Abstract
An understanding of how anthropogenic emissions affect the concentrations and composition of airborne
particulate matter (PM) is fundamental to quantifying the
influence of human activities on climate and air quality.
The central Amazon Basin, especially around the city of
Manaus, Brazil, has experienced rapid changes in the past
decades due to ongoing urbanization. Herein, changes in
the concentration and composition of submicron PM due
to pollution downwind of the Manaus metropolitan region
are reported as part of the GoAmazon2014/5 experiment.
A high-resolution time-of-flight aerosol mass spectrometer
(HR-ToF-AMS) and a suite of other gas- and particle-phase
instruments were deployed at the “T3” research site, 70 km
downwind of Manaus, during the wet season. At this site, organic
components represented 797% of the non-refractory
PM1 mass concentration on average, which was in the same
range as several upwind sites. However, the organic PM1 was
considerably more oxidized at T3 compared to upwind measurements.
Positive-matrix factorization (PMF) was applied
to the time series of organic mass spectra collected at the T3 site, yielding three factors representing secondary processes
(7315% of total organic mass concentration) and
three factors representing primary anthropogenic emissions
(2715 %). Fuzzy c-means clustering (FCM) was applied
to the afternoon time series of concentrations of NOy , ozone,
total particle number, black carbon, and sulfate. Four clusters
were identified and characterized by distinct air mass origins
and particle compositions. Two clusters, Bkgd-1 and Bkgd-
2, were associated with background conditions. Bkgd-1 appeared
to represent near-field atmospheric PM production
and oxidation of a day or less. Bkgd-2 appeared to represent
material transported and oxidized for two or more days, often
with out-of-basin contributions. Two other clusters, Pol-1
and Pol-2, represented the Manaus influence, one apparently
associated with the northern region of Manaus and the other
with the southern region of the city. A composite of the PMF
and FCM analyses provided insights into the anthropogenic
effects on PM concentration and composition. The increase
in mass concentration of submicron PM ranged from 25%
to 200% under polluted compared with background conditions,
including contributions from both primary and secondary
PM. Furthermore, a comparison of PMF factor loadings
for different clusters suggested a shift in the pathways
of PM production under polluted conditions. Nitrogen oxides
may have played a critical role in these shifts. Increased
concentrations of nitrogen oxides can shift pathways of PM
production from HO2-dominant to NO-dominant as well as
increase the concentrations of oxidants in the atmosphere.
Consequently, the oxidation of biogenic and anthropogenic
precursor gases as well as the oxidative processing of preexisting
atmospheric PM can be accelerated. This combined
set of results demonstrates the susceptibility of atmospheric
chemistry, air quality, and associated climate forcing to anthropogenic
perturbations over tropical forests.
