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

Abstract

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.

Probing the limits of sampling gaseous elemental mercury passively in the remote atmosphere

“…GEM has been measured with vapour analyzers 2537A/B (Tekran Inc., Toronto, Canada), running at standard temperature and pressure conditions (273.15 K, 1013.25 hPa), at AMS 11 and DMC 20 since 2012, and at the Maïdo Observatory from 2017 to 2018. 23 These measurements are based on Hg enrichment on a gold cartridge, followed by a thermal desorption and detection by cold vapour atomic fluorescence spectroscopy. Switching between two cartridges allows for alternating sampling and desorption and thus full temporal coverage.…”

Observed in-plume gaseous elemental mercury depletion suggests significant mercury scavenging by volcanic aerosols

“…GEM concentrations at Maïdo are mostly below 1 ng m −3 , but show a marked and regular diurnal variation that has been attributed to mixing processes and surface photo-reemission. 25…”

“…The GEM fluctuation before and afterward does not correspond to a volcanic signal, but the habitual diurnal GEM variation at Ma ïdo. 25 sampling period (compared to the time before) can be attributed near-exclusively to volcanic plumes on the 29th of April.…”

“…GEM concentrations at Maïdo are mostly below 1 ng m −3 , but show a marked and regular diurnal variation that has been attributed to mixing processes and surface photo-reemission. 25 La Réunion has been shaped by volcanic activity and is characterized by its rugged orography. Two volcanoes form most of the island, the dormant Piton de Neiges, whose ∼3070 masl high summit is the highest peak of the island and is located about 10 km east of Maïdo observatory, and the very active Piton de la Fournaise (∼2630 masl), about 40 km southeast of the observatory.…”

Observed in-plume gaseous elemental mercury depletion suggests significant mercury scavenging by volcanic aerosols

“…Summary of observed magnitudes and instrumentation used in this study (see Figure 1). GEM was measured at Maïdo with a 15-minute time resolution with a Tekran 2537A analyzer, as already described in detail in Koenig et al (2023) 20 . The instrument was maintained and regularly calibrated according to standard GMOS (Global Mercury Observation System) procedures.…”