W. T. Sturges

Short‐lived alkyl iodides and bromides at Mace Head, Ireland: Links to biogenic sources and halogen oxide production

Automated in situ gas chromatograph/mass spectrometer (GC/MS) measurements of a range of predominantly biogenic alkyl halides in air, including CHBr3, CHBr2Cl, CH3Br, C2H5Br, CH3I, C2H5I, CH2ICl, CH2I2, 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/CHBr2Cl and CHBr3/CH2IBr and between the monohalomethane pair CH3I/C2H5I, 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, CH2ICl, 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 CH2I2. 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).

Composite global emissions of reactive chlorine from anthropogenic and natural sources: Reactive Chlorine Emissions Inventory

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 incinera- tion). Composite emissions were compared with known sinks to assess budget closure; relative contributions of natural and anthropogenic sources were differentiated. Model cal- culations suggest that conventional acid-displacement reactions involving Sov)+O3, S(Iv)+ H202, and H2SO4 and HNO3 scavenging account for minor fractions of sea-salt dechlorina- tion globally. Other important chemical pathways involving sea-salt aerosol apparently pro- duce 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 tropo- spheric 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 substan- tially higher than the estimated oceanic flux.

Natural emissions of chlorine‐containing gases: Reactive Chlorine Emissions Inventory

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) Cl, while the others together amount to about 120 pptv Cl. 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 Cl/yr for CH3Cl, 320 Gg Cl/yr for CHCl3, 160 Gg Cl/yr for CH2Cl2, and about 20 Gg Cl/yr for each of C2HCl3, and C2Cl4. Land-based emissions are estimated to be 100 Gg Cl/yr for CH3Cl and 200 Gg Cl/yr for CHCl3. 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 1° × 1° latitude-longitude grid for oceanic emissions of methyl chloride.

Distribution of halon‐1211 in the upper troposphere and lower stratosphere and the 1994 total bromine budget

We report here on the details of the first, in situ, real-time measurements of H-1211 (CBrClF2) and sulfur hexafluoride (SF6) mixing ratios in the stratosphere up to 20 km. Stratospheric air was analyzed for these gases and others with a new gas Chromatograph, flown aboard a National Aeronautics and Space Administration ER-2 aircraft as part of the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft mission conducted in 1994. The mixing ratio of SF6, with its nearly linear increase in the troposphere, was used to estimate the mean age of stratospheric air parcels along the ER-2 flight path. Measurements of H-1211 and mean age estimates were then combined with simultaneous measurements of CFC-11 (CCl3F), measurements of brominated compounds in stratospheric whole air samples, and records of tropospheric organic bromine mixing ratios to calculate the dry mixing ratio of total bromine in the lower stratosphere and its partitioning between organic and inorganic forms. We estimate that the organic bromine-containing species were almost completely photolyzed to inorganic species in the oldest air parcels sampled. Our results for inorganic bromine are consistent with those obtained from a photochemical, steady state model for stratospheric air parcels with CFC-11 mixing ratios greater than 150 ppt. For stratospheric air parcels with CFC-11 mixing ratios less than 50 ppt (mean age ≥5 years) we calculate inorganic bromine mixing ratios that are approximately 20% less than the photochemical, steady state model. There is a 20% reduction in calculated ozone loss resulting from bromine chemistry in old air relative to some previous estimates as a result of the lower bromine levels.

Lower stratospheric organic and inorganic bromine budget for the Arctic winter 1998/99

In the Arctic winter 1998/99, two balloon payloads were launched in a co-ordinated study of stratospheric bromine. Vertical profiles (9-28 km) of all known major organic Br species (CH 3 Br, C 2 H 5 Br, CH 2 BrCl, CHBrCl 2 , CH 2 Br 2 , CHBr 2 Cl, CHBr 3 , H1301, H1211, H2402, and H1202) were measured, and total organic Br (henceforth called Br org y ) originating from these organic precursors was inferred as a function of altitude. This was compared with total inorganic reactive Br (henceforth called Br in y ) derived from spectroscopic BrO observations, after accounting for modeled stratospheric Br y partitioning. Within the studied altitude range the two profiles differed by less than the estimated accumulated uncertainties. This good agreement suggests that the lower stratospheric budget and chemistry of Br is well understood for the specified conditions. For early 1999 our data suggest 2 Br in y mixing ratio of 1.5 ppt in air just above the local Arctic tropopause (∼ 9.5 km), whilst at 25 km in air of 5.6 yr mean age it was estimated to be 18.4(+1.8/-1.5) ppt from organic precursor measurements, and (21.5±3.0) ppt from BrO measurements and photochemical modelling, respectively. This suggests a Br in y influx of 3.1(-2.9/+3.5) ppt from the troposphere to the stratosphere.

Bromoform as a source of stratospheric bromine

We have measured bromoform (CHBr3) in the troposphere and lower stratosphere (sea level to 17 km) from balloons, aircraft and ground stations. The concentrations ranged from 2 to 20 ppt at sea level, and 0.1 to 1 ppt in the upper troposphere. Above the tropopause the concentrations declined sharply to less than 0.01 ppt above 14 km. CHBr3 accounted for 3% of total measured organic bromine (Bro) at the extratropical tropopause, and appeared to potentially contribute at least 9% of reactive bromine (Bry) in stratospheric air of mean age less than 1 year. Inclusion of dibromomethane and the mixed bromochloromethanes increased these figures to about 10% of Bro and 20% of Bry respectively. Observations of CHBr3 in both Arctic and mid-latitudinal stratospheric air, and in air masses with mean ages of more than 1.5 years, suggests that the CHBr3 in these locations originates predominantly from extratropical stratosphere-troposphere exchange.

