In situ measurements of molecular iodine in the marine boundary layer: the link to macroalgae and the implications for O<sub>3</sub>, IO, OIO and NO<sub>x</sub>

Abstract

Abstract. Discrete in situ atmospheric measurements of molecular iodine (I 2 ) were carried out at Mace Head and Mweenish Bay on the west coast of Ireland using diffusion denuders in combination with a gas chromatography-mass spectrometry (GC-MS) method. I 2 , IO and OIO were also measured by long-path differential optical absorption spectroscopy (LP-DOAS). The simultaneous denuder and LP-DOAS I 2 measurements were well correlated (R 2 =0.80) but the denuder method recorded much higher concentrations. This can be attributed to the fact that the in situ measurements were made near to macroalgal sources of I 2 in the intertidal zone, whereas the LP-DOAS technique provides distance-averaged mixing ratios of an inhomogeneous distribution along the light-path. The observed mixing ratios of I 2 at Mweenish Bay were significantly higher than that at Mace Head, which is consistent with differences in local algal biomass density and algal species composition. Above algal beds, levels of I 2 were found to correlate inversely with tidal height and positively with the concentrations of O 3 in the surrounding air, indicating a role for O 3 in the production of I 2 from macroalgae, as has been previously suggested from laboratory studies. However, measurements made ∼150 m away from the algal beds showed a negative correlation between O 3 and I 2 during both day and night. We interpret these results to indicate that the released I 2 can also lead to O 3 destruction via the reaction of O 3 with I atoms that are formed by the photolysis of I 2 during the day and via the Correspondence to: T. Hoffmann (hoffmant@uni-mainz.de ) reaction of I 2 with NO 3 radicals at night. The results show that the concentrations of daytime IO are correlated with the mixing ratios of I 2 , and suggest that the local algae sources dominate the inorganic iodine chemistry at Mace Head and Mweenish Bay.

The seaweeds <i>Fucus vesiculosus</i> and <i>Ascophyllum nodosum</i> are significant contributors to coastal iodine emissions

“…Elevated I 2 mixing ratios of up to 302 ppt were observed after the algae had been exposed to air for several hours (Huang et al, 2010b). This behavior differs from the emission characteristics of L. digitata found in laboratory incubation experiments, where I 2 emissions are small or stop completely after prolonged exposure (Dixneuf et al, 2009;Ball et al, 2010;Nitschke et al, 2011).…”

“…In recent years, numerous studies have shown that the coastal particle bursts are closely linked to iodine emission from low-tidal macroalgae exposure (e.g., McFiggans et al, 2004;Sellegri et al, 2005;Saiz-Lopez et al, 2006a;Huang et al, 2010c;McFiggans et al, 2010). A clear negative correlation between IO (the precursor of IOP) and tidal height, and a positive correlation between IO and solar irradiation, have been observed (Carpenter et al, 2001;Saiz-Lopez et al, 2006b;Huang et al, 2010b;Commane et al, 2011). The emission of iodocarbons such as CH 2 I 2 from macroalgae was first proposed to be the source of photolabile iodine (Hoffmann et al, 2001;Carpenter, 2003).…”

Concurrent observations of atomic iodine, molecular iodine and ultrafine particles in a coastal environment

“…In the past, studies at other coastal locations have reported a daytime maximum of 25-29 pptv through integrated long-path differential optical absorption spectroscopy (LP-DOAS) measurements at Mace Head (Saiz-Lopez and Huang et al, 2010), 115 pptv using in situ measurements at Mace Head (Saiz-Lopez et al, 2006), 87 pptv at Mweenish Bay-I and 302 pptv Mweenish Bay-II, both close to Mace Head using in situ techniques (Huang et al, 2010), 32 pptv (integrated) and 50 pptv (in situ) at Roscoff (Mahajan et al, 2009;McFiggans et al, 2010), and 3 pptv (in situ) at Scripps Pier, La Jolla, California (Finley and Saltzman, 2008). The peak I atom mixing ratio of 10 ± 5 pptv is lower than a maximum of 22 pptv reported by Bale et al (2008) at Mace Head.…”

“…Measurements of I 2 have so far been reported at three different mid-latitudinal coastal locations: Mace Head, Ireland (SaizLopez and Peters et al, 2005;Huang et al, 2010), California, USA (Finley and Saltzman, 2008) and Roscoff, France (Mahajan et al, 2009;Leigh et al, 2010). However, detection of I atoms has only been reported by Bale et al (2008) at Mace Head.…”

Iodine monoxide in the north subtropical free troposphere

“…Measurements of tropospheric IO have been mostly located in mid-latitude marine coastal areas such as Mace Head, Ireland (Alicke et al, 1999;Saiz-Lopez and Plane, 2004;Huang et al, 2010) and Roscoff, France (Peters et al, 2005;Wada et al, 2007;Whalley et al, 2007;Mahajan et al, 2009) where its detection is conditioned to the biogenic activity when some species of algae are exposed to the atmosphere, emitting mainly I 2 at low tide (Saiz-Lopez and Plane, 2004). Therefore, the concentrations of IO measured in these field campaigns are highly variable (between 1 and 9.8 pptv).…”