Physical-chemical interaction of Manaus' urban emissions with Amazon forest biogenic emissions: the secondary formation of aerosols in Central Amazon

Abstract:

Surrounded by more than a thousand kilometer of almost pristine Amazon rainforest, Manaus is a metropolis of 2 million inhabitants that has a growing fleet of vehicles and is a symbol of a fast urbanization process. The interaction between urban emissions and pristine biogenic emissions of trace gases and aerosol was the subject of the GoAmazon 2014/5 experiment. Biogenic secondary organic aerosols (BSOA) in the wet season in Central Amazonia comprises most of the PM₁ mass, but, due to very low background concentrations, it’s physical-chemical properties are highly susceptible to change. Understand aerosol sources, processing mechanisms and optical properties is a challenge partly due to the complexity of biogenic and anthropogenic emissions interactions. Aerosols were collected at the 325 meters pristine Amazon Tall Tower Observatory (ATTO; Site T0), northeast of Manaus, at the center of the city of Manaus (T1), and also 17 Km downwind (T2). Here we focus on how fast the urban emissions inside the rainforest atmosphere change the aerosol composition and physical properties from a clean site, to a polluted urban city in the tropics to a rapidly oxidized aerosol downwind site. From Manaus downtown (T1) to 17 Km downwind (T2) it has been found a net growth of particle number concentration of 38%. Organic aerosol (OA) showed a quick large enhancement of 164% and fine mode nitrate showed an enhancement of 200% in PM₁ mass concentration. Aerosol size distribution shows a rapidly shift from nucleation dominated at T1 to Aitken dominated at T2. Single Scattering Albedo in the wet season is 0.93 at 637 nm at the ATTO pristine site, and the effect of the absorbing aerosol from Manaus is huge in optical properties: turns SSA into 0.60 at T1 and 0.66 at T2. The evolution also shows substantial signal of processing, O:C ratio of OA has more than doubled from 0.27 to 0.65 from the same sites. Also, a large development of O₃ concentration of 150% has been found between the sites. These findings show that the photo-chemical processes in a tropical rainforest, enhanced by its natural conditions of high radiation rate, water vapor and temperature, can rapidly alter the profile of urban emissions. The results of this work are a strong impact of urbanization on air quality and aerosol properties in tropical urban areas.