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A
Alexander, B, Vollmer MK, Jackson T, Weiss RF, Thiemens MH.  2001.  Stratospheric CO2 isotopic anomalies and SF6 and CFC tracer concentrations in the Arctic polar vortex. Geophysical Research Letters. 28:4103-4106.   10.1029/2001gl013692   AbstractWebsite

Isotopic measurements (delta O-17 and delta O-18) Of CO2 along with concentration measurements of SF6, CC1(3)F (CFC-11), CC1(2)F(2) (CFC-12) and CC1(2)FCC1F(2) (CFC-113) in stratospheric samples collected within the Arctic polar vortex are reported. These are the first simultaneous measurements of the concentration of fluorinated compounds and the complete oxygen isotopic composition Of CO2 in the middle atmosphere. A mass-independent anomaly in the oxygen isotopic composition Of CO2 is observed that arises from isotopic exchange with stratospheric O(D-1) derived from O-3 photolysis. The data exhibit a strong anti-correlation between the Delta O-17 (the degree of the mass-independent anomaly) and molecular tracer concentrations. The potential ability of tl-ris isotopic proxy to trace mesospheric and stratospheric transport is discussed.

C
Chipperfield, MP, Liang Q, Rigby M, Hossaini R, Montzka SA, Dhomse S, Feng WH, Prinn RG, Weiss RF, Harth CM, Salameh PK, Muhle J, O'Doherty S, Young D, Simmonds PG, Krummel PB, Fraser PJ, Steele LP, Happell JD, Rhew RC, Butler J, Yvon-Lewis SA, Hall B, Nance D, Moore F, Miller BR, Elkins J, Harrison JJ, Boone CD, Atlas EL, Mahieu E.  2016.  Model sensitivity studies of the decrease in atmospheric carbon tetrachloride. Atmospheric Chemistry and Physics. 16:15741-15754.   10.5194/acp-16-15741-2016   AbstractWebsite

Carbon tetrachloride (CCl4) is an ozone-depleting substance, which is controlled by the Montreal Protocol and for which the atmospheric abundance is decreasing. However, the current observed rate of this decrease is known to be slower than expected based on reported CCl4 emissions and its estimated overall atmospheric lifetime. Here we use a three-dimensional (3-D) chemical transport model to investigate the impact on its predicted decay of uncertainties in the rates at which CCl4 is removed from the atmosphere by photolysis, by ocean uptake and by degradation in soils. The largest sink is atmospheric photolysis (74% of total), but a reported 10% uncertainty in its combined photolysis cross section and quantum yield has only a modest impact on the modelled rate of CCl4 decay. This is partly due to the limiting effect of the rate of transport of CCl4 from the main tropospheric reservoir to the stratosphere, where photolytic loss occurs. The model suggests large interannual variability in the magnitude of this stratospheric photolysis sink caused by variations in transport. The impact of uncertainty in the minor soil sink (9% of total) is also relatively small. In contrast, the model shows that uncertainty in ocean loss (17% of total) has the largest impact on modelled CCl4 decay due to its sizeable contribution to CCl4 loss and large lifetime uncertainty range (147 to 241 years). With an assumed CCl4 emission rate of 39 Gg year(-1), the reference simulation with the best estimate of loss processes still underestimates the observed CCl4 (overestimates the decay) over the past 2 decades but to a smaller extent than previous studies. Changes to the rate of CCl4 loss processes, in line with known uncertainties, could bring the model into agreement with in situ surface and remote-sensing measurements, as could an increase in emissions to around 47 Gg year(-1). Further progress in constraining the CCl4 budget is partly limited by systematic biases between observational datasets. For example, surface observations from the National Oceanic and Atmospheric Administration (NOAA) network are larger than from the Advanced Global Atmospheric Gases Experiment (AGAGE) network but have shown a steeper decreasing trend over the past 2 decades. These differences imply a difference in emissions which is significant relative to uncertainties in the magnitudes of the CCl4 sinks.

