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See detailUpdate on Ozone-Depleting Substances (ODSs) and Other Gases of Interest to the Montreal Protocol
Engel, A; Rigby, M; Burkholder, J et al

in Fahey, David W; Newman, Paul A; Pyle, John A (Eds.) et al Scientific Assessment of Ozone Depletion: 2018 (2019)

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See detailModel Sensitivity Studies of the Decrease in Atmospheric Carbon Tetrachloride
Chipperfield, M. P.; Liang, Q.; Rigby, M. et al

in Atmospheric Chemistry and Physics (2016), 16

Carbon tetrachloride 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 ... [more ▼]

Carbon tetrachloride 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 (76% 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 (15% of total) has the largest impact on modelled CCl4 decay due to its sizeable contribution to CCl4 loss and large uncertainty range (157 to 313 years). With an assumed CCl4 emission rate of 39 Gg/yr, the reference simulation with best estimate of loss processes still underestimates the observed CCl4 (overestimates the decay) over the past two 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 45 Gg/yr. Further progress in constraining the CCl4 budget is partly limited by systematic biases between observational datasets. For example, surface observations from the NOAA network are larger than from the AGAGE network but have shown a steeper decreasing trend over the past two decades. These differences imply a difference in emissions which is significant relative to uncertainties in the magnitudes of the CCl4 sinks. [less ▲]

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See detailSPARC Report on the Mystery of Carbon Tetrachloride
Ahmadzai, H; Bock, R P; Burkholder, J B et al

in Liang, Qing; Newman, Paul A; Reimann, Stefan (Eds.) SPARC Report on the Mystery of Carbon Tetrachloride (2016)

The Montreal Protocol (MP) controls the production and consumption of carbon tetrachloride (CCl4 or CTC) and other ozone-depleting substances (ODSs) for emissive uses. CCl4 is a major ODS, accounting for ... [more ▼]

The Montreal Protocol (MP) controls the production and consumption of carbon tetrachloride (CCl4 or CTC) and other ozone-depleting substances (ODSs) for emissive uses. CCl4 is a major ODS, accounting for about 12% of the globally averaged inorganic chlorine and bromine in the stratosphere, compared to 14% for CFC-12 in 2012. In spite of the MP controls, there are large ongoing emissions of CCl4 into the atmosphere. Estimates of emissions from various techniques ought to yield similar numbers. However, the recent WMO/UNEP Scientific Assessment of Ozone Depletion [WMO, 2014] estimated a 2007-2012 CCl4 bottom-up emission of 1-4 Gg/year (1-4 kilotonnes/year), based on country-by-country reports to UNEP, and a global top-down emissions estimate of 57 Gg/ year, based on atmospheric measurements. This 54 Gg/year difference has not been explained. In order to assess the current knowledge on global CCl4 sources and sinks, stakeholders from industrial, governmental, and the scientific communities came together at the “Solving the Mystery of Carbon Tetrachloride” workshop, which was held from 4-6 October 2015 at Empa in Dübendorf, Switzerland. During this workshop, several new findings were brought forward by the participants on CCl4 emissions and related science. • Anthropogenic production and consumption for feedstock and process agent uses (e.g., as approved solvents) are reported to UNEP under the MP. Based on these numbers, global bottom-up emissions of 3 (0-8) Gg/year are estimated for 2007-2013 in this report. This number is also reasonably consistent with this report’s new industry-based bottom-up estimate for fugitive emissions of 2 Gg/year. • By-product emissions from chloromethanes and perchloroethylene plants are newly proposed in this report as significant CCl4 sources, with global emissions estimated from these plants to be 13 Gg/year in 2014. • This report updates the anthropogenic CCl4 emissions estimation as a maximum of ~25 Gg/year. This number is derived by combining the above fugitive and by-product emissions (2 Gg/year and 13 Gg/year, respectively) with 10 Gg/year from legacy emissions plus potential unreported inadvertent emissions from other sources. • Ongoing atmospheric CCl4 measurements within global networks have been exploited for assessing regional emissions. In addition to existing emissions estimates from China and Australia, the workshop prompted research on emissions in the U.S. and Europe. The sum of these four regional emissions is estimated as 21±7.5a Gg/year, but this is not a complete global accounting. These regional top-down emissions estimates also show that most of the CCl4 emissions originate from chemical industrial regions, and are not linked to major population centres. • The total CCl4 lifetime is critical for calculating top-down global emissions. CCl4 is destroyed in the stratosphere, oceans, and soils, complicating the total lifetime estimate. The atmospheric lifetime with respect to stratospheric loss was recently revised to 44 (36-58) years, and remains unchanged in this report. New findings from additional measurement campaigns and reanalysis of physical parameters lead to changes in the ocean lifetime from 94 years to 210 (157-313) years, and in the soil lifetime from 195 years to 375 (288-536) years. • These revised lifetimes lead to an increase of the total lifetime from 26 years in WMO [2014] to 33 (28-41) years. Consequently, CCl4 is lost at a slower rate from the atmosphere. With this new total lifetime, the global top-down emissions calculation decreases from 57 (40-74) Gg/year in WMO [2014] to 40 (25-55) Gg/year. This estimate is relatively consistent with the independent gradient top-down emissions of 30 (25-35) Gg/year, based upon differences between atmospheric measurements of CCl4 in the Northern and Southern Hemispheres. In addition, this new total lifetime implies an upper limit of 3-4 Gg/year of natural emissions, based upon newly reported observations of old air in firn snow. These new CCl4 emissions estimates from the workshop make considerable progress toward closing the emissions discrepancy. The new industrial bottom-up emissions estimate (15 Gg/year total) includes emissions from chloromethanes plants (13 Gg/year) and feedstock fugitive emissions (2 Gg/year). When combined with legacy emissions and unreported inadvertent emissions, this could be up to 25 Gg/year. Top-down emissions estimates are: global 40 (25-55) Gg/year, gradient 30 (25-35) Gg/year, and regional 21 (14-28) Gg/year. While the new bottom-up value is still less than the aggregated top-down values, these estimates reconcile the CCl4 budget discrepancy when considered at the edges of their uncertainties. [less ▲]

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See detailUpdate on Ozone-Depleting Substances (ODSs) and Other Gases of Interest to the Montreal Protocol
Carpenter, L. J.; Reimann, S.; Burkholder, J. B. et al

in Nohende Ajavon, Ayité-Lô; Newman, Paul. A.; Pyle, John A. (Eds.) et al Scientific Assessment of Ozone Depletion: 2014 (2014)

The amended and adjusted Montreal Protocol has continued to reduce emissions and atmospheric abundances of most controlled ozone-depleting substances. By 2012, the total combined abundance of ... [more ▼]

The amended and adjusted Montreal Protocol has continued to reduce emissions and atmospheric abundances of most controlled ozone-depleting substances. By 2012, the total combined abundance of anthropogenic ODSs in the troposphere (measured as Equivalent Chlorine) had decreased by nearly 10% from its peak value in 1994. [less ▲]

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