Modulation of PM2.5 in tropical South America by the upper-level atmospheric circulation and El Nino-Southern Oscillation phases

In the Amazon basin, biomass burning has been identified as a major cause of poor regional air quality and the dominant source of particulate matter (PM). In this study, we analyse the impact of the upper-level jet on PM2.5 (PM with an aerodynamic diameter <= 2.5 mu m) concentrations in tropical South America (SA) from December to February during the period 2003-2022. Furthermore, we investigate the response of air pollutants to the joint modulation by the upper-level jet and El Nin similar to o-Southern Oscillation (ENSO). We find that PM2.5 concentrations in north-eastern Brazil are reduced on days when the subtropical jet (STJ) is absent due to enhanced convection and precipitation over the region. This improvement in air quality is independent of the ENSO phase. Conversely, a prominent STJ inhibits convection and contributes to dry conditions that favour increased biomass burning and elevated pollutant concentrations. Furthermore, the co-occurrence of a prominent STJ with an El Nin similar to o phase acts synergistically to increase pollutant concentrations, as both reduce precipitation in north-eastern Brazil. In combination with La Nin similar to a, this upper-level pattern does not exert any modulation of the PM2.5 concentrations, as the wet conditions favoured by this ENSO phase prevail to reduce biomass burning.