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).
The consumption of methyl chloroform (1,1,1-trichloroethane), an industrial solvent, has been banned by the 1987 Montreal Protocol because of its ozone-depleting potential. During the 1990s, global ...emissions have decreased substantially and, since 1999, near-zero emissions have been estimated for Europe and the United States. Here we present measurements of methyl chloroform that are inconsistent with the assumption of small emissions. Using a tracer transport model, we estimate that European emissions were greater than 20 Gg in 2000. Although these emissions are not significant for stratospheric ozone depletion, they have important implications for estimates of global tropospheric hydroxyl radical (OH) concentrations, deduced from measurements of methyl chloroform. Ongoing emissions therefore cast doubt upon recent reports of a strong and unexpected negative trend in OH during the 1990s and a previously calculated higher OH abundance in the Southern Hemisphere compared to the Northern Hemisphere.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Air samples collected at Cape Grim, Tasmania between 1978 and 2008 and during a series of more recent aircraft sampling programmes have been analysed to determine the atmospheric abundance and trend ...of octafluorocyclobutane (c-C4F8 or PFC-318). c-C4F8 has an atmospheric lifetime in excess of 3000 yr and a global warming potential (GWP) of 10 300 (100 yr time horizon), making it one of the most potent greenhouse gases detected in the atmosphere to date. The abundance of c-C4F8 in the Southern Hemisphere has risen from 0.35 ppt in 1978 to 1.2 ppt in 2010, and is currently increasing at a rate of around 0.03 ppt yr−1. It is the third most abundant perfluorocarbon (PFC) in the present day atmosphere, behind CF4 (~75 ppt) and C2F6 (~4 ppt). Although a number of potential sources of c-C4F8 have been reported, including the electronics and semi-conductor industries, there remains a large discrepancy in the atmospheric budget. Using a 2-D global model to derive top-down global emissions based on the Cape Grim measurements yields a recent (2007) emission rate of around 1.1 Gg yr−1 and a cumulative emission up to and including 2007 of 38.1 Gg. Emissions reported on the EDGAR emissions database for the period 1986–2005 represent less than 1% of the top-down emissions for the same period, which suggests there is a large unaccounted for source of this compound. It is also apparent that the magnitude of this source has varied considerably over the past 30 yr, declining sharply in the late 1980s before increasing again in the mid-1990s.
The PUMA (Pollution of the Urban Midlands Atmosphere) Consortium project involved intensive measurement campaigns in the Summer of 1999 and Winter of 1999/2000, respectively, in which a wide variety ...of air pollutants were measured in the UK West Midlands conurbation including detailed speciation of VOCs and major component analysis of aerosol. Measurements of the OH and HO
2 free radicals by the FAGE technique demonstrated that winter concentrations of OH were approximately half of those measured during the summer despite a factor of 15 reduction in production through the photolysis of ozone. Detailed box modelling of the fast reaction chemistry revealed the decomposition of Criegee intermediates formed from ozone–alkene reactions to be responsible for the majority of the formation of hydroxyl in both the summer and winter campaigns, in contrast to earlier rural measurements in which ozone photolysis was predominant. The main sinks for hydroxyl are reactions with NO
2, alkenes and oxygenates. Concentrations of the more stable hydrocarbons were found to be relatively invariant across the conurbation, but the impacts of photochemistry were evident through analyses of formaldehyde which showed the majority to be photochemical in origin as opposed to emitted from road traffic. Measurements on the upwind and downwind boundaries of the conurbation revealed substantial enhancements in NO
x as a result of emissions within the conurbation, especially during westerly winds which carried relatively clean air. Using calcium as a tracer for crustal particles, it proved possible to reconstruct aerosol mass from the major chemical components with a fairly high degree of success. The organic to elemental carbon ratios showed a far greater influence of photochemistry in summer than winter, presumably resulting mainly from the greater availability of biogenic precursors during the summer campaign. Two urban airshed models were developed and applied to the conurbation, one Eulerian, the other Lagrangian. Both were able to give a good simulation of concentrations of both primary and secondary pollutants at urban background locations.
Reactive halogen species (RHS) such as the halogen oxide radicals IO and BrO influence tropospheric oxidation processes in both polar and temperate regions. Oceanic photolabile halocarbons have been ...shown to be strong sources of RHS in midlatitudes. However, the global source strengths of these halocarbon precursors and the relative importance of the coastal and pelagic oceans on their concentrations are highly uncertain. Here we present atmospheric measurements of the reactive organic halogens CH3I, CH2I2, CH2ClI, CH2IBr, CHIBr2, CHBr3, CH2Br2, and CHBr2Cl made at Mace Head, Ireland, during the Particle Formation and Fate in the Coastal Environment campaign in September 1998 and at Cape Grim, Tasmania, during the Southern Ocean Atmospheric Photochemistry Experiment 2 campaign in January/February 1999. Mace Head is strongly influenced by local macroalgae, whereas Cape Grim, owing to its cliff‐top location, suffers much less impact from seaweeds. The very reactive halocarbons CH2I2, CH2IBr, and CHIBr2 observed at Mace Head were below detection limits at Cape Grim, although CH2ClI was detected at both locations. Mixing ratios of CH3I, CH2ClI, CHBr3, CHBr2Cl, and CH2Br2 at Cape Grim were on average 25–50% of those at Mace Head. Concentrations of the polybromomethanes correlated well at both sites. Using these correlations, we estimate molar source strengths of CHBr2Cl and CH2Br2 to be between 3 and 6% and between 15 and 25% of the global CHBr3 flux, respectively. These values fall within ranges estimated independently from concentration and lifetime data.
