Single-particle characterization of aerosols collected at a remote site in the Amazonian rainforest and an urban site in Manaus, Brazil

Abstract

Abstract. In this study, aerosol samples collected at a remote site in the Amazonian\nrainforest and an urban site in Manaus, Brazil, were investigated on a\nsingle-particle basis using a quantitative energy-dispersive electron probe X-ray\nmicroanalysis\xa0(ED-EPMA). A total of 23 aerosol samples were collected in four\nsize ranges (0.25–0.5, 0.5–1.0, 1.0–2.0, and 2.0–4.0\u2009 µ m) during\nthe wet season in\xa02012 at two Amazon basin sites: 10 samples in Manaus, an\nurban area; and 13\xa0samples at an 80\u2009m high tower, located at the Amazon Tall\nTower Observatory\xa0(ATTO) site in the middle of the rainforest, 150\u2009km\nnortheast of Manaus. The aerosol particles were classified into nine particle\ntypes based on the morphology on the secondary electron images\xa0(SEIs)\ntogether with the elemental concentrations of 3162\xa0individual particles:\n(i)\xa0secondary organic aerosols\xa0(SOA); (ii)\xa0ammonium sulfate\xa0(AS); (iii)\xa0SOA\nand AS mixtures; (iv)\xa0aged mineral dust; (v)\xa0reacted sea salts; (vi)\xa0primary\nbiological aerosol\xa0(PBA); (vii)\xa0carbon-rich or elemental carbon\xa0(EC)\nparticles, such as soot, tarball, and char; (viii)\xa0fly ash; and (ix)\xa0heavy\nmetal (HM, such as Fe, Zn, Ni, and Ti)-containing particles. In submicron\naerosols collected at the ATTO site, SOA and AS mixture particles were\npredominant (50\u2009%–94\u2009% in relative abundance) with SOA and ammonium\nsulfate comprising 73\u2009%–100\u2009%. In supermicron aerosols at the ATTO site,\naged mineral dust and sea salts (37\u2009%–70\u2009%) as well as SOA and ammonium\nsulfate (28\u2009%–58\u2009%) were abundant. PBAs were observed abundantly in the\nPM 2−4 fraction (46\u2009%), and EC and fly ash particles were absent in\nall size fractions. The analysis of a bulk PM 0.25−0.5 aerosol sample\nfrom the ATTO site using Raman microspectrometry and attenuated total\nreflection Fourier-transform infrared spectroscopy (ATR-FTIR) showed that ammonium\nsulfate, organics, and minerals are the major chemical species, which is\nconsistent with the ED-EPMA results. In the submicron aerosols collected in\nManaus, either SOA and ammonium sulfate (17\u2009%–80\u2009%) or EC particles\n(6\u2009%–78\u2009%) were dominant depending on the samples. In contrast, aged\nmineral dust, reacted sea salt, PBA, SOA, ammonium sulfate, and EC particles\ncomprised most of the supermicron aerosols collected in Manaus. The SOA,\nammonium sulfate, and PBAs were mostly of a biogenic origin from the\nrainforest, whereas the EC and HM-containing particles were of an\nanthropogenic origin. Based on the different contents of SOA, ammonium\nsulfate, and EC particles among the samples collected in Manaus, a\nconsiderable influence of the rainforest over the city was observed. Aged\nmineral dust and reacted sea-salt particles, including mineral dust mixed\nwith sea salts probably during long-range transatlantic transport, were\nabundant in the supermicron fractions at both sites. Among the aged mineral\ndust and reacted sea-salt particles, sulfate-containing ones outnumbered\nthose containing nitrates and sulfate\u2009 + \u2009nitrate in the ATTO samples. In\ncontrast, particles containing sulfate\u2009 + \u2009nitrate were comparable in\nnumber to particles containing sulfate only in the Manaus samples, indicating\nthe different sources and formation mechanisms of secondary aerosols, i.e.,\nthe predominant presence of sulfate at the ATTO site from mostly biogenic\nemissions and the elevated influences of nitrates from anthropogenic\nactivities at the Manaus site.