Wet deposition of major ions in a rural area impacted by biomass burning emissions

This work concerns the influence of industrialized agriculture in the tropics on precipitation chemistry. A total of 264 rain events were sampled using a wet-only collector in central São Paulo State, Brazil, between January 2003 and July 2007. Electroneutrality balance calculations (considering H+, K+, Na+, NH 4 + , Ca2+, Mg2+, Cl−, NO 3 − , SO 4 2 − , F−, PO 4 3 − , H3CCOO−, HCOO−, C 2 O 4 2 − and HCO 3 − ) showed that there was an excess of cations (∼15%), which was attributed to the presence of unmeasured organic anion species originating from biomass burning and biogenic emissions. On average, the three ions NH 4 + , NO3− and H+ were responsible for >55% of the total ion concentrations in the rainwater samples. Concentrations (except of H+) were significantly higher (t-test; P = 0.05), by between two to six-fold depending on species, during the winter sugar cane harvest period, due to the practice of pre-harvest burning of the crop. Principal component analysis showed that three components could explain 88% of the variance for measurements made throughout the year: PC1 (52%, biomass burning and soil dust resuspension); PC2 (26%, secondary aerosols); PC3 (10%, road transport emissions). Differences between harvest and non-harvest periods appeared to be mainly due to an increased relative importance of road transport/industrial emissions during the summer (non-harvest) period. lume-weighted mean (VWM) concentrations of ammonium (23.4 μmol L−1) and nitrate (17.5 μmol L−1) in rainwater samples collected during the harvest period were similar to those found in rainwater from São Paulo city, which emphasizes the importance of including rural agro-industrial emissions in regional-scale atmospheric chemistry and transport models. Since there was evidence of a biomass burning source throughout the year, it appears that rainwater composition will continue to be affected by vegetation fires, even after sugar cane burning is phased out as envisaged by recent São Paulo State legislation.