Ppb Yr Research Articles

Interannual variability and trends in atmospheric methane over the western Pacific from 1994 to 2010

[1] We present an analysis of interannual variability (IAV) and trends in atmospheric methane (CH4) mixing ratios over the western Pacific between 55°N and 35°S from 1994 to 2010. Observations were made by the Center for Global Environmental Research (CGER) of the National Institute for Environmental Studies (NIES), using voluntary observation ships sailing between Japan and Australia/New Zealand and between Japan and North America, sampling background maritime air quasi-monthly (∼10 times per year) with high latitudinal resolution. In addition, simulations of CH4 were performed using NIES atmospheric transport model. A large CH4 increase was observed in the tropics (10°N–5°S) during 1997 (between 15 ± 3 and 19 ± 3 ppb yr−1) and during 1998 for other regions (40°N–50°N: 10 ± 2–16 ± 1 ppb yr−1; 10°S–25°S: 12 ± 2–22 ± 4 ppb yr−1). The CH4 increase leveled off from 1999 to 2006 at all latitudes. The CH4 growth rate was enhanced in 2007 (25°N–50°N: 10 ± 1–12 ± 3 ppb yr−1; 15°S–35°S: 7 ± 1–8 ± 1 ppb yr−1) but diminished thereafter; however, a large CH4 growth (10 ± 1–17 ± 1 ppb yr−1) was observed in 2009 over the northern tropics (0°–15°N). These observations, combined with the simulation results, suggest that to explain the CH4 increase in 2007 would require an increase in surface emissions of ∼20 ± 3 Tg-CH4 yr−1 globally and an increase in the Northern Hemisphere (NH) of 4–7 ± 3 Tg-CH4 yr−1 more than that in the Southern Hemisphere (SH), assuming no change in OH concentrations; alternatively, a decrease in OH concentrations of 4.5 ± 0.6%–5.5 ± 0.5% yr−1 globally would be required if we assume no change in surface emissions. Over the western Pacific, the IAV in CH4 within the northern tropics was characterized by a high growth rate in mid-1997 and a reduced growth in 2007. The present data indicate that these events were strongly influenced by the IAV in atmospheric circulation associated with El Nino and La Nina events. Our observations captured the CH4 anomaly in 1997 associated with forest fires in Indonesia. The IAV and trends in CH4 as seen by our data sets capture the global features of background CH4 levels in the northern midlatitudes and the SH, and regional features of CH4 variations in the western tropical Pacific.

Long-term analysis of carbon dioxide and methane column-averaged mole fractions retrieved from SCIAMACHY

Abstract. Carbon dioxide (CO2) and methane (CH4) are the two most important anthropogenic greenhouse gases contributing to global climate change. SCIAMACHY onboard ENVISAT (launch 2002) was the first and is now with TANSO onboard GOSAT (launch 2009) one of only two satellite instruments currently in space whose measurements are sensitive to CO2 and CH4 concentration changes in the lowest atmospheric layers where the variability due to sources and sinks is largest. We present long-term SCIAMACHY retrievals (2003–2009) of column-averaged dry air mole fractions of both gases (denoted XCO2 and XCH4) derived from absorption bands in the near-infrared/shortwave-infrared (NIR/SWIR) spectral region focusing on large-scale features. The results are obtained using an upgraded version (v2) of the retrieval algorithm WFM-DOAS including several improvements, while simultaneously maintaining its high processing speed. The retrieved mole fractions are compared to global model simulations (CarbonTracker XCO2 and TM5 XCH4) being optimised by assimilating highly accurate surface measurements from the NOAA/ESRL network and taking the SCIAMACHY averaging kernels into account. The comparisons address seasonal variations and long-term characteristics. The steady increase of atmospheric carbon dioxide primarily caused by the burning of fossil fuels can be clearly observed with SCIAMACHY globally. The retrieved global annual mean XCO2 increase agrees with CarbonTracker within the error bars (1.80±0.13 ppm yr−1 compared to 1.81±0.09 ppm yr−1). The amplitude of the XCO2 seasonal cycle as retrieved by SCIAMACHY, which is 4.3±0.2 ppm for the Northern Hemisphere and 1.4±0.2 ppm for the Southern Hemisphere, is on average about 1 ppm larger than for CarbonTracker. An investigation of the boreal forest carbon uptake during the growing season via the analysis of longitudinal gradients shows good agreement between SCIAMACHY and CarbonTracker concerning the overall magnitude of the gradients and their annual variations. The analysis includes a discussion of the relative uptake strengths of the Russian and North American boreal forest regions. The retrieved XCH4 results show that after years of stability, atmospheric methane has started to rise again in recent years which is consistent with surface measurements. The largest increase is observed for the tropics and northern mid- and high-latitudes amounting to about 7.5±1.5 ppb yr−1 since 2007. Due care has been exercised to minimise the influence of detector degradation on the quantitative estimate of this anomaly.

