On the Contributions of Photolysis Reactions to the Bottom Side Mesospheric Na Layer Variations.

Decades of Na lidar observations around the globe on the mesospheric Na layer have revealed considerable variations during the night-day and day-night transitions, especially near the bottom half of the layer, where more than an order of magnitude of Na density enhancement has been observed just slightly below 80 km after sunrise. These considerable changes of Na density after the sunrise (decrease after sunset) in the upper mesosphere is the primary feature, but has rarely been investigated comprehensively, while some studies have implicated that such variations cannot be induced by dynamic process alone. In this investigation, the multi-year Na lidar full diurnal cycle observations at Utah State University (USU) (41.8°N, 111.8°W) and the most recent global Na model, WACCM-Na by National Center for Atmospheric Research (NCAR), are utilized to understand the mechanism behind such dramatic change of Na density near the bottom side of the Na layer. The study finds that the photolysis reactions, involving several Na compounds, such as NaO, NaO₂, NaOH and NaHCO₃, play critical role in these daytime variations, increasing (decreasing) the Na density significantly during the sunrise (sunset) due to the changing of solar radiation in the mesopause region ( 80-105 km). The Na production is further facilitated by the increase of atomic H and O mainly due to photolysis of H₂O and ozone during the daytime. The model simulation also suggests that the mesospheric Fe density within the similar altitude range would increase at a much faster pace than Na, mainly due to the considerably larger photolysis rate of its major reservoir FeOH than that of NaHCO₃, the counter part of Na. This finding is further confirmed by the Na lidar observation at USU during the solar eclipse (> 96% totality) in August 21^st, 2017, when considerable decrease (> 10%) and recovering of the Na density were observed mainly in the bottom half of the Na layer, indicating the importance of these photolysis reactions in mesospheric Na chemistry.