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2016
Dalsoren, SB, Myhre CL, Myhre G, Gomez-Pelaez AJ, Sovde OA, Isaksen ISA, Weiss RF, Harth CM.  2016.  Atmospheric methane evolution the last 40 years. Atmospheric Chemistry and Physics. 16:3099-3126.   10.5194/acp-16-3099-2016   AbstractWebsite

Observations at surface sites show an increase in global mean surface methane (CH4) of about 180 parts per billion (ppb) (above 10 %) over the period 1984-2012. Over this period there are large fluctuations in the annual growth rate. In this work, we investigate the atmospheric CH4 evolution over the period 1970-2012 with the Oslo CTM3 global chemical transport model (CTM) in a bottom-up approach. We thoroughly assess data from surface measurement sites in international networks and select a subset suited for comparisons with the output from the CTM. We compare model results and observations to understand causes for both long-term trends and short-term variations. Employing Oslo CTM3 we are able to reproduce the seasonal and year-to-year variations and shifts between years with consecutive growth and stagnation, both at global and regional scales. The overall CH4 trend over the period is reproduced, but for some periods the model fails to reproduce the strength of the growth. The model overestimates the observed growth after 2006 in all regions. This seems to be explained by an overly strong increase in anthropogenic emissions in Asia, having global impact. Our findings confirm other studies questioning the timing or strength of the emission changes in Asia in the EDGAR v4.2 emission inventory over recent decades. The evolution of CH4 is not only controlled by changes in sources, but also by changes in the chemical loss in the atmosphere and soil uptake. The atmospheric CH4 lifetime is an indicator of the CH4 loss. In our simulations, the atmospheric CH4 lifetime decreases by more than 8 % from 1970 to 2012, a significant reduction of the residence time of this important greenhouse gas. Changes in CO and NOx emissions, specific humidity, and ozone column drive most of this, and we provide simple prognostic equations for the relations between those and the CH4 lifetime. The reduced lifetime results in substantial growth in the chemical CH4 loss (relative to its burden) and dampens the CH4 growth.

2014
Patra, PK, Krol MC, Montzka SA, Arnold T, Atlas EL, Lintner BR, Stephens BB, Xiang B, Elkins JW, Fraser PJ, Ghosh A, Hintsa EJ, Hurst DF, Ishijima K, Krummel PB, Miller BR, Miyazaki K, Moore FL, Muhle J, O'Doherty S, Prinn RG, Steele LP, Takigawa M, Wang HJ, Weiss RF, Wofsy SC, Young D.  2014.  Observational evidence for interhemispheric hydroxyl-radical parity. Nature. 513:219-+.   10.1038/nature13721   AbstractWebsite

The hydroxyl radical (OH) is a key oxidant involved in the removal of air pollutants and greenhouse gases from the atmosphere(1-3). The ratio of Northern Hemispheric to Southern Hemispheric (NH/SH) OH concentration is important for our understanding of emission estimates of atmospheric species such as nitrogen oxides and methane(4-6). It remains poorly constrained, however, with a range of estimates from 0.85 to 1.4 (refs 4,7-10). Here we determine the NH/SH ratio of OH with the help of methyl chloroform data (a proxy for OH concentrations) and an atmospheric transport model that accurately describes interhemispheric transport and modelled emissions. We find that for the years 2004-2011 the model predicts an annual mean NH-SH gradient of methyl chloroform that is a tight linear function of the modelled NH/SH ratio in annual mean OH. We estimate a NH/SH OH ratio of 0.97 +/- 0.12 during this time period by optimizing global total emissions and mean OH abundance to fit methyl chloroform data from two surface-measurement networks and aircraft campaigns(11-13). Our findings suggest that top-down emission estimates of reactive species such as nitrogen oxides in key emitting countries in the NH that are based on a NH/SH OH ratio larger than 1 may be overestimated.

