A 13-year study of dissolved organic carbon in rainwater of an agro-industrial region of São Paulo state (Brazil) heavily impacted by biomass burning

Abstract

This work presents the first comprehensive study of DOC in rainwater in a tropical agro-industrial region in central São Paulo State. The DOC concentrations ranged from 15 to 4992 μmol C L− 1, with an overall volume weighted mean (VWM) of 288 ± 17 μmol C L− 1 (n = 881). The number of fire spots accumulated within each year of this study was positively correlated to the VWM concentration of DOC in rainwater. During the whole study period, higher VWM DOC concentrations were found during the dry months, despite the phasing out of agricultural fires in sugar cane plantations. The evidence suggested that inputs of atmospheric soluble organic carbon from biomass burning exceeded those from vehicular fuel combustion and biogenic sources. In most cases, dilution of DOC according to precipitation volume was minimal, showing that in-cloud processes were dominant for this species. In contrast, most of the volatile dissolved organic carbon (VDOC) appeared to be removed from the atmosphere in the first milliliter or so of rain, showing a dominance of below-cloud scavenging. VDOC contributed a significant fraction of the DOC for 62% of the samples analyzed, ranging from 5.1 to 488 μmol C L− 1 (n = 552). 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 in the rainwater was mostly labile (75% on average) and rapidly bioavailable (within days to weeks), in contrast to refractory dissolved carbon found in rainwater from regions where fossil fuel combustion is the dominant source. 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.