Abstract. In this second part of a series of articles dedicated to a detailed analysis of bromine chemistry in the atmosphere we address one (out of two) dominant natural sources of reactive bromine. The two main source categories are the release of bromine from sea salt and the decomposition of bromocarbons by photolysis and reaction with OH. Here, we focus on C1-bromocarbons. We show that the atmospheric chemistry general circulation model ECHAM5/MESSy realistically simulates their emission, transport and decomposition from the boundary layer up to the mesosphere. We included oceanic emission fluxes of the short-lived bromocarbons CH2Br2, CH2ClBr, CHClBr2, CHCl2Br, CHBr3 and of CH3Br. The vertical profiles and the surface mixing ratios of the bromocarbons are in general agreement with the (few available) observations, especially in view of the limited information available and the consequent coarseness of the emission fields. For CHBr3, CHCl2Br and CHClBr2 photolysis is the most important degradation process in the troposphere. In contrast to this, tropospheric CH2Br2, CH3Br and CH2ClBr are more efficiently decomposed by reaction with OH. In the free troposphere approximately 40% of the C1-bromocarbons decompose by reaction with OH. Our results indicate that bromoform contributes substantial amounts of reactive bromine to the lower stratosphere and thus should not be neglected in stratospheric simulations.

Abstract. Airborne measurements of the halogenated trace gases methyl chloride, methyl bromide and chloroform were conducted over the Atlantic Ocean and about 1000 km of pristine tropical rainforest in • N, 51-59 • W) in October 2005. In the boundary layer (0-1.4 km), maritime air masses, advected over the forest by southeasterly trade winds, were measured at various distances from the coast. Since the organohalogens presented here have relatively long atmospheric lifetimes (0.4-1.0 years) in comparison to the advection times from the coast (1-2 days), emissions will accumulate in air traversing the rainforest. The distributions of methyl chloride, methyl bromide and chloroform were analyzed as a function of time the air spent over land and the respective relationship used to determine net fluxes from the rainforest for one week within the long dry season.Net fluxes from the rainforest ecosystem have been calculated for methyl chloride and chloroform as 9.5 (±3.8 2σ ) and 0.35 (±0.15 2σ )µg m −2 h −1 , respectively. No significant flux was observed for methyl bromide within the limits of these measurements.The global budget of methyl chloride contains large uncertainties, in particular with regard to a possible source from tropical vegetation. Our measurements are used in a large-scale approach to determine the net flux from a tropical ecosystem to the planetary boundary layer. The obtained global net flux of 1.5 (±0.6 2σ ) Tg yr −1 for methyl chloride is at the lower end of current estimates for tropical vegetation sources, which helps to constrain the range of tropical sources and sinks (0.82 to 8.2 Tg yr −1 from tropical plants, 0.03 to 2.5 Tg yr −1 from senescent/dead leaves and a sink of 0.1 to 1.6 Tg yr −1 by soil uptake). Nevertheless, these resultsCorrespondence to: J. Williams (williams@mpch-mainz.mpg.de) show that the contribution of the rainforest ecosystem is the major source in the global budget of methyl chloride.For chloroform, the extrapolated global net flux from tropical ecosystems is 56 (±23 2σ ) Gg yr −1 , which is of minor importance compared to the total global sources and might be already contained in the soil emission term.

Methyl bromide (CH3Br) is the most abundant brominated organic compound in the atmosphere. It is known to originate from natural and anthropogenic sources, although many uncertainties remain regarding strengths of both sources and sinks and the processes leading to its formation. In this study a potential new CH3Br source from vegetation has been examined, analogous to the recently discovered abiotic formation of methyl chloride from plant pectin. Several plant samples with known bromine content, including ash (Fraxinus excelsior), saltwort (Batis maritima), tomato reference material (NIST-1573a), hay reference material (IAEA V-10), and also bromine enriched pectin, were incubated in the temperature range of 25-50 degrees C and analyzed for CH3Br emission using gas chromatography/mass spectrometry. All plant samples inspected showed an exponential increase in CH3Br emission as a function of temperature increase, i.e., emissions were observed to approximately double with every 5 degrees C rise in temperature. Next to temperature, it was found that emissions of CH3Br were also dependent on the bromine content of the plants. The highest CH3Br release rates were found for the saltwort which contained the highest bromine concentration. Arrhenius plots confirmed that the observed emissions were from an abiotic origin. The contribution of abiotic CH3Br formation from vegetation to the global budget will vary geographically as a result of regional differences in both temperature and bromide content of terrestrial plants.

Environmental context.The exchange of gases between the atmosphere and oceans impacts Earth’s climate. Over the remote oceans, marine emissions of organic species may have significant impacts on cloud properties and the atmosphere’s oxidative capacity. Quantifying these emissions and their dependence on ocean biology over the global oceans is a major challenge. Here we present a new method which relates atmospheric abundance of several organic chemicals over the South Atlantic Ocean to the exposure of air to ocean biology over several days before its sampling. Abstract.We have used a Lagrangian transport model and satellite observations of oceanic chlorophyll-a concentrations and phytoplankton community structure, to investigate relationships between air mass biological exposure and atmospheric concentrations of organic compounds over the remote South Atlantic Ocean in January and February 2007. Accounting for spatial and temporal exposure of air masses to chlorophyll from biologically active ocean regions upwind of the observation location produces significant correlations with atmospheric organohalogens, despite insignificant or smaller correlations using commonly applied in-situ chlorophyll. Strongest correlations (r = 0.42–0.53) are obtained with chlorophyll exposure over a 2-day transport history for CHBr3, CH2Br2, CH3I, and dimethylsulfide, and are strengthened further with exposure to specific phytoplankton types. Incorporating daylight and wind-speed terms into the chlorophyll exposure results in reduced correlations. The method demonstrates that conclusions drawn regarding oceanic trace-gas sources from in-situ chlorophyll or satellite chlorophyll averages over arbitrary areas may prove erroneous without accounting for the transport history of air sampled.