Reeves and Penkett present material on the photochemical theory of atmospheric peroxides, an overview of the peroxide measurement technique and available global data sets. They also relate what these ...measurements of peroxides tell about the atmospheric photochemistry leading to the production of hydroxyl radicals and the destruction and production of O3.
The nitrate radical (NO
3) was first measured in the atmosphere in the 1970s and suggestions were made that it could play a major role in oxidising many unsaturated hydrocarbons, such as those ...emitted from the biosphere. Analysis of the hydrocarbon mix over the North Atlantic Ocean suggested subsequently that the influence of NO
3 radical chemistry at night was even more extensive, being on a par with hydroxyl radical chemistry at some times of the year.
The paper presents a detailed analysis of an extensive database of many nonmethane hydrocarbons collected at various sites around the North Sea in the mid 1990s during the HANSA project. By comparing the relative rates of oxidation of iso and normal pentane with that of toluene and benzene it clearly shows that the efficiency of NO
3 radical chemistry and hydroxyl radical chemistry over northwest Europe are similar in springtime and predicts an average nighttime NO
3 concentration of the order of 350
pptv, assuming an annual average OH concentration of 0.6×10
6
cm
−3. This value is very dependant on the average emission ratios of the different hydrocarbons and values between 200 and 600
pptv are possible. It is much larger than direct measurements made in Europe at the surface, but is of the same magnitude as concentrations measured recently from aircraft in the boundary layer over the northeast USA, and previously in vertical profiles by remote sounding over Europe.
A simple analytical expression can be derived to calculate the NO
3 concentration at night with the only variables being ozone and the loss rate of N
2O
5, either to the ground or to aerosol surfaces. The concentrations of NO
3 calculated in this manner are similar to those derived from the analysis of the HANSA hydrocarbon database for typical conditions expected over Europe, but they are very dependant on the efficiency of the aerosol sink for N
2O
5.
It is shown that NO
3 oxidation of many unsaturated hydrocarbons can indeed be more efficient than OH oxidation, especially at times of the year outside the summer season. Direct evidence for hydrocarbon oxidation by NO
3 radicals is shown by a series of peroxy radical measurements where the nighttime concentrations can be significantly higher than daytime concentrations in polluted air on occasion. Also the winter/summer (W/S) ratios of many unsaturated hydrocarbons are much lower than those expected from their removal purely by hydroxyl radical chemistry.
The consequences of these findings are profound especially as satellite measurements of NO
2, a major precursor to NO
3, suggest that these high average concentrations of several hundred pptv could be widespread over most of the continents. This needs to be confirmed by direct in-situ measurement of nitrate radicals but it suggests a much larger role for NO
3 chemistry in the oxidation capacity of the atmosphere than realised hitherto.
In this paper we show evidence that isoprene emission from the oceans is strongly seasonally dependent and is correlated with the chlorophyll content of the water from measurements in the North Sea ...and Southern Ocean. We estimate the seasonally averaged flux of isoprene to the atmosphere to be 1.7 × 107 molecules cm−2 s−1, which may be significant for atmospheric chemistry in locations remote from land as it is the only known source of atmospheric isoprene in these regions. We observe a strong seasonal cycle of several other NMHCs in seawater at high latitudes, with a maximum in summer. This will distort current estimates of the annual marine flux of NMHCs to the atmosphere which may need to be reduced by up to an order of magnitude to account for lower emissions in winter.
Bromoform as a source of stratospheric bromine Sturges, William T.; Oram, David E.; Carpenter, Lucy J. ...
Geophysical research letters,
15 July 2000, Letnik:
27, Številka:
14
Journal Article
Recenzirano
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.
Several zero-dimensional box-models with different levels of chemical complexity, based on the Master Chemical Mechanism (MCM), have been used to study the chemistry of OH and HO2 in a coastal ...environment in the Northern Hemisphere. The models were constrained to and compared with measurements made during the NAMBLEX campaign (Mace Head, Ireland) in summer 2002. The base models, which were constrained to measured CO, CH4 and NMHCs, were able to reproduce OH within 25%, but overestimated HO2 by about a factor of 2. Agreement was improved when the models were constrained to oxygenated compounds (acetaldehyde, methanol and acetone), highlighting their importance for the radical budget. When the models were constrained to measured halogen monoxides (IO, BrO) and used a more detailed, measurements-based, treatment to describe the heterogeneous uptake, modelled OH increased by up to 15% and HO2 decreased by up to 30%. The actual impact of halogen monoxides on the modelled concentrations of HOx was dependant on the uptake coefficients used for HOI, HOBr and HO2. Better agreement, within the combined uncertainties of the measurements and of the model, was achieved when using high uptake coefficients for HO2 and HOI (?HO2=1, ?HOI=0.6). A rate of production and destruction analysis of the models allowed a detailed study of OH and HO2 chemistry under the conditions encountered during NAMBLEX, showing the importance of oxygenates and of XO (where X=I, Br) as co-reactants for OH and HO2 and of HOX photolysis as a source for OH.
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