Physical and chemical characterization of the 2019 “black rain” event in the Metropolitan Area of São Paulo, Brazil

Abstract Aerosols emitted from biomass burning in South American tropical forests have been a concern in the last decades. On August 19th, 2019 darkened precipitation was observed over the metropolitan area of Sao Paulo (MASP), in an unprecedented event termed “black rain”, after intense biomass burning episodes in the South American hinterland. Satellite imagery, back trajectory analyses, and meteorological reports showed air masses transporting thick plumes of biomass burning aerosols originated in part from the Amazon Basin and Bolivia. Rainwater samples were collected in different sites of the metropolitan area, during and after the “black rain” event, and both physically and chemically characterized to assess the possible influence of biomass burning aerosols in the event. The collected samples in the “black rain” event presented high turbidity (above 70 NTU), and biomass burning organic tracers (levoglucosan, mannosan, and galactosan) were observed in higher concentrations in the “black rain” samples than in control ones (e.g. average levoglucosan of 0.33 μg mL-1, compared to 0.02 μg mL-1 after the event), with deposition fluxes more than three times higher during the event (1.04 mg m-2 and 0.31 mg m-2, respectively). The detection of glucose, xylose, and mannose, after hydrolysis of the solid material present in the “black rain” samples, strongly suggested the presence of plant cell wall material derived from the partial combustion of wood and grass matter. Total polycyclic aromatic hydrocarbons (PAHs) concentrations were, in general, higher during the event than in post-event control samples collected in the MASP (on average, almost 15 times higher than after the event), with a higher deposition flux (5.1 and 2.7 mg m-2, respectively). Overall, there are strong pieces of evidence that the long-range transport of smoke produced in South American forest fires was connected to the precipitation of darkened rainwater over the Metropolitan Area of Sao Paulo.