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

Abstract

Abstract. Central Amazonia serves as an ideal location to study atmospheric particle formation, since it often represents nearly 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 of volatile organic compounds (VOCs). 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 Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5) field campaign at a site located 70 km southwest of Manaus. A thermal desorption chemical ionization mass spectrometer (TDCIMS) characterized 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 10 d period of analysis. Higher particle number concentrations were measured during the anthropogenic period, and modeled back-trajectory frequencies indicate transport of emissions from the Manaus metropolitan area. During the background period there were much lower number concentrations, and back-trajectory frequencies 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 throughout the 10 d period, the anthropogenic period had higher levels of particulate bisulfate overall. Ammonium and trimethyl ammonium were positively correlated with bisulfate. The background period had distinct diurnal patterns of particulate cyanate and acetate, while oxalate remained relatively constant during the 10 d period. 3-Methylfuran, a thermal 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 ion abundance and aerosol mass spectrometer (AMS) mass concentration data. Two different hierarchical clusters representing unique influences arise: one comprising ultrafine particulate acetate, hydrogen oxalate, cyanate, trimethyl ammonium and 3-methylfuran and another made up of 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 larger submicron-sized particles.

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“…Ns-soot particles that are internally mixed with K-bearing particles can originate from LRT (Saturno et al, 2018) or local biofuel burning for cooking or brick kilns (Martin et al, 2016). Other ns-soot particles can be from anthropogenic sources such as Manaus and neighboring cities (Glicker et al, 2019). Ns-soot particles are attached to or embedded within the mineral, sulfate, organic, and PBA particles (Figs.…”

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“…Chloride and sodium were frequently observed at Mace Head, Ireland, a remote coastal site . These results can also be used to study the emission sources of UFPs, as has been undertaken for the Amazon Basin and northern Oklahoma …”

“…36 Chloride and sodium were frequently observed at Mace Head, Ireland, a remote coastal site. 37 These results can also be used to study the emission sources of UFPs, as has been undertaken for the Amazon Basin 38 and northern Oklahoma. 39 One issue that hinders the wide application of the TDCIMS in atmospheric measurements is the use of radioactive materials (e.g., 210 Po or 241 Am) as ion production sources.…”

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