Emission-dominated gas exchange of elemental mercury vapor over natural surfaces in China

Abstract. Mercury (Hg) emission from natural surfaces plays an important role in global Hg cycling. The present estimate of global natural emission has large uncertainty and remains unverified against field data, particularly for terrestrial surfaces. In this study, a mechanistic model is developed for estimating the emission of elemental mercury vapor (Hg⁰) from natural surfaces in China. The development implements recent advancements in the understanding of air–soil and air–foliage exchange of Hg⁰ and redox chemistry in soil and on surfaces, incorporates the effects of soil characteristics and land use changes by agricultural activities, and is examined through a systematic set of sensitivity simulations. Using the model, the net exchange of Hg⁰ between the atmosphere and natural surfaces of mainland China is estimated to be 465.1 Mg yr⁻¹, including 565.5 Mg yr⁻¹ from soil surfaces, 9.0 Mg yr⁻¹ from water bodies, and −100.4 Mg yr⁻¹ from vegetation. The air–surface exchange is strongly dependent on the land use and meteorology, with 9 % of net emission from forest ecosystems; 50 % from shrubland, savanna, and grassland; 33 % from cropland; and 8 % from other land uses. Given the large agricultural land area in China, farming activities play an important role on the air–surface exchange over farmland. Particularly, rice field shift from a net sink (3.3 Mg uptake) during April–October (rice planting) to a net source when the farmland is not flooded (November–March). Summing up the emission from each land use, more than half of the total emission occurs in summer (51 %), followed by spring (28 %), autumn (13 %), and winter (8 %). Model verification is accomplished using observational data of air–soil/air–water fluxes and Hg deposition through litterfall for forest ecosystems in China and Monte Carlo simulations. In contrast to the earlier estimate by Shetty et al. (2008) that reported large emission from vegetative surfaces using an evapotranspiration approach, the estimate in this study shows natural emissions are primarily from grassland and dry cropland. Such an emission pattern may alter the current understanding of Hg emission outflow from China as reported by Lin et al. (2010b) because a substantial natural Hg emission occurs in West China.