Cui, YY, Vijayan A, Falk M, Hsu YK, Yin DZ, Chen XM, Zhao Z, Avise J, Chen YJ, Verhulst K, Duren R, Yadav V, Miller C, Weiss R, Keeling R, Kim J, Iraci LT, Tanaka T, Johnson MS, Kort EA, Bianco L, Fischer ML, Stroud K, Herner J, Croes B.  2019.  A multiplatform inversion estimation of statewide and regional methane emissions in California during 2014-2016. Environmental Science & Technology. 53:9636-9645.   10.1021/acs.est.9b01769   AbstractWebsite

California methane (CH4) emissions are quantified for three years from two tower networks and one aircraft campaign. We used backward trajectory simulations and a mesoscale Bayesian inverse model, Mbring ratios (VI ' initialized by three inventories, to achieve the emission quantification. Results show total statewide CH4 emissions of 2.05 +/- 0.26 (at 95% confidence) Tg/yr, which is 1.14 to 1.47 times greater than the anthropogenic emission estimates by California Air Resource Board (GARB). Some of differences could be biogenic emissions, superemitter point sources, and other episodic emissions which may not be completely included in the CARB inventory. San Joaquin Valley (SJV) has the largest CH4 emissions (0.94 +/- 0.18 Tg/yr), followed by the South Coast Air Basin, the Sacramento Valley, and the San Francisco Bay Area at 0.39 +/- 0.18, 0.21 +/- 0.04, and 0.16 +/- 0.05 Tg/yr, respectively. The dairy and oil/gas production sources in the SJV contribute 0.44 +/- 0.36 and 0.22 +/- 0.23 Tg CH4/yr, respectively. This study has important policy implications for regulatory programs, as it provides a thorough multiyear evaluation of the emissions inventory using independent atmospheric measurements and investigates the utility of a complementary multiplatform approach in understanding the spatial and temporal patterns of CH4 emissions in the state and identifies opportunities for the expansion and applications of the monitoring network.

E
Elvidge, EL, Bonisch H, Brenninkmeijer CAM, Engel A, Fraser PJ, Gallacher E, Langenfelds R, Muhle J, Oram DE, Ray EA, Ridley AR, Rockmann T, Sturges WT, Weiss RF, Laube JC.  2018.  Evaluation of stratospheric age of air from CF4, C2F6, C3F8, CHF3, HFC-125, HFC-227ea and SF6; implications for the calculations of halocarbon lifetimes, fractional release factors and ozone depletion potentials. Atmospheric Chemistry and Physics. 18:3369-3385.   10.5194/acp-18-3369-2018   AbstractWebsite

In a changing climate, potential stratospheric circulation changes require long-term monitoring. Stratospheric trace gas measurements are often used as a proxy for stratospheric circulation changes via the "mean age of air" values derived from them. In this study, we investigated five potential age of air tracers - the perfluorocarbons CF4, C2F6 and C3F8 and the hydrofluorocarbons CHF3 (HFC-23) and HFC-125 - and compare them to the traditional tracer SF6 and a (relatively) shorter-lived species, HFC-227ea. A detailed uncertainty analysis was performed on mean ages derived from these "new" tracers to allow us to confidently compare their efficacy as age tracers to the existing tracer, SF6. Our results showed that uncertainties associated with the mean age derived from these new age tracers are similar to those derived from SF6, suggesting that these alternative compounds are suitable in this respect for use as age tracers. Independent verification of the suitability of these age tracers is provided by a comparison between samples analysed at the University of East Anglia and the Scripps Institution of Oceanography. All five tracers give younger mean ages than SF6, a discrepancy that increases with increasing mean age. Our findings qualitatively support recent work that suggests that the stratospheric lifetime of SF6 is significantly less than the previous estimate of 3200 years. The impact of these younger mean ages on three policy-relevant parameters - stratospheric lifetimes, fractional release factors (FRFs) and ozone depletion potentials - is investigated in combination with a recently improved methodology to calculate FRFs. Updates to previous estimations for these parameters are provided.

