Carl A. M. Brenninkmeijer

Three-dimensional climatological distribution of tropospheric OH: Update and evaluation

A global climatological distribution of tropospheric OH is computed using observed distributions of O3, H2O, NOt (NO2 +NO + 2N2O5 + NO3 + HNO2 +HNO4), CO, hydrocarbons, temperature, and cloud optical depth. Global annual mean OH is 1.16×106 molecules cm−3 (integrated with respect to mass of air up to 100 hPa within ±32° latitude and up to 200 hPa outside that region). Mean hemispheric concentrations of OH are nearly equal. While global mean OH increased by 33% compared to that from Spivakovsky et al. [1990], mean loss frequencies of CH3CCl3 and CH4 increased by only 23% because a lower fraction of total OH resides in the lower troposphere in the present distribution. The value for temperature used for determining lifetimes of hydrochlorofluorocarbons (HCFCs) by scaling rate constants [Prather and Spivakovsky, 1990] is revised from 277 K to 272 K. The present distribution of OH is consistent within a few percent with the current budgets of CH3CCl3 and HCFC-22. For CH3CCl3, it results in a lifetime of 4.6 years, including stratospheric and ocean sinks with atmospheric lifetimes of 43 and 80 years, respectively. For HCFC-22, the lifetime is 11.4 years, allowing for the stratospheric sink with an atmospheric lifetime of 229 years. Corrections suggested by observed levels of CH2Cl2 (annual means) depend strongly on the rate of interhemispheric mixing in the model. An increase in OH in the Northern Hemisphere by 20% combined with a decrease in the southern tropics by 25% is suggested if this rate is at its upper limit consistent with observations of CFCs and 85Kr. For the lower limit, observations of CH2Cl2 imply an increase in OH in the Northern Hemisphere by 35% combined with a decrease in OH in the southern tropics by 60%. However, such large corrections are inconsistent with observations for 14CO in the tropics and for the interhemispheric gradient of CH3CCl3. Industrial sources of CH2Cl2 are sufficient for balancing its budget. The available tests do not establish significant errors in OH except for a possible underestimate in winter in the northern and southern tropics by 15–20% and 10–15%, respectively, and an overestimate in southern extratropics by ∼25%. Observations of seasonal variations of CH3CCl3, CH2Cl2, 14CO, and C2H6 offer no evidence for higher levels of OH in the southern than in the northern extratropics. It is expected that in the next few years the latitudinal distribution and annual cycle of CH3CCl3 will be determined primarily by its loss frequency, allowing for additional constraints for OH on scales smaller than global.

The use of radiocarbon measurements in atmospheric studies.

(super 14) C measured in trace gases in clean air helps to determine the sources of such gases, their long-range transport in the atmosphere, and their exchange with other carbon cycle reservoirs. In order to separate sources, transport and exchange, it is necessary to interpret measurements using models of these processes. We present atmospheric 14CO (sub 2) measurements made in New Zealand since 1954 and at various Pacific Ocean sites for shorter periods. We analyze these for latitudinal and seasonal variation, the latter being consistent with a seasonally varying exchange rate between the stratosphere and troposphere. The observed seasonal cycle does not agree with that predicted by a zonally averaged global circulation model. We discuss recent accelerator mass spectrometry measurements of atmospheric 14CH (sub 4) and the problems involved in determining the fossil fuel methane source. Current data imply a fossil carbon contribution of ca 25%, and the major sources of uncertainty in this number are the uncertainty in the nuclear power source of 14CH (sub 4) , and in the measured value for delta (super 14) C in atmospheric methane.

Concentration and 13C records of atmospheric methane in New Zealand and Antarctica : evidence for changes in methane sources

