Quantifying the contribution of marine organic gases to atmospheric iodine

Abstract

[1] Oceanic emissions of gaseous organic iodine-atom precursors have the potential to significantly affect atmospheric chemistry and climate, however there is currently considerable uncertainty associated with quantifying their sources. We present sea-air fluxes calculated from simultaneous air and seawater measurements of a comprehensive range of volatile organic iodine compounds (VOICs), including CH 3 I and the less commonly reported dihalomethanes CH 2 ICl, CH 2 IBr and CH 2 I 2 , made during two cruises in the Atlantic Ocean between 15-58°N. The combined dihalomethane flux provides a global iodine source (∼0.33 ± 0.19 Tg I y −1 ) comparable to that of CH 3 I, and a surface iodine atom source 3-4 times higher. However, a 1D atmospheric model reveals that, in the tropical east Atlantic Ocean in the vicinity of Cape Verde, even these combined VOIC fluxes are capable of supporting only ∼10-25% of the observed IO levels, and suggests that a substantial (340-640 nmol I m −2 d −1 ) additional photochemical source of iodine is required.

Detection of iodine monoxide in the tropical free troposphere

“…To convert satellite-retrieved IO SCDs into VCDs, knowledge about vertical profiles is required. Current atmospheric models cannot provide accurate profile information because of gaps in our understanding of the source mechanisms of IO over oceans (11,14). We have simulated the satellite view of our measured vertical profiles by using a radiative transfer model.…”

“…An increasing body of evidence from laboratory experiments (29-32), field observations (11,12,14), and modeling studies (11,14) suggests that very short-lived polyhalogenated iodocarbons, such as diiodomethane (CH 2 I 2 ; photolytic life time of 2-10 min), bromoiodomethane (CH 2 IBr; 1-2.5 h), or chloroiodomethane (CH 2 ICl; 2.4-8 h), as well as molecular iodine (I 2 ; 15 s) contribute significantly to the iodine source flux in the MBL and are needed to sustain elevated IO abundances in the remote MBL (2,16,33). Our airborne observations of ∼0.5-0.6 ppt IO in the central Pacific MBL are slightly lower but generally consistent with ∼1.5 ppt IO at Cape Verde Islands in the tropical Atlantic ocean (3,11), ∼0.9 ppt from ship observations over the Eastern Pacific (12,13), and up to a few ppt over upwelling areas a few hundred kilometers from the Peruvian coast (12).…”

“…More recent studies have measured IO at open ocean sites (3, 11-13), suggesting that there might be reactive iodine chemistry over much of the open ocean. Emissions of reactive iodine species over the remote ocean remain poorly understood (11,14) but are currently thought to be associated with primary production from biological sources (2, 14-16). Satellite maps of IO over tropical oceans (17) support the notion that iodine sources are mostly related to biological activity (2).…”

A nocturnal atmospheric loss of CH2I2 in the remote marine boundary layer

“…However, in certain regions -including at high latitudes where surface ocean iodide levels are low, and in locations where wind speeds are high and surface ozone (O 3 (g)) mixing ratios are relatively low -organic gases may contribute 50 % or more of the iodine source . There are few oceanic flux measurements of CH 2 I 2 and CH 2 ICl, but available data suggest that they contribute~0.11 and 0.17 Tg I yr. −1 to the global atmosphere (Jones et al 2010) similar in total to methyl iodide (CH 3 I) emissions .…”

“…Both CH 2 I 2 and CH 2 ICl are known to have short photolysis lifetimes in the atmosphere of~5 min and~1 h at midday in mid-latitudes, respectively (Rattigan et al 1997;Roehl et al 1997;Mössinger et al 1998), leading to strong diurnal cycles (Carpenter et al 1999;Jones et al 2010). Photolysis is assumed to be the sole driver of atmospheric loss of these compounds, and in the presence of chloride ions likely dominates aqueous loss also (Jones and Carpenter 2005;Martino et al 2005).…”