Oxidized organic molecules in the tropical free troposphere over Amazonia.

New particle formation (NPF) in the tropical free troposphere (FT) is a globally important source of cloud condensation nuclei, affecting cloud properties and climate. Oxidized organic molecules (OOMs) produced from biogenic volatile organic compounds are believed to contribute to aerosol formation in the tropical FT, but without direct chemical observations. We performed in situ molecular-level OOMs measurements at the Bolivian station Chacaltaya at 5240 m above sea level, on the western edge of Amazonia. For the first time, we demonstrate the presence of OOMs, mainly with 4-5 carbon atoms, in both gas-phase and particle-phase (in terms of mass contribution) measurements in tropical FT air from Amazonia. These observations, combined with air mass history analyses, indicate that the observed OOMs are linked to isoprene emitted from the rainforests hundreds of kilometers away. Based on particle-phase measurements, we find that these compounds can contribute to NPF, at least the growth of newly formed nanoparticles, in the tropical FT on a continental scale. Thus, our study is a fundamental and significant step in understanding the aerosol formation process in the tropical FT.

Assessing comparability and uncertainty of analytical methods for methylated mercury species in seawater.

BACKGROUND: The relative distribution and importance of monomethylmercury (MMHg) and dimethylmercury (DMHg) in seawater is still under debate. A lack of comparability between measurements at sub-picomolar levels hampered the further understanding of the biogeochemical Hg cycle. To overcome this, we assessed the relative standard measurement uncertainties (Uex,r) for direct measurements of MMHg and DMHg by species-specific isotope dilution ICP-MS and cryo-focusing GC-ICP-MS at femtomolar concentrations. Furthermore, Uex,r was determined for the indirect determination of DMHg (DMHgcalc = MeHg - MMHg) and MeHg (MeHgcalc = MMHg + DMHg) to compare the two methodologies. RESULTS: Expanded Uex,r (confidence interval of 95%) for cryo-focusing GC-ICP-MS was 14.4 (<50 fM) and 14.2% (>50 fM) and for SS-ID GC-ICP-MS 5.6 (<50 fM) and 3.7% (>50 fM). For concentrations above 50 fM, Uex,r for DMHgcalc was always lower than for direct measurements (14.2%). For MeHgcalc, on the other hand, Uex,r was always higher for concentrations above 115 fM (range: 3.7-13.9%) than for direct measurements (3.7%). We evaluated the comparability of directly measured and calculated DMHg and MeHg concentrations based on Hg speciation measurements for two vertical profiles in the Mediterranean Sea. We show that directly measured and indirectly determined DMHg and MeHg concentrations yield comparable results. SIGNIFICANCE: Our results validate the application of the indirect method for the determination of DMHg if a direct measurement method with a low Uex,r such as isotope dilution is used for MMHg and MeHg measurements. The validation of the indirect measurement approach opens new possibilities to generate more precise and accurate DMHg data in the global ocean.

Dispersion of airborne mercury species emitted from the cement plant.

The cement industry is the second largest source of anthropogenic mercury (Hg) emissions in Europe, accounting for 11% of global anthropogenic Hg emissions. The main objective of this study was to examine the influence of Hg emissions from the Salonit Anhovo cement plant on Hg levels measured in the ambient air at Vodarna, 1 km downwind from the flue gas chimney. The findings reveal that the plant raw mill operational status plays an important role in Hg concentrations in the flue gas emitted from the plant. Emitted total gaseous mercury was, on average, higher (49.4 µg/m3) when raw mills were in the direct mode (both raw mills-off) and lower (23.4 µg/m3) in the combined mode (both raw mills-on). The average Hg concentrations in Vodarna were 3.14 ng/m3 for gaseous elemental mercury, 53.7 pg/m3 for gaseous oxidised mercury, and 41.9 pg/m3 for particulate bound mercury for the whole measurement period. Atmospheric Hg speciation in Vodarna, coupled with plant emissions and wind data, has revealed that the total gaseous mercury emitted from the cement plant is clearly related to all Hg species measured in Vodarna. Wind blowing from the northeastern quadrant (mostly NE, ENE) is responsible for the elevated Hg levels in Vodarna, where gaseous oxidised mercury levels are highly linked to the cement plant emissions. However, elevated levels of Hg species in the absence of northeastern winds indicate potential inputs from other unknown local sources as well as inputs from regional and global transport mechanisms.