Overview of mercury dry deposition, litterfall, and throughfall studies

The current knowledge concerning mercury dry deposition is reviewed, including dry-deposition algorithms used in chemical transport models (CTMs) and at monitoring sites and related deposition calculations, measurement methods and studies for quantifying dry deposition of gaseous oxidized mercury (GOM) and particulate bound mercury (PBM), and measurement studies of litterfall and throughfall mercury. Measured median GOM plus PBM dry deposition in Asia (10.7 µg m−2 yr−1) is almost double that in North America (6.1 µg m−2 yr−1) due to the higher anthropogenic emissions in Asia. The measured mean GOM plus PBM dry deposition in Asia (22.7 µg m−2 yr−1), however, is less than that in North America (30.8 µg m−2 yr−1). The variations between the median and mean values reflect the influences that single extreme measurements can have on the mean of a data set. Measured median litterfall and throughfall mercury are, respectively, 34.8 and 49.0 µg m−2 yr−1 in Asia, 12.8 and 16.3 µg m−2 yr−1 in Europe, and 11.9 and 7.0 µg m−2 yr−1 in North America. The corresponding measured mean litterfall and throughfall mercury are, respectively, 42.8 and 43.5 µg m−2 yr−1 in Asia, 14.2 and 19.0 µg m−2 yr−1 in Europe, and 12.9 and 9.3 µg m−2 yr−1 in North America. The much higher litterfall mercury than GOM plus PBM dry deposition suggests the important contribution of gaseous elemental mercy (GEM) to mercury dry deposition to vegetated canopies. Over all the regions, including the Amazon, dry deposition, estimated as the sum of litterfall and throughfall minus open-field wet deposition, is more dominant than wet deposition for Hg deposition. Regardless of the measurement or modelling method used, a factor of 2 or larger uncertainties in GOM plus PBM dry deposition need to be kept in mind when using these numbers for mercury impact studies