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

Abstract. Central Amazonia serves as an ideal location to study atmospheric particle formation since it often can be characterized as representing natural, pre-industrial conditions but can also experience periods of anthropogenic influence due to the presence of emissions from large metropolitan areas like Manaus, Brazil. Ultrafine (sub-100 nm diameter) particles are often observed in this region, although new particle formation events seldom occur near the ground despite being readily observed in other forested regions with similar emissions. This study focuses on identifying the chemical composition of ultrafine particles as a means of determining the chemical species and mechanisms that may be responsible for new particle formation and growth in the region. These measurements were performed during the wet season as part of the GoAmazon2014/5 field campaign at a site located 70 km southwest of Manaus. A Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS) measured the concentrations of the most abundant compounds detected in ultrafine particles. Two time periods representing distinct influences on aerosol composition, which we label as anthropogenic and background periods, were studied as part of a larger ten-day period of analysis. The anthropogenic period saw higher particle number concentrations and modeled back-trajectories indicate transport of emissions from the Manaus metropolitan area. The background period saw much lower number concentrations and back-trajectories showed that air masses arrived at the site predominantly from the forested regions to the north and northeast. TDCIMS-measured constituents also show distinct differences between the two observational periods. Although bisulfate was detected in particles during the ten-day period, the anthropogenic period had increased levels of particulate bisulfate overall. Additionally, with larger fractions of bisulfate observed, increased fractions of ammonium and trimethyl ammonium were observed. The background period had distinct diurnal patterns of particulate organic nitrogen species and acetate, while oxalate remained relatively constant during the ten-day period. 3-Methylfuran, a thermal decomposition product of 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 ion abundance and Aerosol Mass Spectrometer (AMS) mass concentration data. Two different hierarchical clusters representing unique influences arise: one relating ultrafine particulate acetate, hydrogen oxalate, organic nitrogen species, trimethyl ammonium and 3-methylfuran with each other and ultrafine particulate bisulfate, chloride, ammonium and potassium. A third cluster separated AMS-measured species from the two TDCIMS-derived clusters, indicating different sources or processes in ultrafine aerosol particle formation compared to submicron-sized particles.