C033-02
Rectifier Effect due to Seasonality in Convective Mixing in Firn
Thursday, 10 December 2020: 10:33
Virtual
Cathy M Trudinger1, David M Etheridge1, Christo Buizert2, Benjamin Hmiel3, Paul B Krummel1, Ray Leonard Langenfelds1, Martin Manning4, Blagoj Mitrevski1, Peter D Neff5, Vasilii V Petrenko3, Jeffrey P Severinghaus6, Andrew Milford Smith7 and Martin K Vollmer8, (1)CSIRO, Oceans and Atmosphere, Aspendale, VIC, Australia, (2)Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR, United States, (3)University of Rochester, Department of Earth and Environmental Sciences, Rochester, NY, United States, (4)Victoria University of Wellington, Wellington, New Zealand, (5)University of Minnesota Twin Cities, Department of Soil, Water, and Climate, Minneapolis, MN, United States, (6)Scripps Institution of Oceanography, La Jolla, CA, United States, (7)Australian Nuclear Science and Technology Organisation (ANSTO), Kirrawee DC, Australia, (8)EMPA, Duebendorf, Switzerland
Abstract:
Air bubbles trapped in polar ice can be used to infer historical variations in atmospheric composition. The trapped air is influenced not only by past atmospheric composition but also by processes that occur in the firn (the porous layer of compacted snow in the upper part of an ice sheet). An understanding of these processes is necessary to accurately interpret ice core measurements. Diffusive mixing is important throughout the firn, but convective mixing (e.g. caused by pressure gradients due to surface wind or buoyancy due to seasonal temperature gradients) can dominate near the surface at some sites. Covariation of convective mixing in the upper firn and seasonality of the atmospheric abundance of some tracers has the potential to shift the mean level of a tracer in the firn and ice relative to the mean of the atmospheric record, a so-called ‘rectifier effect’.
We have found evidence that a rectifier effect has influenced carbon monoxide (CO) at the Antarctic DE08-OH site sampled in December 2018, causing an offset of about 2 ppb in CO. While the possibility of such rectifier effects has been suggested in the past, to date there has been no confirmation. Measurements of CO at the DE08-OH site provide a unique opportunity to confirm and quantify rectifier effects because the direct atmospheric measurements of CO from 1997 at the nearby Casey Station a) cover most of the time period of the DE08-OH firn record and b) were measured at the same laboratory as the firn air. We use a numerical model of the firn processes to investigate rectifier effects at DE08-OH for CO and several other trace gases with significant seasonality in the atmosphere, and to look for evidence of rectifier effects at other firn sites. We also try to understand the likely mechanisms responsible for seasonality in convective mixing. While the rectifier effect leads to offsets that are small compared to the measurement and modelling uncertainties for most trace gases, our modelling suggests that it may be a significant effect in some circumstances, such as for 14CO.
