Detection of iodine monoxide in the tropical free troposphere

Abstract

Atmospheric iodine monoxide (IO) is a radical that catalytically destroys heat trapping ozone and reacts further to form aerosols. Here, we report the detection of IO in the tropical free troposphere (FT). We present vertical profiles from airborne measurements over the Pacific Ocean that show significant IO up to 9.5 km altitude and locate, on average, two-thirds of the total column above the marine boundary layer. IO was observed in both recent deep convective outflow and aged free tropospheric air, suggesting a widespread abundance in the FT over tropical oceans. Our vertical profile measurements imply that most of the IO signal detected by satellites over tropical oceans could originate in the FT, which has implications for our understanding of iodine sources. Surprisingly, the IO concentration remains elevated in a transition layer that is decoupled from the ocean surface. This elevated concentration aloft is difficult to reconcile with our current understanding of iodine lifetimes and may indicate heterogeneous recycling of iodine from aerosols back to the gas phase. Chemical model simulations reveal that the iodine-induced ozone loss occurs mostly above the marine boundary layer (34%), in the transition layer (40%) and FT (26%) and accounts for up to 20% of the overall tropospheric ozone loss rate in the upper FT. Our results suggest that the halogen-driven ozone loss in the FT is currently underestimated. More research is needed to quantify the widespread impact that iodine species of marine origin have on free tropospheric composition, chemistry, and climate.

Fig. 1.

Spectral proof of the detection of IO along the flight track. (A) The measured IO signals at 0.3 (MBL), 1.6 (TL), and 9.5 km (FT) altitude are overlaid on the noise level of the instrument and show the unique (fingerprint) absorption of IO as it varies with altitude. Spectra were recorded between 00:49 Coordinated Universal Time (UTC) and 01:02 UTC at 158.6° W and 6.9°–8.0° N. SCDs and rms noise values are SCD(9.5 km) = 0.7 ± 0.14 × 10¹³ molecules/cm² (molec/cm²), RMS(9.5 km) = 1.1 × 10⁻⁴; SCD(1.6 km) = 1.4 ± 0.16 × 10¹³ molec/cm², RMS(1.6 km) = 1.3 × 10⁻⁴; and SCD(0.3 km) = 2.0 ± 0.16 × 10¹³ molec/cm², RMS(0.3 km) = 1.2 × 10⁻⁴. The fit uncertainty is indicated by the SCD error. (B) The flight track is overlaid on a GOES-11 IR satellite image (Geostationary Operational Environmental Satellites, www.ncdc.noaa.gov/gibbs) from January 30, 2010 at 00:00 UTC; it shows a high rising cloud cover (dark blue and green) that is indicative of deep convection. Locations where IO was detected in the FT are shown; altitudes below 1.8 km are shaded orange. During the beginning and end of the flight, no high-sensitivity spectra were recorded.

Fig. 2.

Vertical profiles of IO and H₂O (in volume mixing ratio; A and B) and potential temperature and aerosol extinction (C). Profiles were retrieved for two different locations labeled descent and ascent (Fig. 1B). Open symbols denote data points below detection limit. The corresponding descent and ascent IO VCDs are 2.91 ± 0.78 × 10¹² and 2.49 ± 0.72 × 10¹² molec/cm². Error bars indicate the retrieval uncertainty.

Fig. 3.

Ozone loss simulations. (A) The total ozone loss rate and percent contributions constrained by IO observations and simulated bromine, HO_x, photolysis, and NO_y chemistry during the ascent profile (base case simulation; conditions in SI Text). (B) Sensitivity of the percentage contribution of iodine-induced ozone loss to the ozone mixing ratio (base case, O₃-varied). The shaded orange line indicates base case ozone levels (A).