Abstract
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.
Generated Summary
This research calculates a semiempirical lifetime of nitrous oxide (N2O), a significant greenhouse gas, using satellite measurements from the Microwave Limb Sounder (MLS) instrument, stratospheric profiles of N2O, ozone, and temperature data. The study employs laboratory cross-section data for ozone and molecular oxygen, observed solar spectra, and a radiative transfer model. The resulting lifetime is 116 ± 9 years. The research further investigates the monthly-to-biennial variations in lifetime and tropical abundance by utilizing four independent chemistry-transport models, driven by reanalysis meteorological fields, for the period of observation (2005-2010). The models are evaluated against MLS observations and are used to estimate the reduction in lifetime when N2O increases, as well as changes in lifetime since the preindustrial era. The study then compares the semiempirical lifetime with model estimates, analyzing the sensitivity of N2O lifetime to its burden and the feedback factor. This study has rigorously tested four global chemistry-transport models’ ability to simulate the observed distribution and loss patterns of N2O based on 5 years of MLS observations.
Key Findings & Statistics
- The study determines a semiempirical N2O lifetime of 116 ± 9 years.
- The study finds the mean burden over the MLS period is calculated as 1539 Tg N, and the mean loss is 13.2 Tg N/yr, giving a lifetime of ~116.5 years.
- A quasi-biennial signal is clear over the 5 years of MLS data, and the minimum-to-maximum amplitude is about 10%.
- The study reveals that 81% of N2O loss occurs between 24 and 40 km including all latitudes, while 76% occurs between 30°S and 30°N including all heights.
- The research finds that the N2O lifetime varies with solar activity, and MLS-G and MLS-U evaluated this by changing the solar spectrum and thus the photolysis rates, giving a span of 7% (109 years at solar maximum to 117 years at solar minimum).
- The study shows that the solar-minimum calculation gives a slightly longer lifetime of 119 years.
- The interannual variation in the N2O lifetime for 1997-2010 is shown in Figure 5.
- The models using IFS forecast or MERRA-assimilated met data show realistic variations when compared with the MLS-G/U results.
- The mean column of NOy is 0.39 DU with a March minimum and July maximum and with a minimum-to-maximum amplitude of ~12%.
- The models show similar seasonal patterns and fall within ±16% of the MIPAS value.
- The study found that the best estimate for preindustrial conditions gives a longer lifetime of 123 years.
- The lifetime sensitivity of N₂O to its burden is s = -0.065 ± 0.010, where the ±0.010 is a 1 sigma uncertainty.
- The feedback factor is ff=0.94±0.01, and the effective residence time of an N₂O perturbation is 109 ± 10 years.
- The evaluation of emissions based on observed abundances and lifetime (both PI and present) gave Pl emissions of 9.1 ± 1.0 Tg N/yr and present anthropogenic emissions of 6.5 ± 1.3 Tg N/yr.
- The full Pl lifetime is estimated from these three models to be +4% to +8% larger than present day, with the upper range including CO2 dynamical effects (G2d only).
- The research showed that by combining the interannual variability and other uncertainties, we derive the N2O lifetime of 116 ± 9 years for the period of 2005-2010.
- The 2005-2010 period covers an extended solar minimum period.
Other Important Findings
- The study compares four chemistry-transport models (CTMs) with the MLS observations of stratospheric N2O and O3 to understand the cause of model errors in N2O lifetime.
- The critical region for N2O loss is identified in the tropical middle stratosphere, with photochemical loss peaking at the equator near 32 km.
- The research shows that monthly zonal-mean N₂O abundances at 10 hPa have large seasonal and interannual variations.
- The upwelling region with highest N₂O levels, and hence greatest loss, occurs in the tropics.
- The N2O lifetime varies with solar activity.
- The critical region for modeling the N2O lifetime thus centers on the region 30°S-30°N and 24 km to 40 km.
- Four models are used to estimate the reduction in lifetime when N2O increases and the change in lifetime since the preindustrial era.
- The study emphasizes the core tropics more than in Figure 2, and the spread across models widens.
- Both ECMWF and MERRA meteorology produce a modeled 10 hPa N₂O surface that is biased but highly correlated with the MLS observations, indicating that transport variations in the tropical midstratosphere on a month-by-latitude basis are well represented.
- The study shows that the G2d model is an exception with a very weak seasonal cycle in NOy columns.
- The differences in the O3 profiles (Figure 2b) can help explain why the lifetimes listed in Table 2 are not inversely related to the 10 hPa N₂O abundances.
- The models show that the G2d models’ lifetimes are smaller than the GMI lifetime, even though their N₂O abundances are 10-20% smaller.
- The best estimate for preindustrial conditions gives a longer lifetime of 123 years.
Limitations Noted in the Document
- The primary measurement uncertainty is associated with the N2O profile in the 30-3 hPa altitude range (N2O~40-240 ppb).
- The possible bias in the MLS N2O measurements near 10 hPa has been assessed as being less than 10%.
- The uncertainty in the photochemical data leads to a 1 sigma uncertainty of 3% in the photochemically modeled lifetime.
- The models tend to overestimate the semiempirical lifetime, 116±9 years, by a wide range, but agree on the negative sensitivity of lifetime to burden.
- The uncertainty in the N2O burden is negligible because the mass of the atmosphere is well known.
- The SPARC models may not be representative.
- The MERRA fields are known to have excessive subtropical mixing in the lower stratosphere, which reduces N2O in the tropics.
- The IFS fields for cycle 36 appear to have more stagnant vertical transport in the midstratosphere than cycle 29.
- The study acknowledges that the analysis does not include possible circulation changes and ozone depletion from the CIO catalytic cycle (40-50 km altitudes).
- The study acknowledges limitations of the data sets and the need for more research.
Conclusion
The study underscores the crucial role of N2O in the global climate system, identifying its long lifetime and the sensitivity of its atmospheric behavior to changes. The semiempirical N2O lifetime of 116 ± 9 years is a significant finding, offering valuable insights into the gas’s atmospheric dynamics. The analysis of four chemistry-transport models (CTMs) against the MLS observations provides a robust method for understanding N2O loss and variability. The research indicates that the models, while generally capturing the observed patterns, tend to overestimate the lifetime, which warrants further scrutiny. The study’s estimation of a longer lifetime in preindustrial conditions underscores the impact of human activities on the balance of emissions and atmospheric loss. This research enhances the accuracy of current projections of N2O increases throughout this century. The study suggests that the best estimate for the current N2O lifetime, which incorporates the slow increase in burden, is 116 ± 9 years, based on the MLS-G/U calculations. The analysis reveals that the dominant factor influencing this value is the N2O abundance, with the addition of a further uncertainty related to O3 and T. The study also highlights the importance of the quasi-biennial cycle in understanding the interannual variability in N2O lifetime. The research recognizes that the overall magnitude of the loss rate presents the greatest difference among the models and emphasizes the need for more detailed analyses to improve the simulations and the understanding of the factors. The study highlights the need for a more rigorous protocol of model studies to assess separately, for example, the decline in ODS, future stratospheric temperatures, and the role of changes in transport.