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

[摘要] Atmospheric mercury (Hg) observations in the lower free troposphere (LFT)can give important insights into Hg redox chemistry and can help constrainHg background concentrations on a regional level. Relatively continuoussampling of LFT air, inaccessible to most ground-based stations, can beachieved at high-altitude observatories. However, such high-altitudeobservatories are rare, especially in the Southern Hemisphere (SH), andatmospheric Hg in the SH LFT is unconstrained. To fill this gap, wecontinuously measured gaseous elemental mercury (GEM; hourly) and reactivemercury (RM; integrated over ∼  6–14 d) for 9 months atMaïdo mountain observatory (2160 m a.s.l.) on remote Réunion Island(21.1 ∘  S, 55.5 ∘  E) in the tropical Indian Ocean. GEMexhibits 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 islikely driven by the interplay of important GEM photo-reemission from theislands' 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 onnighttime 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 toNovember (mean: 0.79 ng m −3 ; SD: 0.09 ng m −3 ). Such a clear GEMseasonality 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 LFTmight be about twice as high, as weekly–biweekly sampled RM observations are likely diluted by low-RM contributions from the boundary layer and clouds.

[学科分类] 医学（综合）