F
Fang, XK, Park S, Saito T, Tunnicliffe R, Ganesan AL, Rigby M, Li SL, Yokouchi Y, Fraser PJ, Harth CM, Krummel PB, Muhle J, O'Doherty S, Salameh PK, Simmonds PG, Weiss RF, Young D, Lunt MF, Manning AJ, Gressentl A, Prinn RG.  2019.  Rapid increase in ozone-depleting chloroform emissions from China. Nature Geoscience. 12:89-+.   10.1038/s41561-018-0278-2   AbstractWebsite

Chloroform contributes to the depletion of the stratospheric ozone layer. However, due to its short lifetime and predominantly natural sources, it is not included in the Montreal Protocol that regulates the production and uses of ozone-depleting substances. Atmospheric chloroform mole fractions were relatively stable or slowly decreased during 1990-2010. Here we show that global chloroform mole fractions increased after 2010, based on in situ chloroform measurements at seven stations around the world. We estimate that the global chloroform emissions grew at the rate of 3.5% yr(-1) between 2010 and 2015 based on atmospheric model simulations. We used two regional inverse modelling approaches, combined with observations from East Asia, to show that emissions from eastern China grew by 49 (41-59) Gg between 2010 and 2015, a change that could explain the entire increase in global emissions. We suggest that if chloroform emissions continuously grow at the current rate, the recovery of the stratospheric ozone layer above Antarctica could be delayed by several years.

G
Gordon, AL, Weiss RF, Smethie WM, Warner MJ.  1992.  Thermocline and intermediate water communication between the south Atlantic and Indian oceans. Journal of Geophysical Research-Oceans. 97:7223-7240.   10.1029/92jc00485   AbstractWebsite

A conductivity-temperature-depth and tracer chemistry section in the southeast South Atlantic in December 1989 and January 1990 presents strong evidence that there is a significant interocean exchange of thermocline and intermediate water between the South Atlantic and Indian oceans. Eastward flowing water at 10-degrees-W composed of South Atlantic Central (thermocline) Water is too enriched with chlorofluoromethanes 11 and 12 and oxygen to be the sole source of similar theta-S water within the northward flowing Benguela Current. About two thirds of the Benguela Current thermocline transport is drawn from the Indian Ocean; the rest is South Atlantic water that has folded into the Benguela Current in association with the Agulhas eddy-shedding process. South Atlantic Central water passes in the Indian Ocean by a route to the south of the Agulhas Return Current. The South Atlantic water loops back to the Atlantic within the Indian Ocean, perhaps mostly within the Agulhas recirculation cell of the southwest Indian Ocean. Linkage of Atlantic and Indian Ocean water diminishes with increasing depth; it extends through the lower thermocline into the Antarctic Intermediate Water (AAIW) (about 50% is derived from the Indian Ocean) but not into the deep water. While much of the interocean exchange remains on an approximate horizontal "isopycnal" plane, as much as 10 x 10(6) m3 s-1 of Indian Ocean water within the 25 x 10(6) m3 s-1 Benguela Current, mostly derived from the lower thermocline and AAIW, may balance deeper Atlantic export of North Atlantic Deep Water (NADW). The addition of salt water from the evaporative Indian Ocean into the South Atlantic Ocean thermocline and AAIW levels may precondition the Atlantic for NADW formation. While AAIW seems to be the chief feed for NADW, the bulk of it enters the subtropical South Atlantic, spiked with Indian Ocean salt, within the Benguela Current rather than along the western boundary of the South Atlantic.

N
Nevison, CD, Kinnison DE, Weiss RF.  2004.  Stratospheric influences on the tropospheric seasonal cycles of nitrous oxide and chlorofluorocarbons. Geophysical Research Letters. 31   10.1029/2004gl020398   AbstractWebsite

The stratospheric influence on the tropospheric seasonal cycles of N2O, CFC-11 ( CCl3F), CFC-12 (CCl2F2) and CFC-113 (CCl2FCClF2) is investigated using observations from the AGAGE global trace gas monitoring network and the results of the Whole Atmosphere Community Climate Model (WACCM). WACCM provides the basis for a number of predictions about the relative amplitudes of N2O and CFC seasonal cycles and about the relative magnitude and phasing of seasonal cycles in the northern and southern hemispheres. These predictions are generally consistent with observations, suggesting that the stratosphere exerts a coherent influence on the tropospheric seasonal cycles of trace gases whose primary sinks are in the stratosphere. This stratospheric influence may complicate efforts to validate estimated source distributions of N2O, an important greenhouse gas, in atmospheric transport model studies.

