The ability to reliably estimate CO2 fluxes from current in situ atmospheric CO2 measurements and future satellite CO2 measurements is dependent on transport model performance at synoptic and shorter timescales The TransCom continuous experiment was designed to evaluate the performance of forward transport model simulations at hourly, daily, and synoptic timescales, and we focus on the latter two in this paper Twenty-five transport models or model variants submitted hourly time series of nine predetermined tracers (seven for CO2) at 280 locations We extracted synoptic-scale variability from daily averaged CO2 time series using a digital filter and analyzed the results by comparing them to atmospheric measurements at 35 locations The correlations between modeled and observed synoptic CO2 variabilities were almost always largest with zero time lag and statistically significant for most models and most locations Generally, the model results using diurnally varying land fluxes were closer to the observations compared to those obtained using monthly mean or daily average fluxes, and winter was often better simulated than summer Model results at higher spatial resolution compared better with observations, mostly because these models were able to sample closer to the measurement site location The amplitude and correlation of model-data variability is strongly model and season dependent Overall similarity in modeled synoptic CO2 variability suggests that the first-order transport mechanisms are fairly well parameterized in the models, and no clear distinction was found between the meteorological analyses in capturing the synoptic-scale dynamics

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2007GB003081

TransCom model simulations of hourly atmospheric CO2 : Analysis of synoptic-scale variations for the period 2002-2003

Long-term monitoring of data of ambient mercury (Hg) on a global scale to assess its emission, transport, atmospheric chemistry, and deposition processes is vital to understanding the impact of Hg pollution on the environment. The Global Mercury Observation System (GMOS) project was funded by the European Commission (http://www.gmos.eu) and started in November 2010 with the overall goal to develop a coordinated global observing system to monitor Hg on a global scale, including a large network of ground-based monitoring stations, ad hoc periodic oceanographic cruises and measurement flights in the lower and upper troposphere as well as in the lower stratosphere. To date, more than 40 ground-based monitoring sites constitute the global network covering many regions where little to no observational data were available before GMOS. This work presents atmospheric Hg concentrations recorded worldwide in the framework of the GMOS project (2010-2015), analyzing Hg measurement results in terms of temporal trends, seasonality and comparability within the network. Major findings highlighted in this paper include a clear gradient of Hg concentrations between the Northern and Southern hemispheres, confirming that the gradient observed is mostly driven by local and regional sources, which can be anthropogenic, natural or a combination of both.

/pdf/atmospheric-mercury-concentrations-observed-at-ground-based-22u6u1w5g6.pdf

Atmospheric mercury concentrations observed at ground-based monitoring sites globally distributed in the framework of the GMOS network

. Concern about the adverse effects of mercury on human health and ecosystems has led to tightening emission controls since the mid 1980s. But the resulting mercury emissions reductions in many parts of the world are believed to be offset or even surpassed by the increasing emissions in rapidly industrializing countries. Consequently, concentrations of atmospheric mercury are expected to remain roughly constant. Here we show that the worldwide atmospheric mercury concentrations have decreased by about 20 to 38 % since 1996 as indicated by long-term monitoring at stations in the Southern and Northern Hemispheres combined with intermittent measurements of latitudinal distribution over the Atlantic Ocean. The total reduction of the atmospheric mercury burden of this magnitude within 14 years is unusually large among most atmospheric trace gases and is at odds with the current mercury emission inventories with nearly constant anthropogenic emissions over this period. This suggests a major shift in the biogeochemical cycle of mercury including oceans and soil reservoirs. Decreasing reemissions from the legacy of historical mercury emissions are the most likely explanation for this decline since the hypothesis of an accelerated oxidation rate of elemental mercury in the atmosphere is not supported by the observed trends of other trace gases. Acidification of oceans, climate change, excess nutrient input and pollution may also contribute by their impact on the biogeochemistry of ocean and soils. Consequently, models of the atmospheric mercury cycle have to include soil and ocean mercury pools and their dynamics to be able to make projections of future trends.

/pdf/worldwide-trend-of-atmospheric-mercury-since-1995-575h1i3bqt.pdf

Worldwide trend of atmospheric mercury since 1995

. Our knowledge of the distribution of mercury concentrations in air of the Southern Hemisphere was until recently based mostly on intermittent measurements made during ship cruises. In the last few years continuous mercury monitoring has commenced at several sites in the Southern Hemisphere, providing new and more refined information. In this paper we compare mercury measurements at several remote sites in the Southern Hemisphere made over a period of at least 1 year at each location. Averages of monthly medians show similar although small seasonal variations at both Cape Point and Amsterdam Island. A pronounced seasonal variation at Troll research station in Antarctica is due to frequent mercury depletion events in the austral spring. Due to large scatter and large standard deviations of monthly average median mercury concentrations at Cape Grim, no systematic seasonal variation could be found there. Nevertheless, the annual average mercury concentrations at all sites during the 2007–2013 period varied only between 0.85 and 1.05 ng m−3. Part of this variability is likely due to systematic measurement uncertainties which we propose can be further reduced by improved calibration procedures. We conclude that mercury is much more uniformly distributed throughout the Southern Hemisphere than the distributions suggested by measurements made onboard ships. This finding implies that smaller trends can be detected in shorter time periods. We also report a change in the trend sign at Cape Point from decreasing mercury concentrations in 1996–2004 to increasing concentrations since 2007.

