508 research outputs found
Analysis of non-methane hydrocarbons in air samples collected aboard the CARIBIC passenger aircraft
The CARIBIC project (Civil Aircraft for the Regular Investigation of the
atmosphere Based on an Instrument Container) is a long-term monitoring
program making regular atmospheric measurements from an instrument container
installed monthly aboard a passenger aircraft. Typical cruising altitudes of
the aircraft allow for the study of the free troposphere and the
extra-tropical upper troposphere as well as the lowermost stratosphere.
CARIBIC measurements include a number of real time analyses as well as the
collection of aerosol and whole air samples. These whole air samples are
analyzed post-flight for a suite of trace gases, which includes non-methane
hydrocarbons (NMHC).<br>
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The NMHC measurement system and its analytical performance are described
here. Precision was found to vary slightly by compound, and is less than
2% for the C<sub>2</sub>âC<sub>6</sub> alkanes and ethyne, and between 1% and
6% for C<sub>7</sub>âC<sub>8</sub> alkanes and aromatic compounds. Preliminary
results from participation in a Global Atmospheric Watch (WMO) VOC audit
indicate accuracies within the precision of the system. Limits of detection
are 1 pptv for most compounds, and up to 3 pptv for some aromatics. These
are sufficiently low to measure mixing ratios typically observed in the
upper troposphere and lowermost stratosphere for the longer-lived NMHC,
however, in air samples from these regions many of the compounds with
shorter lifetimes (<5 days) were frequently below the detection limit.
Observed NMHC concentrations span several orders of magnitude, dependent on
atmospheric region and air mass history, with concentrations typically
decreasing with shorter chemical lifetimes
Gaseous elemental mercury depletion events observed at Cape Point during 2007â2008
Gaseous mercury in the marine boundary layer has been measured with a 15 min temporal resolution at the Global Atmosphere Watch station Cape Point since March 2007. The most prominent features of the data until July 2008 are the frequent occurrences of pollution (PEs) and depletion events (DEs). Both types of events originate mostly within a short transport distance (up to about 100 km), which are embedded in air masses ranging from marine background to continental. The Hg/CO emission ratios observed during the PEs are within the range reported for biomass burning and industrial/urban emissions. The depletion of gaseous mercury during the DEs is in many cases almost complete and suggests an atmospheric residence time of elemental mercury as short as a few dozens of hours, which is in contrast to the commonly used estimate of approximately 1 year. The DEs observed at Cape Point are not accompanied by simultaneous depletion of ozone which distinguishes them from the halogen driven atmospheric mercury depletion events (AMDEs) observed in Polar Regions. Nonetheless, DEs similar to those observed at Cape Point have also been observed at other places in the marine boundary layer. Additional measurements of mercury speciation and of possible mercury oxidants are hence called for to reveal the chemical mechanism of the newly observed DEs and to assess its importance on larger scales
222Rn calibrated mercury fluxes from terrestrial surfaces of southern Africa derived from observations at Cape Point, South Africa
Gaseous elemental mercury (GEM) and 222Rn, a radioactive gas of primarily terrestrial origin with a half-life of 3.8 days, have been measured simultaneously at Cape Point, South Africa, since March 2007. Between March 2007 and December 2009 altogether 59 events with high 222Rn concentrations were
identified. GEM correlated with 222Rn in 41 of the events and was constant during the remaining events without significant correlation. The average GEM/222Rn emission ratio of all events was -0.0047 ± 0.0054 pg
mBq-1, with ± 0.0054 being the standard error of the average. With an emission rate of 1.1 222Rn atoms cm-2 s-1 and a correction for the transport duration, this emission ratio corresponds to a radon calibrated flux of about -0.53 ± 0.62 ng m-2 h-1 which is statistically not distinguishable from zero. With wet deposition, which is not included in this estimate, the terrestrial surface of southern Africa appears to be a net mercury sink. © Owned by the authors, published by EDP Sciences, 201
222Rn-calibrated mercury fluxes from terrestrial surface of southern Africa
Gaseous elemental mercury (GEM) and 222Rn, a radioactive gas of primarily terrestrial origin with a half-life of 3.8 days, have been measured simultaneously at Cape Point, South Africa, since March 2007. Between March 2007 and December 2011, altogether 191 events with high 222Rn concentrations were identified. GEM correlated with 222Rn in 94 of the events and was constant during almost all the remaining events without significant correlation. The average GEM / 222Rn flux ratio of all events including the non-significant ones was â0.0001 with a standard error of ±0.0030 pg mBqâ1. Weighted with the event duration, the average GEM / 222Rn flux ratio was â0.0048 ± 0.0011 pg mBqâ1. With an emission rate of 1.1 222Rn atoms cmâ2 sâ1 and a correction for the transport time, this flux ratio corresponds to a radon-calibrated flux of about â0.54 ng GEM mâ2 hâ1 with a standard error of ±0.13 ng GEM mâ2 hâ1 (n = 191). With wet deposition, which is not included in this estimate, the terrestrial surface of southern Africa seems to be a net mercury sink of about â1.55 ng mâ2 hâ1. The additional contribution of an unknown but presumably significant deposition of reactive gaseous mercury would further increase this sink.© 2013, European Geosciences Unio
