THg

Table 1 Coefficients for 7[ (a ) for third harmonic generation (THG), degenerate four wave mixing (DFWM), electric field induced second harmonic generation (ESHG), and Kerr effect in methane at the experimental geometry rcH = 2.052 a.u. A CCSD wavefunction and the t-aug-cc-pVDZ basis were used. (Results given in atomic units, the number in parentheses indicate powers of ten.)...

If we compare results obtained with the same basis sets with the three coupled cluster models CCS, CC2 and CCSD, we find similar trends as observed in Refs. [22,45] The CCS model underestimates strongly the static hyperpolarizabilities and their dispersion. The results are usually of similar quality as those obtained with SCF. For methane, the CCS static hyperpolarizabilities are intermediate between the SCF and the CCSD values obtained in the same basis set. In Ref. [45] the CCS percentage dispersion contribution to the third harmonic generation (THG) hyperpolarizability of methane was found to be slightly smaller than for SCF, both underestimating significantly the dispersion obtained with the correlated coupled cluster models CC2 and CCSD. Accordingly the CCS dispersion coeflBcients listed in Table 3 are substantially smaller than the respective CCSD results obtained in the same basis sets. 

Watersheds are sinks for atmospheric Hg deposition (Grigal 2002). However, they are highly variable in their ability to retain inputs of total Hg (THg), convert ionic Hg (Hg(ll)) to bioavailable methylmercury (MeHg), and snpply Hg(II) and MeHg to downstream aqnatic ecosystems, ultimately influencing exposure to sensitive biota and hnmans. 

FIGURE 2.5 a) Wet THg deposition at the Mercury Deposition Network (MDN) sites for 2004 and b) temporal patterns in the concentration of THg in precipitation at a MDN site, based on weekly observations. 

Wet deposition of THg from 4 Swedish and 1 Finnish station is presented in Figme 2.6. The stations are located in a south-to-north gradient with increasing distance from the main source regions. 

FIGURE 2.6 Wet deposition of THg over a south to north gradient in Sweden and Finland. Vavihill (southernmost) and Pallas (northernmost). Data are shown for 6 years. 

FIGURE 2.8 Inputs and losses of a) THg and b) MeHg for watersheds in Europe and North America. 

Frescholtz 2002). Although ongoing and new planned field and laboratory studies are designed to further test this hypothesis, we feel that it is warranted at this time to develop a pilot-scale network of aimual ecosystem fluxes of THg in TF and LF as indicators of total atmospheric deposition. These fluxes can then be compared with measured wet plus modeled diy deposition based on both inferential and regional-scale models to develop independent estimates of total atmospheric deposition for forested catchments. We also believe that this approach could eventually be applied to a national network, such as the MDN. Although this method is best aimed at forested sites, ongoing research will address methods appropriate for other ecosystems. 

Estimates of THg loading (pg/m ) can be determined for a dated snowpack interval by the product of the interval Hg concentration and the snow water equivalent, which is determined from the interval density and thickness. Seasonal and, at many sites, near annual (-90%) loadings can then be estimated by summing the interval loadings (USEPA 2003). 

Mercury in soil is not only likely to have a different potential for evasion and methylation than Hg in runoff, but soil Hg may be perturbed by land disturbance. Land disturbances that are particularly relevant to Hg cycling include the formation of wetlands and flooding of reservoirs (Rudd 1995 see Chapter 3). Disturbances such as clear-cutting can also result in marked increases in the release of THg and MeHg from soils (Munthe and Hultbeig 2003 Porvari et al. 2003). Fire can result in large Hg losses by volatilization (Grigal 2002). 

At an intensive watershed Hg monitoring site, it is envisioned that THg and MeHgwonld be measured in ambient concentrations of atmospheric Hg species (i.e.,... 

Summary of collections and measurements of THg and MeHg that should be made at intensive watershed Hg monitoring sites... 

Hg(0), PHg, RGHg), wet deposition, throughfall, and litterfall, as discussed in the program to determine total ecosystem deposition (see Section 2.2.8). A summary of the measurements of Hg species that should be made in an intensive watershed Hg monitoring program is provided in Table 2.4. We envision that stream water measurements of total and dissolved THg and total and dissolved MeHg would also be made. 

As inputs of THg appear to be strongly retained in the forest floor, we reconunend a forest floor survey be conducted in areas that are receiving elevated Hg deposition and where it is expected that deposition will change markedly. In this site, permanent sampling sites would be established. We envision that forest floor samples would be resurveyed at appropriate time intervals (e.g., 10 years). A forest floor survey would help clarify current patterns of total Hg deposition and potentially quantify the response of forests to decreases in Hg emissions and deposition. 

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