S. A. Penkett

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).

The seasonal variation of nonmethane hydrocarbons in the free troposphere over the North Atlantic Ocean: Possible evidence for extensive reaction of hydrocarbons with the nitrate radical

Detailed observations have been made of the seasonal variation in concentration of a wide range of nonmethane hydrocarbons over the North Atlantic Ocean in air with a predominantly polar maritime origin. The results bear out many of the findings of our previous studies [Lightman et al., 1990] with the observation of a concentration maximum in the winter and a minimum in the summer. The more recent results indicate that the amplitude of the seasonal cycle is remarkably constant. Also the total concentration of reactive carbon in the various forms of nonmethane hydrocarbons probably exceeds 20 ppbC. This constitutes a substantial reservoir of material which could take part in ozone production in the relatively remote troposphere, if sufficient nitrogen oxides are copresent. The hydrocarbon composition of air in winter is strongly dependent on its origin, with tropical air having a composition similar to polar air in the spring months. The relative magnitude of the seasonal cycles for some hydrocarbons is proportional to their rates of reaction with hydroxyl radicals. This is true for straight chain paraffins up to C5, acetylene and benzene, and it suggests that the removal of these molecules from the atmosphere occurs predominantly by reaction with hydroxyl radicals. For other molecules, particularly branched chain paraffins and substituted aromatic molecules, there is evidence that other removal processes are operating in competition with the hydroxyl radical, especially in winter. Arguments are advanced that in the case of the branched chain paraffins this may be caused by reaction with nitrate radicals.

Seasonal emissions of isoprene and other reactive hydrocarbon gases from the ocean

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.

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.

Modeling OH, HO2, and RO2 radicals in the marine boundary layer: 1. Model construction and comparison with field measurements

An observationally constrained box model has been constructed in order to investigate the chemistry of the marine boundary layer at Mace Head, a remote location on the west coast of Ireland. The primary aim of the model is to reproduce concentrations of the hydroxyl (OH) and hydroperoxy (HO2) radicals measured by an in situ fluorescence assay by gas expansion (PAGE) instrument, and the sum of peroxy radicals ∑([HO2]+[RO2]) determined by a peroxy radical chemical amplification (PERCA) instrument. The model has been constructed based on observed concentrations of a suite of non-methane hydrocarbons, measured in situ by gas chromatography. The chemical mechanism for the model is a subset of a comprehensive master chemical mechanism (MCM). This paper describes in detail the construction of the model, as well as the underlying approach. Comparisons of modeled and measured concentrations of radical species, from a recent field campaign held at the Mace Head Atmospheric Observatory during July and August 1996 (EASE 96), are also presented. For the limited OH data available from this campaign, the model tends to overestimate the observations by about 40%, although this discrepancy is within the uncertainties of the model (±31%, 2σ) and the PAGE measurements (±75% on average, 2σ). For HO2 the model reproduces the concentrations well on one day but less well on another. Low HOx concentrations compared to model results have been observed previously, with greater than expected heterogeneous losses invoked to explain the differences. Comparisons between measurements of peroxy radicals made by chemical amplification and model predictions show good agreement over a wide range of conditions.

The effect of aircraft emissions on tropospheric ozone in the northern hemisphere

Abstract The effect of aircraft emissions on tropospheric ozone was studied using a two-dimensional zonal model, of longitude vs altitude, extending between 30°–60°N. An emission inventory for the 1987 civil aircraft fleet was constructed based on fuel usage and civil aviation statistics. The addition of the 1987 civil emissions to a modelled standard atmosphere caused increases in concentrations of O3 (12%; 10 ppbv), NOx (40%; 20 pptv) and OH (10%) between 8 and 12 km altitude. A doubling and tripling of the emissions corresponding to a present day inclusion of military aircraft and a future scenario, respectively, caused the increase in O3 to double (19%) and almost triple (25%). The lightning source of NOx was found to be an important parameter. When this source was ommitted the change in O3 at the cruise flight altitude increased to 16%, with respect to a standard atmosphere not containing a lightning source of NOx.

