Aerosol Inorganic Composition at a Tropical Site: Discrepancies Between Filter-Based Sampling and a Semi-Continuous Method

Abstract

The concentrations of the water-soluble inorganic aerosol species, ammonium (NH₄ ⁺), nitrate (NO₃ ⁻), chloride (Cl⁻), and sulfate (SO₄ ²⁻), were measured from September to November 2002 at a pasture site in the Amazon Basin (Rondônia, Brazil) (LBA-SMOCC). Measurements were conducted using a semi-continuous technique (Wet-annular denuder/Steam-Jet Aerosol Collector: WAD/SJAC) and three integrating filter-based methods, namely (1) a denuder-filter pack (DFP: Teflon and impregnated Whatman filters), (2) a stacked-filter unit (SFU: polycarbonate filters), and (3) a High Volume dichotomous sampler (HiVol: quartz fiber filters). Measurements covered the late dry season (biomass burning), a transition period, and the onset of the wet season (clean conditions). Analyses of the particles collected on filters were performed using ion chromatography (IC) and Particle-Induced X-ray Emission spectrometry (PIXE). Season-dependent discrepancies were observed between the WAD/SJAC system and the filter-based samplers. During the dry season, when PM_2.5 (D_p ≤ 2.5 μm) concentrations were ∼100 μg m⁻³, aerosol NH₄ ⁺ and SO₄ ²⁻ measured by the filter-based samplers were on average two times higher than those determined by the WAD/SJAC. Concentrations of aerosol NO₃ ⁻ and Cl⁻ measured with the HiVol during daytime, and with the DFP during day- and nighttime also exceeded those of the WAD/SJAC by a factor of two. In contrast, aerosol NO₃ ⁻ and Cl⁻ measured with the SFU during the dry season were nearly two times lower than those measured by the WAD/SJAC. These differences declined markedly during the transition period and towards the cleaner conditions during the onset of the wet season (PM_2.5∼ 5 μg m⁻³); when filter-based samplers measured on average 40–90% less than the WAD/SJAC. The differences were not due to consistent systematic biases of the analytical techniques, but were apparently a result of prevailing environmental conditions and different sampling procedures. For the transition period and wet season, the significance of our results is reduced by a low number of data points. We argue that the observed differences are mainly attributable to (a) positive and negative filter sampling artifacts, (b) presence of organic compounds and organosulfates on filter substrates, and (c) a SJAC sampling efficiency of less than 100%.

Acknowledgments

The data presented in this study were acquired within the framework of the Large Scale Biosphere Atmosphere Experiment in Amazonia (LBA). The authors gratefully acknowledge financial support by the European Commission (contract No. EVK2-CT-2001-00110 SMOCC) and by the Max Planck Society. The research at Ghent University was supported by the Belgian Federal Science Policy Office. The authors are indebted to Magda Claeys, András Hoffer, and András Gelencer for discussion of the results.

Notes

(*) MPI-C: Max Planck Institute for Chemistry (Mainz, Germany).

⁽¹⁾only concentrations determined on the Teflon filters are presented.

⁽²⁾HiVol samples were analyzed for PM_2.5 only (coarse fraction of TSP samples is not included).

⁽³⁾elemental S is presented as the SO₄ ^{2 −} equivalent by assuming that the measured S concentration is entirely attributable to aerosol SO₄ ^{2 −}.