Contributions of biomass-burning, urban, and biogenic emissions tothe concentrations and light-absorbing properties of particulatematter in central Amazonia during the dry season

Abstract. Urbanization and deforestation have important impacts on atmospheric particulate matter (PM) over Amazonia. This study presents observations and analysis of submicron PM 1 concentration, composition, and optical properties in central Amazonia during the dry season. The focus is on delineating the anthropogenic impact on the observed quantities, especially as related to the organic PM 1 . The primary study site was located 70 km to the west of Manaus, a city of over two million people in Brazil. As part of the GoAmazon2014/5 experiment, datasets from a large suite of instrumentation were employed. A high-resolution time-of-flight aerosol mass spectrometer (AMS) provided data on PM 1 composition, and aethalometer measurements were used to derive the absorption coefficient b abs,BrC of brown carbon (BrC) at 370 nm. The relationships of b abs,BrC with AMS-measured quantities showed that the absorption was associated with less-oxidized, nitrogen-containing organic compounds. Atmospheric processing appeared to bleach the BrC components. The organic PM 1 was separated into different classes by positive-matrix factorization (PMF). Estimates of the effective mass absorption efficiency associated with each PMF factor were obtained. Biomass burning and urban emissions appeared to contribute at least 80 % of b abs,BrC while accounting for 30 to 40 % of the organic PM 1 mass concentration. In addition, a comparison of organic PM 1 composition between wet and dry seasons revealed that only a fraction of the nine-fold increase in mass concentration between the seasons was due to biomass burning. An eight-fold increase in biogenic secondary organic PM 1 was observed. A combination of decreased wet deposition and increased emissions and oxidant concentrations, as well as a positive feedback on larger mass concentrations are thought to play a role in the observed increases. Fuzzy c-means clustering identified three clusters to represent different pollution influences during the dry season, including baseline (dry season background, which includes biomass burning), event (increased influence of biomass burning and long-range transport of African volcanic emissions), and urban (Manaus influence on top of the background). The baseline cluster was associated with a mean mass concentration of 9 ± 3 μg m −3 . This concentration increased on average by 3 μg m −3 for both the urban and the event clusters. The event cluster was characterized by remarkably high sulfate concentrations. Differences in the organic PM 1 composition for the urban cluster compared to the other two clusters suggested a shift in oxidation pathways as well as an accelerated oxidation cycle due to urban emissions, in agreement with findings for the wet season.