 4 contributed with high loadings for Mg2+, Sr, Ba and Rb, Cl-, Cd, Mn, Co, Cu and Pb. Factor 5 had the highest loadings for Na+ and K+. Factor 6 accounted for 69.6%, 52.8%, 47.8%, 40.3%, 36.6%, and 31.2% of the total concentrations of TC, Fe, Co, Cu, Zn, and Al, respectively. The average contribution of sources from the PMF analysis was crustal (48%), followed by secondary aerosol (26%), biogenic (22%), and industrial emissions (4%).
Factor profiles (% of species sum) of source species measured in rainwater collected in Cuieiras Biological Reserve from 2008 to 2010 according to EPA PMF 5.0 output.
Source apportionment profiles of chemical components of rainwater collected in the Cuieiras Biological Reserve during 2008-2010.
DISCUSSION
Rainfall, pH, conductivity and rainwater composition
The conductivity reflects the total dissolved ions in rainfall (Gioda et al. 2013Gioda, A.; Mayol-Bracero, O.L.; Scatena, F.N.; Weathers, K.C.; Mateus, V.L.; McDowell, W.H. 2013. Chemical constituents in clouds and rainwater in the Puerto Rican rainforest: Potential sources and seasonal drivers. Atmospheric Environment 68: 208-220.). Lower conductivity in the wet season may be ascribed to the dilution factor or higher water content in the rain droplets. The average conductivity in CRB (3.4 µS cm-1) was lower than that in Lake Calado, located 80 km west of Manaus, in the 1990s (6.5 µS cm-1) (Williams et al. 1997Williams, M.R.; Fisher, T.R.; Melack, J.M. 1997. Chemical composition and deposition of rain in the central Amazon, Brazil. Atmospheric Environment 31: 207-217.), and in an open area in Manaus city (7.9 µS cm-1) (Honório et al. 2010Honório, B. a D.; Horbe, a. M.C.; Seyler, P. 2010. Chemical composition of rainwater in western Amazonia - Brazil. Atmospheric Research 98: 416-425.). Rainfall samples from other Amazonian cities showed small variations in the mean conductivity such as in Boa Vista, in Roraima state (4.4), and in Apuí (4.5), Tabatinga (4.8), Itapiranga (5.2) and Paritins (6.4) in Amazonas state (Honório et al. 2010) and in Rio Branco, in Acre state (6.7) (Duarte et al. 2013Duarte, A.F.; Gioda, A.; Ziolli, R.; Duó, D. 2013. Contaminación atmosférica y deposición húmeda en la Amazonia brasileña. Revista Cubana de Salud Publica 39: 627-639.).
Unpolluted rainfall normally has a pH between 5.0 and 5.5, which is considered slightly acidic (EPA 2020EPA. 2020. Acid Rain. United States Environmental Protection Agency. ( (https://www3.epa.gov/acidrain/education/site_students/phscale.html#:~:text=Normal%2C clean rain has a,a pH value of 4.0 .). Accessed on 15 Dec 2022.
https://www3.epa.gov/acidrain/education/...
). Our samples showed pH within the normal range in both seasons, with no statistical difference between the seasons. This suggests that the anthropogenic influence is small in the study site.
Similarly to our study, an average TOC of 4.2 mg L-1 was measured in rainwater collected from 2005 to 2010 in the southwestern state of Acre, with a higher concentration in the dry than in the wet season (Duarte et al. 2013Duarte, A.F.; Gioda, A.; Ziolli, R.; Duó, D. 2013. Contaminación atmosférica y deposición húmeda en la Amazonia brasileña. Revista Cubana de Salud Publica 39: 627-639.). In Mato Grosso state (also in the southwest) an average TOC of 6.7 mg L-1 (15.6 and 3.3 mg L-1 at the beginning of the dry and rainy season, respectively) was reported 2007-2008 (Neu et al. 2016Neu, V.; Ward, N.D.; Krusche, A. V.; Neill, C. 2016. Dissolved organic and inorganic carbon flow paths in an amazonian transitional forest. Frontiers in Marine Science 3: 114. doi: 10.3389/fmars.2016.00114
https://doi.org/10.3389/fmars.2016.00114...