Continuing emissions of methyl chloroform from Europe

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.

Simultaneous observations of nitrate and peroxy radicals in the marine boundary layer

This paper describes the most extensive set of simultaneous measurements of the concentrations of nitrate (NO3) and peroxy (sum of HO2 + RO2, R = alkyl and acyl) radicals to date. The measurements were made in the coastal marine boundary layer over the North Sea, at the Weybourne Atmospheric Observatory on the North Norfolk coast during the spring and autumn of 1994. In spring the average nighttime concentration of NO3 measured by differential optical absorption spectroscopy, was about 10 parts per trillion (ppt) (maximum 25 ppt). The corresponding peroxy radical concentration, measured by the chemical amplifier technique, averaged about 2 ppt (maximum 6 ppt), although this is likely to be an underestimate of the total radical concentration. There is a significant positive correlation between the two sets of radicals, which has not been reported previously. A box model of the marine boundary layer is used to show that this correlation arises from the processing of reactive organic species by NO3. During spring the relatively long lifetime of NO3 (up to 18 min) at night is controlled by reaction with dimethyl sulfide (DMS), and the model predicts significant production of HNO3, methyl tiomethylen (CH3SCH2O2) and other peroxy radicals, HCHO, and eventually sulfate. A nighttime production rate for the hydroxyl (OH) of about 2 x 10(4) molecules cm(-3) s(-1) is estimated. During one night in autumn the NO3 lifetime of about 3 min is too short to be explained by reaction with unsaturated hydrocarbons, but is satisfactorily accounted for by the heterogeneous loss of N2O5 on deliquesced aerosols in relatively polluted conditions.

A study of peroxy radicals and ozone photochemistry at coastal sites in the northern and southern hemispheres

Peroxy radicals and other important species relevant to ozone photochemistry, including ozone, its photolysis rate coefficient jO(1D), NOx (NO + NO2), and peroxides, were measured at the coastal sites of Cape Grim, Tasmania, in January/February 1995 during the Southern Ocean Atmospheric Photochemistry Experiment (SOAPEX 1) and Mace Head, Western Ireland, in May 1995 during the Atlantic Atmospheric Photochemistry Experiment (ATAPEX 1). At both sites it was observed that the relationship between peroxy radical (HO2 + RO2) concentrations and jO(1D) switched from a square root dependence in clean oceanic or “baseline” air to a first-order dependence in more polluted air. Simple algorithms derived from a photochemical reaction scheme indicate that this switch-over occurs when atmospheric NO levels are sufficient for peroxy radical reaction with NO to compete with radical recombination reactions. At this crucial point, net tropospheric ozone production is expected to occur and was observed in the ozone diurnal cycles when the peroxy radical/jO(1D) dependencies became first order. The peroxy radical/jO(1D) relationships imply that ozone production exceeds destruction at NO levels of 55±30 parts per trillion by volume (pptv) at Mace Head during late spring and 23±20 pptv at Cape Grim during summer, suggesting that the tropospheric ozone production potential of the southern hemisphere is more responsive to the availability of NO than that of the northern hemisphere.

Estimates of atmospheric hydroxyl radical concentrations from the observed decay of many reactive hydrocarbons in well-defined urban plumes

An analytical system has been developed which allows the confident identification and measurement of 35 hydrocarbons of different reactivities in air samples collected in many locations. This paper describes the application of the technique to follow the differential decay of hydrocarbons in urban plumes spreading from London during the summer. The data have been used to determine atmospheric hydroxyl radical concentrations, averaged over several hours, on the assumption that the decay of the hydrocarbons is entirely due to reaction with these radicals. Hydroxyl radical concentrations were derived from the measured decay of several alkenes which agreed with theoretical estimates. There are strong indications, however, that substituted aromatic molecules decay much faster than could be accounted for solely by reaction with hydroxyl radicals; this may indicate the presence of a further chemical removal mechanism. 18 refs., 9 figs., 5 tabs.