An Intensive Study of Aerosol Optical Properties in the Outflow of the Manaus Urban Plume, in Central Amazon

Abstract:

In this study, one year of ground-based observations of aerosol optical properties from a site impacted by urban emissions in Central Amazon of Brazil are assessed as part of results from GoAmazon2014/5 experiment. The aerosol absorption (σ_a) and scattering (σ_s) coefficients, as well as single scattering albedo (SSA) are analyzed to aid in characterizing Manaus' urban aerosol at GoAmazon T2 site. There is a distinct diurnal variation for (σ_a) it was mainly attributed to the severe emission of particulate pollutants and black carbon during the morning and evening traffic rush hours. The decrease of (σ_a) nearly at noon (12:00-14:00 LT) was a result of strong atmospheric mixing and dilution due to the elevated height of atmospheric planetary boundary layer (PBL). After sunset (18:00 LT), the formation of stable nocturnal PBL even in atmospheric inversion led to a low atmospheric diffusion ability to aerosols and thus relatively high (σ_a) and (σ_s) throughout the night. Indeed, it was observed a strong dependence on local wind confirmed by simulated back trajectories in all two seasons. Overall, the wind dependence results provide valuable information about the locations of aerosol pollution sources and suggest that the air pollution in dry season is a regional problem but in the wet season it is mainly affected by local urban emissions. We have also seen an interesting difference in variability of (σ_s) and (σ_a) during 8:00-13:00 LT in wet season. A clear decrease was observed for (σ_a), while a smooth increase during 11:00–13:00 LT was presented for (σ_s). This is possibly a consequence of secondary aerosol production. (σ_a) is controlled to a large degree by primary aerosols such as black carbon that are emitted directly from pollution sources like vehicles, while (σ_s) is related to secondary aerosols such as sulfate and nitrate that contribute the most to light scattering. SSA was relatively low around 7:00-08:00 LT, which reflected that (σ_a) increased more than (σ_s) during traffic rush hours. The peak at noon (around 13:00 LT) could be explained by a large number of secondary aerosols born out of photochemistry.The results shown that while (σ_a) decreased during 8:00–12:00 LT, SSA increased, indicating that light absorbing aerosols like BC diluted at a greater speed than scattering-related particles.