Abstract
Abstract. The Green Ocean Amazon experiment – GoAmazon 2014–2015 – explored the interactions between natural biogenic forest emissions from central Amazonia and urban air pollution from Manaus. Previous GoAmazon 2014–2015 studies showed that nitrogen oxide (NOx = NO + NO2) and sulfur oxide (SOx) emissions from Manaus strongly interact with biogenic volatile organic compounds (BVOCs), affecting secondary organic aerosol (SOA) formation. In previous studies, ground-based and aircraft measurements provided evidence of SOA formation and strong changes in aerosol composition and properties. Aerosol optical properties also evolve, and their impacts on the Amazonian ecosystem can be significant. As particles age, some processes, such as SOA production, black carbon (BC) deposition, particle growth and the BC lensing effect change the aerosol optical properties, affecting the solar radiation flux at the surface. This study analyzes data and models SOA formation using the Weather Research and Forecasting with Chemistry (WRF-Chem) model to assess the spatial variability in aerosol optical properties as the Manaus plumes interact with the natural atmosphere. The following aerosol optical properties are investigated: single scattering albedo (SSA), asymmetry parameter (gaer), absorption Ångström exponent (AAE) and scattering Ångström exponent (SAE). These simulations were validated using ground-based measurements at three experimental sites, namely the Amazon Tall Tower Observatory – ATTO (T0a), downtown Manaus (T1), Tiwa Hotel (T2) and Manacapuru (T3), as well as the U.S. Department of Energy (DOE) Gulfstream 1 (G-1) aircraft flights. WRF-Chem simulations were performed over 7 d during March 2014. Results show a mean biogenic SOA (BSOA) mass enrichment of 512 % at the T1 site, 450 % in regions downwind of Manaus, such as the T3 site, and 850 % in areas north of the T3 site in simulations with anthropogenic emissions. The SOA formation is rather fast, with about 80 % of the SOA mass produced in 3–4 h. Comparing the plume from simulations with and without anthropogenic emissions, SSA shows a downwind reduction of approximately 10 %, 11 % and 6 % at the T1, T2 and T3 sites, respectively. Other regions, such as those further downwind of the T3 site, are also affected. The gaer values increased from 0.62 to 0.74 at the T1 site and from 0.67 to 0.72 at the T3 site when anthropogenic emissions are active. During the Manaus plume-aging process, a plume tracking analysis shows an increase in SSA from 0.91 close to Manaus to 0.98 160 km downwind of Manaus as a result of SOA production and BC deposition.
Highlights
Aerosol particles are present in the atmosphere in highly variable types and concentrations, which contribute differently to climate forcing, cloud formation and development, as well as ecosystem impacts
Numerical simulations with the WRF-Chem model were performed in order to investigate the impact of the Manaus plume on secondary organic aerosol production and aerosol optical properties downwind of Manaus
We used the simulations to investigate the impact of anthropogenic emissions on secondary organic aerosol (SOA) formation over the Amazon region during the wet season and the effect of anthropogenic NOx on O3 production from volatile organic compounds (VOCs) precursors emitted by the forest
Summary
Aerosol particles are present in the atmosphere in highly variable types and concentrations, which contribute differently to climate forcing, cloud formation and development, as well as ecosystem impacts. Direct effects are related to scattering and absorption of solar radiation by aerosol particles. These effects tend to dominate under clear-sky conditions. Semidirect effects involve the absorption (by black carbon, BC, and other absorbing aerosol compounds) of solar radiation affecting temperature, humidity, atmospheric stability and cloud formation (Forkel et al, 2012; Boucher, 2013). Outside of local emissions plumes, average observed BC values are influenced by biogenic aerosol absorption, the global BC background and by long-range transport of BC from Saharan dust and African biomass burning. As we have several years of BC background measurements at the ATTO tower, it is possible to separate African episodic events from the rather constant regional BC concentrations that are relevant when comparing with modeled values not under anthropogenic influences (Artaxo et al, 2020). Air mass transport from Africa during the wet season occurs when the ITCZ is shifted to the south of the central Amazonian basin, allowing air masses from the Northern Hemisphere to reach the central portion of the basin
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