Abstract. 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 high-altitude 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 ∼ 6–14 d) for 9 months at
Maïdo mountain observatory (2160 m a.s.l.) on remote Réunion Island
(21.1∘ S, 55.5∘ E) in the tropical Indian Ocean. GEM
exhibits a marked diurnal variation characterized by a midday peak (mean:
0.95 ng m−3; SD: 0.08 ng m−3) and a nighttime low (mean: 0.78 ng m−3; SD: 0.11 ng m−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 + soil) during daylight hours
(8–22 ng m−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 ng m−3; SD: 0.07 ng m−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. Maïdo 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.