Eight-year atmospheric mercury deposition to a tropical high mountain background site downwind of the East Asian continent
Guey-Rong Sheu1, Ly Sy Phu Nguyen2, Leiming Zhang3, Da-Wei Lin4 and Neng-Huei Lin5
Atmospheric deposition has been identified as an important source of mercury (Hg) to terrestrial and aquatic ecosystems. Although East Asia is the major atmospheric Hg emission source region, very few studies have been conducted to quantify atmospheric Hg deposition in its downwind region. In this study, 8-year (2009–2016) atmospheric Hg deposition was reported at the Lulin Atmospheric Background Station (LABS; 120.87ºE, 23.47ºN, 2862 m a.s.l.), a high mountain forest site in central Taiwan. Dry deposition of speciated Hg was estimated using a bi-directional air-surface flux exchange model for gaseous elemental mercury (GEM) and dry deposition models for gaseous oxidized mercury (GOM) and particulate-bound mercury (PBM), making use of the monitored speciated atmospheric Hg concentrations. Wet deposition of Hg was calculated as the product of rainwater Hg concentration and rainfall depth. Annual total Hg dry deposition ranged from 51.9 to 84.9 µg m-2 yr-1 with a multi-year average of 66.1 µg m-2 yr-1. Among the three forms of atmospheric Hg, GEM was the main contributor to the total dry deposition, contributing about 77.8% to the total, due to the high density of forest canopy as well as the much higher concentration of GEM than GOM and PBM at LABS. Mercury dry deposition is higher in winter and spring than in summer and fall, partly due to the elevated Hg concentrations associated with air masses from East and Southeast Asia where with high atmospheric Hg emissions. Annual wet Hg deposition ranged from 10.9 to 48.9 µg m-2 yr-1 with a multi-year average of 27.7 µg m-2 yr-1. The mean annual dry/wet deposition ratio of 2.8 at LABS indicated that Hg deposition to its surrounding forest landscape was governed by dry rather than wet deposition. The modeled dry and measured wet depositions at this tropical mountain site were significantly higher than those reported for remote sites in temperate regions. Such a finding has important implications in Taiwan and other tropical Southeast Asian countries since most land covers of mountains in this region are forests.
1National Central University, grsheu@atm.ncu.edu.tw 2National Central University 3Environment and Climate Change Canada 4National Central University 5National Central University