Greenhouse gas observations from Cabauw Tall Tower (1992–2010)

Abstract. Since 1992 semi-continuous in-situ observations of greenhouse gas concentrations have been performed at the tall tower of Cabauw (4.927° E, 51.971° N, −0.7 m a.s.l.). Through 1992 up to now, the measurement system has been gradually extended and improved in precision, starting with CO2 and CH4 concentrations from 200 m a.g.l. in 1992 to vertical gradients at 4 levels of the gases CO2, CH4, SF6, N2O, H2, CO and gradients at 2 levels for 222Rn. In this paper the measurement systems and measurement results are described for the main greenhouse gases and CO, for the whole period. The automatic measurement system now provides half-hourly concentration gradients with a precision better than or close to the WMO recommendations. The observations at Cabauw show a complex pattern caused by the influence of sources and sinks from a large area around the tower with significant contributions of sources and sinks at distances up to 500–700 km. The concentration footprint area of Cabauw is one the most intensive and complex source areas of greenhouse gases in the world. Despite this, annual mean trends for the most important greenhouse gases, compatible with the values derived using the global network, can be reproduced from the measured concentrations at Cabauw over the entire measurement period, with a measured increase in the period 2000–2009 for CO2 of 1.90 ± 0.1 ppm yr−1, for CH4 of 4.4 ± 0.6 ppb yr−1, for N2O of 0.86 ± 0.04 ppb yr−1, and for SF6 of 0.27 ± 0.01 ppt yr−1; for CO no significant trend could be detected. The influences of strong local sources and sinks are reflected in the amplitude of the mean seasonal cycles observed at Cabauw, that are larger than the mean Northern Hemisphere average; Cabauw mean seasonal amplitude for CO2 is 25–30 ppm (higher value for lower sampling levels). The observed CH4 seasonal amplitude is 50–110 ppb. All gases except N2O show highest concentrations in winter and lower concentrations in summer, N2O observations show two additional concentration maxima in early summer and in autumn. Seasonal cycles of the day-time mean concentrations show that surface concentrations or high elevation concentrations alone do not give a representative value for the boundary layer concentrations, especially in winter time, but that the vertical profile data along the mast can be used to construct a useful boundary layer mean value. The variability at Cabauw in the atmospheric concentrations of CO2 on time scales of minutes to hours is several ppm and is much larger than the precision of the measurements (0.1 ppm). The diurnal and synoptical variability of the concentrations at Cabauw carry information on the sources and sinks in the footprint area of the mast, that will be useful in combination with inverse atmospheric transport model to verify emission estimates and improve ecosystem models. For this purpose a network of tall tower stations like Cabauw forms a very useful addition to the existing global observing network for greenhouse gases.

Tropospheric halocompounds and nitrous oxide monitored at a remote site in the Mediterranean