). Higher TOC concentrations in the dry season are due to the accumulation of carbon-rich PM in the atmosphere when there is minimal rainfall, while PM is depleted during rainfall events in the rainy, because a fraction of particulate organic matter (POM) is solubilized (Monteith et al. 2007Monteith, D.T.; Stoddard, J.L.; Evans, C.D.; de Wit, H.A.; Forsius, M.; Høgåsen, T.; et al. 2007. Dissolved organic carbon trends resulting from changes in atmospheric deposition chemistry. Nature 450: 537-540.). Correspondingly, our results suggest that TOC is accumulated during the dry season and depleted during subsequent rainfall events.
The overall contribution of cations and anions to the ionic strength in our samples indicate dominance of alkaline components, especially NH4 +, Na+, and K+. The average concentrations of ions were of the same order of magnitude as those reported in Lake Calado, 80 km west of Manaus in 1984 (0.7 µmol L-1 for K+ to 9.0 µmol L-1 for SO4 2-) (Lesack and Melack 1991Lesack, L.F.W.; Melack, J.M. 1991. The deposition, composition, and potential sources of major ionic solutes in rain of the central Amazon Basin. Water Resources Research 27: 2953-2977.) and in 1990 (0.8 µmol L-1 for K+ to 4.6 µmol L-1 for Cl- (Williams et al. 1997Williams, M.R.; Fisher, T.R.; Melack, J.M. 1997. Chemical composition and deposition of rain in the central Amazon, Brazil. Atmospheric Environment 31: 207-217.). However, the concentrations were two to three times higher than in this study when measured in an open area in Manaus city (2.1 µmol L-1 for Mg2+ to 17 µmol L-1 for 17.1 µmol L-1 for Ca2+) (Honório et al. 2010Honório, B. a D.; Horbe, a. M.C.; Seyler, P. 2010. Chemical composition of rainwater in western Amazonia - Brazil. Atmospheric Research 98: 416-425.). As expected, anthropogenically impacted areas present higher concentrations of some ions. The lack of more recent studies makes it difficult to assess the trend in concentrations.
NH4 + is present even in air considered to be unpolluted, because of natural chemical and biochemical processes (Spataru 2022Spataru, P. 2022. Influence of organic ammonium derivatives on the equilibria between NH4+, NO2− and NO3− ions in the Nistru River water. Scientific Reports 12: 13505.). However, like NO3 - and SO4 2-, NH4 + is a main component of secondary aerosols and has a share coming from anthropic activities (Migliavacca et al. 2004Migliavacca, D.; Teixeira, E.C.; Pires, M.; Fachel, J. 2004. Study of chemical elements in atmospheric precipitation in South Brazil. Atmospheric Environment 38: 1641-1656.). The presence of SO4 2- and NO3 - mainly indicates anthropogenic origin due to emissions of SO2 and NOx gases through the burning of fossil fuels and industries (contribution received from long distances by winds) (Sudalma et al. 2015Sudalma, S.; Purwanto, P.; Santoso, L. 2015. The effect of SO2 and NO2 from transportation and stationary emissions sources to SO42− and NO3− in rain water in Semarang. Procedia Environmental Sciences 23: 247-252. ). Furthermore, the association between these three ions shows that the aerosols NH4NO3 and (NH4)2SO4 were generated through chemical reactions of NH3 with HNO3 and H2SO4.
