Abstract
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.
Highlights
The Amazonian rainforest is regarded as one of the primitive continental regions and atmospheric aerosol particles over the region are expected to be influenced minimally by anthropogenic activities, during the wet season (Andreae, 2007; Martin et al, 2010b; Chen et al, 2015)
The analyzed particles were classified based on their X-ray spectral and secondary electron images (SEIs) data, where nine different particle types were observed in the samples collected at the Amazon Tall Tower Observatory (ATTO) and Manaus sites during the wet season in 2012: i.e., (i) secondary organic aerosols (SOA); (ii) ammonium sulfate (AS) particles; (iii) SOA and AS mixture particles; (iv) aged sea salt; (v) aged mineral dust; (vi) primary biological aerosol (PBA) particles; (vii) carbon-rich or elemental carbon (EC) particles such as soot, tarball, and char or coal dust; (viii) fly ash particles; and (ix) heavy-metal-containing (HM) particles
Ammonium sulfate and SOA particles are dominant in the sub- and supermicron aerosol fractions collected at the ATTO site with some mineral particles and aged sea salts in the supermicron fractions, whereas the aerosol samples collected at the Manaus site are composed of various types of particles of anthropogenic and/or natural origin
Summary
The Amazonian rainforest is regarded as one of the primitive continental regions and atmospheric aerosol particles over the region are expected to be influenced minimally by anthropogenic activities, during the wet season (Andreae, 2007; Martin et al, 2010b; Chen et al, 2015). The atmosphere in the Amazon basin is rich in PBA particles (Andreae, 2007; Artaxo et al, 1998, 2013; Martin et al, 2010a) Their unique morphology and elemental compositions of major C and O with minor S, K, P, Na, N, Cl, and/or Mg obtained by SEM/EDX are characteristic of individual PBAs like fungal spores (China et al, 2016). 23 aerosol samples collected at the ATTO site and at an urban site in Manaus during the wet season in 2012 were examined on a single-particle basis using a quantitative energy-dispersive electron probe X-ray microanalysis (EDEPMA), which provided information on the morphology and chemical compositions of aerosols containing both light and heavy elements. Wu et al.: Single-particle characterization of aerosols collected at a remote site in the Amazonian rainforest 1223
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