Coastal measurements of short-lived reactive iodocarbons and bromocarbons at Roscoff, Brittany during the RHaMBLe campaign

Atmospheric concentrations of the volatile reactive iodocarbons C[subscript 2]H[subscript 5]I, 1-C[subscript 3]H[subscript 7]I, 2-C[subscript 3]H[subscript 7]I, CH[subscript 2]ICl, CH[subscript 2]IBr, CH[subscript 2]I[subscript 2] and bromocarbons CH[subscript 2]Br[subscript 2] and CHBr[subscript 3] were determined by GC/MS analysis of marine boundary layer air at Roscoff, Brittany on the northwest coast of France during September 2006. Comparison with other coastal studies suggests that emissions of these trace gases are strongly influenced by site topography, seaweed populations and distribution, as well as wind speed and direction and tide height. Concentrations of the very short-lived dihalomethanes CH[subscript 2]IBr and CH[subscript 2]I[subscript 2] in particular showed evidence of tidal dependence, with higher concentrations observed at low tide during maximum exposure of seaweed beds. We also present a limited number of halocarbon measurements in surface seawater and estimate sea-air fluxes based on these and simultaneous air measurements. CH[subscript 2]Br[subscript 2] and CHBr[subscript 3] were strongly correlated both in air and in seawater, with CH[subscript 2]Br[subscript 2]/CHBr[subscript 3] ratios of 0.19 in air and 0.06 in water. The combined midday I atom flux from the photolabile diahlomethanes CH[subscript 2]I[subscript 2], CH[subscript 2]IBr and CH[subscript 2]ICl of ~5×10[superscript 3] molecules cm[superscript −3] s[superscript −1] is several orders of magnitude lower than the estimated I atom flux from I[subscript 2] based on coinciding measurements at the same site, which indicates that at Roscoff the major I atom precursor was I[subscript 2] rather than reactive iodocarbons

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

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[subscript 2] emission rates previously measured in the laboratory) and in situ point and line measurements of I[subscript 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[subscript 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[subscript 2] concentrations during RHaMBLe. Total I[subscript 2] emissions over the 100 km[superscript 2] region around Roscoff are calculated to be 1.7×10[superscript 19] molecules per second during the lowest tides. During the night, the model replicates I[subscript 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[subscript 2] concentrations up to 50 pptv are also calculated at the measurement site, in broad agreement with the BBCRDS observations. Daytime measured concentrations of I[subscript 2] at the site correlate with modelled production and transport processes. However substantial recycling of the photodissociated I[subscript 2] is required for the model to quantitatively match measured concentrations. This result corroborates previous modelling of iodine and NO[subscript x] chemistry in the semi-polluted marine boundary layer which proposed a mechanism for recycling I[subscript 2] via the formation, transport and subsequent reactions of the IONO[subscript 2] reservoir compound. The methodology presented in this paper provides a tool for linking spatially distinct measurements to inhomogeneous and temporally varying emission fields

Quantifying the magnitude of a missing hydroxyl radical source in a tropical rainforest

The lifetime of methane is controlled to a very large extent by the abundance of the OH radical. The tropics are a key region for methane removal, with oxidation in the lower tropical troposphere dominating the global methane removal budget (Bloss et al., 2005). In tropical forested environments where biogenic VOC emissions are high and NO[subscript x] concentrations are low, OH concentrations are assumed to be low due to rapid reactions with sink species such as isoprene. New, simultaneous measurements of OH concentrations and OH reactivity, k'[subscript OH'], in a Borneo rainforest are reported and show much higher OH than predicted, with mean peak concentrations of ~2.5×10[superscript 6] molecule cm[superscript −3] (10 min average) observed around solar noon. Whilst j(O[superscript 1]D) and humidity were high, low O[subscript 3] concentrations limited the OH production from O[subscript 3] photolysis. Measured OH reactivity was very high, peaking at a diurnal average of 29.1±8.5 s[superscript −1], corresponding to an OH lifetime of only 34 ms. To maintain the observed OH concentration given the measured OH reactivity requires a rate of OH production approximately 10 times greater than calculated using all measured OH sources. A test of our current understanding of the chemistry within a tropical rainforest was made using a detailed zero-dimensional model to compare with measurements. The model over-predicted the observed HO[subscript 2] concentrations and significantly under-predicted OH concentrations. Inclusion of an additional OH source formed as a recycled product of OH initiated isoprene oxidation improved the modelled OH agreement but only served to worsen the HO2 model/measurement agreement. To replicate levels of both OH and HO[subscript 2], a process that recycles HO[subscript 2] to OH is required; equivalent to the OH recycling effect of 0.74 ppbv of NO. This recycling step increases OH concentrations by 88% at noon and has wide implications, leading to much higher predicted OH over tropical forests, with a concomitant reduction in the CH[subscript 4] lifetime and increase in the rate of VOC degradation