05 Oct 2022
05 Oct 2022
Status: this preprint is open for discussion.

Mercury in the Free Troposphere and Bidirectional Atmosphere-Vegetation Exchanges – Insights from Maïdo Mountain Observatory in the Southern Hemisphere Tropics

Alkuin Maximilian Koenig1, Olivier Magand1, Bert Verreyken2,3,4,a, Jerome Brioude4, Crist Amelynck 2,3, Niels Schoon2, Aurélie Colomb5, Beatriz Ferreira Araujo6, Michel Ramonet7, Mahesh Kumar Sha7, Jean-Pierre Cammas8, Jeroen E. Sonke6, and Aurélien Dommergue1 Alkuin Maximilian Koenig et al.
  • 1Institut des Géosciences de l'Environnement, Univ Grenoble Alpes, CNRS, IRD, Grenoble INP, Grenoble, France
  • 2Royal Belgian Institute for Space Aeronomy, BIRA-IASB, Brussels, Belgium
  • 3Department of Chemistry, Ghent University, Ghent, Belgium
  • 4Laboratoire de l’Atmosphère et des Cyclones, UMR 8105, CNRS, Université de La Réunion, Météo France, 97744 Saint-Denis, La Réunion, France
  • 5Laboratoire de Météorologie Physique, UMR6016, CNRS, Université Clermont Auvergne, 63178 Aubière, France
  • 6Géosciences Environnement Toulouse, CNRS/IRD/Université Paul Sabatier Toulouse 3, Toulouse, France
  • 7Laboratoire des Sciences du Climat et de l’Environnement, Université Paris-Saclay, CEA-CNRS-UVSQ, UMR8212, Gif-sur-Yvette, France
  • 8Observatoire des Sciences de l'Univers à La Réunion (OSU-R), UAR 3365, CNRS, Université de La Réunion, Météo France, 97744 Saint-Denis, La Réunion, France
  • anow at: NOAA Chemical Sciences Laboratory, Boulder, CO 80305, United States of America

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 days) for 9 months at Maïdo mountain observatory (2160 masl) 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 surface 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 airmasses 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 the weak seasonal variation reported for the SH marine boundary layer, but goes 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.

Alkuin Maximilian Koenig et al.

Status: open (until 03 Dec 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-903', Anonymous Referee #1, 13 Nov 2022 reply
  • RC2: 'Comment on egusphere-2022-903', Anonymous Referee #2, 29 Nov 2022 reply

Alkuin Maximilian Koenig et al.

Alkuin Maximilian Koenig et al.


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Short summary
The global distribution of mercury, a potent neurotoxin, depends on atmospheric transport, chemistry, and interactions between the earth’s surface and the air. Our understanding of these processes is still hampered by insufficient observations. Here, we present new data from a mountain observatory in the Southern Hemisphere. We give insights into mercury concentrations in air masses coming from aloft, and we show that tropical mountain vegetation may be a daytime source of mercury to the air.