1 Chemical composition of ultrafine aerosol particles in central 1 Amazonia during the wet season 2 Hayley S. Glicker 1 , Michael J. Lawler 1 , John Ortega 1 , Suzane S. de Sá 2 , Scot T. Martin 2,3 , Paulo 3 Artaxo 4 , Oscar Vega Bustillos 5 , Rodrigo de Souza 6 , Julio Tota 7 , Annmarie Carlton 1 , and James N. 4 Smith 1* 5 1 Department of Chemistry, University of California, Irvine, CA 92697 USA 6 2 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138 USA 7 3 Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138 USA 8 4 Institute of Physics, University of São Paulo, Rua do Matão 1371, 05508-090, São Paulo, Brazil 9 5 Instituto de Pesquisas Energéticas e Nucleares, São Paulo, Brazil 10 6 Universidade do Estado do Amazonas, Manaus, AM, Brazil 11 7 Institute of Engineering and Geoscience, Federal University of West Pará, Santarém, PA, Brazil 12 Correspondence to: James N. Smith (jimsmith@uci.edu) 13 Abstract Central Amazonia serves as an ideal location to study atmospheric particle formation since it often can be 14 characterized as representing natural, pre-industrial conditions but can also experience periods of anthropogenic 15 influence due to the presence of emissions from large metropolitan areas like Manaus, Brazil. Ultrafine (sub-100 nm 16 diameter) particles are often observed in this region, although new particle formation events seldom occur near the 17 ground despite being readily observed in other forested regions with similar emissions. This study focuses on 18 identifying the chemical composition of ultrafine particles as a means of determining the chemical species and 19 mechanisms that may be responsible for new particle formation and growth in the region. These measurements were 20 performed during the wet season as part of the GoAmazon2014/5 field campaign at a site located 70 km southwest of 21 Manaus. A Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS) measured the concentrations of 22 the most abundant compounds detected in ultrafine particles. Two time periods representing distinct influences on 23 aerosol composition, which we label as “anthropogenic” and “background” periods, were studied as part of a larger 24 ten-day period of analysis. The anthropogenic period saw higher particle number concentrations and modeled back- 25 trajectories indicate transport of emissions from the Manaus metropolitan area. The background period saw much 26 lower number concentrations and back-trajectories showed that air masses arrived at the site predominantly from the 27 forested regions to the north and northeast. TDCIMS-measured constituents also show distinct differences between 28 the two observational periods. Although bisulfate was detected in particles during the ten-day period, the 29 anthropogenic period had increased levels of particulate bisulfate overall. Additionally, with larger fractions of 30 bisulfate observed, increased fractions of ammonium and trimethyl ammonium were observed. The background period 31 had distinct diurnal patterns of particulate organic nitrogen species and acetate, while oxalate remained relatively 32 constant during the ten-day period. 3-Methylfuran, a thermal decomposition product of particulate phase isoprene 33 epoxydiol (IEPOX), was the dominant species measured in the positive ion mode. Principal Component Analysis 34 (PCA) was performed on the TDCIMS-measured ion abundance and Aerosol Mass Spectrometer (AMS) mass 35 concentration data. Two different hierarchical clusters representing unique influences arise: one relating ultrafine 36 particulate acetate, hydrogen oxalate, organic nitrogen species, trimethyl ammonium and 3-methylfuran with each 37 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2019-299 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 24 April 2019 c  Author(s) 2019. CC BY 4.0 License.