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. Central Amazonia serves as an ideal location to studyatmospheric particle formation, since it often represents nearly natural,pre-industrial conditions but can also experience periods of anthropogenicinfluence due to the presence of emissions from large metropolitan areaslike Manaus, Brazil. Ultrafine (sub-100 nm diameter) particles are oftenobserved in this region, although new particle formation events seldom occurnear the ground despite being readily observed in other forested regionswith similar emissions of volatile organic compounds (VOCs). This study focuses onidentifying the chemical composition of ultrafine particles as a means ofdetermining the chemical species and mechanisms that may be responsible fornew particle formation and growth in the region. These measurements wereperformed during the wet season as part of the Observations andModeling of the Green Ocean Amazon (GoAmazon2014/5) field campaignat a site located 70 km southwest of Manaus. A thermal desorption chemicalionization mass spectrometer (TDCIMS) characterized the most abundantcompounds detected in ultrafine particles. Two time periods representingdistinct influences on aerosol composition, which we label as“anthropogenic” and “background” periods, were studied as part of alarger 10 d period of analysis. Higher particle number concentrationswere measured during the anthropogenic period, and modeled back-trajectoryfrequencies indicate transport of emissions from the Manaus metropolitanarea. During the background period there were much lower numberconcentrations, and back-trajectory frequencies showed that air massesarrived at the site predominantly from the forested regions to the north andnortheast. TDCIMS-measured constituents also show distinct differencesbetween the two observational periods. Although bisulfate was detected inparticles throughout the 10 d period, the anthropogenic period had higherlevels of particulate bisulfate overall. Ammonium and trimethyl ammoniumwere positively correlated with bisulfate. The background period haddistinct diurnal patterns of particulate cyanate and acetate, while oxalateremained relatively constant during the 10 d period. 3-Methylfuran, athermal decomposition product of a particulate-phase isoprene epoxydiol(IEPOX), was the dominant species measured in the positive-ion mode.Principal component analysis (PCA) was performed on the TDCIMS-measured ionabundance and aerosol mass spectrometer (AMS) mass concentration data. Twodifferent hierarchical clusters representing unique influences arise: onecomprising ultrafine particulate acetate, hydrogen oxalate, cyanate,trimethyl ammonium and 3-methylfuran and another made up of ultrafineparticulate bisulfate, chloride, ammonium and potassium. A third clusterseparated AMS-measured species from the two TDCIMS-derived clusters,indicating different sources or processes in ultrafine aerosol particleformation compared to larger submicron-sized particles.

Chemical composition of ultrafine aerosol particles in central Amazonia during the wet season