Abstract
Abstract. We report weekly observations of volatile organic iodocarbons (CH3I, CH2ClI and CH2I2) over the time period May 2015 to December 2017 from four depths in Bedford Basin, a coastal fjord (70 m deep) on the Atlantic coast of Canada. The fjord is subject to wintertime mixing, seasonal stratification and bloom dynamics, subsurface oxygen depletion, local input of freshwater, and occasional intrusions of higher-density water from the adjacent continental shelf. Near-surface concentrations showed strong seasonal and sub-seasonal variability, which is compared with other coastal time series. The vertical variation of CH2I2 and CH2ClI within the upper 10 m is consistent with rapid photolysis of CH2I2. Average annual sea-to-air fluxes (46.7 nmol m−2 day−1) of total volatile organic iodine were similar to those observed in other coastal and shelf time series, and polyiodinated compounds contributed 80 % of the total flux. Fluxes were subject to strong interannual variability (a factor of 2) mainly due to wind speed variability. Near-surface net production of CH3I averaged 1 pmol L−1 day−1 and was similar to rates in the English Channel but an order of magnitude higher than in shallow waters of the Kiel Fjord, Germany, possibly due to higher microbial degradation in the latter. The near-bottom (60 m) time series showed evidence of CH3I production associated with organic matter degradation and a possible “switch” from the production of CH3I via an alkylation pathway to the production of CH2I2 by a haloform-type reaction. Near-bottom CH3I production varied strongly between years but was generally ca. 20 times lower than near-surface production.
Highlights
Volatile organic iodocarbons (VOIs) such as methyl iodide (CH3I), chloroiodomethane (CH2ClI) and diiodomethane (CH2I2) have a predominantly oceanic source and supply a significant amount of iodine to the atmosphere
In the following we discuss the Bedford Basin data in comparison with other studies that have reported concentrations of multiple iodocarbons, especially those that have reported time series covering an annual cycle. All of these time series are from midlatitude (40– 60◦ N) nearshore or continental shelf environments subject to strong seasonal variations of light, temperature and biological productivity
We investigated this in July 2017 by sampling at five nearshore sites around Bedford Basin (Fig. 1) and comparing nearshore concentrations with values measured at the regular sampling site in the center of the basin (Table 4)
Summary
Volatile organic iodocarbons (VOIs) such as methyl iodide (CH3I), chloroiodomethane (CH2ClI) and diiodomethane (CH2I2) have a predominantly oceanic source and supply a significant amount of iodine to the atmosphere (see review by Saiz-Lopez and Von Glasow, 2012) These gases, referred to as VSLSs (very short-lived halogenated substances) due to their reactivity and short atmospheric lifetimes, have been implicated in supporting catalytic ozone destruction in the troposphere (Davis et al, 1996; McFiggans et al, 2000) and potentially in the lower stratosphere (Solomon et al, 1994) as well as aerosol formation in the marine boundary layer (McFiggans et al, 2000, 2004; O’Dowd et al, 2002). The total supply of organically bound iodine to the atmosphere is sev-
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