Biomass burning smoke transport and radiative impact over the city of Sao Paulo: An extreme event case study

Abstract. Biomass burning is a worldwide practice applied to deforestation which can have disastrous consequences to local and regional environments. This paper describes a case study of an extreme event of biomass burning smoke transport toward the São Paulo metropolitan area (MASP), on 19 August 2019, when the city experienced an uncommon completely dark sky around 3:00 PM. A synergy between air mass back trajectories, satellite retrieved aerosol fields and surface radiometric measurements was used to find the origin of the smoke plume affecting the city and to analyse the radiative impact of the transport of the smoke toward the city. Results showed that the MASP atmosphere was affected by the transport of a dense smoke plume with aerosol optical depth at 550 nm above 1. Air mass back trajectories and auxiliary data indicated that most of the smoke was emitted two days before arrival. The smoke plume in combination with clouds, associated with a frontal system, produced a strong radiative impact, as observed by a regional network of pyranometers. During the darkness day, the diurnal clearness index was below 0.1 in all five MASP stations and a maximum of the cloud optical depth higher than 300 was retrieved producing irradiances at surface dropped to 0 during approximately 40 minutes. The strong radiative efficiency (cloud radiative effect per cloud optical depth unit) of this extreme event, was 7 % higher than other overcast days observed in a two-year period.