Analysis of time series and trends of nitrous oxide (N 2O) and halocompounds weekly monitored at the Mediterranean island of Lampedusa are discussed. Atmospheric N 2O levels showed a linear upward growth rate of 0.78 ppb yr −1 and mixing ratios comparable with Northern Hemisphere global stations. CFC-11 and CFC-12 time series displayed a decline consistent with their phase-out. Chlorofluorocarbons (CFCs) replacing compounds and SF 6 exhibited an increasing temporal behaviour. The most rapid growth rate was recorded for HFC-134a with a value of 9.6% yr −1. The industrial solvents CCl 4 and CH 3CCl 3, banned by the Montreal Protocol, showed opposite trends. While CH 3CCl 3 reported an expected decay of −1.8 ppt yr −1, an increasing rate of 5.7 ppt yr −1 was recorded for CCl 4 and it is probably related to its relatively long lifetime and persisting emissions. Chlorinated halomethanes showed seasonality with a maximum in early April and a minimum at the end of September. Halon-1301 and Halon-1211 displayed a decreasing trend consistent with industry emission estimates. An interspecies correlation analysis gave positive high correlations between HCFC-22 and HFC-134a (+0.84) highlighting the common extensive employment as refrigerants. Sharing sources inferred the high coupling between CH 3Cl and CH 3Br (+0.73) and between CHCl 3 and CH 2Cl 2 (+0.77). A singular strong relationship (+0.55) between HFC-134a and CH 3I suggested the influence of an unknown anthropogenic source of CH 3I. Constraining of source and sink distribution was carried out by transport studies. Results were compared with the European Environment Agency (EEA) emission database. In contrast with the emission database results, our back trajectory analysis highlighted the release of large amounts of HFC-134a and SF 6 from Eastern Europe. Observations also showed that African SF 6 emissions may be considerable. Leakages from SF 6 insulated electrical equipments located in the industrialized Northern African areas justify our observations.

Atmospheric methane in the Mediterranean: Analysis of measurements at the island of Lampedusa during 1995–2005

Measurements of atmospheric methane and other greenhouse gases are routinely carried out at the island of Lampedusa (35.5°N, 12.6°E), in the Mediterranean Sea. The CH 4 mixing ratio record obtained during the period June 1995–September 2005 shows a distinct annual cycle characterized by a maximum in March and a minimum in late summer. The cycle peak-to-peak amplitude is about 30 ppb. In the period of investigation the CH 4 growth rate (GR) shows two positive peaks, the first in 1998 with 19 ppb yr −1, and the second at the end of 2001 with 14 ppb yr −1. A sharp minimum of −13 ppb yr −1 occurs in April 2000. The GR remains close to zero after mid-2002. The evolution of methane GR and global temperature anomaly appears well correlated ( R=0.71). The methane removal by reaction with OH, acting both on a global and on smaller scales, seems to play a large role. Significant differences exist among the GR at stations located within or close to the Mediterranean basin, indicating that regional and smaller scale processes must be taken into account to explain the methane evolution. The weekly measurements have been combined with backward airmass trajectories to study the relationship between CH 4 mixing ratio and transport. On average, the methane mixing ratio in airmasses from Eastern Europe is about 8 ppb higher than in airmasses from Western Europe, 15 ppb higher than in airmasses from oceanic regions, and about 20 ppb higher than in airmasses from Africa. Among the airmasses from Africa, those from Northern Algeria display the largest CH 4 mixing ratio, possibly because of emissions from gas and oil production and leakage from the gas pipelines. A North-to-South latitude gradient in CH 4 mixing ratio is also apparent.

On the changing seasonal cycles and trends of ozone at Mace Head, Ireland

Abstract. A seasonal-trend decomposition technique based on a locally-weighted regression smoothing (Loess) approach has been used to decompose monthly ozone concentrations at Mace Head (Ireland) into trend, seasonal and irregular components. The trend component shows a steady increase from 1990–2004, which is confirmed by statistical testing which shows that ozone concentrations at Mace Head have increased at the p=0.06 level by 0.18±0.04 ppb yr−1. By considering different air mass origins using a trajectory analysis, it has been possible to separate air masses into "polluted" and "unpolluted" origins. The seasonal-trend decomposition technique confirms the different seasonal cycles of these air mass origins with unpolluted air mass maxima in April and polluted air mass maxima in July/August. A detailed consideration of the seasonal component reveals different behaviour depending on the air mass origin. For baseline unpolluted air arriving at Mace Head there has been a gradual increase in the seasonal amplitude, driven by a declining summertime component. The amplitude of the seasonal component of baseline air is controlled by a maximum in April and a minimum in July. For polluted air mass trajectories, there was a substantial reduction in the amplitude of the seasonal component from 1990–1997. However, post-1997 results indicate that the seasonal amplitude in polluted air masses arriving at Mace Head is increasing. Furthermore, there has been a shift in the months controlling the size of the seasonal amplitude in polluted air from a maximum in May and minimum in January in 1990 to a maximum in April and a minimum in July by 2001. This finding suggests that there has been a steadily decreasing influence of polluted air masses arriving from Europe. These air masses have therefore increasingly taken on the attributes of baseline air.