Nevison, CD, Mahowald NM, Weiss RF, Prinn RG.  2007.  Interannual and seasonal variability in atmospheric N2O. Global Biogeochemical Cycles. 21   10.1029/2006gb002755   AbstractWebsite

The increase in atmospheric N2O observed over the last century reflects large- scale human perturbations to the global nitrogen cycle. High- precision measurements of atmospheric N2O over the last decade reveal subtle signals of interannual variability (IAV) superimposed upon the more prominent growth trend. Anthropogenic sources drive the underlying growth in N2O, but are probably too monotonic to explain most of the observed IAV. The causes of both seasonal and interannual variability in atmospheric N2O are explored on the basis of comparisons of a 1993 - 2004 atmospheric transport simulation to observations of N2O at five stations of the Advanced Global Atmospheric Gases Experiment (AGAGE). The complementary tracers chlorofluorocarbons (CFCs) 11 and 12 and SF6 also are examined. The model simulation does not include a stratospheric sink and thus isolates the effects of surface sources and tropospheric transport. Both model and observations yield correlations in seasonal and interannual variability among species, but only in a few cases are model and observed variability correlated to each other. The results suggest that tropospheric transport contributes substantially to observed variability, especially at Samoa station. However, some features of observed variability are not explained by the model simulation and appear more consistent with a stratospheric influence. At Mace Head, Ireland, N2O and CFC growth rate anomalies are weakly correlated to IAV in polar winter lower stratospheric temperature, a proxy for the strength of the mean meridional stratospheric circulation. Seasonal and interannual variability in the natural sources of N2O may also contribute to observed variability in atmospheric N2O.

P
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.

R
Rigby, M, Muhle J, Miller BR, Prinn RG, Krummel PB, Steele LP, Fraser PJ, Salameh PK, Harth CM, Weiss RF, Greally BR, O'Doherty S, Simmonds PG, Vollmer MK, Reimann S, Kim J, Kim KR, Wang HJ, Olivier JGJ, Dlugokencky EJ, Dutton GS, Hall BD, Elkins JW.  2010.  History of atmospheric SF6 from 1973 to 2008. Atmospheric Chemistry and Physics. 10:10305-10320.   10.5194/acp-10-10305-2010   AbstractWebsite

We present atmospheric sulfur hexafluoride (SF(6)) mole fractions and emissions estimates from the 1970s to 2008. Measurements were made of archived air samples starting from 1973 in the Northern Hemisphere and from 1978 in the Southern Hemisphere, using the Advanced Global Atmospheric Gases Experiment (AGAGE) gas chromatographic-mass spectrometric (GC-MS) systems. These measurements were combined with modern high-frequency GC-MS and GC-electron capture detection (ECD) data from AGAGE monitoring sites, to produce a unique 35-year atmospheric record of this potent greenhouse gas. Atmospheric mole fractions were found to have increased by more than an order of magnitude between 1973 and 2008. The 2008 growth rate was the highest recorded, at 0.29 +/- 0.02 pmol mol(-1) yr(-1). A three-dimensional chemical transport model and a minimum variance Bayesian inverse method was used to estimate annual emission rates using the measurements, with a priori estimates from the Emissions Database for Global Atmospheric Research (EDGAR, version 4). Consistent with the mole fraction growth rate maximum, global emissions during 2008 were also the highest in the 1973-2008 period, reaching 7.4 +/- 0.6 Gg yr(-1) (1-sigma uncertainties) and surpassing the previous maximum in 1995. The 2008 values follow an increase in emissions of 48 +/- 20% since 2001. A second global inversion which also incorporated National Oceanic and Atmospheric Administration (NOAA) flask measurements and in situ monitoring site data agreed well with the emissions derived using AGAGE measurements alone. By estimating continent-scale emissions using all available AGAGE and NOAA surface measurements covering the period 2004-2008, with no pollution filtering, we find that it is likely that much of the global emissions rise during this five-year period originated primarily from Asian developing countries that do not report detailed, annual emissions to the United Nations Framework Convention on Climate Change (UNFCCC). We also find it likely that SF(6) emissions reported to the UNFCCC were underestimated between at least 2004 and 2005.

S
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.

