Especiação química do carbono dissolvido na água de chuva de Araraquara (SP): variabilidade interanual e sazonal
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Data
2017-11-22
Autores
Silva, Daniely de Godoy
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Universidade Estadual Paulista (Unesp)
Resumo
No estado de São Paulo, desde 2006, a queima da palha de cana-de-açúcar para colheita manual foi gradativamente substituída pela colheita mecanizada. Apesar dessa mudança na prática agrícola, a queima de biomassa no Brasil, seja esta intencional ou acidental, ainda é intensa, contribuindo para a emissão de uma diversidade de espécies gasosas e de material particulado para a atmosfera. Uma característica particular do Brasil é a elevada utilização de etanol como combustível, sendo que essa demanda vem aumentando dentro e fora do país, ocasionando uma maior emissão atmosférica de etanol e de espécies orgânicas correlatas, com consequências ainda pouco conhecidas. Neste contexto, este trabalho apresenta o primeiro estudo ao longo de 13 anos (2004 a 2016) de carbono orgânico dissolvido (COD) na água de chuva das cidades de Araraquara e Ribeirão Preto, localizadas numa região tipicamente agroindustrial do estado de São Paulo. As concentrações de metanol, etanol, formaldeído, acetaldeído, ácido fórmico e ácido acético foram determinadas em água de chuva, a fim de melhor compreender suas fontes, as possíveis oscilações sazonais e ao longo dos anos. A média ponderada pelo volume (MPV) de COD em água de chuva foi de 288 ± 17 μmol C L-1. As concentrações MPV no período seco (safra; abril a novembro) foram de 373 ± 29 μmol C L-1 (n = 485) sendo esta significativamente maior (teste-t, P = 0,05) do que aquela obtida no período chuvoso que abrange os meses de dezembro a março (180 ± 12 μmol C L-1; n = 396). Apesar da eliminação progressiva da queima da palha de cana-de-açúcar no estado de São Paulo, o número de focos de incêndio continuou elevado, apresentando uma correlação positiva e significativa com a MPV de COD na água de chuva, sugerindo que a queima de biomassa é fonte predominante de carbono orgânico solúvel. Na maioria dos casos, a diluição do COD de acordo com a precipitação foi pequena, mostrando predominância desta espécie no processo de formação da nuvem. Em contraste, a maior parte do carbono orgânico dissolvido volátil (CODV) foi removida da atmosfera nos primeiros milímetros de chuva, mostrando um predomínio da remoção por lavagem abaixo da nuvem. O CODV contribuiu com uma fração significativa de COD para 62% das amostras analisadas (n = 552), variando de 5,1 a 488 μmol C L-1 (n = 340). O fluxo médio de deposição úmida de COD foi de 49 kg C ha-1 ano-1, sendo a concentração média de CODV responsável por 10% do total. Este fluxo de carbono dissolvido é maior do que a média mundial estimada (34 kg C ha-1 ano-1). O COD da água de chuva mostrou ser predominantemente lábil (75% em média) e rapidamente biodisponível (de dias a semanas), em contraste com o carbono dissolvido refratário encontrado na água da chuva com predomínio de combustão fóssil. Os resultados de carbono orgânico indicam que a queima de biomassa pode levar a importantes entradas atmosféricas de matéria orgânica prontamente disponível para deposição na terra e no oceano aberto. As amostras de água de chuva coletadas de agosto de 2013 a dezembro de 2016 em Araraquara apresentaram um valor MPV de pH de 5,7 (n = 122), sendo que 45% das amostras apresentaram valores menores que 5,6, indicando um excesso de acidez. As concentrações MPV das espécies orgânicas analisadas foram: 6,12 ± 0,68 μmol L-1 formaldeído (FA) (n = 135); 0,35 ± 0,03 μmol L-1 acetaldeído (AA) (n = 135); 16,4 ± 1,2 μmol L-1 metanol (n = 127); 4,49 ± 0,34 μmol L-1 etanol (n = 127); 5,9 ± 1,0 μmol L-1 ácido fórmico (n = 122); e 5,2 ± 1,3 μmol L-1 ácido acético (n = 122). O FA e ácido acético apresentaram variação sazonal com maior concentração no período seco em relação ao chuvoso possivelmente devido às emissões pela queima de biomassa. A variação sazonal da concentração de etanol e de AA não foi clara, sugerindo que a emissão veicular foi similar durante todo o ano. Ainda que tenha sido observada uma sazonalidade significativa entre a concentração de algumas espécies, fontes pontuais são de difícil identificação, havendo uma competição dinâmica e simultânea entre vários processos de emissão e de perdas, incluindo fontes biogênicas e antropogênicas e reações fotoquímicas. As espécies orgânicas determinadas neste trabalho foram correlacionadas com alguns íons inorgânicos majoritários, a fim de tentar identificar algumas fontes predominantes. O COD teve correlação significativa com todas as espécies estudadas, exceto com o AA, sugerindo múltiplas fontes. O etanol apresentou correlações significativas com FA e AA, sugerindo que a emissão veicular é uma fonte comum dessas espécies, confirmada pela correlação com os íons nitrato e sulfato.
