Major Regional-Scale Production of O₃ and Secondary Organic Aerosol in Remote Amazon Regions from the Dynamics and Photochemistry of Urban and Forest Emissions

Abstract

The Amazon rainforest suffers increasing pressure from anthropogenic activities. A key aspect not fully understood is how anthropogenic atmospheric emissions within the basin interact with biogenic emissions and impact the forest’s atmosphere and biosphere. We combine a high-resolution atmospheric chemical transport model with an improved emissions inventory and in-situ measurements to investigate a surprisingly high concentration of ozone (O₃) and secondary organic aerosol (SOA) 150–200 km downwind of Manaus city in an otherwise pristine forested region. We show that atmospheric dynamics and photochemistry determine a gross production of secondary pollutants seen in the simulation. After sunrise, the erosion of the nocturnal boundary layer mixes natural forest emissions, rich in biogenic volatile organic compounds, with a lofted pollution layer transported overnight, rich in nitrogen oxides and formaldehyde. As a result, O₃ and SOA concentrations greater than ∼47 ppbv and 1.8 μg m⁻³, respectively, were found, with maximum concentrations occurring at 2 pm LT, 150–200 km downwind of Manaus city. These high concentrations affect a large primary forested area of about 11,250 km². These oxidative areas are under a NOx-limited regime so that changes in NOx emissions from Manaus have a significant impact on O₃ and SOA production.