We recorded an excess of cations relative to anions, which is compatible with other studies (Galloway et al. 1982Galloway, J.N.; Likens, G.E.; Keene, W.C.; Miller, J.M. 1982. The composition of precipitation in remote areas of the world. Journal of Geophysical Research: Oceans 87: 8771-8786.; Williams et al. 1997Williams, M.R.; Fisher, T.R.; Melack, J.M. 1997. Chemical composition and deposition of rain in the central Amazon, Brazil. Atmospheric Environment 31: 207-217.) with an anion deficit of 14 µmol L-1. It is worth highlighting the average contribution of acetic and formic acids with 2.7 µmol L-1 and inorganic acids (Cl-+NO3 -+SO4 2-) with 7.4 µmol L-1. Overall, the mean concentration of major ions was in the same range as reported around the same period for a nearby location with influence of the plume from Manaus and road emissions (Pauliquevis et al. 2012Pauliquevis, T.; Lara, L.L.; Antunes, M.L.; Artaxo, P. 2012. Aerosol and precipitation chemistry measurements in a remote site in Central Amazonia: the role of biogenic contribution. Atmospheric Chemistry and Physics 12: 4987-5015.), although some species such as acetic acid, Ca2+, and Mg2+ had higher concentrations. Because it is 1,200 km from the Atlantic coast, CBR has a low VWM for Na+ when compared to regions of direct marine influence, which is owed to the very high rainfall rate in the central Amazon, resulting in the efficient removal of the sea salt component (Pauliquevis et al. 2012).
Although there was no significant difference in VWM concentration for some ions, the deposition in the dry season was higher, differing from another study in the same region and similar to results from the southwestern Amazon state of Rondônia around the same period (Artaxo et al. 2009Artaxo, P.; Rizzo, L. V.; Paixão, M.; de Lucca, S.; Oliveira, P.H.; Lara, L.L.; et al. 2009. Aerosol particles in Amazonia: Their composition, role in the radiation balance, cloud formation, and nutrient cycles. In: Keller, M.; Bustamente, M.; Gash, J.; Silva Dias, P. (Ed.). Amazonia and Global Change. American Geophysical Union, Geophysical Monograph Series # 186, p.233-250. ).
Wet deposition fluxes
A similar behavior to our wet deposition fluxes, with highest concentrations for Al, Fe and Zn, was observed in rainwater collected in six Amazonian cities (Honório et al. 2010Honório, B. a D.; Horbe, a. M.C.; Seyler, P. 2010. Chemical composition of rainwater in western Amazonia - Brazil. Atmospheric Research 98: 416-425.). This is expected, since Al, Fe, and Zn are generally the most abundant metals worldwide in aerosols, which are responsible for raindrop formation (Li et al. 2017Li, T.; Wang, Y.; Zhou, J.; Tao, W.; Ding, A.; Nie, W.; Xue, L.; Wang, X.; Wang, W. 2017. Evolution of trace elements in the planetary boundary layer in southern China: Effects of dust storms and aerosol-cloud interactions. Journal of Geophysical Research: Atmospheres 122: 3492-3506.). However, the average concentrations found in our samples (30-100 nmol L-1) were much lower than those in Honório et al. (2010) (300-950 nmol L-1). This indicates that the region in this study is less affected in terms of atmospheric pollutants than the cities located in the central regions of the Amazon, where air quality is poorer.
For most elements higher concentrations occurred during the dry season, mainly Al, Mn, Co, Cu, Rb, Cd and Ba, which had ratios more than two times higher than in the wet season. Most of the metals were enriched in the rainfall, suggesting that their charges originate almost entirely from local and long-range anthropogenic sources (Da Silva et al. 2020Da Silva, G.N.; Alves, L.D.; Dos Santos, I.E.; Bila, D.M.; Júnior, A.O.; Corrêa, S.M. 2020. An assessment of atmospheric deposition of metals and the physico - chemical parameters of a rainwater harvesting system in Rio de Janeiro Brazil, by means of statistical multivariate analysis. Ambiente & Água 15: 1-31. doi.org/10.4136/ambi-agua.2522
https://doi.org/10.4136/ambi-agua.2522...
