The lifetime of nitrous oxide, the third‐most‐important human‐emitted greenhouse gas, is based to date primarily on model studies or scaling to other gases. This work calculates a semiempirical ...lifetime based on Microwave Limb Sounder satellite measurements of stratospheric profiles of nitrous oxide, ozone, and temperature; laboratory cross‐section data for ozone and molecular oxygen plus kinetics for O(1D); the observed solar spectrum; and a simple radiative transfer model. The result is 116 ± 9 years. The observed monthly‐to‐biennial variations in lifetime and tropical abundance are well matched by four independent chemistry‐transport models driven by reanalysis meteorological fields for the period of observation (2005–2010), but all these models overestimate the lifetime due to lower abundances in the critical loss region near 32 km in the tropics. These models plus a chemistry‐climate model agree on the nitrous oxide feedback factor on its own lifetime of 0.94 ± 0.01, giving N2O perturbations an effective residence time of 109 years. Combining this new empirical lifetime with model estimates of residence time and preindustrial lifetime (123 years) adjusts our best estimates of the human‐natural balance of emissions today and improves the accuracy of projected nitrous oxide increases over this century.
Key Points
Nitrous oxide lifetime is computed empirically from MLS satellite data
Empirical N2O lifetimes compared with models including interannual variability
Results improve values for present anthropogenic and preindustrial emissions
The lifetime of nitrous oxide, the third-most-important human-emitted greenhouse gas, is based to date primarily on model studies or scaling to other gases. This work calculates a semiempirical ...lifetime based on Microwave Limb Sounder satellite measurements of stratospheric profiles of nitrous oxide, ozone, and temperature; laboratory cross-section data for ozone and molecular oxygen plus kinetics for O(1D); the observed solar spectrum; and a simple radiative transfer model. The result is 116 ± 9 years. The observed monthly-to-biennial variations in lifetime and tropical abundance are well matched by four independent chemistry-transport models driven by reanalysis meteorological fields for the period of observation (2005–2010), but all these models overestimate the lifetime due to lower abundances in the critical loss region near 32 km in the tropics. These models plus a chemistry-climate model agree on the nitrous oxide feedback factor on its own lifetime of 0.94 ± 0.01, giving N2O perturbations an effective residence time of 109 years. Combining this new empirical lifetime with model estimates of residence time and preindustrial lifetime (123 years) adjusts our best estimates of the human-natural balance of emissions today and improves the accuracy of projected nitrous oxide increases over this century.
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.
The vapour phase reactions of 2-fluoroethanol, 2,2-difluoroethanol, and 2,2,2-trifluoroethanol with OH radicals and Cl atoms were studied at 298
K and 1013
mbar using long-path FTIR detection. The ...following reaction rate coefficients were determined by the relative rate method:
k
298(OH+CH
2FCH
2OH)=(1.42±0.11)×10
−12,
k
298(OH+CHF
2CH
2OH)=(4.51±0.06)×10
−13,
k
298(OH+CF
3CH
2OH)=(1.23±0.06)×10
−13,
k
298(Cl+CH
2FCH
2OH)=(2.67±0.3)×10
−11,
k
298(Cl+CHF
2CH
2OH)=(3.12±0.06)×10
−12, and
k
298(Cl+CF
3CH
2OH)=(7.42±0.12)×10
−13
cm
3 molecule
−1
s
−1; the reported uncertainties represent
3
σ
from the statistical analyses and do not include any systematic errors or uncertainties in the reference rate coefficients. Quantitative infrared cross-sections of the title compounds at 298
K are reported in the 4000–50
cm
−1 region.
A 3D chemistry transport model was applied to calculate the atmospheric distributions and lifetimes of the title compounds; the global and yearly averaged lifetimes were calculated as 20 days for CH
2FCH
2OH, 40 days for CHF
2CH
2OH, and 117 days for CF
3CH
2OH. Radiative forcing calculations were carried out assuming either constant vertical profiles or the distribution derived from the chemistry transport model. The Global Warming Potentials for the title compounds are insignificant compared to, e.g. CFC-11 (CCl
3F).
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