Arnold, T, Ivy DJ, Harth CM, Vollmer MK, Muhle J, Salameh PK, Steele LP, Krummel PB, Wang RHJ, Young D, Lunder CR, Hermansen O, Rhee TS, Kim J, Reimann S, O'Doherty S, Fraser PJ, Simmonds PG, Prinn RG, Weiss RF.  2014.  HFC-43-10mee atmospheric abundances and global emission estimates. Geophysical Research Letters. 41:2228-2235.   10.1002/2013gl059143   AbstractWebsite

We report in situ atmospheric measurements of hydrofluorocarbon HFC-43-10mee (C5H2F10; 1,1,1,2,2,3,4,5,5,5-decafluoropentane) from seven observatories at various latitudes, together with measurements of archived air samples and recent Antarctic flask air samples. The global mean tropospheric abundance was 0.210.05ppt (parts per trillion, dry air mole fraction) in 2012, rising from 0.040.03ppt in 2000. We combine the measurements with a model and an inverse method to estimate rising global emissionsfrom 0.430.34Ggyr(-1) in 2000 to 1.130.31Ggyr(-1) in 2012 (similar to 1.9TgCO(2)-eqyr(-1) based on a 100year global warming potential of 1660). HFC-43-10meea cleaning solvent used in the electronics industryis currently a minor contributor to global radiative forcing relative to total HFCs; however, our calculated emissions highlight a significant difference from the available reported figures and projected estimates.

2013
Simmonds, PG, Manning AJ, Athanassiadou M, Scaife AA, Derwent RG, O'Doherty S, Harth CM, Weiss RF, Dutton GS, Hall BD, Sweeney C, Elkins JW.  2013.  Interannual fluctuations in the seasonal cycle of nitrous oxide and chlorofluorocarbons due to the Brewer-Dobson circulation. Journal of Geophysical Research-Atmospheres. 118:10694-10706.   10.1002/jgrd.50832   AbstractWebsite

The tropospheric seasonal cycles of N2O, CFC-11 (CCl3F), and CFC-12 (CCl2F2) are influenced by atmospheric dynamics. The interannually varying summertime minima in mole fractions of these trace gases have been attributed to interannual variations in mixing of stratospheric air (depleted in CFCs and N2O) with tropospheric air with a few months lag. The amount of wave activity that drives the stratospheric circulation and influences the winter stratospheric jet and subsequent mass transport across the tropopause appears to be the primary cause of this interannual variability. We relate the observed seasonal minima of species at three Northern Hemisphere sites (Mace Head, Ireland; Trinidad Head, U.S.; and Barrow, Alaska) with the behavior of the winter stratospheric jet. As a result, a good correlation is obtained between zonal winds in winter at 10 hPa, 58°N–68°N, and the detrended seasonal minima in the stratosphere-influenced tracers. For these three tracers, individual Pearson correlation coefficients (r) between 0.51 and 0.71 were found, with overall correlations of between 0.67 and 0.77 when “composite species” were considered. Finally, we note that the long-term observations of CFCs and N2O in the troposphere provide an independent monitoring method complementary to satellite data. Furthermore, they could provide a useful observational measure of the strength of stratosphere-troposphere exchange and, thus, could be used to monitor any long-term trend in the Brewer-Dobson circulation which is predicted by climate models to increase over the coming decades.

2012
Kim, J, Li S, Muhle J, Stohl A, Kim SK, Park S, Park MK, Weiss RF, Kim KR.  2012.  Overview of the findings from measurements of halogenated compounds at Gosan (Jeju Island, Korea) quantifying emissions in East Asia. Journal of Integrative Environmental Sciences. 9:71-80.   10.1080/1943815x.2012.696548   AbstractWebsite

With increased economic growth in East Asia, regional emissions of many anthropogenic halogenated compounds now constitute a substantial fraction of the global totals. Here, we summarize recently reported findings from measurements of a wide range of chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur hexafluoride (SF6), and other halogenated compounds at Gosan (Jeju Island, Korea) within the advanced global atmospheric gases experiment (AGAGE). General wind patterns at Gosan bring air masses from the surrounding areas, allowing the monitoring of both clean baseline and polluted air masses. We have analyzed our measurements since November 2007 both with an interspecies correlation method and with an inversion method based on the FLEXPART Lagrangian particle dispersion model to estimate these regional emissions. The results show that emissions of halogenated compounds in East Asia account for over 20% of global emissions, both in terms of ozone depletion potential (ODP) and global warming potential (GWP), and emphasize the importance of atmospheric measurements for quantifying emissions of these compounds in this region.

