Latitudinal distribution of reactive iodine in the Eastern Pacific and its link to open ocean sources

Abstract

Ship-based Multi-Axis Differential Optical Absorption Spectroscopy measurements of iodine monoxide (IO) and atmospheric and seawater Gas Chromatography – Mass Spectrometer observations of methyl iodide (CH<sub>3</sub>I) were made in the Eastern Pacific marine boundary layer during April 2010 as a part of the HaloCarbon Air Sea Transect-Pacific (HaloCAST-P) scientific cruise. The presence of IO in the open ocean environment was confirmed, with a maximum differential slant column density of 5 × 10<sup>13</sup> molecules cm<sup>−2</sup> (corresponding to approximately 1 pptv) measured in the oligotrophic region of the Southeastern Pacific. Such low IO mixing ratios and their observed geographical distribution are inconsistent with satellite estimates and with previous understanding of oceanic sources of iodine. A strong correlation was observed between reactive iodine (defined as IO + I) and CH<sub>3</sub>I, suggesting common sources. In situ measurements of meteorological parameters and physical ocean variables, along with satellite-based observations of Chlorophyll <i>a</i> (Chl <i>a</i>) and Chromophoric Dissolved Organic Matter (CDOM) were used to gain insight into the possible sources of iodine in this remote environment. Surprisingly, reactive iodine showed a negative correlation (>99% confidence) to Chl <i>a</i> and CDOM across the cruise transect. However, a significant positive correlation (>99% confidence) with sea surface temperature (SST) and salinity instead suggests a widespread abiotic source related to the availability of aqueous iodine and to temperature

Reactive halogen chemistry in the troposphere

“…They also concluded that the measured iodocarbon precursors were not enough to explain the measured IO mixing ratios and suggested that I 2 could be an additional iodine precursor possibly produced from the deposition of O 3 on the ocean. 226 Mahajan et al 229 have recently reported the latitudinal distribution of reactive iodine in the Eastern Pacific MBL confirming the presence of IO in the open ocean MBL. The IO peak (approximately 1 ppt) was measured over the oligotrophic region of the southeastern Pacific.…”

Tropospheric Halogen Chemistry: Sources, Cycling, and Impacts

“…Measurements on a cruise in the East Pacific by Mahajan and co-workers 192 showed maximum IO mixing ratios of 1.2 pmol/mol. They observed a positive correlation with sea surface temperature and salinity but a negative correlation with organic matter in the surface ocean, chlorophyll, and atmospheric ozone.…”

“…192,193 At all these locations, comparisons of model results with field data suggest that organic iodine precursors are insufficient to explain the observed levels of IO, 154,177,191 pointing at the importance of I 2 and especially HOI as a source for reactive iodine from the reaction of O 3 on the ocean’s surface. 85,174−176 At Cape Verde, I 2 was measured recently 66 showing an increase in its mixing ratio at sunset, a leveling off around midnight and rapid decrease at sunrise.…”

Iodine chemistry in the eastern Pacific marine boundary layer

“…Laboratory and field data exist, linking very short‐lived iodocarbons and phytoplankton activity [ Smythe‐Wright et al ., ; Hill and Manley , ; Arnold et al ., ; Brownell et al ., ; Lai et al ., ]. However, there is increasing evidence that biological sources of iodine precursors are not the most significant [ Garland and Curtis , ; Reeser et al ., ; Martino et al ., ; Sakamoto et al ., ; Mahajan et al ., ]. Furthermore, it has been shown that iodocarbons are not the major contributor to active iodine in the open ocean MBL but rather inorganic molecules such as I 2 [ Jones et al ., ; Mahajan et al ., ].…”