Alpine ice evidence of a three-fold increase in atmospheric iodine deposition since 1950 in Europe due to increasing oceanic emissions

Abstract

Iodine is an important nutrient and a significant sink of tropospheric ozone, a climate-forcing gas and air pollutant. Ozone interacts with seawater iodide, leading to volatile inorganic iodine release that likely represents the largest source of atmospheric iodine. Increasing ozone concentrations since the preindustrial period imply that iodine chemistry and its associated ozone destruction is now substantially more active. However, the lack of historical observations of ozone and iodine means that such estimates rely primarily on model calculations. Here we use seasonally resolved records from an Alpine ice core to investigate 20th century changes in atmospheric iodine. After carefully considering possible postdepositional changes in the ice core record, we conclude that iodine deposition over the Alps increased by at least a factor of 3 from 1950 to the 1990s in the summer months, with smaller increases during the winter months. We reproduce these general trends using a chemical transport model and show that they are due to increased oceanic iodine emissions, coupled to a change in iodine speciation over Europe from enhanced nitrogen oxide emissions. The model underestimates the increase in iodine deposition by a factor of 2, however, which may be due to an underestimate in the 20th century ozone increase. Our results suggest that iodine's impact on the Northern Hemisphere atmosphere accelerated over the 20th century and show a coupling between anthropogenic pollution and the availability of iodine as an essential nutrient to the terrestrial biosphere.

Keywords: Alpine ice core; GEOS-Chem; iodine; trend; tropospheric ozone.

Fig. 1.

(A) Time series of iodine in Col du Dome ice core in summer (red) and winter (blue). Dots are yearly values; solid lines are the first component of a single spectra analysis with a 7-y time window (summer) and robust spline (winter). Vertical bars show the modeled mean deposition for summer (brown) and winter (purple) for 1850 (shown at 1890), 1950, 1980, 1995, and 2005. The dashes and arrow on the bars show the deposition of inorganic iodine minus HOI (lower dash) and minus half of HOI deposition (middle dash), and the total (top arrow). (B and C) Modeled seasonality of surface iodine species mixing ratio in 1850 (PI) and 2005 (PD); other iodine is composed of HI, I₂, I_xO_y, IBr, ICl, IONO, and aerosol phase iodine. (D and E) Modeled seasonality of iodine species deposition. (F–H) Seasonal cycle of iodine measured in the CDD ice cores averaged for three time periods: 1930–1950, 1965–1980, and 1980–1995. Outlier values in winter 1994/1995 were not considered, as well as values observed in the 1948 summer layers (SI Appendix). Data obtained in firn layers were also not considered (SI Appendix).

Fig. 2.

Modeled percentage change between PI (1850) and PD (2005) changes in December and July (A and D) iodine emissions, (B and E) surface inorganic iodine concentrations, and (C and F) inorganic iodine deposition. Concentric green circles give the location of CDD.