| dc.description | 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. | |
