Abstract. Automated in situ gas chromatograph/mass spectrometer (GC/MS) measurements of a range of predominantly biogenic alkyl halides in air, including CHBr3, CHBr2C1, CH3Br, C2HsBr, CH3I, C2HsI, CH2IC1, CH212, and the hitherto unreported CH2IBr were made at Mace Head during a 3-week period in May 1997. C3H7I and CH3CHICH3 were monitored but not detected. Positive correlations were observed between the polyhalomethane pairs CHBr3/C•r2C1 and CHBr3/CH2IBr and between the monohalomethane pair CH3I/C2HsI, which are interpreted in terms of common or linked marine sources. During periods when air masses were affected by emissions from local seaweed beds, the concentrations of CHBr3, CH2IC1, and CH2IBr not only showed remarkable correlation but also maximized at low water. These are the first field observations to provide evidence for a link between the tidal cycle, polyhalomethanes in air, and potential marine production. The calculated total flux of iodine atoms into the boundary layer at Mace Head from organic gaseous precursors was dominated by photolytic destruction of CH212. Photolysis of CH3I contributed less than 3%. The calculated peak flux of iodine atoms during the campaign coincided with the highest measured levels of iodine oxide radicals, as determined using Differential Optical Absorption Spectrometry (DOAS).

Abstract. Emission inventories for major reactive tropospheric CI species (particulate CI, HC1, C1NO2, CH3CI, CHCI3, CH3CCI3, C2C14, C2HC13, CH2C12, and CHCIF2) were integrated across source types (terrestrial biogenic and oceanic emissions, sea-salt production and dechlorination, biomass burning, industrial emissions, fossil-fuel combustion, and incineration). Composite emissions were compared with known sinks to assess budget closure; relative contributions of natural and anthropogenic sources were differentiated. Model calculations suggest that conventional acid-displacement reactions involving Sov)+O3, S(Iv)+ H202, and H2SO4 and HNO3 scavenging account for minor fractions of sea-salt dechlorination globally. Other important chemical pathways involving sea-salt aerosol apparently produce most volatile chlorine in the troposphere. The combined emissions of CH3CI from known sources account for about half of the modeled sink, suggesting fluxes from known sources were unde:estimated, the OH sink was overestimated, or significant unidentified sources exist. Anthropogenic activities (primarily biomass burning) contribute about half the net CH3CI emitted from known sources. Anthropogenic emissions account for only about 10% of the modeled CHCl3 sink. Although poorly constrained, significant fractions of tropospheric CH2C12 (25%), C2HC13 (10%), and C2C14 (5%) are emitted from the surface ocean; the combined contributions of C2C14 and C2HC13 from all natural sources may be substantially higher than the estimated oceanic flux.

Air was sampled from the porous firn layer at the NEEM site in Northern Greenland. We use an ensemble of ten reference tracers of known atmospheric history to characterise the transport properties of the site. By analysing uncertainties in both data and the reference gas atmospheric histories, we can objectively assign weights to each of the gases used for the depth-diffusivity reconstruction. We define an objective root mean square criterion that is minimised in the model tuning procedure. Each tracer constrains the firn profile differently through its unique atmospheric history and free air diffusivity, making our multiple-tracer characterisation method a clear improvement over the commonly used single-tracer tuning. Six firn air transport models are tuned to the NEEM site; all models successfully reproduce the data within a 1s Gaussian distribution. A comparison between two replicate boreholes drilled 64 m apart shows differences in measured mixing ratio profiles that exceed the experimental error. We find evidence that diffusivity does not vanish completely in the lock-in zone, as is commonly assumed. The ice age- gas age difference (?age) at the firn-ice transition is calculated to be 182+3-9 yr. We further present the first intercomparison study of firn air models, where we introduce diagnostic scenarios designed to probe specific aspects of the model physics. Our results show that there are major differences in the way the models handle advective transport. Furthermore, diffusive fractionation of isotopes in the firn is poorly constrained by the models, which has consequences for attempts to reconstruct the isotopic composition of trace gases back in time using firn air and ice core records

Abstract. Although there are many chlorine-containing trace gases in the atmosphere, only those with atmospheric lifetimes of 2 years or fewer appear to have significant natural sources. The most abundant of these gases are methyl chloride, chloroform, dichloromethane, perchloroethylene, and trichloroethylene. Methyl chloride represents about 540 parts per trillion by volume (pptv) C1, while the others together amount to about 120 pptv C1. For methyl chloride and chloroform, both oceanic and land-based natural emissions have been identified. For the other gases, there is evidence of oceanic emissions, but the roles of the soils and land are not known and have not been studied. The global annual emission rates from the oceans are estimated to be 460 Gg C1/yr for CH3C1, 320 Gg C1/yr for CHC13, 160 Gg C1/yr for CH2C12, and about 20 Gg C1/yr for each of C2HC13, and C2C14. Land-based emissions are estimated to be 100 Gg C1/yr for CH3C1 and 200 Gg C1/yr for CHC13. These results suggest that the oceans account for about 12% of the global annual emissions of methyl chloride, although until now oceans were thought to be the major source. For chloroform, natural emissions from the oceans and lands appear to be the major sources. For further research, the complete database compiled for this work is available from the archive, which includes a monthly emissions inventory on a 1øxl ø latitude-longitude grid for oceanic emissions of methyl chloride.

We detected a compound previously unreported in the atmosphere, trifluoromethyl sulfur pentafluoride (SF(5)CF(3)). Measurements of its infrared absorption cross section show SF(5)CF(3) to have a radiative forcing of 0.57 watt per square meter per parts per billion. This is the largest radiative forcing, on a per molecule basis, of any gas found in the atmosphere to date. Antarctic firn measurements show it to have grown from near zero in the late 1960s to about 0.12 part per trillion in 1999. It is presently growing by about 0.008 part per trillion per year, or 6% per year. Stratospheric profiles of SF(5)CF(3) suggest that it is long-lived in the atmosphere (on the order of 1000 years).