Changes of daily surface ozone maxima in Switzerland in all seasons from 1992 to 2002 and discussion of summer 2003

Abstract. An Analysis of Covariance (ANCOVA) was used to derive the influence of the meteorological variability on the daily maximum ozone concentrations at 12 low-elevation sites north of the Alps in Switzerland during the four seasons in the 1992–2002 period. The afternoon temperature and the morning global radiation were the variables that accounted for most of the meteorological variability in summer and spring, while other variables that can be related to vertical mixing and dilution of primary pollutants (afternoon global radiation, wind speed, stability or day of the week) were more significant in winter. In addition, the number of days after a frontal passage was important to account for ozone build-up in summer and ozone destruction in winter. The statistical model proved to be a robust tool for reducing the impact of the meteorological variability on the ozone concentrations. The explained variance of the model, averaged over all stations, ranged from 60.2% in winter to 71.9% in autumn. The year-to-year variability of the seasonal medians of daily ozone maxima was reduced by 85% in winter, 60% in summer, and 50% in autumn and spring after the meteorological adjustment. For most stations, no significantly negative trends (at the 95% confidence level) of the summer medians of daily O3 or Ox (O3+NO2) maxima were found despite the significant reduction in the precursor emissions in Central Europe. However, significant downward trends in the summer 90th percentiles of daily Ox maxima were observed at 6 sites in the region around Zürich (on average −0.73 ppb yr-1 for those sites). The lower effect of the titration by NO as a consequence of the reduced emissions could partially explain the significantly positive O3 trends in the cold seasons (on average 0.69 ppb yr-1 in winter and 0.58 ppb yr-1 in autumn). The increase of Ox found for most stations in autumn (on average 0.23 ppb yr-1) and winter (on average 0.39 ppb yr-1) could be due to increasing European background ozone levels, in agreement with other studies. The statistical model was also able to explain the very high ozone concentrations in summer 2003, the warmest summer in Switzerland for at least ~150 years. On average, the measured daily ozone maximum was 15 ppb (nearly 29%) higher than in the reference period summer 1992–2002, corresponding to an excess of 5 standard deviations of the summer means of daily ozone maxima in that period.

Influences of boreal fire emissions on Northern Hemisphere atmospheric carbon and carbon monoxide

There were large interannual variations in burned area in the boreal region (ranging between 3.0 and 23.6 × 106 ha yr−1) for the period of 1992 and 1995–2003 which resulted in corresponding variations in total carbon and carbon monoxide emissions. We estimated a range of carbon emissions based on different assumptions on the depth of burning because of uncertainties associated with the burning of surface‐layer organic matter commonly found in boreal forest and peatlands, and average total carbon emissions were 106–209 Tg yr−1 and CO emissions were 33–77 Tg CO yr−1. Burning of ground‐layer organic matter contributed between 46 and 72% of all emissions in a given year. CO residuals calculated from surface mixing ratios in the high Northern Hemisphere (HNH) region were correlated to seasonal boreal fire emissions in 8 out of 10 years. On an interannual basis, variations in area burned explained 49% of the variations in HNH CO, while variations in boreal fire emissions explained 85%, supporting the hypotheses that variations in fuels and fire severity are important in estimating emissions. Average annual HNH CO increased by an average of 7.1 ppb yr−1 between 2000 and 2003 during a period when boreal fire emissions were 26 to 68 Tg CO−1 higher than during the early to mid‐1990s, indicating that recent increases in boreal fires are influencing atmospheric CO in the Northern Hemisphere.

