Mercury in the free troposphere and bidirectional atmosphere-vegetation exchanges - insights from Maido mountain observatory in the Southern Hemisphere tropics

Atmospheric mercury (Hg) observations in the lower free troposphere (LFT) can give important insights into Hg redox chemistry and can help constrain Hg background concentrations on a regional level. Relatively continuous sampling of LFT air, inaccessible to most ground-based stations, can be achieved at highaltitude observatories. However, such high-altitude observatories are rare, especially in the Southern Hemisphere (SH), and atmospheric Hg in the SH LFT is unconstrained. To fill this gap, we continuously measured gaseous elemental mercury (GEM; hourly) and reactive mercury (RM; integrated over similar to 6-14 d) for 9 months at Maido mountain observatory (2160ma.s.l.) on remote Reunion Island (21.1 degrees S, 55.5 degrees E) in the tropical Indian Ocean. GEM exhibits a marked diurnal variation characterized by a midday peak (mean: 0.95 ngm(-3); SD: 0.08 ngm(-3)) and a nighttime low (mean: 0.78 ngm(-3); SD: 0.11 ngm(-3)). We find that this diurnal variation is likely driven by the interplay of important GEM photo-reemission from the islands' vegetated surfaces (i.e. vegetation C soil) during daylight hours (8-22 ngm(-2) h(-1)), boundary layer influences during the day, and predominant LFT influences at night. We estimate GEM in the LFT based on nighttime observations in particularly dry air masses and find a notable seasonal variation, with LFT GEM being lowest from December to March (mean 0.66 ngm 3; SD: 0.07 ngm 3) and highest from September to November (mean: 0.79 ng m(-3); SD: 0.09 ng m(-3)). Such a clear GEM seasonality contrasts with the weak seasonal variation reported for the SH marine boundary layer but is in line with modeling results, highlighting the added value of continuous Hg observations in the LFT. Maido RM is 10.6 pg m(-3) (SD: 5.9 pg m(-3)) on average, but RM in the cloud-free LFT might be about twice as high, as weekly-biweekly sampled RM observations are likely diluted by low-RM contributions from the boundary layer and clouds.