09:30 〜 09:45
[PCG19-03] Impact of a global dust storm event on the CO VMR in the Martian mesosphere and lower thermosphere measured by TGO/NOMAD
キーワード：火星大気、CO大気、ダストストーム、大気大循環、大気化学反応
On Mars, dust storms are unique phenomena appearing in the southern summer season. The dust is lifted into the atmosphere, and it affects the atmosphere. It has been found that water vapor ascends above 60 km altitude after the onset of global and regional dust storm events (Heavens et al., 2018; Fedorova et al., 2018, 2020; Aoki et al., 2019; Vandaele et al., 2019). The enhancement of the general circulation is suggested by general circulation models (Shaposhinikov et al., 2019; Neary et al., 2020). The impact of global dust storm events can appear within a few days. For the measurement of CO, depletion of CO VMR after the onset of a global dust storm event appeared in the lower atmosphere and the upper atmosphere (Olsen et al., 2021; Modak et al., 2023; Fedorova et al., 2022). These studies proposed that the decrease of CO VMR in the lower atmosphere could be explained as an enhanced catalytic cycle between CO and OH due to enhanced water vapor during the dust storm event (Aoki et al., 2019; Fedorova et al., 2020). They also explained that the same reaction is accelerated in the upper atmosphere because the hygropause altitude also increased due to the warming and intensified general circulation (Neary et al., 2020). However, the lifetime of CO due to the reaction between CO and OH is still 109 s around 80 km under the enhanced water vapor in the mesosphere (Daerden et al., 2022b). This fact implies the difficulty in suddenly reducing the CO density via photochemistry after the onset of the dust storm. To identify the mechanisms for decreasing the CO VMR during the dust storm event, it is important to investigate the impacts on the CO and CO2 densities, respectively. However, previous literature studies do not distinguish their variations. Thus, the purpose of this study is to understand the impact of the global dust storm on the CO VMR by separating the variability of the CO and CO2 densities.
The solar occultation (SO) channel of NOMAD is used to derive the CO VMR and CO density. We retrieved the CO density using the CO 2-0 band transmittance measured in diffraction orders 186 (4180.06 – 4213.37 cm-1) and 190 (4269.95 – 4303.99 cm-1) with the radiative transfer code, named ASIMUT (Vandaele et al., 2006). The CO absorption line and continuum level are fitted with an optimal estimation method (Rodgers, 2000). The fit is performed independently for each spectrum at each tangential altitude. The temperature and pressure simulated by the Global Environmental Multiscale Mars (GEM-Mars) model (Daerden et al., 2019) are inputted for retrieval. Thus, the variability of CO2 density comes from the GEM-Mars model. The vertical profile of CO VMR is obtained by the combination of diffraction orders 186 and 190 because of differences in the line intensities of those orders. We already confirmed that the retrieved CO VMR agrees well with the GEM-Mars simulation, except for the dust storm event in MY 34. The distribution of CO VMR is directly linked to the transport in the atmosphere.
The impact of the global dust storm on CO VMR is investigated from Ls = 190 to 240 in MYs 34 and 35 between 80 and 110 km altitude. The measured CO VMR and density and model simulated CO2 density showed that the decrease of CO VMR is restricted to high latitudes (> 50 deg.), and it is caused by the increase of CO2 density at that region. The decrease of CO VMR is by a factor of 2. We consider that the global circulation intensified by the dust storm could lead to atmospheric heating in the polar regions and decrease CO VMR rather than photochemistry.
The comparison of CO density between NOMAD and GEM-Mars showed that the GEM-Mars model underestimated the CO density by a factor of 2 during the dust storm season. This suggests the possibility of overestimation of the general circulation during the dust storm. This would imply that an improved general circulation model could modify the distributions of minor species during the dust storm event.
The solar occultation (SO) channel of NOMAD is used to derive the CO VMR and CO density. We retrieved the CO density using the CO 2-0 band transmittance measured in diffraction orders 186 (4180.06 – 4213.37 cm-1) and 190 (4269.95 – 4303.99 cm-1) with the radiative transfer code, named ASIMUT (Vandaele et al., 2006). The CO absorption line and continuum level are fitted with an optimal estimation method (Rodgers, 2000). The fit is performed independently for each spectrum at each tangential altitude. The temperature and pressure simulated by the Global Environmental Multiscale Mars (GEM-Mars) model (Daerden et al., 2019) are inputted for retrieval. Thus, the variability of CO2 density comes from the GEM-Mars model. The vertical profile of CO VMR is obtained by the combination of diffraction orders 186 and 190 because of differences in the line intensities of those orders. We already confirmed that the retrieved CO VMR agrees well with the GEM-Mars simulation, except for the dust storm event in MY 34. The distribution of CO VMR is directly linked to the transport in the atmosphere.
The impact of the global dust storm on CO VMR is investigated from Ls = 190 to 240 in MYs 34 and 35 between 80 and 110 km altitude. The measured CO VMR and density and model simulated CO2 density showed that the decrease of CO VMR is restricted to high latitudes (> 50 deg.), and it is caused by the increase of CO2 density at that region. The decrease of CO VMR is by a factor of 2. We consider that the global circulation intensified by the dust storm could lead to atmospheric heating in the polar regions and decrease CO VMR rather than photochemistry.
The comparison of CO density between NOMAD and GEM-Mars showed that the GEM-Mars model underestimated the CO density by a factor of 2 during the dust storm season. This suggests the possibility of overestimation of the general circulation during the dust storm. This would imply that an improved general circulation model could modify the distributions of minor species during the dust storm event.