Hydrogen sulfide and carbonyl sulfide in the museum environment?Part 1

Abstract This paper discusses H 2 S and OCS measurements in a variety of museums. It addresses several issues of interpretation of those results, such seasonal and spatial variation, effects of sources and sinks local to the measurement. Typical indoor H 2 S concentrations found vary from 86 to 600 ppt, whereas OCS varies from 400 to 850 ppt. A lowest observed adverse effect dose (LOAED) for H 2 S and silver is defined (3.85 μg m −3 yr −1 ∼2.5 ppb yr −1 ). All museum H 2 S concentration data collected by the authors thus far is compared to this LOAED and in the main found to be considerably lower. Therefore tarnishing is not solely a function of H 2 S, instead is the result of multiple factors.

Ten years of atmospheric methane observations at a high elevation site in Western China

Abstract In this paper, the continuous (1994–2001) and discrete air sample (1991–2001) measurements of atmospheric CH 4 from the Waliguan Baseline Observatory located in western China (36°17′N, 100°54′E, 3816 m asl) are presented and characterized. The CH 4 time series show large episodic events on the order of 100 ppb throughout the year. During spring, a diurnal cycle with average amplitude of 7 ppb and a morning maximum and late afternoon minimum is observed. In winter, a diurnal cycle with average amplitude of 14 ppb is observed with an afternoon maximum and morning minimum. Unlike most terrestrial observational sites, no obvious diurnal patterns are present during the summer or autumn. A background data selection procedure was developed based on local horizontal and vertical winds. A selected hourly data set representative of “baseline” conditions was derived with approximately 50% of the valid hourly data. The range of CH 4 mixing ratios, annual means, annual increases and mean annual cycle at Waliguan during the 1992–2001 were derived from discrete and continuous data representative of “baseline” conditions and compared to air samples collected at other Northern Hemisphere sites. The range of CH 4 monthly means of 1746–1822 ppb, average annual means of 1786.7±10.8 ppb and mean annual increase of 4.5±4.2 ppb yr −1 at Waliguan were inline with measurements from sites located between 30° and 60°N. There were variations observed in the CH 4 annual increase patterns at Waliguan that were slightly different from the global pattern. The mean CH 4 annual cycle at Waliguan shows an unusual pattern of two gentle peaks in summer and February along with two small valleys in early winter and spring and a mean peak-to-peak amplitude of ∼11 ppb, much smaller than amplitudes observed at most other mid- and high-northern latitude sites. The Waliguan CH 4 data are strongly influenced by continental Asian CH 4 emissions and provide key information for global atmospheric CH 4 models.

Three years of trace gas observations over the EuroSiberian domain derived from aircraft sampling — a concerted action

A three-year trace gas climatology of CO2 and its stable isotopic ratios, as well as CH4, N2O and SF6, derived from regular vertical aircraft sampling over the Eurasian continent is presented. The four sampling sites range from about 1°E to 89°E in the latitude belt from 48°N to 62°N. The most prominent features of the CO2 observations are an increase of the seasonal cycle amplitudes of CO2 andδ13C–CO2 in the free troposphere (at 3000 m a.s.l.) by more than 60% from Western Europe to Western and Central Siberia. δ18O–CO2 shows an even larger increase of the seasonal cycle amplitude by a factor of two from Western Europe towards the Ural mountains, which decreases again towards the most eastern site, Zotino. These data reflect a strong influence of carbon exchange fluxes with the continental biosphere. In particular, during autumn and winter δ18O–CO2 shows a decrease by more than 0.5% from Orléans (Western Europe) to Syktyvkar (Ural mountains) and Zotino (West Siberia), mainly caused by soil respiration fluxes depleted inδ18O with respect to atmospheric CO2. CH4 mixing ratios in the free troposphere at 3000 m over Western Siberia are higher by about 20–30 ppb if compared to Western Europe. Wetland emissions seem to be particularly visible in July–September, with largest signals at Zotino in 1998. Annual mean CH4 mixing ratios decrease slightly from 1998 to 1999 at all Russian sites. In contrast to CO2 and CH4, which show significant vertical gradients between 2000 and 3000 m a.s.l., N2O mixing ratios are vertically very homogeneous and show no significant logitudinal gradient between the Ural mountains and Western Siberia, indicating insignificant emissions of this trace gas from boreal forest ecosystems in Western Siberia. The growth rate of N2O (1.2–1.3 ppb yr−1) and the seasonal amplitude (0.5–1.1 ppb) are similar at both aircraft sites, Syktyvkar and Zotino. For SF6 an annual increase of 5% is observed, together with a small seasonal cycle which is in phase with the N2O cycle, indicating that the seasonality of both trace gases are most probably caused by atmospheric transport processes with a possible contribution from stratosphere–troposphere exchange.

