Abstract
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 (CH3I) 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 × 1013 molecules cm−2 along the 1° elevation angle (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 CH3I, suggesting common sources. In situ measurements of meteorological parameters and physical ocean variables, along with satellite-based observations of Chlorophyll a (Chl a) 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 a 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.
Highlights
The recent observation of significant slant column densities (SCDs) of iodine monoxide (IO) over a large region of the Eastern Pacific from the satellite-borne instrument SCIAMACHY (Schonhardt et al, 2008) has raised the question of the potential climatic impact of iodine chemistry on a regional-global scale, given its well known link with ozone (O3) depletion, changes to the oxidizing capacity of the marine boundary layer (MBL) by affecting HOx and NOx, and new particle formation
Surface IO mixing ratios were retrieved from the MAXDOAS DSCDs using the O4 slant columns measured during the study by the “O4 method”, similar to other groups (e.g., Wagner et al, 2004; Sinreich et al, 2010) and validated by the NIMO fully spherical Monte Carlo radiative transfer model (Hay et al, 2012) where possible, which is described in detail in the SI
The model has been validated in the past (Saiz-Lopez et al, 2012a) and was revalidated recently for O3 mixing ratios at the surface at latitude 1◦ S, where year-long ground based measurements of O3 were made at the Galapagos Islands and agree well with the model simulation for April 2011
Summary
The recent observation of significant slant column densities (SCDs) of iodine monoxide (IO) over a large region of the Eastern Pacific from the satellite-borne instrument SCIAMACHY (Schonhardt et al, 2008) has raised the question of the potential climatic impact of iodine chemistry on a regional-global scale, given its well known link with ozone (O3) depletion, changes to the oxidizing capacity of the marine boundary layer (MBL) by affecting HOx and NOx, and new particle formation (see Saiz-Lopez et al, 2012b and references therein). Mahaja et al.: Latitudinal distribution of reactive iodine in the lower latitudes is poor (Schonhardt et al, 2008) Observation of such enhanced IO slant columns above the biologically active region off the coast of Peru would be consistent with the source of IO being the production of iodinecontaining species by marine algae due to the high Chlorophyll a (Chl a) content in upwelling regions. (i) assess the geographical distribution of RIS in the Eastern Pacific along a latitudinal transect and provide information for validation of IO satellite retrievals; and (ii) discuss emission mechanisms of precursors of RIS by studying correlations with potentially related oceanic variables such as sea surface temperature (SST), Chl a, salinity, Chromophoric DOM (CDOM) and other meteorological data
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