Photochemistry on the under side of the mesospheric Na layer

Lidar observations of the mesospheric Na layer have revealed considerable diurnal variations 10 particularly on the underside of the main layer where more than an order-of-magnitude increase of Na 11 density has been observed below 80 km after sunrise. In this paper, multi-year Na lidar observations over 12 a full diurnal cycle at Utah State University (USU) (41.8°N, 111.8°W) and a global atmospheric model of 13 Na with 0.5 km vertical resolution in the mesosphere lower thermosphere (WACCM-Na) are utilized to 14 explore the dramatic changes of Na density on the layer underside. Photolysis of the principal reservoir 15 NaHCO3 is shown to be primarily responsible for the increase in Na after sunrise, amplified by the 16 increased rate of reaction of NaHCO3 with atomic H, which is mainly produced from the photolysis of 17 H2O and the reaction of OH with O3. This finding is further supported by Na lidar observation at USU during the solar eclipse (> 96% totality) event on August 21st 18 , 2017, when a decrease and recovery of the 19 Na density on the underside of the layer were observed. Lastly, the model simulation shows that the Fe 20 density underside around 80 km increases larger and earlier than observed Na changes during sunrise 21 because of the considerably faster photolysis rate of its major reservoir FeOH.