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

Assessment of the performance of a compact concentric spectrometer system for Atmospheric Differential Optical Absorption Spectroscopy

A breadboard demonstrator of a novel UV/VIS grating spectrometer has been developed based upon a concentric arrangement of a spherical meniscus lens, concave spherical mirror and curved diffraction grating suitable for a range of atmospheric remote sensing applications from the ground or space. The spectrometer is compact and provides high optical efficiency and performance benefits over traditional instruments. The concentric design is capable of handling high relative apertures, owing to spherical aberration and comma being near zero at all surfaces. The design also provides correction for transverse chromatic aberration and distortion, in addition to correcting for the distortion called "smile", the curvature of the slit image formed at each wavelength. These properties render this design capable of superior spectral and spatial performance with size and weight budgets significantly lower than standard configurations. This form of spectrometer design offers the potential for exceptionally compact instrument for differential optical absorption spectroscopy (DOAS) applications from LEO, GEO, HAP or ground-based platforms. The breadboard demonstrator has been shown to offer high throughput and a stable Gaussian line shape with a spectral range from 300 to 450 nm at 0.5 nm resolution, suitable for a number of typical DOAS applications

Isoprene oxidation mechanisms: Measurements and modelling of OH and HO[subscript 2] over a South-East Asian tropical rainforest during the OP3 field campaign

Forests are the dominant source of volatile organic compounds into the atmosphere, with isoprene being the most significant species. The oxidation chemistry of these compounds is a significant driver of local, regional and global atmospheric composition. Observations made over Borneo during the OP3 project in 2008, together with an observationally constrained box model are used to assess our understanding of this oxidation chemistry. In line with previous work in tropical forests, we find that the standard model based on MCM chemistry significantly underestimates the observed OH concentrations. Geometric mean observed to modelled ratios of OH and HO[subscript 2] in airmasses impacted with isoprene are 5.32[subscript −4.43,superscript +3.68] and 1.18[subscript −0.30,superscript +0.30] respectively, with 68 % of the observations being within the specified variation. We implement a variety of mechanistic changes into the model, including epoxide formation and unimolecular decomposition of isoprene peroxy radicals, and assess their impact on the model success. We conclude that none of the current suggestions can simultaneously remove the bias from both OH and HO[subscript 2] simulations and believe that detailed laboratory studies are now needed to resolve this issue

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