Measurements of 13C in atmospheric methane made at Baring Head, New Zealand (41°S), over the 4-year period, 1989–1993, display a persistent but highly variable seasonal cycle. Values for δ13C peak in summer at about −46.9‰ and drop to around −47.5‰ in the late winter. Methane concentration shows a similar cycle, with winter peaks and summer minima. Similar features are observed at the New Zealand Antarctic station, Scott Base, at 78°S. While the phase of the δ13C cycle is consistent with a kinetic isotope effect that preferentially leaves methane enriched in 13C in the atmosphere after oxidation by OH, the amplitude of the cycle is much larger than expected from published laboratory measurements of the effect. We interpret the origin of this cycle and its inter-annual variability to be due to episodic southward transport of isotopically heavy methane from large-scale tropical biomass burning, possibly in conjunction with changes in the rate of interhemispheric transport in the troposphere. The Baring Head 13C data show no significant secular trend from 1989 to mid-1991, followed by a rapid trend toward methane less enriched in 13C. This indicates a major shift in the balance of the sources of atmospheric methane and precludes an increased sink strength. The trend in 13C since mid-1991 coincided with significant changes to the methane growth rate observed at Baring Head and at Scott Base: an elevated growth rate of about 15 parts per billion by volume (ppbv) during 1991 gave way to less than 3 ppbv yr−1 thereafter. A 2-box model of atmospheric methane (one box per hemispheric reservoir) suggests that (1) the recent decline in 13C in methane observed at Baring Head and Scott Base cannot have a solely northern hemispheric origin and (2) the most plausible origin is a recent reduction in methane released by biomass burning in the southern hemisphere, combined with a lower release rate of fossil methane in the northern hemisphere.

Carbon 13 and D kinetic isotope effects in the reactions of CH4 with O(1 D) and OH: New laboratory measurements and their implications for the isotopic composition of stratospheric methane

Measurements of the 13C and D kinetic isotope effects (KIE) in methane, 13CKIE = k(12CH4)/k(13CH4) and DKIE = k(12CH4)/k(12CH3D), in the reactions of these atmospherically important methane isotopomers with O(1D) and OH have been undertaken using mass spectrometry and tunable diode laser absorption spectroscopy to determine isotopic composition. For the carbon kinetic isotope effect in the reaction with the OH radical, 13CKIEOH = 1.0039 (±0.0004, 2σ) was determined at 296 K, which is significantly smaller than the presently accepted value of 1.0054 (±0.0009, 2 σ). For DKIEOH we found 1.294 (± 0.018, 2σ) at 296 K, consistent with earlier observations. The carbon kinetic isotope effect in the reaction with O(1D) 13CKIEO(1D), was determined to be 1.013, whereas the deuterium kinetic isotope effect is given by DKIEO(1D) = 1.06. Both values are approximately independent of temperature between 223 and 295 K. The room temperature fractionation effect 1000(KIE-1) in the reaction of O(1D) with 12CH4 versus CH4 is thus ≈ 13‰, which is an order of magnitude greater than the previous value of 1‰. In combination with recent results from our laboratory on 13CKIE and DKIE for the reaction of CH4 with Cl, these new measurements were used to simulate the effective kinetic isotope effect for the stratosphere with a two-dimensional, time dependent chemical transport model. The model results show reasonable agreement with field observations of the 13CH4/12CH4 ratio in the lowermost stratosphere, and also reproduce the observed CH3D/CH4 ratio.

Measurement of the abundance of 14CO in the atmosphere and the 13C/12C and 18O/16O ratio of atmospheric CO with applications in New Zealand and Antarctica

Equipment and method for accurate and precise concentration, 14C, 13C, and 18O isotope analysis for CO in background air is presented along with new results for Antarctica and New Zealand. High flow rate cryogenic extraction systems which separate CO after its oxidation to CO2 are used, incorporating a novel ultraefficient cryogenic trap. Air in quantities from a few hundred liters to 2 m3 with CO concentrations from 20 part per billion by volume (ppbv) to 1000 ppbv can be analyzed. The absolute CO concentration is determined volumetrically. The 13C/12C and 18O/16O ratios are determined by mass spectrometry. For 18O a correction is applied for the oxygen in the CO2 derived from the oxidant. Carbon 14 is determined by accelerator mass spectrometry. Prior to this the very small CO-derived samples with their high specific activity are diluted accurately. For polluted air the proportional decrease in specific activity with increasing CO levels is confirmed. The 14CO abundance and CO concentration in background air in New Zealand and Antarctica are not much different, and both follow a distinct seasonal pattern, in particular 14CO which is mainly forced by OH seasonality. The 14CO abundance swings between its February minimum of about 6 and its August maximum of about 13 molecules per cm3 air (STP). CO has a smaller seasonality and shows a larger scatter due to local CO sources. The impact of changes in solar activity on 14CO for the period considered has been small. Most of the short-term variability in 14CO is due to the sampling of different air masses. It appears that interannual OH variations may be reflected in 14CO variations. Both 13C/12C and 18O/16O at Scott Base show large seasonal variation, and the impact of biomass burning and isotopic fractionation in CO destruction are used to try to explain the respective isotopic compositions.