). The degree of enrichment depends on the type, proximity, and extent of individual sources. Atmospheric Co can be attributed to coal combustion, mining activities, and automobile traffic, in a mix of crustal and anthropogenic factors (Uygur et al. 2010Uygur, N.; Karaca, F.; Alagha, O. 2010. Prediction of sources of metal pollution in rainwater in Istanbul, Turkey using factor analysis and long-range transport models. Atmospheric Research 95: 55-64.). Diesel powered machinery, manure, pesticide and fertilizer application in agriculture, and sewage wastes cause entry of Cd into the soil (Tabelin et al. 2018Tabelin, C.B.; Igarashi, T.; Villacorte-tabelin, M.; Park, I.; Opiso, E.M.; Ito, M.; et al. 2018. Science of the Total Environment Arsenic, selenium, boron, lead, cadmium, copper, and zinc in naturally contaminated rocks: A review of their sources, modes of enrichment, mechanisms of release, and mitigation strategies. Science of the Total Environment 645: 1522-1553.; Hocaoglu-ozyigit and Genc 2020Hocaoglu-ozyigit, A.; Genc, B.N. 2020. Cadmium in plants, humans and the environment. Frontiers in Life Sciences and Related Technologies 1: 12-21.). This metal in rainwater is a reliable fingerprint for evaluating aerial pollution (Adegunwa et al. 2019Adegunwa, A.O.; Adebiyi, F.M.; Asubiojo, O.I. 2019. Heavy metal analysis in rainwaters as indicator of aerial pollution. Research and Reviews: Journal of Environmental Sciences 1: 1-13. ). No clear trend of increase or decrease in the concentration of these elements was observed over the years of study. EF values may result in metals, their pollution loads, as well as speciation forms of trace elements (Gopal et al. 2023Gopal, V.; Ramasamy, R.; Vignesh, R. 2023. Assessment of heavy metal contamination in the surface sediments of the Vedaranyam coast, Southern India. Regional Studies in Marine Science 65: 103081.).
Origin of pollutants
The rainwater in the Amazon has a chemically heterogeneous composition, which is driven primarily by the continuous cycle of rainfall and evaporation (Honório et al. 2010Honório, B. a D.; Horbe, a. M.C.; Seyler, P. 2010. Chemical composition of rainwater in western Amazonia - Brazil. Atmospheric Research 98: 416-425.). Some íons such as Cl- and Na+ are predominantly from the ocean, while Mg2+, K+, and Ca2+ may have marine, soil or anthropogenic origin. K+ and Ca2+ presented similar profiles, originated predominantly from anthropogenic sources (90%), followed by seawater (7%) and soil (3%). Sr, Rb, V, Mn, Co, and Ba enrichment may be ascribed to a terrestrial or biogenic influence related to the Sahara dust (Honório et al. 2010). A small fraction of Sr also could be related to seawater (Lebrato et al. 2020Lebrato, M.; Garbe-Schönberg, D.; Müller, M.N.; Blanco-Ameijeiras, S.; Feely, R.A.; Lorenzoni, L.; et al. 2020. Global variability in seawater Mg:Ca and Sr:Ca ratios in the modern ocean. Proceedings of the National Academy of Sciences 117: 22281-22292.). The local biomass burning and forest fire emission during the dry season alters profoundly the composition of the atmosphere in most of the Amazon because higher amounts (until 30 times higher compared to the rainy season) of particles and gases are emitted (Artaxo et al. 2005Artaxo, P.; Gatti, L. V.; Leal, A.M.C.; Longo, K.M.; Freitas, S.R. De; Lara, L.L.; et al. 2005. Química atmosférica na Amazônia: a floresta e as emissões de queimadas controlando a composição da atmosfera amazônica. Acta Amazonica 35: 185-196.). As the Amazon is heavily influenced by forest burning, K+ and Cl- content, as well as higher enrichment of Cu, Pb, Zn, and Sb in our study, is probably influenced by fires (Yamasoe et al. 2000Yamasoe, M. a.; Artaxo, P.; Miguel, A.H.; Allen, A.G. 2000. Chemical composition of aerosol particles from direct emissions of vegetation fires in the Amazon Basin: water-soluble species and trace elements. Atmospheric Environment 34: 1641-1653.). As the city of Manaus lies 60 km away from CBR, its rapid urban development and major use of fossil fuel likely contributes to the increase in the chemical load of rainwater in the study area. The dominant species of fossil fuel originate from transformation of the precursors NOx and SO2 (Lin 2020Lin, W. 2020. Change in SO42-, NO3- and NH4+ levels in PM2.5 in Beijing from 1999 to 2016. Environmental Science Current Research 3: 1-9. doi:10.24966/ESCR-5020/100018
https://doi.org/10.24966/ESCR-5020/10001...