Stohl, A, Kim J, Li S, O'Doherty S, Muhle J, Salameh PK, Saito T, Vollmer MK, Wan D, Weiss RF, Yao B, Yokouchi Y, Zhou LX.  2010.  Hydrochlorofluorocarbon and hydrofluorocarbon emissions in East Asia determined by inverse modeling. Atmospheric Chemistry and Physics. 10:3545-3560.   10.5194/acp-10-3545-2010   AbstractWebsite

The emissions of three hydrochlorofluorocarbons, HCFC-22 (CHClF(2)), HCFC-141b (CH(3)CCl(2)F) and HCFC-142b (CH(3)CClF(2)) and three hydrofluorocarbons, HFC-23 (CHF(3)), HFC-134a (CH(2)FCF(3)) and HFC-152a (CH(3)CHF(2)) from four East Asian countries and the Taiwan region for the year 2008 are determined by inverse modeling. The inverse modeling is based on in-situ measurements of these halocarbons at the Japanese stations Cape Ochi-ishi and Hateruma, the Chinese station Shangdianzi and the South Korean station Gosan. For every station and every 3 h, 20-day backward calculations were made with the Lagrangian particle dispersion model FLEXPART. The model output, the measurement data, bottom-up emission information and corresponding uncertainties were fed into an inversion algorithm to determine the regional emission fluxes. The model captures the observed variation of halocarbon mixing ratios very well for the two Japanese stations but has difficulties explaining the large observed variability at Shangdianzi, which is partly caused by small-scale transport from Beijing that is not adequately captured by the model. Based on HFC-23 measurements, the inversion algorithm could successfully identify the locations of factories known to produce HCFC-22 and emit HFC-23 as an unintentional byproduct. This lends substantial credibility to the inversion method. We report national emissions for China, North Korea, South Korea and Japan, as well as emissions for the Taiwan region. Halocarbon emissions in China are much larger than the emissions in the other countries together and contribute a substantial fraction to the global emissions. Our estimates of Chinese emissions for the year 2008 are 65.3 +/- 6.6 kt/yr for HCFC-22 (17% of global emissions extrapolated from Montzka et al., 2009), 12.1 +/- 1.6 kt/yr for HCFC-141b (22%), 7.3 +/- 0.7 kt/yr for HCFC-142b (17%), 6.2 +/- 0.7 kt/yr for HFC-23 (> 50%), 12.9 +/- 1.7 kt/yr for HFC-134a (9% of global emissions estimated from Velders et al., 2009) and 3.4 +/- 0.5 kt/yr for HFC-152a (7%).

Stohl, A, Seibert P, Arduini J, Eckhardt S, Fraser P, Greally BR, Lunder C, Maione M, Muhle J, O'Doherty S, Prinn RG, Reimann S, Saito T, Schmidbauer N, Simmonds PG, Vollmer MK, Weiss RF, Yokouchi Y.  2009.  An analytical inversion method for determining regional and global emissions of greenhouse gases: Sensitivity studies and application to halocarbons. Atmospheric Chemistry and Physics. 9:1597-1620.   10.5194/acp-9-1597-2009   AbstractWebsite

A new analytical inversion method has been developed to determine the regional and global emissions of long-lived atmospheric trace gases. It exploits in situ measurement data from three global networks and builds on backward simulations with a Lagrangian particle dispersion model. The emission information is extracted from the observed concentration increases over a baseline that is itself objectively determined by the inversion algorithm. The method was applied to two hydrofluorocarbons (HFC-134a, HFC-152a) and a hydrochlorofluorocarbon (HCFC-22) for the period January 2005 until March 2007. Detailed sensitivity studies with synthetic as well as with real measurement data were done to quantify the influence on the results of the a priori emissions and their uncertainties as well as of the observation and model errors. It was found that the global a posteriori emissions of HFC-134a, HFC-152a and HCFC-22 all increased from 2005 to 2006. Large increases (21%, 16%, 18%, respectively) from 2005 to 2006 were found for China, whereas the emission changes in North America (-9%, 23%, 17%, respectively) and Europe (11%, 11%,-4%, respectively) were mostly smaller and less systematic. For Europe, the a posteriori emissions of HFC-134a and HFC-152a were slightly higher than the a priori emissions reported to the United Nations Framework Convention on Climate Change (UNFCCC). For HCFC-22, the a posteriori emissions for Europe were substantially (by almost a factor 2) higher than the a priori emissions used, which were based on HCFC consumption data reported to the United Nations Environment Programme (UNEP). Combined with the reported strongly decreasing HCFC consumption in Europe, this suggests a substantial time lag between the reported time of the HCFC-22 consumption and the actual time of the HCFC-22 emission. Conversely, in China where HCFC consumption is increasing rapidly according to the UNEP data, the a posteriori emissions are only about 40% of the a priori emissions. This reveals a substantial storage of HCFC-22 and potential for future emissions in China. Deficiencies in the geographical distribution of stations measuring halocarbons in relation to estimating regional emissions are also discussed in the paper. Applications of the inversion algorithm to other greenhouse gases such as methane, nitrous oxide or carbon dioxide are foreseen for the future.

