Risch Martin R, DeWild John F, Gay David A, Zhang Leiming, Boyer Elizabeth W, Krabbenhoft David P
U.S. Geological Survey, 5957 Lakeside Blvd., Indianapolis, IN 46278, United States.
U.S. Geological Survey, 8505 Research Way, Middleton, WI, 53562, United States.
Environ Pollut. 2017 Sep;228:8-18. doi: 10.1016/j.envpol.2017.05.004. Epub 2017 May 11.
Atmospheric mercury (Hg) deposition to forests is important because half of the land cover in the eastern USA is forest. Mercury was measured in autumn litterfall and weekly precipitation samples at a total of 27 National Atmospheric Deposition Program (NADP) monitoring sites in deciduous and mixed deciduous-coniferous forests in 16 states in the eastern USA during 2007-2014. These simultaneous, uniform, repeated, annual measurements of forest Hg include the broadest area and longest time frame to date. The autumn litterfall-Hg concentrations and litterfall mass at the study sites each year were combined with annual precipitation-Hg data. Rates of litterfall-Hg deposition were higher than or equal to precipitation-Hg deposition rates in 70% of the annual data, which indicates a substantial contribution from litterfall to total atmospheric-Hg deposition. Annual litterfall-Hg deposition in this study had a median of 11.7 μg per square meter per year (μg/m/yr) and ranged from 2.2 to 23.4 μg/m/yr. It closely matched modeled dry-Hg deposition, based on land cover at selected NADP Hg-monitoring sites. Mean annual atmospheric-Hg deposition at forest study sites exhibited a spatial pattern partly explained by statistical differences among five forest-cover types and related to the mapped density of Hg emissions. Forest canopies apparently recorded changes in atmospheric-Hg concentrations over time because litterfall-Hg concentrations decreased year to year and litterfall-Hg concentrations were significantly higher in 2007-2009 than in 2012-2014. These findings reinforce reported decreases in Hg emissions and atmospheric elemental-Hg concentrations during this same time period. Methylmercury (MeHg) was detected in all litterfall samples at all sites, compared with MeHg detections in less than half the precipitation samples at selected sites during the study. These results indicate MeHg in litterfall is a pathway into the terrestrial food web where it can accumulate in the prey of songbirds, bats, and raptors.
大气汞（Hg）沉降到森林中很重要，因为美国东部一半的土地被森林覆盖。2007年至2014年期间，在美国东部16个州的落叶林和落叶针叶混交林的27个国家大气沉降计划（NADP）监测站点，对秋季凋落物和每周降水样本中的汞进行了测量。这些对森林汞的同步、统一、重复的年度测量，涵盖了迄今为止最广的区域和最长的时间范围。每年研究站点的秋季凋落物汞浓度和凋落物质量与年度降水汞数据相结合。在70%的年度数据中，凋落物汞沉降速率高于或等于降水汞沉降速率，这表明凋落物对总大气汞沉降有很大贡献。本研究中每年的凋落物汞沉降中位数为每年每平方米11.7微克（μg/m²/yr），范围为2.2至23.4μg/m²/yr。它与基于选定NADP汞监测站点土地覆盖情况模拟的干汞沉降密切匹配。森林研究站点的年平均大气汞沉降呈现出一种空间格局，部分原因是五种森林覆盖类型之间的统计差异，并与汞排放的地图密度有关。森林冠层显然记录了大气汞浓度随时间的变化，因为凋落物汞浓度逐年下降，且2007 - 2009年的凋落物汞浓度显著高于2012 - 2014年。这些发现强化了同期报告的汞排放和大气元素汞浓度下降的情况。在所有站点的所有凋落物样本中都检测到了甲基汞（MeHg），而在研究期间，选定站点不到一半的降水样本中检测到了MeHg。这些结果表明，凋落物中的甲基汞是进入陆地食物网的一条途径，它可以在鸣禽、蝙蝠和猛禽的猎物中积累。
