On the distribution of CO₂ and CO in the mesosphere and lower thermosphere

Abstract

We have used the Whole Atmosphere Community Climate Model (WACCM) to calculate the distribution of CO 2 and CO in the mesosphere and lower thermosphere (MLT), and we have compared the results with observations, mainly from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer and Michelson Interferometer for Passive Atmospheric Sounding satellite-borne instruments. We find that WACCM can reproduce the observed distribution of CO 2 in the MLT and the rapid falloff of CO 2 above about 80 km. Analysis of the principal terms in the calculated budget of CO 2 shows that its global-mean vertical profile is determined mainly by the competition between molecular diffusive separation and eddy mixing by gravity waves. The model underestimates somewhat the mixing ratio of CO 2 in the thermosphere compared to that in the observations, but we show that the discrepancy may be eliminated by a reasonable adjustment of the Prandtl number used to calculate the diffusivity due to gravity waves. Simulated CO is also consistent with observations, except that in the standard version of the model, its mixing ratio is uniformly lower than observed above about 100 km. We conclude that WACCM likely underestimates the rate of production of CO in the lower thermosphere from photolysis of CO 2 at wavelengths < 121 nm, and we show that this stems from the use of a very large absorption cross section for O 2 in the wavelength range 105-121 nm. When a smaller cross section is used, photolysis of CO 2 increases by a factor of 2-3 at~95-115 km and, as a result, CO mixing ratios become larger and agree much more closely with observations. We emphasize that the increase in CO 2 photolysis implies only minor changes in the vertical profile of CO 2 because photolytic loss is a minor term in the budget of CO 2 in the MLT.

Determination of the atmospheric lifetime and global warming potential of sulfur hexafluoride using a three-dimensional model

“…This can be explained by diffusive separation, which becomes pronounced in the upper mesosphere because SF 6 is a relatively heavy molecule (molar mass 0.146 kg) compared to the mean mass of air molecules (mean molar mass 0.0288 kg; cf. Garcia et al 2014, where similar behaviour is seen for CO 2 , another relatively heavy molecule). Panels a-c of the figure all show SF 6 decreasing above ∼ 80 km, and panels a and c are almost identical, while in panel b the decrease begins about 4 km lower.…”

“…The model contains a detailed treatment of middle atmosphere chemistry including interactive treatments of Na and K . We use the specified dynamics (SD) version of the model to allow comparison with observations (see Garcia et al, 2014, for details). The SF 6 surface emission flux and initial global vertical profiles were taken from a CCMI (Chemistry Climate Model Initiative) simulation using the same version of SD-WACCM with the same nudging analyses (D. Kinnison, personal communication, 2013).…”

Stratospheric and mesospheric HO&lt;sub&gt;2&lt;/sub&gt; observations from the Aura Microwave Limb Sounder

“…Lastly, (3) these discrepancies might be related to the WACCM representation of the mean meridional circulation which has been shown to have some deficiencies (Smith et al, 2011;Smith, 2012), suggesting that the gravity wave parametrization needs to be modified. In addition, Garcia et al (2014) have shown that adjusting the Prandtl number, used to calculate the diffusivity due to gravity waves, significantly alters the CO 2 SD-WACCM simulations, improving the agreement with satellite measurements. Such adjustment should also affect the H 2 O and hence the HO x chemistry.…”

Assessing the ability to derive rates of polar middle-atmospheric descent using trace gas measurements from remote sensors

“…This vertical branch of the meridional circulation can transport gases with high thermospheric volume mixing ratios (VMRs), and change the composition of the middle atmosphere (e.g. Solomon et al, 1985;Allen et al, 2000;Hauchecorne et al, 2007;Smith et al, 2011;Garcia et al, 2014;Manney et al, 2009;2015;Funke et al, 2014aFunke et al, , b, 2017. The downward transport of nitrogen oxide (NO), produced at around 110 km through energetic particle precipitation (EPP) events (e.g.…”