Temporal variation of ethanol in rainwater from the sugar cane belt of São Paulo State (Brazil)

Abstract

This work presents the first comprehensive assessment of ethanol dissolved in rainwater of São Paulo State's sugar cane belt, considering the period from 2012 to 2017. Ethanol concentrations ranged from 0.25 to 20.9 μmol L−1, with a volume-weighted mean (VWM) of 4.67 ± 0.13 μmol L−1 (n = 410). Despite the increasing use of ethanol fuel in Brazil, during the study period, the ethanol concentration in rainwater showed an average decrease of 0.54 μmol L−1 per year, attributed to technological improvements in vehicular fuel combustion and catalysts. Anthropogenic emissions appeared to overwhelm possible seasonal fluctuations of ethanol in rainwater due to biogenic sources or meteorological conditions such as temperature, solar radiation, rainfall amount, rainfall rate, and air mass origin. Higher VWM concentrations were generally observed in dry months, when the production of ethanol in the distilleries was highest and biomass burning increased. Consideration of ethanol oxidation in aqueous media by OH radicals indicated that secondary formation of highly toxic aldehydes is likely to increase with greater use of this biofuel. Dissolution of gaseous ethanol during cloud formation appeared to be a more important mechanism of atmospheric removal of this alcohol, compared to below-cloud processes. The wet flux of ethanol was ca. 3 kg ha−1 year−1 and there were evidences that atmospheric replenishment of this species was fast. Brazil has used ethanol fuel on a large scale since the 1980s, so better understanding of the behavior of this species in the lower troposphere of the study region can provide valuable information to assist in predicting future worldwide impacts of increasing use of this biofuel.