Single-particle characterization of aerosols collected at a remote site in the Amazonian rainforest and an urban site in Manaus, Brazil
Authors: Wu Li , Li Xue , Kim HyeKyeong , Geng Hong , Godoi Ricardo H. M. , Barbosa Cybelli G. G. , Godoi Ana F. L. , Yamamoto Carlos I. , de Souza Rodrigo A. F. , Pöhlker Christopher , Andreae Meinrat O. , Ro Chul-Un 
Published: January 2019

Abstract. In this study, aerosol samples collected at a remote site in the Amazonian
rainforest and an urban site in Manaus, Brazil, were investigated on a
single-particle basis using a quantitative energy-dispersive electron probe X-ray
microanalysis (ED-EPMA). A total of 23 aerosol samples were collected in four
size ranges (0.25–0.5, 0.5–1.0, 1.0–2.0, and 2.0–4.0 µm) during
the wet season in 2012 at two Amazon basin sites: 10 samples in Manaus, an
urban area; and 13 samples at an 80 m high tower, located at the Amazon Tall
Tower Observatory (ATTO) site in the middle of the rainforest, 150 km
northeast of Manaus. The aerosol particles were classified into nine particle
types based on the morphology on the secondary electron images (SEIs)
together with the elemental concentrations of 3162 individual particles:
(i) secondary organic aerosols (SOA); (ii) ammonium sulfate (AS); (iii) SOA
and AS mixtures; (iv) aged mineral dust; (v) reacted sea salts; (vi) primary
biological aerosol (PBA); (vii) carbon-rich or elemental carbon (EC)
particles, such as soot, tarball, and char; (viii) fly ash; and (ix) heavy
metal (HM, such as Fe, Zn, Ni, and Ti)-containing particles. In submicron
aerosols collected at the ATTO site, SOA and AS mixture particles were
predominant (50 %–94 % in relative abundance) with SOA and ammonium
sulfate comprising 73 %–100 %. In supermicron aerosols at the ATTO site,
aged mineral dust and sea salts (37 %–70 %) as well as SOA and ammonium
sulfate (28 %–58 %) were abundant. PBAs were observed abundantly in the
PM2−4 fraction (46 %), and EC and fly ash particles were absent in
all size fractions. The analysis of a bulk PM0.25−0.5 aerosol sample
from the ATTO site using Raman microspectrometry and attenuated total
reflection Fourier-transform infrared spectroscopy (ATR-FTIR) showed that ammonium
sulfate, organics, and minerals are the major chemical species, which is
consistent with the ED-EPMA results. In the submicron aerosols collected in
Manaus, either SOA and ammonium sulfate (17 %–80 %) or EC particles
(6 %–78 %) were dominant depending on the samples. In contrast, aged
mineral dust, reacted sea salt, PBA, SOA, ammonium sulfate, and EC particles
comprised most of the supermicron aerosols collected in Manaus. The SOA,
ammonium sulfate, and PBAs were mostly of a biogenic origin from the
rainforest, whereas the EC and HM-containing particles were of an
anthropogenic origin. Based on the different contents of SOA, ammonium
sulfate, and EC particles among the samples collected in Manaus, a
considerable influence of the rainforest over the city was observed. Aged
mineral dust and reacted sea-salt particles, including mineral dust mixed
with sea salts probably during long-range transatlantic transport, were
abundant in the supermicron fractions at both sites. Among the aged mineral
dust and reacted sea-salt particles, sulfate-containing ones outnumbered
those containing nitrates and sulfate + nitrate in the ATTO samples. In
contrast, particles containing sulfate + nitrate were comparable in
number to particles containing sulfate only in the Manaus samples, indicating
the different sources and formation mechanisms of secondary aerosols, i.e.,
the predominant presence of sulfate at the ATTO site from mostly biogenic
emissions and the elevated influences of nitrates from anthropogenic
activities at the Manaus site.