Contribution of long and mid-range transport on the sodium and potassium concentrations in rainwater samples, Sمo Paulo megacity, Brazil

The surface winds and backward trajectories were analyzed to investigate sources of sodium and potassium ions measured in rainwater in the metropolitan area of São Paulo (MASP), Brazil, from July-2002 to December-2005. In this study, we evaluated the chemical composition of 315 rainwater samples with focus on precipitation events with high concentrations of sodium and potassium, with cations concentrations, at least 1.5 times higher than the sulfate and nitrate concentrations. In the results from the total samples, the Na+ and K+ volume weighted mean (VWM) concentrations were 10.4 and 3.2 μmol L−1, respectively, while for the excess events, the concentrations were 32.4 and 11.1 μmol L−1. From the 31 sodium excess events, 45% showed an influence from ocean air masses by the wind direction evaluation while 57% of backward trajectories for 7 potassium excess events indicated a contribution of biomass burning air masses from the northwestern region of the São Paulo State. Although some excess events pointed the pattern of long-range transport, they represented less than 15% of total samples. The results suggest that the megacity itself is predominantly involved in the process of emission and removal of the compounds and the influence of external components are limited to circumstantial events.