Analysis and presentation of in situ atmospheric methane measurements from Cape Ochi‐ishi and Hateruma Island

In situ measurements of atmospheric methane (CH4) from the monitoring stations at Cape Ochi‐ishi (latitude 43°10′N, longitude 145°30′E) during the period from July 1995 to July 2000 and at Hateruma Island (latitude 24°03′N, longitude 123°48′E) during the period from January 1996 to January 2000 are presented. At each station a fully automated gas chromatograph equipped with a flame ionization detector measures the CH4 mixing ratios at a frequency of more than 82 air samples per day. Over the above observation periods, average growth rates of CH4 mixing ratios were 4.5 parts per billion (ppb) yr−1 for Ochi‐ishi and 4.7 ppb yr−1 for Hateruma, but there were considerable fluctuations in the instantaneous growth rate. The peak‐to‐peak amplitudes in the seasonal cycles determined from the smooth curve fits were 71 ppb for Ochi‐ishi and 94 ppb for Hateruma. These seasonal amplitudes are larger than those at the sampling network sites of the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory. Analysis of the back trajectories indicated that the air masses at both sites are transported from continental Asia in winter and from the Pacific Ocean in summer. Because there are strong CH4 sources in continental Asia but no strong sources in the Pacific Ocean, the seasonality of airflow patterns can explain the enhancement of the seasonal cycles. The significantly large seasonality, especially at Hateruma, is a result of air masses originating from a wide range of latitudes, the clear seasonality of airflows, and the steep latitudinal gradient of the CH4 mixing ratio.

Trends in near-surface ozone concentrations in Switzerland: the 1990s

A time series analysis of ozone monitoring data from several locations in Switzerland from 1991 to 1999 is presented. Different methods are used to address changes in the ozone level during these years and to account for the influence of changing meteorological conditions. The results show a slight decrease of the peaks but a highly significant increase of the mean value of around 0.5–0.9 ppb yr −1. The frequency distribution has changed in the sense that very low values have become less frequent and that there is a strong increase in frequency of occurrence of half-hourly mean values between about 45 and 55 ppb. A selection procedure reveals slight tendencies towards different trends of afternoon ozone peaks in summer depending on weather and pollution situations. Ozone peaks tend to decrease on fair weather days at rural sites (but increase at urban sites) and show a small increase on cloudy and windy days. A non-linear regression model is used to estimate trends of summertime afternoon ozone peaks in the presence of meteorological variability. In the model, the long-term signal is additively split into a linear part and a part which is modulated by global radiation. The coefficients for both terms are statistically significant at many sites, with an increasing linear trend at the sites north of the Alps of around 1 ppb yr −1 and a decrease of ozone peaks under fair weather conditions relative to cloudy conditions. When additionally considering the effect of precursor concentrations in the regression models, both trends are weakened, which means that they can partly be explained by changes in local to regional emissions. However, at the sites north of the Alps remains a tendency towards a positive linear “base trend” of around 0.4 ppb yr −1. This could possibly be due to increasing background ozone concentrations.

Western European N2O emissions: A top‐down approach based on atmospheric observations

