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​​​​​​​At the altitudes of the mesosphere–lower thermosphere, OH and HO 2 play a significant role in many physicochemical processes. Thus, monitoring their spatiotemporal evolution, together with other chemically active trace gases, is one of the most important problems for this atmosphere region, in which direct measurements are difficult. This paper studies the nighttime OH and HO 2 chemical equilibria using the 3D chemical transport modeling within the general approach, which includes the identification of the main sources and sinks in the equilibrium space–time areas and the derivation of analytical criteria for equilibrium validity. The presented analysis shows that there are extended areas where nighttime HO 2 and OH are close to their local equilibrium concentrations, determined mainly by the reaction between HOx and Ox components among themselves and with H 2 O 2 , N, NO, NO 2 , and CO. In the upper mesosphere–lower thermosphere, the equilibrium expressions can be shortened so that they include the HOx – Ox chemistry only. These expressions describe the HO 2 and OH equilibria from the top down to some boundaries, the altitude positions of which vary in the interval between 72–73 and 85 km and depend essentially on season and latitude. The developed analytical criteria reproduce the main features of these boundaries well almost everywhere. Due to weak sensitivity to uncertainties of reaction rates and other parameters, the criteria can be regarded as a robust instrument for HO 2 and OH equilibrium validation. The obtained results allow us to extend previously proposed methods for the retrieval of poorly measured components from measurement data and to develop new approaches. [ABSTRACT FROM AUTHOR]

At the altitudes of the mesosphere – lower thermosphere, OH and HO2 play a significant role in many physicochemical processes. Thus, monitoring of their spatiotemporal evolution together with other chemically active trace gases is one of the most important problems for this atmosphere region, in which direct measurements are difficult. The paper studies the nighttime OH and HO2 chemical equilibria using the 3D chemical transport modeling within the general approach including the extraction of the main sources and sinks in the equilibrium space-time areas and derivation of analytical criteria for equilibrium validity. The presented analysis shows that there are extended areas, where nighttime HO2 and OH are close to their local equilibrium concentrations determined mainly by the reaction between HOx – Ox components themselves and with H2O2, N, NO, NO2, and CO. In the upper mesosphere – lower thermosphere, the equilibrium expressions can be shortened, including the HOx – Ox chemistry only. These conditions describes the HO2 and OH equilibrium from the top to the some lower borders, the altitude position of which vary in the interval between 72–73 and 85 km and depends essentially on season and latitude. The developed analytical criteria almost everywhere well reproduce the main features of these borders. The obtained results allow to extend the abilities of previously proposed methods for the retrieval of poorly measured components from measurement data and to develop new approaches. [ABSTRACT FROM AUTHOR]

The assumption of nighttime ozone chemical equilibrium (NOCE) is widely used for retrieval of the Оx-НOx components in the mesopause from rocket and satellite measurements. In this work, recently developed analytical criterion of determining the NOCE boundary is applied (1) to study of connection of this boundary with O and H spatiotemporal variability basing on the 3D chemical transport modeling, and (2) to retrieve and analyze the spatiotemporal evolution of the NOCE boundary in 2002–2021 from SABER/TIMED data set. It was revealed, first, that the NOCE boundary well reproduces the transition zone dividing deep and weak diurnal oscillations of O and H at the low and middle latitudes. Second, the NOCE boundary is sensitive to sporadic abrupt changes in the middle atmosphere dynamics, in particular, due to powerful sudden stratospheric warmings leading to events of elevated (up to ~80 km) stratopause, which took place in January–February 2004, 2006, 2009, 2010, 2012, 2013, 2018 and 2019. Third, the space-time evolution of this characteristics expressed via pressure-height contains a clear signal of 11-year solar cycle in the range of 55º S–55º N. In particular, average annual the NOCE boundary averaged in this range of latitudes anticorrelates well with F10.7 index with the coefficient of -0.96. Moreover, it shows a weak linear trend of 49.2±36.2 m/decade. [ABSTRACT FROM AUTHOR]

Ground-based passive measurements of downwelling atmospheric radiation at ~110.836 GHz allow extracting the spectra of ozone self-radiation (rotational transition J = 61,5–60,6) coming from the low stratosphere–mesosphere and retrieving vertical profiles of ozone concentration at these altitudes. There is a notable (several hundred kHz) ambiguity in the determination of the resonance frequency of this important ozone line. We carried out long-term ground-based measurements of atmospheric microwave radiation in this range using upgraded apparatus with high technical accuracy and spectral resolution (~12 kHz). The obtained brightness temperature spectra allowed us to determine the frequency of this ozone line to be 110,835.909 ± 0.016 MHz. We verified that the Doppler frequency shift by horizontal wind as well as the variations of the tropospheric absorption had little effect on the obtained result. The found value was 131 ± 16 kHz less than that measured in the laboratory and differed from modern model calculations. At the same time, it was close to the results of early semiempirical calculations made more than 40 years ago. The applications where precise knowledge about the resonance frequency of this ozone line can be important were discussed in this paper. [ABSTRACT FROM AUTHOR]

This Technical Note presents a statistical approach to evaluating simultaneous measurements of several atmospheric components under the assumption of photochemical equilibrium. We consider simultaneous measurements of OH, HO2, and O3 at the altitudes of the mesosphere as a specific example and their daytime photochemical equilibrium as an evaluating relationship. A simplified algebraic equation relating local concentrations of these components in the 50-100 km altitude range has been derived. The parameters of the equation are temperature, neutral density, local zenith angle, and the rates of eight reactions. We have performed a one-year simulation of the mesosphere and lower thermosphere using a 3-D chemical-transport model. The simulation shows that the discrepancy between the calculated evolution of the components and the equilibrium value given by the equation does not exceed 3-4% in the full range of altitudes independent of season or latitude. We have developed a statistical Bayesian evaluation technique for simultaneous measurements of OH, HO2, and O3 based on the equilibrium equation taking into account the measurement error. The first results of the application of the technique to MLS/Aura data (Microwave Limb Sounder) are presented in this Technical Note. It has been found that the satellite data of the HO2 distribution regularly demonstrate lower altitudes of this component's mesospheric maximum. This has also been confirmed by model HO2 distributions and comparison with offline retrieval of HO2 from the daily zonal means MLS radiance. [ABSTRACT FROM AUTHOR]

The Technical Note presents a statistically correct approach to evaluating simultaneous measurements of several atmospheric components under the assumption of photochemical equilibrium. We consider simultaneous measurements of OH, HO2, and O3 at the altitudes of the mesosphere as a specific example and their daytime photochemical equilibrium as an evaluating relationship. A simplified algebraic equation relating local concentrations of these components in the 50-100 km altitude range has been derived. The parameters of the equation are air temperature, air concentration, local zenith angle, and the rates of 9 reactions. We have performed one-year simulation of the mesosphere and lower thermosphere using a 3D chemical-transport model. The simulation shows that the discrepancy between the calculated evolution of the components and the equilibrium value given by the equation does not exceed 3-4 % in the full range of altitudes independent of season or latitude. We have developed the technique of statistic Bayesian evaluation of simultaneous measurements of OH, HO2 and O3 based on the equilibrium equation taking into account the measurement error. The first results of application of the technique to MLS/Aura data are presented in this Technical Note. It has been found that the satellite data of HO2 distribution regularly demonstrates essentially lower altitudes of mesospheric maximum of this component. This has also been confirmed by offline retrieval of HO2 from the MLS primary data. [ABSTRACT FROM AUTHOR]