T
Thompson, RL, Ishijima K, Saikawa E, Corazza M, Karstens U, Patra PK, Bergamaschi P, Chevallier F, Dlugokencky E, Prinn RG, Weiss RF, O'Doherty S, Fraser PJ, Steele LP, Krummel PB, Vermeulen A, Tohjima Y, Jordan A, Haszpra L, Steinbacher M, Van der Laan S, Aalto T, Meinhardt F, Popa ME, Moncrieff J, Bousquet P.  2014.  TransCom N2O model inter-comparison - Part 2: Atmospheric inversion estimates of N2O emissions. Atmospheric Chemistry and Physics. 14:6177-6194.   10.5194/acp-14-6177-2014   AbstractWebsite

This study examines N2O emission estimates from five different atmospheric inversion frameworks based on chemistry transport models (CTMs). The five frameworks differ in the choice of CTM, meteorological data, prior uncertainties and inversion method but use the same prior emissions and observation data set. The posterior modelled atmospheric N2O mole fractions are compared to observations to assess the performance of the inversions and to help diagnose problems in the modelled transport. Additionally, the mean emissions for 2006 to 2008 are compared in terms of the spatial distribution and seasonality. Overall, there is a good agreement among the inversions for the mean global total emission, which ranges from 16.1 to 18.7 TgN yr(-1) and is consistent with previous estimates. Ocean emissions represent between 31 and 38% of the global total compared to widely varying previous estimates of 24 to 38%. Emissions from the northern mid- to high latitudes are likely to be more important, with a consistent shift in emissions from the tropics and subtropics to the mid- to high latitudes in the Northern Hemisphere; the emission ratio for 0-30A degrees N to 30-90A degrees N ranges from 1.5 to 1.9 compared with 2.9 to 3.0 in previous estimates. The largest discrepancies across inversions are seen for the regions of South and East Asia and for tropical and South America owing to the poor observational constraint for these areas and to considerable differences in the modelled transport, especially inter-hemispheric exchange rates and tropical convective mixing. Estimates of the seasonal cycle in N2O emissions are also sensitive to errors in modelled stratosphere-to-troposphere transport in the tropics and southern extratropics. Overall, the results show a convergence in the global and regional emissions compared to previous independent studies.

W
Waugh, DW, Vollmer MK, Weiss RF, Haine TWN, Hall TM.  2002.  Transit time distributions in Lake Issyk-Kul. Geophysical Research Letters. 29   10.1029/2002gl016201   AbstractWebsite

[1] Measurements of sulfur hexafluoride (SF6)and chlorofluorocarbons (CFCs) are used to constrain the timescales for deep-water renewal in Lake Issyk-Kul. As these tracers have different tropospheric histories their combination provides more transport information than one tracer alone. In particular, from these measurements the mean, Gamma, and standard deviation, sigma, of the distributions of transit times since water made last contact with the surface can be tightly constrained. Gamma is older than the age determined from SF6 and younger than the ages from the CFCs, and increases from around 4 yrs at 200 m to around 10.5 yrs at the deepest location (655 m). sigma also increases with depth and equals around 0.7 to 0.8 Gamma, which corresponds to large ranges of transit times, and implies mixing processes play a major role in the transport. The approach used can also be applied to similar tracer measurements in the oceans and groundwaters to constrain transport in these geophysical systems.

