Measurements and modelling of molecular iodine emissions, transport and photodestruction in the coastal region around Roscoff

Abstract

Abstract. Iodine emissions from the dominant six macroalgal species in the coastal regions around Roscoff, France, have been modelled to support the Reactive Halogens in the Marine Boundary Layer Experiment (RHaMBLe) undertaken in September 2006. A two-dimensional model is used to explore the relationship between geographically resolved regional emissions (based on maps of seaweed beds in the area and seaweed I 2 emission rates previously measured in the laboratory) and in situ point and line measurements of I 2 performed respectively by a broadband cavity ringdown spectroscopy (BBCRDS) instrument sited on the shoreline and a long-path differential optical absorption spectroscopy (LP-DOAS) instrument sampling over an extended light path to an off-shore island. The modelled point and line I 2 concentrations compare quantitatively with BBCRDS and LP-DOAS measurements, and provide a link between emission fields and the different measurement geometries used to quantify atmospheric I 2 concentrations during RHaMBLe. Total I 2 emissions over the 100 km 2 region around Roscoff are calculated to be 1.7×10 19 molecules per second during the lowest tides.During the night, the model replicates I 2 concentrations up to 50 pptv measured along the LP-DOAS instrument's line of sight, and predicts spikes of several hundred pptv in certain conditions. Point I 2 concentrations up to 50 pptv are also calculated at the measurement site, in broad agreementCorrespondence to: R. J. Leigh (r.j.leigh@leicester.ac.uk) with the BBCRDS observations. Daytime measured concentrations of I 2 at the site correlate with modelled production and transport processes. However substantial recycling of the photodissociated I 2 is required for the model to quantitatively match measured concentrations. This result corroborates previous modelling of iodine and NO x chemistry in the semi-polluted marine boundary layer which proposed a mechanism for recycling I 2 via the formation, transport and subsequent reactions of the IONO 2 reservoir compound.The methodology presented in this paper provides a tool for linking spatially distinct measurements to inhomogeneous and temporally varying emission fields.

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

“…This necessary flux is higher than previously estimated fluxes, e.g. 1.2 × 10 11 molecule cm −2 in Roscoff (Mahajan et al, 2009;Leigh et al, 2010). Figure 4 shows the I 2 /I ratio predicted by the model for the above 4 scenarios.…”

“…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.…”

Light extinction by secondary organic aerosol: an intercomparison of three broadband cavity spectrometers

“…The BBCRDS technique is based on the simultaneous measurement of wavelength-resolved ring-down transients in a high-finesse optical cavity containing the sample (Bitter et al, 2005;Ball and Jones, 2003;Leigh et al, 2010). Light from a pulsed broadband dye laser (662 nm emission maximum with FWHM of 16 nm) pumped at 20 Hz by a frequency-doubled Nd:YAG laser was directed into a 183 cm long ring-down cavity formed by two highly reflective mirrors (maximum reflectivity, R max = 99.996 % at 680 nm).…”

In situ detection of atomic and molecular iodine using Resonance and Off-Resonance Fluorescence by Lamp Excitation: ROFLEX

“…However, in the mid-latitude coastal MBL, where the spatial distribution of iodine emissions is likely to be heterogeneous, in situ measurements near the iodine emission sources are very valuable. In situ techniques such as online detection by Broadband Cavity Ring Down Spectroscopy (BBCRDS) (Bitter et al, 2005;Leigh et al, 2010), and Atmospheric Pressure Chemical Ionization with Tandem Mass Spectrometry (APCI/MS/MS) (Finley and Saltzman, 2008), and analysis of diffusion denuder air samples with Inductively Coupled Plasma Mass Spectrometry (ICP-MS) (Saiz-Lopez et al, 2006) or Gas Chromatography Mass Spectrometry (GC-MS) (Huang et al, 2010), have also been applied to the quantitative detection of atmospheric I 2 . h Mixing ratio detection limit in pptv for S/N = 1.…”