Chemical composition of ultrafine aerosol particles in central Amazonia during the wet season
Authors: Glicker Hayley S. , Lawler Michael J. , Ortega John , de Sá Suzane S. , Martin Scot T. , Artaxo Paulo , Vega Bustillos Oscar , de Souza Rodrigo , Tota Julio , Carlton Annmarie , Smith James N. 
Published: October 2019

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.