/pdf/comparison-of-mercury-concentrations-measured-at-several-3p4t09y6a5.pdf

Comparison of mercury concentrations measured at several sites in the Southern Hemisphere

. Knowledge of aerosol size and composition is important for determining radiative forcing effects of aerosols, identifying aerosol sources and improving aerosol satellite retrieval algorithms. The ability to extrapolate aerosol size and composition, or type, from intensive aerosol optical properties can help expand the current knowledge of spatiotemporal variability in aerosol type globally, particularly where chemical composition measurements do not exist concurrently with optical property measurements. This study uses medians of the scattering Angstrom exponent (SAE), absorption Angstrom exponent (AAE) and single scattering albedo (SSA) from 24 stations within the NOAA/ESRL Federated Aerosol Monitoring Network to infer aerosol type using previously published aerosol classification schemes. Three methods are implemented to obtain a best estimate of dominant aerosol type at each station using aerosol optical properties. The first method plots station medians into an AAE vs. SAE plot space, so that a unique combination of intensive properties corresponds with an aerosol type. The second typing method expands on the first by introducing a multivariate cluster analysis, which aims to group stations with similar optical characteristics and thus similar dominant aerosol type. The third and final classification method pairs 3-day backward air mass trajectories with median aerosol optical properties to explore the relationship between trajectory origin (proxy for likely aerosol type) and aerosol intensive parameters, while allowing for multiple dominant aerosol types at each station. The three aerosol classification methods have some common, and thus robust, results. In general, estimating dominant aerosol type using optical properties is best suited for site locations with a stable and homogenous aerosol population, particularly continental polluted (carbonaceous aerosol), marine polluted (carbonaceous aerosol mixed with sea salt) and continental dust/biomass sites (dust and carbonaceous aerosol); however, current classification schemes perform poorly when predicting dominant aerosol type at remote marine and Arctic sites and at stations with more complex locations and topography where variable aerosol populations are not well represented by median optical properties. Although the aerosol classification methods presented here provide new ways to reduce ambiguity in typing schemes, there is more work needed to find aerosol typing methods that are useful for a larger range of geographic locations and aerosol populations.

/pdf/classifying-aerosol-type-using-in-situ-surface-spectral-2vbo2mswpl.pdf

Classifying aerosol type using in situ surface spectral aerosol optical properties

Baseline air mass selection at Cape Point, South Africa: application of 222Rn and other filter criteria to CO2

Cape Point GAW Station 222Rn detector: factors affecting sensitivity and accuracy

Specific factors of a baseline 222 Rn detector installed at Cape Point, South Africa, were studied with the aim of improving its performance. Direct sunlight caused air turbulence within the instrument, resulting in 13.6% variability of the calibration factor. Shading the instrument eliminated this effect. A residual temperature dependence of the calibration factor was reduced to negligible levels with an improved photomultiplier tube. A superior detector head permits field servicing of the instrument, and has reduced one component of the instrumental background by a factor of 2. The other component probably constitutes thoron emissions from the stainless steel walls. The detection limit of the instrument could be reduced from its current 33 to 20 mBq m � 3 if the thoron were to be eliminated. r 2002 Elsevier

Technical note Cape Point GAW Station 222 Rn detector: factors affecting sensitivity and accuracy

Overview: The manuscript entitled “222Rn calibrated mercury fluxes from terrestrial surface of southern Africa derived from observations at Cape Point, South Africa”, by F. Slemr et al. describes observationally constrained estimates of terrestrial Hg fluxes from southern Africa. The analysis draws upon nearly 5 years (2007–2011) of measurements of gaseous elemental mercury (GEM) and 222Rn made at the Global Atmospheric Watch (GAW) station Cape Point. Estimated terrestrial 222Rn fluxes are combined with correlations between GEM and 222Rn during periods with elevated 222Rn to derive terrestrial GEM fluxes. The derived fluxes are corrected for 222Rn decay using an assumed transport time. As the authors point out, terrestrial Hg fluxes are poorly constrained by existing observations, particularly outside of northern mid-

/pdf/222-rn-calibrated-mercury-fluxes-from-terrestrial-surface-of-1ph5lgi3wi.pdf

S. Whittlestone

Papers

Baseline air mass selection at Cape Point, South Africa: application of 222Rn and other filter criteria to CO2

Cape Point GAW Station 222Rn detector: factors affecting sensitivity and accuracy

Technical note Cape Point GAW Station 222 Rn detector: factors affecting sensitivity and accuracy

222 Rn calibrated mercury fluxes from terrestrial surface of southern Africa derived from observations at Cape Point, South Africa