Convective forcing of mercury and ozone in the Arctic boundary layer induced by leads in sea ice

Abstract

The ongoing regime shift of Arctic sea ice from perennial to seasonal ice is associated with more dynamic patterns of opening and closing sea-ice leads (large transient channels of open water in the ice)1,2,3, which may affect atmospheric and biogeochemical cycles in the Arctic4. Mercury and ozone are rapidly removed from the atmospheric boundary layer during depletion events in the Arctic5,6,7, caused by destruction of ozone along with oxidation of gaseous elemental mercury (Hg(0)) to oxidized mercury (Hg(ii)) in the atmosphere and its subsequent deposition to snow and ice5. Ozone depletion events can change the oxidative capacity of the air by affecting atmospheric hydroxyl radical chemistry8, whereas atmospheric mercury depletion events can increase the deposition of mercury to the Arctic6,9,10,11, some of which can enter ecosystems during snowmelt12. Here we present near-surface measurements of atmospheric mercury and ozone from two Arctic field campaigns near Barrow, Alaska. We find that coastal depletion events are directly linked to sea-ice dynamics. A consolidated ice cover facilitates the depletion of Hg(0) and ozone, but these immediately recover to near-background concentrations in the upwind presence of open sea-ice leads. We attribute the rapid recoveries of Hg(0) and ozone to lead-initiated shallow convection in the stable Arctic boundary layer, which mixes Hg(0) and ozone from undepleted air masses aloft. This convective forcing provides additional Hg(0) to the surface layer at a time of active depletion chemistry, where it is subject to renewed oxidation. Future work will need to establish the degree to which large-scale changes in sea-ice dynamics across the Arctic alter ozone chemistry and mercury deposition in fragile Arctic ecosystems.

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Figure 1: Time series of Hg(0) and O3 concentrations.
Figure 2: Impact of sea-ice leads on Hg(0) and O3 in 2012.
Figure 3: Impact of sea-ice leads on Hg(0) and O3 in 2009.

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Acknowledgements

This research was supported in part by the National Aeronautics and Space Administration (NASA) Cryospheric Sciences Program (CSP) and by the Desert Research Institute. The Science and Technology Branch of Environment Canada helped fund Hg measurements in 2012 along with the Canadian International Polar Year government programme in 2009. The research at the Jet Propulsion Laboratory, California Institute of Technology, was supported by NASA CSP. We thank Umiaq for field logistic assistance, the Barrow whaling community for beneficial interactions, and the National Oceanic and Atmosphere Administration (NOAA), Global Monitoring Division for the Barrow Observatory data. We gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for provision of the HYSPLIT transport and dispersion model and READY Website (http://www.ready.noaa.gov) used in this publication. We thank K. Pratt and R. Kreidberg for feedback on the manuscript, B. Hatchett and T. Malamakal for help with radiosonde and WRF data, D. Hall and J. Schmaltz for MODIS imagery support, and J. Deary for outstanding field technical support.

Author information

C.W.M. and D.O. performed data analysis, participated in the 2012 field campaign, and prepared an initial version of the manuscript. A.S., R.M.S. and T.A.D. were instrumental in both the 2009 and 2012 field campaigns along with data analysis and interpretation. A.R. was responsible for satellite BrO column data retrieval and interpretation. S.V.N. was responsible for satellite data retrieval and interpretation for 2009 and 2012 and participated in the 2012 field campaign. All authors participated in the writing and editing of the manuscript.

Correspondence to Christopher W. Moore.

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The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 A second example of the impact of sea-ice leads on Hg(0) and O3 in 2012.

Time series of Hg(0) and O3 concentrations near Barrow between 26 March 2012 and 29 March 2012. Bold numbers correspond to time periods, as numbered on the corresponding satellite images. 24-hour HYSPLIT back-trajectories were generated every four hours from 26 March to 29 March 2012. Satellite images were taken at approximately 16:00 utc each day. Colours represent trajectory arrival times near Barrow: orange, 04:00 utc; blue, 08:00 utc; red, 12:00 utc; pink, 16:00 utc; yellow, 20:00 utc; black, 00:00 utc (the next day); and purple, 04:00 utc (the next day). Original satellite images from Google Earth, Terrametrics.

Extended Data Figure 2 A third example of the impact of sea-ice leads on Hg(0) and O3 in 2012.

Time series of Hg(0) and O3 concentrations near Barrow between 29 March 2012 and 1 April 2012. Bold numbers correspond to time periods, as numbered on the corresponding satellite images. 24-hour HYSPLIT back-trajectories were generated every four hours from 29 March 2012 to 31 March 2012. Satellite images were taken at approximately 16:00 utc each day. Colours represent trajectory arrival times near Barrow: orange, 04:00 utc; blue, 08:00 utc; red, 12:00 utc; pink, 16:00 utc; yellow, 20:00 utc; black, 00:00 utc (the next day); and purple, 04:00 utc (the next day). Original satellite images from Google Earth, Terrametrics.

Extended Data Figure 3 Meteorological and radiosonde data for 22–24 March 2012.

The lower 1,000 m of a radiosonde launch from Barrow Airport at 05:30 utc on 22 March 2012 (a), 23 March 2012 (b) and 24 March 2012 (c). The change in potential temperature with height (dΦ/dz) indicates a boundary layer height near 100 m for 22 March and 23 March (air mass over consolidated sea ice) and grows to 250 m on 24 March (the lead influence). d, Wind rose for 22 March 2012 0:00 to 26 March 2012 0:00; wind directions are consistently from the east to the northeast, in support of the calculated air-mass trajectories in Fig. 2.

Extended Data Figure 4 Atmospheric BrO around Barrow.

BrO columns estimated from the Global Ozone Monitoring Experiment-2 (GOME-2) spectrometer on the Meteorological Operational satellite-Metop-A17 during the time period corresponding to patterns shown in Fig. 2. The image domain is 60°–80° N and 170°–270° E.

Extended Data Figure 5 Low wind speed associated with an opening lead.

O3 and wind speed from 26 March 2012 to 30 March 2012. The bold number corresponds to case 13, when an opening lead influences concentrations of O3 and Hg(0) that quickly recover to near-background levels. During this time period, wind speeds remained low (below 3 m s−1), indicating that concentration recoveries were not linked to increased wind speed through increased wind shear.

Extended Data Table 1 Key variable means during the two campaigns

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Moore, C., Obrist, D., Steffen, A. et al. Convective forcing of mercury and ozone in the Arctic boundary layer induced by leads in sea ice. Nature 506, 81–84 (2014). https://doi.org/10.1038/nature12924

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