Global atmospheric model for mercury including oxidation by bromine atoms
Global models of atmospheric mercury generally assume that gas-phase OH and ozone are the main oxidants converting Hg<sup>0</sup> to Hg<sup>II</sup> and thus driving mercury deposition to ecosystems. However, thermodynamic considerations argue against the importance of these reactions. We demonstrate here the viability of atomic bromine (Br) as an alternative Hg<sup>0</sup> oxidant. We conduct a global 3-D simulation with the GEOS-Chem model assuming gas-phase Br to be the sole Hg<sup>0</sup> oxidant (Hg + Br model) and compare to the previous version of the model with OH and ozone as the sole oxidants (Hg + OH/O<sub>3</sub> model). We specify global 3-D Br concentration fields based on our best understanding of tropospheric and stratospheric Br chemistry. In both the Hg + Br and Hg + OH/O<sub>3</sub> models, we add an aqueous photochemical reduction of Hg<sup>II</sup> in cloud to impose a tropospheric lifetime for mercury of 6.5 months against deposition, as needed to reconcile observed total gaseous mercury (TGM) concentrations with current estimates of anthropogenic emissions. This added reduction would not be necessary in the Hg + Br model if we adjusted the Br oxidation kinetics downward within their range of uncertainty. We find that the Hg + Br and Hg + OH/O<sub>3</sub> models are equally capable of reproducing the spatial distribution of TGM and its seasonal cycle at northern mid-latitudes. The Hg + Br model shows a steeper decline of TGM concentrations from the tropics to southern mid-latitudes. Only the Hg + Br model can reproduce the springtime depletion and summer rebound of TGM observed at polar sites; the snowpack component of GEOS-Chem suggests that 40% of Hg<sup>II</sup> deposited to snow in the Arctic is transferred to the ocean and land reservoirs, amounting to a net deposition flux to the Arctic of 60 Mg a<sup>â1</sup>. Summertime events of depleted Hg<sup>0</sup> at Antarctic sites due to subsidence are much better simulated by the Hg + Br model. Model comparisons to observed wet deposition fluxes of mercury in the US and Europe show general consistency. However the Hg + Br model does not capture the summer maximum over the southeast US because of low subtropical Br concentrations while the Hg + OH/O<sub>3</sub> model does. Vertical profiles measured from aircraft show a decline of Hg<sup>0</sup> above the tropopause that can be captured by both the Hg + Br and Hg + OH/O<sub>3</sub> models, except in Arctic spring where the observed decline is much steeper than simulated by either model; we speculate that oxidation by Cl species might be responsible. The Hg + Br and Hg + OH/O<sub>3</sub> models yield similar global budgets for the cycling of mercury between the atmosphere and surface reservoirs, but the Hg + Br model results in a much larger fraction of mercury deposited to the Southern Hemisphere oceans
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Air-Sea Exchange in the Global Mercury Cycle
We present results from a new global atmospheric mercury model coupled with a mixed layer slab ocean. The ocean model describes the interactions of the mixed layer with the atmosphere and deep ocean, as well as conversion between elemental, divalent, and nonreactive mercury species. Our global mean aqueous concentrations of 0.07 pM elemental, 0.80 pM reactive, and 1.51 pM total mercury agree with observations. The ocean provides a 14.1 Mmol yrâ1 source of mercury to the atmosphere, at the upper end of previous estimates. Re-emission of previously deposited mercury constitutes 89% of this flux. Ocean emissions are largest in the tropics and downwind of industrial regions. Midlatitude ocean emissions display a large seasonal cycle induced by biological productivity. Oceans contribute 54% (36%) of surface atmospheric mercury in the Southern (Northern) Hemisphere. We find a large net loss of mercury to the deep ocean (8.7 Mmol yrâ1), implying a âŒ0.7%/year increase in deep ocean concentrations.Earth and Planetary SciencesEngineering and Applied Science
El Niño-Southern Oscillation influence on tropospheric mercury concentrations
The El Nino-Southern Oscillation (ENSO) affects the tropospheric concentrations of many trace gases. Here we investigate the ENSO influence on mercury concentrations measured in the upper troposphere during Civil Aircraft for the Regular Investigation of the atmosphere Based on an instrumented Container flights and at ground at Cape Point, South Africa, and Mace Head, Ireland. Mercury concentrations cross-correlate with Southern Oscillation Index (SOI) with a lag of 8 +/- 2 months. Highest mercury concentrations are always found at the most negative SOI values, i.e., 8 months after El Nino, and the amplitude of the interannual variations fluctuates between similar to 5 and 18%. The time lag is similar to that of CO whose interannual variations are driven largely by emissions from biomass burning (BB). The amplitude of the interannual variability of tropospheric mercury concentrations is consistent with the estimated variations in mercury emissions from BB. We thus conclude that BB is a major factor driving the interannual variation of tropospheric mercury concentrations
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