Photochemistry on the bottom side of the mesospheric Na layer
2018
Abstract. Lidar observations of the mesospheric Na layer have revealed considerable
diurnal variations, particularly on the bottom side of the layer, where more
than an order-of-magnitude increase in Na density has been observed below 80 km
after sunrise. In this paper, multi-year Na lidar observations are
utilized over a full diurnal cycle at Utah State University (USU) (41.8 ∘ N,
111.8 ∘ W) and a global atmospheric model of Na with 0.5 km
vertical resolution in the mesosphere and lower thermosphere (WACCM-Na) to explore
the dramatic changes of Na density on the bottom side of the layer. Photolysis of the principal reservoir NaHCO 3 is shown to be
primarily responsible for the increase in Na after sunrise, amplified by the
increased rate of reaction of NaHCO 3 with atomic H, which is mainly
produced from the photolysis of H 2 O and the reaction of OH with
O 3 . This finding is further supported by Na lidar observation at USU
during the solar eclipse ( >96 % totality) event on 21 August 2017, when a decrease and recovery of the Na density on the
bottom side of the layer were observed. Lastly, the model simulation shows
that the Fe density below around 80 km increases more strongly and earlier
than observed Na changes during sunrise because of the considerably faster
photolysis rate of its major reservoir of FeOH.
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