揮散損失を低減した溶存揮発性水銀捕集用海水サンプラーの開発と揮散損失が水銀放出フラックスの見積もりに与える影響

摘要

海水中の溶存揮発性水銀（dissolved gaseous mercury，以下DGM）を観測する際の揮散損失による系統的な誤差を防止するため，試料採取からDGMの捕集までの作業を試料の移し換えなしに実施できる新しい海水サンプラーを開発した．試料の移し換えを必要とする従来の海水採取方法との比較検討を行った結果，新規海水サンプラーを使用した方が従来法に比べてDGM濃度が最大27％高かった．また，1～2回の試料の移し換えにより平均で約15±10％（N＝12）のDGMが損失していることが明らかとなった．本研究で開発したサンプラーを用いて瀬戸内海東部及び水俣湾においてDGM濃度を測定した．その結果，平均濃度は瀬戸内海で61±19pg L~(-1)，水俣湾で］182±95pg L~(-1)であり，他の海域に比べて同等もしくはやや高かった．また，DGM濃度のデータ等を用いて水銀放出フラックスを計算し，DGMの揮散損失が計算値に与える影響を評価したところ，海水中DGM濃度が低いほど，DGMの揮散損失割合以上に水銀放出フラツタスが大幅に過小評価されることが分かった．%In this study, a new sampler for the determination of dissolved gaseous mercury (DGM) in seawater was developed. Using this sampler, the analytical procedures from seawater sampling to purge and trap of DGM can be handled without any decanting processes, by which the volatilization loss of DGM from samples is driven. The DGM concentration was a maximum of 27 % higher using the new sampler compared to that with other sampling methods that had decanting processes. It was clear that the average loss of DGM caused by the decanting processes was - 15 ± 10 % (N = 12). Thus, more accurate measurements of DGM in seawater were achieved using the new sampler with no decanting processes. The concentrations of DGM in the surface seawater in the eastern part of the Seto Inland Sea and Minamata Bay were measured using the new sampler. The results indicate that the DGM concentrations in the Seto Island Sea and Minamata Bay were 61 ± 19 pg L~(-1) and 182 ± 95 pg L~~(-1), respectively. On the basis of the observed data, mercury emission fluxes were calculated using an existing gas exchange model for sea surface. Then, the influences of the loss of DGM due to the decanting processes in sampling on the estimating mercury fluxes were evaluated.