Growth of fluoroform (CHF3, HFC‐23) in the background atmosphere

There is growing concern over the emission and accumulation of very long-lived fluorinated trace gases in the atmosphere, due to their large global warming potentials (GWPs). Unlike CFCs and other ozone-depleting, chlorinated and brominated chemicals, consumption of these fluorinated compounds is not controlled by the Montreal Protocol or any other international agreement. Of all the known and potential trace ‘greenhouse’ gases, the two with the highest GWPs are sulfur hexafluoride (SF6) and fluoroform (CHF3, HFC-23). Whereas several studies have reported the detection and accumulation of SF6 in the atmosphere, the presence of HFC-23 has remained unreported. We have found that present-day HFC-23 concentrations (c. 11 pptv in late 1995) exceed those of SF6 by a factor of three. Concentrations have steadily increased in the atmosphere since at least 1978, and are continuing to do so at a present rate of 5% per year. Furthermore, HFC-23 appears to be long-lived in the atmosphere, with a stratospheric lifetime of at least 1000 years, and a modelled tropospheric lifetime of 230 years. In terms of global warming, the cumulative emissions of HFC-23 up to, and including, 1995 are equivalent to 1.6 billion tonnes of CO2.

Methyl bromide, other brominated methanes, and methyl iodide in polar firn air

We report measurements of brominated, bromochlorinated, and iodinated methanes in air extracted from deep firn at three polar locations (two Antarctic and one Arctic). Using a firn diffusion model, we are able to reconstruct a consistent temporal trend for methyl bromide from the two Antarctic sites. This indicates a steady increase by about 2 ppt from the midtwentieth century to 8 ppt today. The Arctic firn, however, contained extremely high levels of methyl bromide as well as numerous other organic gases, which are evidently produced in situ. The other brominated species (dibromomethane, bromochloromethane, bromodichloromethane, dibromochloromethane, and bromoform) showed little or no long-term trend in Antarctic firn and therefore are evidently of entirely natural origin in the Southern Hemisphere. A clear seasonal trend was observed in the upper firn for the shortest-lived halocarbons (notably bromoform and methyl iodide). The same species were present at lower abundance at the higher altitude and more inland Antarctic site, possibly due to their origin from more distant oceanic sources.

Oxidized nitrogen chemistry and speciation in the Antarctic troposphere

Understanding the NOy budget at high latitudes is important for our knowledge of present-day clean air chemistry and essential for reliable interpretation of existing ice core nitrate data. However, measurements of NOy components at high latitudes have been limited, and no measurements have attempted to address the budget of NOy. Here we report on a campaign conducted in the austral summer of 1997 at the German Antarctic research station, Neumayer, with first Antarctic measurements for NOy in addition to light alkyl nitrates, NO, HNO3 and p−NO3−. Inorganic nitrate has generally been assumed to be the dominant component of NOy in Antarctica, although this idea has not previously been tested. However, our results show that for this coastal station, methyl nitrate was present in much higher concentration than inorganic nitrate (median CH3ONO2 = 38 pptv, HNO3 = 5 pptv). It has been suggested earlier that some alkyl nitrates might have a marine source. If this suggestion is correct, the implication arises that the oceans are an important source of NOy to the Antarctic troposphere and that their role in determining nitrate concentrations in ice must be considered.

Dimethyl sulfide in the Arctic atmosphere

Dimethyl sulfide (DMS), sulfur dioxide, non-sea-salt sulfate, and various aerosol properties were measured during three field programs (two airborne and one ground-based) near Barrow and Deadhorse (Prudhoe Bay), Alaska. The two airborne sampling programs took place in spring and early summer, and the ground-based measurements spanned an entire summer. DMS concentrations in the Arctic atmosphere ranged from a few parts per trillion by volume (pptv) in spring and fall to higher values in summer (generally a few tens of pptv with occasional peaks of 100 to 300 pptv). In addition, DMS concentrations were measured during the spring near Resolute in seawater below the ice and in ice-algae and kelp cultures. The seawater samples taken from below the ice in spring had DMS concentrations comparable to those in other oceanic regions. Taken together, these measurements show that the Arctic Ocean is potentially a substantial source of DMS, which likely becomes important as sea ice melts in the early summer. Local atmospheric concentrations increased throughout the summer, peaking in August. In regions where accumulation mode aerosols have been scavenged (e.g., by low-level stratus clouds, which are common during the Arctic summer), evidence of rapid new particle production was observed. The seasonal cycle of atmospheric DMS closely resembles that of fine particles observed at Barrow, Alaska, and Alert, Northwest Territories, Canada. This finding indicates that DMS is likely an important precursor to the types of particles that dominate the background arctic aerosol in summertime. These results, together with those from several recently published studies of arctic aerosol, are combined to yield a consistent picture of the role of locally emitted DMS in the production of atmospheric aerosols in the Arctic in summer.