Abstract. Urbanization and deforestation have important impacts on atmospheric
particulate matter (PM) over Amazonia. This study presents observations and
analysis of PM1 concentration, composition, and optical
properties in central Amazonia during the dry season, focusing on the
anthropogenic impacts. The primary study site was located 70 km downwind of
Manaus, a city of over 2 million people in Brazil, as part of the
GoAmazon2014/5 experiment. A high-resolution time-of-flight aerosol mass
spectrometer (AMS) provided data on PM1 composition, and aethalometer
measurements were used to derive the absorption coefficient babs,BrC of
brown carbon (BrC) at 370 nm. Non-refractory PM1 mass concentrations
averaged 12.2 µg m−3 at the primary study site, dominated by
organics (83 %), followed by sulfate (11 %). A decrease in
babs,BrC was observed as the mass concentration of nitrogen-containing
organic compounds decreased and the organic PM1 O:C ratio increased,
suggesting atmospheric bleaching of the BrC components. The organic PM1
was separated into six different classes by positive-matrix factorization
(PMF), and the mass absorption efficiency Eabs associated with each
factor was estimated through multivariate linear regression of
babs,BrC on the factor loadings. The largest Eabs values were
associated with urban (2.04±0.14 m2 g−1) and biomass-burning
(0.82±0.04 to 1.50±0.07 m2 g−1) sources. Together, these sources contributed at least 80 % of
babs,BrC while accounting for 30 % to 40 % of the organic PM1 mass
concentration. In addition, a comparison of organic PM1 composition
between wet and dry seasons revealed that only part of the 9-fold
increase in mass concentration between the seasons can be attributed to
biomass burning. Biomass-burning factor loadings increased by 30-fold,
elevating its relative contribution to organic PM1 from about 10 % in
the wet season to 30 % in the dry season. However, most of the PM1
mass (>60 %) in both seasons was accounted for by biogenic
secondary organic sources, which in turn showed an 8-fold seasonal
increase in factor loadings. A combination of decreased wet deposition and
increased emissions and oxidant concentrations, as well as a positive
feedback on larger mass concentrations are thought to play a role in the
observed increases. Furthermore, fuzzy c-means clustering identified three
clusters, namely “baseline”, “event”, and “urban” to represent
different pollution influences during the dry season. The baseline cluster,
representing the dry season background, was associated with a mean mass
concentration of 9±3 µg m−3. This concentration increased
on average by 3 µg m−3 for both the urban and the event clusters.
The event cluster, representing an increased influence of biomass burning
and long-range transport of African volcanic emissions, was characterized by
remarkably high sulfate concentrations. The urban cluster, representing the
influence of Manaus emissions on top of the baseline, was characterized by
an organic PM1 composition that differed from the other two clusters.
The differences discussed suggest a shift in oxidation pathways as well as
an accelerated oxidation cycle due to urban emissions, in agreement with
findings for the wet season.
Supplementary material to "Contributions of biomass-burning, urban, and biogenic emissions to
the concentrations and light-absorbing properties of particulate
matter in central Amazonia during the dry season"