). This increase may explain the NO3 - and SO4 2- content, as well as the more acidic pH in the dry season. The intensification of positive anthropogenic emission sources results in the acidification of the samples. However, SO4 2- may have part of its origin in seawater, as this ion is found in the form of CaSO4 and MgSO4.
Regarding natural sources, neutralization occurs through the presence of alkaline species in the atmosphere such as Ca2+ and Mg2+, derived mostly from soil dust and resuspension, and NH4 +, mainly released from wastewater treatment, animal waste and fertilizers, and K+ from biomass burning (Tiwari et al. 2016Tiwari, S.; Hopke, P.K.; Thimmaiah, D.; Dumka, U.C.; Srivastava, A.K.; Bisht, D.S.; et al. 2016. Nature and sources of ionic species in precipitation across the Indo-Gangetic Plains, India. Aerosol and Air Quality Research 16: 943-957.). Differently from SO2 and NO2, NH3 reacts rapidly with acidic components to form NH4 + in the source area and is not transported over long distances (Asman et al. 1998Asman, W.A.H.; Sutton, M.A.; Schjorring, J.K. 1998. Ammonia: emission, atmospheric transport and deposition. New Phytologist 139: 27-48.). NH4 + acts as a basic agent neutralizing HNO3 (Alves et al. 2007Alves, C.; Pio, C.; Campos, E.; Barbedo, P. 2007. Size distribution of atmospheric particulate ionic species at a coastal site in Portugal. Quimica Nova 30: 1938-1944.). Since the main economic activities in the region of CBR are agriculture (cassava, sugar cane, rice, beans, and fruits), livestock (cattle, equine, goats, and pigs), mineral extraction, and poultry farming (EcuRed 2017), ammonium sources in this area may be related to anaerobic digestion of animal and human waste and the use of agricultural fertilizers.
Finally, the biogenic factor was formed by HCOO-, although these organic ions were underestimated. Some ions may have been more underestimated than others. Ca2+ and Mg2+ are markers of local calcareous soils and suspended road dust (Rao et al. 2016Rao, P.S.P.; Tiwari, S.; Matwale, J.L.; Pervez, S.; Tunved, P.; Safai, P.D.; et al. 2016. Sources of chemical species in rainwater during monsoon and non-monsoonal periods over two mega cities in India and dominant source region of secondary aerosols. Atmospheric Environment 146: 90-99.). Suspended soil dust, vegetation, and the metabolism of microorganisms constituted the main natural sources of these acids in the atmosphere.
