[1] The air-surface exchange of atmospheric ammonia (NH 3 ) and measurements of the canopy and stomatal compensation points (c cp and c st , respectively) and the stomatal and soil emission potentials (G st and G g , respectively) are reviewed. A database of these values has been developed, to be used for the development of input parameters and for model evaluation. The compensation points are dependent on canopy type, nitrogen (N) status, temperature, growth stage, and meteorological conditions. Canopies that receive high atmospheric nitrogen input generally have high c cp values. c cp values also tend to be higher over intensively managed vegetated surfaces than semi-natural vegetation, due to the higher nitrogen content in these surfaces. Increased nitrogen concentrations from fertilization and cutting practices have been observed to increase the compensation points and therefore the emission from these canopies. The decomposition of litter leaves has been found to play a dominant role and significantly increase the values of c cp over agricultural vegetation and fertilized grasslands. By modifying an existing big-leaf dry deposition model to allow NH 3 emission from leaf stomata and soil surfaces, a bi-directional air-surface exchange model has been developed for applications in regional-scale air-quality models. The model predicts c cp values that vary with canopy type, nitrogen content, and meteorological conditions. c cp values predicted by the model are around 1-2 mg m −3 over forest canopies and 4-10 mg m −3 over grasslands and agricultural canopies during a typical summer daytime; c cp values are ∼10 and ∼3 times lower over forests and agricultural lands, respectively, in nighttime and/or winter conditions. These results are similar to the range of measured values. The new bi-directional air-surface exchange model will reduce the dry deposition fluxes by 20-100 ng m −2 s −1 compared to the original dry deposition model over low-N forests and agricultural lands during typical summer daytime conditions, which can be the difference between whether deposition or emission occurs. For example, this new model produces emissions fluxes of 0-90 ng m −2 s −1 over croplands when the atmospheric NH 3 concentrations are below 10 mg m −3 .

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Abstract. 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.

Dry deposition of atmospheric mercury (Hg) to various land covers surrounding 24 sites in North America was estimated for the years 2009 to 2014. Depending on location, multiyear mean annual Hg dry deposition was estimated to range from 5.1 to 23.8 μg m yr to forested canopies, 2.6 to 20.8 μg m yr to nonforest vegetated canopies, 2.4 to 11.2 μg m yr to urban and built up land covers, and 1.0 to 3.2 μg m yr to water surfaces. In the rural or remote environment in North America, annual Hg dry deposition to vegetated surfaces is dominated by leaf uptake of gaseous elemental mercury (GEM), contrary to what was commonly assumed in earlier studies which frequently omitted GEM dry deposition as an important process. Dry deposition exceeded wet deposition by a large margin in all of the seasons except in the summer at the majority of the sites. GEM dry deposition over vegetated surfaces will not decrease at the same pace, and sometimes may even increase with decreasing anthropogenic emissions, suggesting that Hg emission reductions should be a long-term policy sustained by global cooperation.

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The bidirectional air-surface exchange for gaseous elemental mercury (GEM) and existing measurements of the compensation points over a variety of canopy types are reviewed. Deposition and emission of GEM are dependent on several factors such as the type of canopy, temperature, season, atmospheric GEM concentrations, and meteorological conditions, with compensation points varying between 0.5 and 33 ng m 23 . Emissions tend to increase from the spring to summer seasons, as the GEM accumulates in the foliage of the vegetation. A strong dependence on solar radiation has been observed, with higher emissions under light conditions. A bidirectional air-surface exchange flux model is proposed for estimating GEM fluxes at a two-hourly time resolution for the National Atmospheric Deposition Program's, Atmospheric Mercury Network (AMNet) sites. Compared to the unidirectional dry deposition model used in Zhang et al. (2012), two additional parameters, stomatal and soil emission potential, were needed in the bidirectional model and were chosen based on knowledge gained in the literature review and model sensitivity test results. Application of this bidirectional model to AMNet sites have produced annual net deposition fluxes comparable to those estimated in Zhang et al. (2012) at the majority of the sites. In this study, the net GEM dry deposition has been estimated separately for each dominant land use type surrounding each site, and this approach is also recommended for future calculations for easy application of the results to assessments of the mercury effects on various ecosystems.

<p><strong>Abstract.</strong> 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<sup>−2</sup> yr<sup>−1</sup>) almost double that in North America (6.1 μg m<sup>−2</sup> yr<sup>−1</sup>) due to the higher anthropogenic emissions in Asia. Measured median litterfall and throughfall mercury are 22.3 and 56.5 μg m<sup>−2</sup> yr<sup>−1</sup>, respectively, in Asia, 12.8 and 16.3 μg m<sup>−2</sup> yr<sup>−1</sup> in Europe, and 11.9 and 7.0 μg m<sup>−2</sup> yr<sup>−1</sup> 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 two or larger uncertainties in GOM plus PBM dry deposition need to be kept in mind when using these numbers for mercury impact studies.</p>

Modeled and measured bi-directional fluxes (BDFs) of ammonia (NH 3) were compared over fertilized soybean and corn canopies for three intensive sampling periods: the first, during the summer of 2002 in Warsaw, North Carolina (NC), USA; and the second and third during the summer of 2007 in Lillington, NC. For the first and the third experimental periods, the BDF model produced reasonable diurnal flux patterns. The model also produced correct flux directions (emission and dry deposition) and magnitudes under dry and wet canopy conditions and during day and nighttime for these two periods. However, the model fails to produce the observed very high upward fluxes from the second sampling period due to the fertilization application (and thus being much higher soil emission potentials in the field than the default model values), although this can be improved by adjusting model input of soil emission potentials. Model-measurement comparison results suggest that the model is likely capable for improving long-term or regional scale ammonia predictions if implemented in chemical transport models replace the traditional dry deposition models, although modifications are needed when applying to specific situations.