Mass Spectrometry of the Intramolecular Nitrogen Isotope Distribution of Environmental Nitrous Oxide Using Fragment-ion Analysis

Mass spectrometry is applied to measure the intramolecular distribution of (15)N in N(2)O samples of near natural isotopic composition. The method is relatively straightforward and based on the analysis of the (14)NO and (15)NO fragment ion beams at mass 30 and 31, respectively, in combination with the standard analysis of the masses 44, 45, and 46 of the non-fragmented N(2)O. Various complications in the application, not all of which are resolved at present, are discussed. Copyright 1999 John Wiley & Sons, Ltd.

CARIBIC—Civil Aircraft for Global Measurement of Trace Gases and Aerosols in the Tropopause Region

Abstract The deployment of measurement equipment in passenger aircraft for the observation of atmospheric trace constituents is described. The package of automated instruments that is installed in a one-ton-capacity aircraft freight container positioned in the forward cargo bay of a Boeing 767 ER can register a vast amount of atmospheric data during regular long-distance flights. The air inlet system that is mounted on the fuselage directly below the container comprises an aerosol inlet, a separate inlet for trace-gas sampling, and an air exhaust. All instruments, the central computer, and power supply are mounted in aviation-approved racks that slide into the reinforced container. The current instrument package comprises a fast-response chemiluminescence sensor and a conventional UV absorption detector for O3; a gas chromatograph for CO; two condensation nuclei counters for particles larger than 5 and 12 nm; and a 12-canister large-capacity whole air sampler for laboratory trace-gas analysis and isotopic...

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.

The 13C, 14C, and 18O isotopic composition of CO, CH4, and CO2 in the higher southern latitudes lower stratosphere

Large air samples were collected in the lower stratosphere (10–12 km) from 43° to 85°S in June 1993, using a special compressor system. For the important trace gases CO, CH4 and CO2, concentration and isotopic analyses were carried out and significant correlations were discovered. The 14CO isotope is considerably in excess of tropospheric levels with very high values from 40 to 120 14CO molecules/cm3 STP (corresponding to 12,500 percent modern carbon, at 30 ppbv), and is negatively correlated with CO. The linear relationship is used to estimate OH to be 2.9×105 cm−3. The 18O/16O ratios for CO are the lowest ever measured and reflect the inverse kinetic isotope effect in the oxidation of CO by OH. The 13C/12C ratios for CO are not much different from tropospheric values and confirm that fractionation is small but also that the in situ contribution from CH4 oxidation is minor. For CH4 a correlation between δ13C and concentration exists from which a fractionation factor for the sink reaction (k12/k13) of about 1.012 is calculated, well in excess of results from laboratory experiments for OH +CH4. The most plausible explanation presently is the removal of approximately 9% of CH4 by Cl atoms, which, as laboratory experiments have just confirmed, induces a very large fractionation. We also reveal a linear correlation between 14CO and 14CO2, precursor and product. Finally, an analysis of potential vorticity shows a structure that seems to give an overall agreement with the trace gas variations.

Determination of the isotopic composition of atmospheric methane and its application in the Antarctic

A procedure for the determination of the 13C/12C ratio and the 14C abundance in atmospheric methane is presented. The method is based on the collection of air samples in stainless steel tanks at a pressure of 7 bar. The air is processed in the laboratory by cryogenic removal of condensibles, followed by oxidation of the methane content after which the resulting CO2 is collected. Also CO is removed prior to oxidation. The 13C/12C ratio is determined on the CO2 sample by stable isotope ratio mass spectrometry. The 14C content is determined by means of accelerator mass spectrometry. The overall precision of the technique is 0.1‰ for δ13C and 1.5 pMC for 14C. The method has been used to determine the carbon isotopic composition of methane in air collected at Baring Head, New Zealand, and also in air collected on aircraft flights between New Zealand and Antarctica. No gradient in the carbon isotopic composition between Baring Head and South Pole station was detected. The annual mean δ13C value at Baring Head was −47.13±0.20‰ for 1989 which includes seasonal effects probably due to OH variations and local meteorology. The annual mean 14C value at Baring Head in 1989 was 118.3 percent modern (pMC).