We present a 3 year record of continuous gas chromatographic nitrous oxide (N2O) observations performed at the urban station Heidelberg (Germany) together with weekly flask data from a remote continental site, Schauinsland (Black Forest, Germany), and two‐weekly integrated data from the maritime background station Izaña (Canary Islands). These data are supplemented by continuous atmospheric radon 222 observations. Mean rates of increase of N2O of 0.70–0.78 ppb yr−1 were observed over the continent and in maritime background air (Izaña). The well‐mixed continental mixing ratio was found to be higher by only 1.1 ppb (Schauinsland) and 2.4 ppb (Heidelberg) than for maritime background air. Specially tailored data selection of the Heidelberg record allowed the changing influence of a regional N2O point source (adipic acid production, BASF AG) to be clearly identified. The radon (222Rn) tracer method was applied to nighttime N2O observations at Heidelberg to estimate mean regional emissions, which changed from (161 ± 32) μg N2O‐N m−2 h−1 in 1996–1997 to (77 ± 10) μg N2O‐N m−2 h−1 in 1998 as a consequence of 90% emission reduction from BASF. An estimate of the continental N2O flux from southwestern Europe based on further selected Heidelberg data (only well‐mixed, late afternoon situations) and observations from the Schauinsland station yielded mean N2O fluxes of (43 ± 5) μg N2O‐N m−2 h−1 and (42 ± 4) μg N2O‐N m−2 h−1. These results compare well with statistical emissions inventories, when taking into account possible systematic errors of the radon tracer method of 30–35%.

Continuous high‐frequency observations of hydrogen at the Mace Head baseline atmospheric monitoring station over the 1994–1998 period

Continuous high‐frequency (every 40‐min) automatic measurements of hydrogen have been made at the Mace Head atmospheric research station on the Atlantic Ocean coast of Ireland throughout 1994–1998. These observations represent one the most comprehensive in situ records of a trace gas that has received comparatively little attention. Individual measurements have been sorted by four independent methods to separate clean, maritime air masses from regionally polluted European air masses. Hydrogen concentrations in midlatitude Northern Hemisphere baseline air show a distinct seasonal cycle with highest concentrations during spring and lowest concentrations during late autumn, with a peak‐to‐trough amplitude of 38±6 ppb, averaged over the observed seasonal cycles from 1994 to 1998. The mean hydrogen concentration in midlatitude Northern Hemisphere baseline air on January 1, 1995, was estimated as 496.5 ppb with an upward trend of 1.2±0.8 ppb yr−1. Evidence has also been obtained for European pollution sources with source strength of about 0.8 Tg yr−1 and for deposition of hydrogen to soils. The observation of slightly elevated hydrogen concentrations relative to baseline levels in tropical maritime air masses points to a latitudinal gradient in hydrogen with higher concentrations in lower latitudes of the Northern Hemisphere and in the Southern Hemisphere. This is confirmed by comparable hydrogen observations at Cape Grim, Tasmania, which are consistently higher than measurements recorded at Mace Head. Mean hemispheric concentrations of 504 and 520 ppb have been estimated for the Northern and Southern Hemispheres, respectively, for January 1, 1996, corresponding to a total atmospheric hydrogen burden of 182 Tg.

Meteorologically adjusted trends in UK daily maximum surface ozone concentrations

A methodology to meteorologically adjust daily UK surface ozone data is presented which reveals the impact of longer-term variations in precursor emissions more clearly. Based on this approach a general site-dependant decline in meteorologically adjusted summer daily maximum ozone concentrations has occurred between 1994 and 1998 and is between 0.7 and 2.3 ppb yr −1.

Distributions and recent changes of carbon monoxide in the lower troposphere

Since 1988, the distribution of carbon monoxide (CO) in the lower troposphere has been determined using a globally distributed air sampling network. Site locations range from 82°N to 90°S, with wide longitudinal coverage, and represent the marine boundary layer, regionally polluted atmospheres, and the free troposphere. These measurements present a unique, intercalibrated, and internally consistent data set that are used to better define the global temporal and spatial distribution of CO. In this paper, times series from 49 sites are discussed. With an average lifetime of ∼2 months, CO showed significant concentration gradients. In the marine boundary layer, mixing ratios were greatest in the northern winter (200–220 ppb) and lowest in the southern summer (35–45 ppb). The interhemispheric gradient showed strong seasonality with a maximum difference between the high latitudes of the northern and southern hemispheres (160–180 ppb) in February and March and a minimum in July and August (10–20 ppb). Higher CO was found in regions near human development relative to those over more remote areas. The distributions provide additional evidence of the widespread pollution of the lower atmosphere. Remote areas in the high northern hemisphere are polluted by anthropogenic activities in the middle latitudes, and those in the southern hemisphere are heavily influenced by the burning of biomass in the tropics. While tropospheric concentrations of CO exhibit periods of increase and decrease, the globally averaged CO mixing ratio over the period from 1990 through 1995 decreased at a rate of approximately 2 ppb yr−1.

