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Title: Contributions of biomass-burning, urban, and biogenic emissions to the concentrations and light-absorbing properties of particulate matter in central Amazonia during the dry season

Abstract. Urbanization and deforestation have important impacts on atmosphericparticulate matter (PM) over Amazonia. This study presents observations andanalysis of PM1 concentration, composition, and opticalproperties in central Amazonia during the dry season, focusing on theanthropogenic impacts. The primary study site was located 70 km downwind ofManaus, a city of over 2 million people in Brazil, as part of theGoAmazon2014/5 experiment. A high-resolution time-of-flight aerosol massspectrometer (AMS) provided data on PM1 composition, and aethalometermeasurements were used to derive the absorption coefficient babs,BrC ofbrown carbon (BrC) at 370 nm. Non-refractory PM1 mass concentrationsaveraged 12.2 µg m−3 at the primary study site, dominated byorganics (83 %), followed by sulfate (11 %). A decrease inbabs,BrC was observed as the mass concentration of nitrogen-containingorganic compounds decreased and the organic PM1 O:C ratio increased,suggesting atmospheric bleaching of the BrC components. The organic PM1was separated into six different classes by positive-matrix factorization(PMF), and the mass absorption efficiency Eabs associated with eachfactor was estimated through multivariate linear regression ofbabs,BrC on the factor loadings. The largest Eabs values wereassociated with urban (2.04±0.14 m2 g−1) and biomass-burning(0.82±0.04 to 1.50±0.07 m2 g−1) sources. Together, these sources contributed at least 80 % ofbabs,BrC while accounting for 30 % to 40 % of the organic PM1 massconcentration. In addition, a comparison of organic PM1 compositionbetween wet and dry seasons revealed that only part of the 9-foldincrease in mass concentration between the seasons can be attributed tobiomass burning. Biomass-burning factor loadings increased by 30-fold,elevating its relative contribution to organic PM1 from about 10 % inthe wet season to 30 % in the dry season. However, most of the PM1mass (>60 %) in both seasons was accounted for by biogenicsecondary organic sources, which in turn showed an 8-fold seasonalincrease in factor loadings. A combination of decreased wet deposition andincreased emissions and oxidant concentrations, as well as a positivefeedback on larger mass concentrations are thought to play a role in theobserved increases. Furthermore, fuzzy c-means clustering identified threeclusters, namely “baseline”, “event”, and “urban” to representdifferent pollution influences during the dry season. The baseline cluster,representing the dry season background, was associated with a mean massconcentration of 9±3 µg m−3. This concentration increasedon average by 3 µg m−3 for both the urban and the event clusters.The event cluster, representing an increased influence of biomass burningand long-range transport of African volcanic emissions, was characterized byremarkably high sulfate concentrations. The urban cluster, representing theinfluence of Manaus emissions on top of the baseline, was characterized byan organic PM1 composition that differed from the other two clusters.The differences discussed suggest a shift in oxidation pathways as well asan accelerated oxidation cycle due to urban emissions, in agreement withfindings for the wet season.

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Journal Name:
Atmospheric Chemistry and Physics
Page Range / eLocation ID:
7973 to 8001
Medium: X
Sponsoring Org:
National Science Foundation
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