2011
Vollmer, MK, Miller BR, Rigby M, Reimann S, Muhle J, Krummel PB, O'Doherty S, Kim J, Rhee TS, Weiss RF, Fraser PJ, Simmonds PG, Salameh PK, Harth CM, Wang RHJ, Steele LP, Young D, Lunder CR, Hermansen O, Ivy D, Arnold T, Schmidbauer N, Kim KR, Greally BR, Hill M, Leist M, Wenger A, Prinn RG.  2011.  Atmospheric histories and global emissions of the anthropogenic hydrofluorocarbons HFC-365mfc, HFC-245fa, HFC-227ea, and HFC-236fa. Journal of Geophysical Research-Atmospheres. 116   10.1029/2010jd015309   AbstractWebsite

We report on ground-based atmospheric measurements and emission estimates of the four anthropogenic hydrofluorocarbons (HFCs) HFC-365mfc (CH(3)CF(2)CH(2)CF(3), 1,1,1,3,3-pentafluorobutane), HFC-245fa (CHF(2)CH(2)CF(3), 1,1,1,3,3-pentafluoropropane), HFC-227ea (CF(3)CHFCF(3), 1,1,1,2,3,3,3-heptafluoropropane), and HFC-236fa (CF(3)CH(2)CF(3), 1,1,1,3,3,3-hexafluoropropane). In situ measurements are from the global monitoring sites of the Advanced Global Atmospheric Gases Experiment (AGAGE), the System for Observations of Halogenated Greenhouse Gases in Europe (SOGE), and Gosan (South Korea). We include the first halocarbon flask sample measurements from the Antarctic research stations King Sejong and Troll. We also present measurements of archived air samples from both hemispheres back to the 1970s. We use a two-dimensional atmospheric transport model to simulate global atmospheric abundances and to estimate global emissions. HFC-365mfc and HFC-245fa first appeared in the atmosphere only similar to 1 decade ago; they have grown rapidly to globally averaged dry air mole fractions of 0.53 ppt (in parts per trillion, 10(-12)) and 1.1 ppt, respectively, by the end of 2010. In contrast, HFC-227ea first appeared in the global atmosphere in the 1980s and has since grown to similar to 0.58 ppt. We report the first measurements of HFC-236fa in the atmosphere. This long-lived compound was present in the atmosphere at only 0.074 ppt in 2010. All four substances exhibit yearly growth rates of >8% yr(-1) at the end of 2010. We find rapidly increasing emissions for the foam-blowing compounds HFC-365mfc and HFC-245fa starting in similar to 2002. After peaking in 2006 (HFC-365mfc: 3.2 kt yr(-1), HFC-245fa: 6.5 kt yr(-1)), emissions began to decline. Our results for these two compounds suggest that recent estimates from long-term projections (to the late 21st century) have strongly overestimated emissions for the early years of the projections (similar to 2005-2010). Global HFC-227ea and HFC-236fa emissions have grown to average values of 2.4 kt yr(-1) and 0.18 kt y(r-)1 over the 2008-2010 period, respectively.

2009
O'Doherty, S, Cunnold DM, Miller BR, Muhle J, McCulloch A, Simmonds PG, Manning AJ, Reimann S, Vollmer MK, Greally BR, Prinn RG, Fraser PJ, Steele LP, Krummel PB, Dunse BL, Porter LW, Lunder CR, Schmidbauer N, Hermansen O, Salameh PK, Harth CM, Wang RHJ, Weiss RF.  2009.  Global and regional emissions of HFC-125 (CHF2CF3) from in situ and air archive atmospheric observations at AGAGE and SOGE observatories. Journal of Geophysical Research-Atmospheres. 114   10.1029/2009jd012184   AbstractWebsite

