09:00 〜 10:30
[PPS04-P08] Local time dependence of Venusian cloud-top SO2 obtained from Akatsuki UV images
キーワード：金星、大気、放射輸送、二酸化硫黄
The distribution of H2SO4 clouds in the Venusian atmosphere is an important factor that influences the solar energy absorbed by Venus. Understanding how SO2, the precursor of H2SO4, is transported from the lower layers to the cloud top where the cloud particles are formed from SO2 photochemistry is essential for understanding the climate system of Venus.
The UV imager onboard Akatsuki takes images from the orbit around Venus to observe the spatial and temporal distribution of SO2. Retrieval of SO2 distribution from these data sets will be important in understanding not only planetary scale but also finer temporal and spatial SO2 transport. The 283-nm UV images taken by Akatsuki reflect the amount of SO2 as an absorber at first, but they also include the effects of H2SO4 aerosols, unidentified UV absorbers and CO2 which is the main component of the atmosphere. That makes quantitative discussions difficult.
In this study, we developed a new method to estimate the SO2 mixing ratio at the cloud top assuming that all UV absorption is due to SO2 using a newly developed radiative transfer code from UV images taken by UVI under various conditions and estimated the SO2 mixing ratio during the period from 2016 to 2020. The total number of images analysed in this study is 11243, and we focused only on the results over low latitudes (< 30 degrees) in order to use the same atmospheric model.
From the retrieved SO2 maps, we derived the local time and latitude mean field of the amount of SO2. The mean value of the SO2 volume mixing ratio is from 100 to 200 ppb at the cloud top, which is consistent with the previous study (Belyaev et al., 2012). We found the local time variation of SO2 has a single peak in the afternoon, which is not consistent with that of Venus Express nadir observation (Marcq et al., 2022), which has two peaks both in the morning and afternoon. We considered that the reason is due to unidentified UV absorbers and recalculated the SO2 distribution using the mean local time-latitude distribution of the imaginary part of the refractive index of cloud particles obtained by Marcq et al. (2022). However, the major structure remained unchanged and still did not agree with their results. Our results are qualitatively consistent with the vertical SO2 transport induced by thermal tides, based on the waves’ structures reproduced by the GCM (Takagi et al., 2018).
The 365-nm channel of UVI is mostly affected by the absorption by the unidentified absorbers. We are also investigating methods to separately obtain the distributions of SO2 and unidentified absorbers using both 283-nm and 365-nm images.
The UV imager onboard Akatsuki takes images from the orbit around Venus to observe the spatial and temporal distribution of SO2. Retrieval of SO2 distribution from these data sets will be important in understanding not only planetary scale but also finer temporal and spatial SO2 transport. The 283-nm UV images taken by Akatsuki reflect the amount of SO2 as an absorber at first, but they also include the effects of H2SO4 aerosols, unidentified UV absorbers and CO2 which is the main component of the atmosphere. That makes quantitative discussions difficult.
In this study, we developed a new method to estimate the SO2 mixing ratio at the cloud top assuming that all UV absorption is due to SO2 using a newly developed radiative transfer code from UV images taken by UVI under various conditions and estimated the SO2 mixing ratio during the period from 2016 to 2020. The total number of images analysed in this study is 11243, and we focused only on the results over low latitudes (< 30 degrees) in order to use the same atmospheric model.
From the retrieved SO2 maps, we derived the local time and latitude mean field of the amount of SO2. The mean value of the SO2 volume mixing ratio is from 100 to 200 ppb at the cloud top, which is consistent with the previous study (Belyaev et al., 2012). We found the local time variation of SO2 has a single peak in the afternoon, which is not consistent with that of Venus Express nadir observation (Marcq et al., 2022), which has two peaks both in the morning and afternoon. We considered that the reason is due to unidentified UV absorbers and recalculated the SO2 distribution using the mean local time-latitude distribution of the imaginary part of the refractive index of cloud particles obtained by Marcq et al. (2022). However, the major structure remained unchanged and still did not agree with their results. Our results are qualitatively consistent with the vertical SO2 transport induced by thermal tides, based on the waves’ structures reproduced by the GCM (Takagi et al., 2018).
The 365-nm channel of UVI is mostly affected by the absorption by the unidentified absorbers. We are also investigating methods to separately obtain the distributions of SO2 and unidentified absorbers using both 283-nm and 365-nm images.