In the state of São Paulo, since 2006, the burning of the sugar cane for manual harvesting has gradually been substituted by the mechanized harvest. Despite this change in agricultural practice, the biomass burning in Brazil, whether intentional or accidental, is still intense, contributing to the emission of a diversity of gaseous species and particulate matter into the atmosphere. A particular characteristic of Brazil is the intense use of ethanol as a fuel, and this demand has been increasing both inside and outside the country, resulting in a higher atmospheric emission of ethanol and correlates organic species, with consequences yet unknown. In this context, this work presents the first study over 13 years (2004 to 2016) of dissolved organic carbon (DOC) in the rainwater of the cities of Araraquara and Ribeirão Preto located in a typically agroindustrial region of the state of São Paulo. The concentrations of methanol, ethanol, formaldehyde, acetaldehyde, formic acid and acetic acid were determined in rainwater in order to better understand their sources, the possible seasonal fluctuations and yearly. The volume weighted average (VWA) of DOC in rainwater was 288 ± 17 μmol C L-1. VWA concentrations in the dry period (harvest; April to November) were 373 ± 29 μmol C L-1 (n = 485) and this was significantly higher (t-test, P = 0.05) than that obtained in the rainy season which covers the months from December to March (180 ± 12 μmol C L-1; n = 396). Despite the progressive elimination of the burning of sugar cane straw in the state of São Paulo, the number of fire spots continued high, presenting a positive and significant correlation with VWA DOC in rainwater, suggesting that the burning of biomass is the predominant source of soluble organic carbon. In most cases, DOC dilution according to rainfall height was small, showing predominance of this species in the cloud formation process. In contrast, most of the volatile dissolved organic carbon (VDOC) was removed from the atmosphere in the first millimeters of rain, showing a predominance of washout. VDOC contributed a significant fraction of DOC to 62% of the analyzed samples (n = 552), ranging from 5.1 to 488 μmol C L-1 (n = 340). The average wet deposition flux of DOC was 49 kg C ha−1 yr−1, with VDOC accounting for 10% of the total. This dissolved carbon flux is higher than the estimated world average (34 kg C ha−1 yr−1). The DOC of rainwater showed to be predominantly labile (75% on average) and rapidly bioavailable (from days to weeks), in contrast to the refractory dissolved carbon found in rainwater with a predominance of fossil fuel combustion. The findings of this work indicate that biomass burning can lead to important atmospheric inputs of readily available organic matter to land and to the open ocean. Rainwater samples collected from August 2013 to December 2016 in Araraquara had an VWA pH value of 5.7 (n = 122), and 45% of the samples presented values lower than 5.6, indicating an excess of acidity. The MPV concentrations of the organic species analyzed were: 6.12 ± 0.68 μmol L-1 formaldehyde (FA) (n = 135); 0.35 ± 0.03 μmol L-1 acetaldehyde (AA) (n = 135); 16.4 ± 1.2 μmol L-1 methanol (n = 127); 4.49 ± 0.34 μmol L-1 ethanol (n = 127); 5.9 ± 1.0 μmol L-1 formic acid (n = 122); and 5.2 ± 1.3 μmol L-1 acetic acid (n = 122). The FA and acetic acid showed seasonal variation with a higher concentration in the dry period in relation to the rainy season, possibly due to the emissions by the biomass burning. The seasonal variation of ethanol and AA concentration was not clear, suggesting that vehicular emission was similar throughout the year. Although a significant seasonality has been observed among the concentrations of some species, point sources are difficult to identify, and there is a simultaneous and dynamic competition between several emission and loss processes, including biogenic and anthropogenic sources and photochemical reactions. The organic species determined in this work were correlated with some major inorganic ions, in order to try to identify some predominant sources. COD had a significant correlation with all species studied except AA, suggesting multiple sources. Ethanol had significant correlations with FA and AA, suggesting that the vehicular emission is a common source of these species, confirmed by the correlation with the nitrate and sulfate ions.