High-frequency, in situ observations from the Advanced Global Atmospheric Gases Experiment (AGAGE) and System for Observation of halogenated Greenhouse gases in Europe (SOGE) networks for the period 1998 to 2008, combined with archive flask measurements dating back to 1978, have been used to capture the rapid growth of HFC-125 (CHF(2)CF(3)) in the atmosphere. HFC-125 is the fifth most abundant HFC, and it currently makes the third largest contribution of the HFCs to atmospheric radiative forcing. At the beginning of 2008 the global average was 5.6 ppt in the lower troposphere and the growth rate was 16% yr(-1). The extensive observations have been combined with a range of modeling techniques to derive global emission estimates in a top-down approach. It is estimated that 21 kt were emitted globally in 2007, and the emissions are estimated to have increased 15% yr(-1) since 2000. These estimates agree within approximately 20% with values reported to the United Nations Framework Convention on Climate Change (UNFCCC) provided that estimated emissions from East Asia are included. Observations of regionally polluted air masses at individual AGAGE sites have been used to produce emission estimates for Europe (the EU-15 countries), the United States, and Australia. Comparisons between these top-down estimates and bottom-up estimates based on reports by individual countries to the UNFCCC show a range of approximately four in the differences. This process of independent verification of emissions, and an understanding of the differences, is vital for assessing the effectiveness of international treaties, such as the Kyoto Protocol.

Petrenko, VV, Smith AM, Brook EJ, Lowe D, Riedel K, Brailsford G, Hua Q, Schaefer H, Reeh N, Weiss RF, Etheridge D, Severinghaus JP.  2009.  14CH4 measurements in Greenland ice: investigating last glacial termination CH4 sources. Science. 324:506-508.   10.1126/science.1168909   AbstractWebsite

The cause of a large increase of atmospheric methane concentration during the Younger Dryas-Preboreal abrupt climatic transition (similar to 11,600 years ago) has been the subject of much debate. The carbon-14 (C-14) content of methane ((CH4)-C-14) should distinguish between wetland and clathrate contributions to this increase. We present measurements of (CH4)-C-14 in glacial ice, targeting this transition, performed by using ice samples obtained from an ablation site in west Greenland. Measured (CH4)-C-14 values were higher than predicted under any scenario. Sample (CH4)-C-14 appears to be elevated by direct cosmogenic C-14 production in ice. C-14 of CO was measured to better understand this process and correct the sample (CH4)-C-14. Corrected results suggest that wetland sources were likely responsible for the majority of the Younger Dryas-Preboreal CH4 rise.

2005
Vollmer, MK, Bootsma HA, Hecky RE, Patterson G, Halfman JD, Edmond JM, Eccles DH, Weiss RF.  2005.  Deep-water warming trend in Lake Malawi, East Africa. Limnology and Oceanography. 50:727-732. AbstractWebsite

We use historic water temperature measurements to define a deep-water warming trend in Lake Malawi, East Africa. Over the past six decades, the temperature of the deep water below 300 m has increased by similar to 0.7 degrees C. The warming trend is due mainly to the reduction of cold-water deep convection over this period, which is associated with milder winters in the region. Despite deep-water warming, density stratification was maintained at depths below 100 in. The observed warming trend was interrupted at least twice by abyssal cooling events that were associated with the wettest years on record. We propose that rainfall and cool river inflow are critical factors that control thermal structure and the rate of deep-water recharge in this deep, tropical lake.

2001
Prinn, RG, Huang J, Weiss RF, Cunnold DM, Fraser PJ, Simmonds PG, McCulloch A, Harth C, Salameh P, O'Doherty S, Wang RHJ, Porter L, Miller BR.  2001.  Evidence for substantial variations of atmospheric hydroxyl radicals in the past two decades. Science. 292:1882-1888.   10.1126/science.1058673   AbstractWebsite

The hydroxyl radical (OH) is the dominant oxidizing chemical in the atmosphere. It destroys most air pollutants and many gases involved in ozone depletion and the greenhouse effect. Global measurements of 1,1,1-trichloroethane (CH3CCl3, methyl chloroform) provide an accurate method for determining the global and hemispheric behavior of OH. Measurements show that CH3CCl3 Levels rose steadily from 1978 to reach a maximum in 1992 and then decreased rapidly to levels in 2000 that were Lower than the levels when measurements began in 1978. Analysis of these observations shows that global OH Levels were growing between 1978 and 1988, but the growth rate was decreasing at a rate of 0.23 +/- 0.18% year(-2), so that OH Levels began declining after 1988. Overall, the global average OH trend between 1978 and 2000 was -0.64 +/- 0.60% year(-1). These variations imply important and unexpected gaps in current understanding of the capability of the atmosphere to cleanse itself.