ICMGP - Conference Presentation

Abstract Information :

Mountain sites can give important information about atmospheric mercury (Hg) concentrations and dynamics in the free troposphere (FT). However, while several high-elevation Hg monitoring sites exist in the northern hemisphere, these are nearly absent in the southern hemisphere. To fill this gap, we continuously measured atmospheric Hg0 at Ma‹do observatory between September 2017 and May 2018. Ma‹do is located at 2160 MASL on a remote island in the tropical Indian ocean, around 700 km east of Madagascar. This site receives mostly FT air during nighttime while being influenced by the boundary layer (BL; both planetary and marine) during daytime. We use the predominantly FT character of nighttime air and observations on air mass humidity to derive a time series of Hg0 in the FT. Maido FT Hg0 was quite low on average (mean: 0.76 ng/m3; SD: 0.11), suggesting the presence of an important chemical or physical Hg0 sink in the tropical FT. We observed a notable seasonality, with higher FT Hg0 from September to November (mean: 0.82 ng/m3; SD: 0.10) and lower FT Hg0 from December to March (mean: 0.65 ng/m3; SD: 0.06). The diurnal variation of Ma‹do Hg0 was marked, being lowest during nighttime (mean: 0.78 ng/m3; SD: 0.11) and peaking around noon (mean: 0.95 ng/m3; SD: 0.06), following closely the diurnal variation of solar radiation. Based on air mass origin and source receptor relationships modelled with FLEXPART-AROME, we find that this Hg0 diurnal variation cannot be explained by FT-BL mixing alone. We suggest that diurnal GEM variation with its midday peak could be well explained if strong Hg0 photo-reemission from the island?s densely vegetated mountain slopes is assumed. Under this assumption, we estimate through inverse modelling that the vegetated mountain slopes may release on average a net 18 (12-23) ng/m2/h of Hg0 to the atmosphere during daylight hours.