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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.

Deeds, DA, Kulongoski JT, Mühle J, Weiss RF.  2015.  Tectonic activity as a significant source of crustal tetrafluoromethane emissions to the atmosphere: Observations in groundwaters along the San Andreas Fault. Earth and Planetary Science Letters. 412:163-172.   10.1016/j.epsl.2014.12.016   AbstractWebsite

Tetrafluoromethane (CF4) concentrations were measured in 14 groundwater samples from the Cuyama Valley, Mil Potrero and Cuddy Valley aquifers along the Big Bend section of the San Andreas Fault System (SAFS) in California to assess whether tectonic activity in this region is a significant source of crustal CF4 to the atmosphere. Dissolved CF4 concentrations in all groundwater samples but one were elevated with respect to estimated recharge concentrations including entrainment of excess air during recharge ( C r e ; ∼30 fmol kg−1 H2O), indicating subsurface addition of CF4 to these groundwaters. Groundwaters in the Cuyama Valley contain small CF4 excesses (0.1–9 times C r e ), which may be attributed to an in situ release from weathering and a minor addition of deep crustal CF4 introduced to the shallow groundwater through nearby faults. CF4 excesses in groundwaters within 200 m of the SAFS are larger (10–980 times C r e ) and indicate the presence of a deep crustal flux of CF4 that is likely associated with the physical alteration of silicate minerals in the shear zone of the SAFS. Extrapolating CF4 flux rates observed in this study to the full extent of the SAFS (1300 km × 20–100 km) suggests that the SAFS potentially emits ( 0.3 – 1 ) × 10 − 1 kg CF4 yr−1 to the Earth's surface. For comparison, the chemical weathering of ∼ 7.5 × 10 4 km 2 of granitic rock in California is estimated to release ( 0.019 – 3.2 ) × 10 − 1 kg CF4 yr−1. Tectonic activity is likely an important, and potentially the dominant, driver of natural emissions of CF4 to the atmosphere. Variations in preindustrial atmospheric CF4 as observed in paleo-archives such as ice cores may therefore represent changes in both continental weathering and tectonic activity, including changes driven by variations in continental ice cover during glacial–interglacial transitions.

Deeds, DA, Muhle J, Weiss RF.  2008.  Tetrafluoromethane in the deep North Pacific Ocean. Geophysical Research Letters. 35   10.1029/2008gl034355   AbstractWebsite

Dissolved tetrafluoromethane (CF(4)) has been measured for the first time in the North Pacific Ocean. Surface water collected during calm weather is near equilibrium with the modern atmosphere. Deep water, isolated from atmospheric exchange for centuries, is near equilibrium with the preindustrial atmosphere, after accounting for an expected 5% addition of this low-solubility gas due to air injection during high-latitude deep-water formation. These results strongly suggest that dissolved CF(4) is conservative in seawater and that the oceanic imprint of anthropogenic increases in atmospheric CF(4) can be used as a time-dependent tracer of ocean ventilation and subsurface circulation processes. Although the continental lithosphere is a source of natural atmospheric CF(4), we find no evidence of an oceanic lithospheric CF(4) input into deep Pacific waters. The estimated upper limit of a potential oceanic lithospheric CF(4) flux to the global atmosphere is on the order of 4% of that from the continental lithosphere.

Deeds, DA, Vollmer MK, Kulongoski JT, Miller BR, Muhle J, Harth CM, Izbicki JA, Hilton DR, Weiss RF.  2008.  Evidence for crustal degassing of CF4 and SF6 in Mojave Desert groundwaters. Geochimica Et Cosmochimica Acta. 72:999-1013.   10.1016/j.gca.2007.11.027   AbstractWebsite

Dissolved tetrafluoromethane (CF(4)) and sulfur hexafluoride (SF(6)) concentrations were measured in groundwater samples from the Eastern Morongo Basin (EMB) and Mojave River Basin (MRB) located in the southern Mojave Desert, California. Both CF(4) and SF(6) are supersaturated with respect to equilibrium with the preindustrial atmosphere at the recharge temperatures and elevations of the Mojave Desert. These observations provide the first in situ evidence for a flux of CF(4) from the lithosphere. A gradual basin-wide enhancement in dissolved CF(4) and SF(6) concentrations with groundwater age is consistent with release of these gases during weathering of the surrounding granitic alluvium. Dissolved CF(4) and SF(6) concentrations in these groundwaters also contain a deeper crustal component associated with a lithospheric flux entering the EMB and MRB through the underlying basement. The crustal flux of CF(4), but not of SF(6), is enhanced in the vicinity of local active fault systems due to release of crustal fluids during episodic fracture events driven by local tectonic activity. When fluxes of CF(4) and SF(6) into Mojave Desert groundwaters are extrapolated to the global scale they are consistent, within large uncertainties, with the fluxes required to sustain the preindustrial atmospheric abundances of CF(4) and SF(6). (c) 2007 Elsevier Ltd. All rights reserved.