Seven years of continuous methane observations at a remote boreal site in Ontario, Canada

A 7‐year record (1990–1996) of continuous atmospheric methane (CH4) measurements is presented from a remote midcontinental monitoring station at Fraserdale, Ontario (49°53′N, 81°34′W). Ninety‐six air samples per day were measured with a fully automated gas chromatograph with flame ionization detection. Five‐day Lagrangian back trajectories over the 7‐year period were used to establish a climatology in the region of the station. The site is predominantly influenced by air flow from northern and high‐latitude regions and therefore uniquely positioned to monitor wetland emissions. During winter, CH4 concentration time series from Fraserdale often match the short‐term variability observed at the high Arctic monitoring station at Alert, Northwest Territories (82°27′N, 61°31′W). During summer, due to diurnal changes of vertical mixing in the boundary layer, large diurnal cycles in CH4 mixing ratio up to 150 ppb are observed. The data selected for the afternoon, when the boundary layer is well‐mixed, are representative of a larger spatial scale. The mean annual cycle of CH4 at Fraserdale determined using these selected data is significantly different from annual cycles at other mid‐ and high‐northern latitude sites thus providing key information for global atmospheric CH4 models. In late summer the annual cycle at Fraserdale shows a distinct secondary maximum in CH4. This is the result of advection of air with enhanced CH4 due to emissions from the extensive wetland areas to the north and northwest. The average growth rate (using selected data) for the period was 5.6 ppb yr−1 with a growth rate pattern that is slightly different and out of phase with growth rate changes observed at other high‐latitude observing sites by 2 to 6 months.

Atmospheric methane between 1000 A.D. and present: Evidence of anthropogenic emissions and climatic variability

Atmospheric methane mixing ratios from 1000 A.D. to present are measured in three Antarctic ice cores, two Greenland ice cores, the Antarctic firn layer, and archived air from Tasmania, Australia. The record is unified by using the same measurement procedure and calibration scale for all samples and by ensuring high age resolution and accuracy of the ice core and firn air. In this way, methane mixing ratios, growth rates, and interpolar differences are accurately determined. From 1000 to 1800 A.D. the global mean methane mixing ratio averaged 695 ppb and varied about 40 ppb, contemporaneous with climatic variations. Interpolar (N‐S) differences varied between 24 and 58 ppb. The industrial period is marked by high methane growth rates from 1945 to 1990, peaking at about 17 ppb yr−1 in 1981 and decreasing significantly since. We calculate an average total methane source of 250 Tg yr−1 for 1000–1800 A.D., reaching near stabilization at about 560 Tg yr−1 in the 1980s and 1990s. The isotopic ratio, δ13CH4, measured in the archived air and firn air, increased since 1978 but the rate of increase slowed in the mid‐1980s. The combined CH4 and δ13CH4 trends support the stabilization of the total CH4 source.

The 1991–1992 atmospheric methane anomaly: Southern hemisphere 13C decrease and growth rate fluctuations

Measurements of atmospheric methane from 1989–1996 at Baring Head, New Zealand, and at Scott Base, Antarctica show a seasonal cycle in the mixing ratio with a peak to peak amplitude of 28 ppb. This is superposed on a trend varying between 16 ppb yr−1 and near zero. δ13C values also show a seasonal cycle, with an amplitude of 0.1–0.3‰, approximately 6 months out of phase with the mixing ratio cycle. A pronounced negative anomaly in δ13C occurred in 1992 with annual average values dropping from −47.08‰ to −47.28‰. From 1992 to 1996, average δ13C values recovered slowly at an average rate of about 0.04‰ yr−1. The simultaneous changes in the mixing ratio growth rate and δ13C together with the rapid drop and slow recovery in the latter provide a stringent test of possible causes. Although a combination of causes cannot be ruled out, decreased emissions from an isotopically heavy source such as biomass burning best meet the constraints of the data.