In the state of São Paulo, since 2006, the burning of the sugar cane for manual harvesting has gradually been substituted by the mechanized harvest. Despite this change in agricultural practice, the biomass burning in Brazil, whether intentional or accidental, is still intense, contributing to the emission of a diversity of gaseous species and particulate matter into the atmosphere. A particular characteristic of Brazil is the intense use of ethanol as a fuel, and this demand has been increasing both inside and outside the country, resulting in a higher atmospheric emission of ethanol and correlates organic species, with consequences yet unknown. In this context, this work presents the first study over 13 years (2004 to 2016) of dissolved organic carbon (DOC) in the rainwater of the cities of Araraquara and Ribeirão Preto located in a typically agroindustrial region of the state of São Paulo. The concentrations of methanol, ethanol, formaldehyde, acetaldehyde, formic acid and acetic acid were determined in rainwater in order to better understand their sources, the possible seasonal fluctuations and yearly. The volume weighted average (VWA) of DOC in rainwater was 288 ± 17 μmol C L-1. VWA concentrations in the dry period (harvest; April to November) were 373 ± 29 μmol C L-1 (n = 485) and this was significantly higher (t-test, P = 0.05) than that obtained in the rainy season which covers the months from December to March (180 ± 12 μmol C L-1; n = 396). Despite the progressive elimination of the burning of sugar cane straw in the state of São Paulo, the number of fire spots continued high, presenting a positive and significant correlation with VWA DOC in rainwater, suggesting that the burning of biomass is the predominant source of soluble organic carbon. In most cases, DOC dilution according to rainfall height was small, showing predominance of this species in the cloud formation process. In contrast, most of the volatile dissolved organic carbon (VDOC) was removed from the atmosphere in the first millimeters of rain, showing a predominance of washout. VDOC contributed a significant fraction of DOC to 62% of the analyzed samples (n = 552), ranging from 5.1 to 488 μmol C L-1 (n = 340). The average wet deposition flux of DOC was 49 kg C ha−1 yr−1, with VDOC accounting for 10% of the total. This dissolved carbon flux is higher than the estimated world average (34 kg C ha−1 yr−1). The DOC of rainwater showed to be predominantly labile (75% on average) and rapidly bioavailable (from days to weeks), in contrast to the refractory dissolved carbon found in rainwater with a predominance of fossil fuel combustion. The findings of this work indicate that biomass burning can lead to important atmospheric inputs of readily available organic matter to land and to the open ocean. Rainwater samples collected from August 2013 to December 2016 in Araraquara had an VWA pH value of 5.7 (n = 122), and 45% of the samples presented values lower than 5.6, indicating an excess of acidity. The MPV concentrations of the organic species analyzed were: 6.12 ± 0.68 μmol L-1 formaldehyde (FA) (n = 135); 0.35 ± 0.03 μmol L-1 acetaldehyde (AA) (n = 135); 16.4 ± 1.2 μmol L-1 methanol (n = 127); 4.49 ± 0.34 μmol L-1 ethanol (n = 127); 5.9 ± 1.0 μmol L-1 formic acid (n = 122); and 5.2 ± 1.3 μmol L-1 acetic acid (n = 122). The FA and acetic acid showed seasonal variation with a higher concentration in the dry period in relation to the rainy season, possibly due to the emissions by the biomass burning. The seasonal variation of ethanol and AA concentration was not clear, suggesting that vehicular emission was similar throughout the year. Although a significant seasonality has been observed among the concentrations of some species, point sources are difficult to identify, and there is a simultaneous and dynamic competition between several emission and loss processes, including biogenic and anthropogenic sources and photochemical reactions. The organic species determined in this work were correlated with some major inorganic ions, in order to try to identify some predominant sources. COD had a significant correlation with all species studied except AA, suggesting multiple sources. Ethanol had significant correlations with FA and AA, suggesting that the vehicular emission is a common source of these species, confirmed by the correlation with the nitrate and sulfate ions.
Descrição
Palavras-chave
Chuvas, Compostos orgânicos, Aldeídos, Álcoois, Ácidos carboxílicos