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Centre for Atmospheric Science

RHaMBLe - Reactive Halogens in the Marine Boundary Layer.

Programme Summary
RHaMBLe is an integrated programme aiming to quantify impacts of marine halogen emissions on atmospheric composition by the direct observation of a range of reactive halogen species (RHS) in the marine atmosphere. Emphasis is placed on the roles of halogens in oxidative processes and on secondary aerosol formation and transformations resulting from the cycling of RHS. RHaMBLe has extensive links to UK and Internationally funded programmes providing a package within UK SOLAS to link oceanic fluxes to atmospheric impacts.

The RHaMBLe open ocean studies combine two deployments targeted at investigating the magnitude and effects of multiphase halogen cycling. The two deployments link with the long-term studies in the tropical Atlantic Ocean on Sao Vicente in the Cape Verde Islands:
i) an intensive campaign and supplementation of the continuous measurements already funded at the Cape Verde observatory and
ii) ii) RRS Discovery Cruise D319, simultaneously transecting through the biologically-active upwelling region west of Mauritania.
The aim of these coupled experiments is to provide possibly the most comprehensive characterisation of halogen compounds in the marine atmosphere to date, including both organic and inorganic reactive species and providing substantial spatial and temporal coverage. A coastal field project has already been conducted in Septembet 2006 in Roscoff, providing direct observational linkage between new particle formation and RHS.

Programme Objectives

i) to quantify the spatial and temporal variability of RHS in the tropical and sub-tropical Atlantic, to assess the cycling mechanisms of reactive halogens in the marine atmosphere and hence quantify the regional and global effect on oxidative chemistry through perturbation of the HOx and NOx cycles, coupling to the sulphur cycle and the effect on ozone and on aqueous oxidation.

ii) to describe the formation and growth mechanisms and assess the regional importance of iodine-containing particles recently formed in the coastal and remote marine boundary layers and compare this with predicted contributions due to DMS oxidation.

Measurement campaigns
RHaMBLe comprises 3 core activities. The first two aim to quantify the remote ocean contribution of halogen chemistry to MBL processes and the third to quantify the coastal contribution. Cruise D319 is one of the two remote marine experiments in Work Packages 1 and 2 taking place in the tropical North Atlantic. The tropical Atlantic is of major importance for the Earth’s climate, ecology and biogeochemistry, yet is poorly characterised; large radiative fluxes and high water vapour concentrations lead to a high oxidative turnover. In addition, significant concentrations of reactive bromine have been observed in the tropical northeast Atlantic possibly due to emissions from the large ocean upwelling area off Mauritania (though reactions involving mineral aerosols or biomass-burning products transported from Africa may be involved). During late spring and summer, the region near the Cape Verde islands is typically influenced by “clean” North Atlantic marine air along the NNE trade winds, allowing study of background Atlantic air, expected to be frequently influenced by the Mauritanian upwelling and associated oceanic gases. During autumn and winter, CV is situated in the direct transport pathway of westerly dust and biomass burning plumes from Africa to the North Atlantic. The figure shows the probability of an airmass having passed over a region 5 days before arrival at CV for Jun-Aug 2000 and for the whole of 2000, using ECMWF data. Warm colours represent higher probability. A summer project will maximise the frequency of airmass trajectories passing over the Mauritanian upwelling region prior to arrival at the station, increasing the potential role of halogen chemistry; and minimise the dust input from Saharan outflow which dominates other periods and may significantly affect heterogeneous processes and perturb the radiative balance and hence photochemistry. At higher altitude, there may be significant dust outflows but MBL chemistry is less likely to be directly affected. The two experiments in WP 1 and 2focus on characterising and understanding halogen chemistry in the remote tropical MBL during this period.