Abstract. The Green Ocean Amazon (GoAmazon 2014/5) campaign, conducted from
January 2014 to December 2015 in the vicinity of Manaus, Brazil, was designed
to study the aerosol life cycle and aerosol–cloud interactions in both
pristine and anthropogenically influenced conditions. As part of this
campaign, the U.S. Department of Energy (DOE) Gulfstream 1 (G-1) research aircraft was
deployed from 17 February to 25 March 2014 (wet season) and 6 September to
5 October 2014 (dry season) to investigate aerosol and cloud properties
aloft. Here, we present results from the G-1 deployments focusing on
measurements of the aerosol chemical composition and secondary organic
aerosol (SOA) formation and aging. In the first portion of the paper, we provide an overview of the data
and compare and contrast the data from the wet and dry season. Organic aerosol (OA) dominates the deployment-averaged chemical composition,
comprising 80 % of the non-refractory PM1 aerosol mass, with sulfate
comprising 14 %, nitrate 2 %, and ammonium 4 %. This product
distribution was unchanged between seasons, despite the fact that total
aerosol loading was significantly higher in the dry season and that regional
and local biomass burning was a significant source of OA mass in the dry,
but not wet, season. However, the OA was more oxidized in the dry season,
with the median of the mean carbon oxidation state increasing from −0.45 in
the wet season to −0.02 in the dry season. In the second portion of the paper, we discuss the evolution of the
Manaus plume, focusing on 13 March 2014, one of the exemplary days in the
wet season. On this flight, we observe a clear increase in OA concentrations
in the Manaus plume relative to the background. As the plume is transported
downwind and ages, we observe dynamic changes in the OA. The mean carbon
oxidation state of the OA increases from −0.6 to −0.45 during the 4–5 h
of photochemical aging. Hydrocarbon-like organic aerosol (HOA) mass is lost,
with ΔHOA∕ΔCO values
decreasing from 17.6 µg m−3 ppmv−1 over Manaus to 10.6 µg m−3 ppmv−1 95 km downwind.
Loss of HOA is balanced out by formation of oxygenated organic aerosol (OOA),
with ΔOOA∕ΔCO increasing from 9.2 to 23.1 µg m−3 ppmv−1.
Because hydrocarbon-like organic aerosol (HOA) loss is balanced by OOA formation, we observe
little change in the net Δorg∕ΔCO values;
Δorg∕ΔCO averages 31 µg m−3 ppmv−1 and does not
increase with aging. Analysis of the Manaus plume evolution using data from
two additional flights in the wet season showed similar trends in Δorg∕ΔCO
to the 13 March flight; Δorg∕ΔCO values
averaged 34 µg m−3 ppmv−1 and showed little change over
4–6.5 h of aging. Our observation of constant Δorg∕ΔCO
are in contrast to literature studies of the outflow of several North
American cities, which report significant increases in
Δorg∕ΔCO for the first day of plume aging. These observations suggest that SOA
formation in the Manaus plume occurs, at least in part, by a different
mechanism than observed in urban outflow plumes in most other literature
studies. Constant Δorg∕ΔCO with plume aging has been
observed in many biomass burning plumes, but we are unaware of reports of
fresh urban emissions aging in this manner. These observations show that
urban pollution emitted from Manaus in the wet season forms less particulate
downwind as it ages than urban pollution emitted from North American cities.
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