Wells, KC, Millet DB, Bousserez N, Henze DK, Chaliyakunnel S, Griffis TJ, Luan Y, Dlugokencky EJ, Prinn RG, O'Doherty S, Weiss RF, Dutton GS, Elkins JW, Krummel PB, Langenfelds R, Steele LP, Kort EA, Wofsy SC, Umezawa T.  2015.  Simulation of atmospheric N2O with GEOS-Chem and its adjoint: evaluation of observational constraints. Geoscientific Model Development. 8:3179-3198.   10.5194/gmd-8-3179-2015   AbstractWebsite

We describe a new 4D-Var inversion framework for nitrous oxide (N2O) based on the GEOS-Chem chemical transport model and its adjoint, and apply it in a series of observing system simulation experiments to assess how well N2O sources and sinks can be constrained by the current global observing network. The employed measurement ensemble includes approximately weekly and quasicontinuous N2O measurements (hourly averages used) from several long-term monitoring networks, N2O measurements collected from discrete air samples onboard a commercial aircraft (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container; CARIBIC), and quasi-continuous measurements from the airborne HIAPER Pole-to-Pole Observations (HIPPO) campaigns. For a 2-year inversion, we find that the surface and HIPPO observations can accurately resolve a uniform bias in emissions during the first year; CARIBIC data provide a somewhat weaker constraint. Variable emission errors are much more difficult to resolve given the long lifetime of N2O, and major parts of the world lack significant constraints on the seasonal cycle of fluxes. Current observations can largely correct a global bias in the stratospheric sink of N2O if emissions are known, but do not provide information on the temporal and spatial distribution of the sink. However, for the more realistic scenario where source and sink are both uncertain, we find that simultaneously optimizing both would require unrealistically small errors in model transport. Regardless, a bias in the magnitude of the N2O sink would not affect the a posteriori N2O emissions for the 2-year timescale used here, given realistic initial conditions, due to the timescale required for stratosphere-troposphere exchange (STE). The same does not apply to model errors in the rate of STE itself, which we show exerts a larger influence on the tropospheric burden of N2O than does the chemical loss rate over short (< 3 year) timescales. We use a stochastic estimate of the inverse Hessian for the inversion to evaluate the spatial resolution of emission constraints provided by the observations, and find that significant, spatially explicit constraints can be achieved in locations near and immediately upwind of surface measurements and the HIPPO flight tracks; however, these are mostly confined to North America, Europe, and Australia. None of the current observing networks are able to provide significant spatial information on tropical N2O emissions. There, averaging kernels (describing the sensitivity of the inversion to emissions in each grid square) are highly smeared spatially and extend even to the midlatitudes, so that tropical emissions risk being conflated with those elsewhere. For global inversions, therefore, the current lack of constraints on the tropics also places an important limit on our ability to understand extratropical emissions. Based on the error reduction statistics from the inverse Hessian, we characterize the atmospheric distribution of unconstrained N2O, and identify regions in and downwind of South America, central Africa, and Southeast Asia where new surface or profile measurements would have the most value for reducing present uncertainty in the global N2O budget.

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Yadav, V, Duren R, Mueller K, Verhulst KR, Nehrkorn T, Kim J, Weiss RF, Keeling R, Sander S, Fischer ML, Newman S, Falk M, Kuwayama T, Hopkins F, Rafiq T, Whetstone J, Miller C.  2019.  Spatio-temporally resolved methane fluxes from the Los Angeles megacity. Journal of Geophysical Research-Atmospheres. 124:5131-5148.   10.1029/2018jd030062   AbstractWebsite

We combine sustained observations from a network of atmospheric monitoring stations with inverse modeling to uniquely obtain spatiotemporal (3-km, 4-day) estimates of methane emissions from the Los Angeles megacity and the broader South Coast Air Basin for 2015-2016. Our inversions use customized and validated high-fidelity meteorological output from Weather Research Forecasting and Stochastic Time-Inverted Lagrangian model for South Coast Air Basin and innovatively employ a model resolution matrix-based metric to disentangle the spatiotemporal information content of observations as manifested through estimated fluxes. We partially track and constrain fluxes from the Aliso Canyon natural gas leak and detect closure of the Puente Hills landfill, with no prior information. Our annually aggregated fluxes and their uncertainty excluding the Aliso Canyon leak period lie within the uncertainty bounds of the fluxes reported by the previous studies. Spatially, major sources of CH4 emissions in the basin were correlated with CH4-emitting infrastructure. Temporally, our findings show large seasonal variations in CH4 fluxes with significantly higher fluxes in winter in comparison to summer months, which is consistent with natural gas demand and anticorrelated with air temperature. Overall, this is the first study that utilizes inversions to detect both enhancement (Aliso Canyon leak) and reduction (Puente Hills) in CH4 fluxes due to the unintended events and policy decisions and thereby demonstrates the utility of inverse modeling for identifying variations in fluxes at fine spatiotemporal resolution.