The PMF analysis flagged six possible sources of the chemical species found in the rainwater. Na± and K+ indicate the contribution of sea salt and/or fungal spores and biomass burning (factor 5) to the rainwater composition. Sodium salts in the Amazon Basin are mainly attributed to marine aerosols transported from the Atlantic Ocean (Moran-Zuloaga et al. 2018Moran-Zuloaga, D.; Ditas, F.; Walter, D.; Saturno, J.; Brito, J.; Carbone, S.; et al. 2018. Long-term study on coarse mode aerosols in the Amazon rain forest with the frequent intrusion of Saharan dust plumes. Atmospheric Chemistry and Physics 18: 10055-10088.). However, it has been demonstrated that fungal spores can account for 69% of the total sodium mass during the wet season (China et al. 2018China, S.; Burrows, S.M.; Wang, B.; Harder, T.H.; Weis, J.; Tanarhte, M.; et al. 2018. Fungal spores as a source of sodium salt particles in the Amazon basin. Nature Communications 9: 4793. doi.org/10.1038/s41467-018-07066-4
https://doi.org/10.1038/s41467-018-07066...
). The analysis managed to separate the part of Na± originating from sea salts (factor 4) from that originating from fungi (factor 5). The proportion of Na± in fungal spores coincided with that of the study (China et al. 2018). Besides, the Brazilian Amazon is yearly impacted by forest fires that produce large amounts of smoke particles that linger in the atmosphere and affect local health of the population, which, in small quantities, can contribute to Na± and K± levels (De Oliveira Alves et al. 2015).
The significant concentration of NH4 + and NO3 - obtained are indicators of anthropic influence. Long-range transport likely increased the concentrations of these ions, as biomass fires are frequent during the dry season throughout Amazonia, and air masses from the city of Manaus may have reached the sampling site.
There are no recent studies published on the chemical composition of rainwater in the CBR, so it is difficult to draw a scenario. But, considering that the population in Manaus went from 1.8 million in 2010 (final collection period) to 2.1 million in 2024 and that the vehicle fleet went from 452 thousand vehicles (2010) to 855 thousand (2024), we can infer that pollution has increased considerably. Therefore, there is a tendency for the chemical composition of rainwater in CRB to also be influenced. Our findings corroborate the complex composition of rainwater in the Amazon, influenced by a mixture of different local and distant sources, and continuous cycles of rainfall and evaporation in this region.
CONCLUSIONS
The assessment of the chemical composition of rainwater from the Cueiras Biological Reserve, in central Amazonia, a primary forest 60 km from a large urban center, indicated a highly heterogeneous composition, reflecting the contribution mainly from crustal and biogenic emissions. However, sources such as biomass burning, as well as long-distance Saharan dust and fossil fuel combustion in the nearby urban center were also observed. The characteristic continuous cycles of evaporation and rainfall in the region also played a role. The high concentration of chemical components in the dry season were owed directly to the winds blowing from the interior of the country to the Amazon region between May and October, together with the intensification of the biomass burning. The concentration of elements of anthropogenic origin was detected even in places far from the Amazon. This is the longest monitoring of the chemical composition of rainfall ever carried out at CBR. Continuous monitoring of rainfall is essential for several reasons, including providing data to analyze weather patterns and changes in the climate, in addition to detecting the presence of pollutants in rainwater, which is important for public health and the environment.
ACKNOWLEDGMENTS
This work was carried out with the support of Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Brazil - Financing Code 001. The authors acknowledge the support received from Fundação de Amparo à pesquisa do Estado do Rio de Janeiro (FAPERJ), and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Brazil. Adriana Gioda thanks FAPERJ (Auxílio Cientista do Nosso Estado) and CNPq (Bolsa de Produtividade).
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CITE AS:
De La Cruz, A.R.H.; Pedreira, M.F. de S.; Godoy, J.M.; Artaxo, P.; Gioda, A. 2024. Chemical characterization and source apportionment of rainwater in Cuieiras Biological Reserve, central Amazon, Brazil. Acta Amazonica 54: e54es23131.
Data availability
The data that support the findings of this study are not publicly available.
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Publication Dates
-
Publication in this collection
30 Sept 2024 -
Date of issue
Jul-Sep 2024
History
-
Received
21 May 2023 -
Accepted
22 May 2024