Mesospheric Ozone Depletion at 77 km During the 2020 Solar Eclipse

Abstract The annular solar eclipse of 21 June 2020 traverses Africa, Asia, and the Pacific region, producing an abrupt reduction in solar radiation that provides a distinct natural experiment to investigate atmospheric photochemical responses. Based on WACCM‐X simulations and Aura MLS observations, this study identifies and explains mesospheric ozone (O 3 ) depletion at 77 km during the eclipse, a feature rarely reported in previous studies that mainly described O 3 enhancement. In contrast to the sustained enhancement seen at 66 km, the O 3 mixing ratio at 77 km increases initially and then declines to 25.1% below non‐eclipse levels. This depletion results from a temporal lag between key chemical reactions. The recovery of the three‐body ozone formation reaction lags approximately 1 hr behind that of ozone photolysis, which begins once the eclipse center moves away. Consequently, the ozone depletion caused by photolysis recovers while chemical production remains suppressed. This imbalance consumes the ozone accumulated earlier and causes its mixing ratio to fall below the non‐eclipse background level. Once O 3 depletion begins, the hydrogen‐ozone reaction plays an important role in recovery. Its rate decreases after the eclipse maximum, which reduces ozone loss and helps overcome the negative effect from the net response of the ozone formation reaction and photolysis, thereby promoting O 3 recovery. Transport contributes up to 76% of the O 3 enhancement near the maximum eclipse, but only within about 10° of the eclipse center at 07:00 UT (30.63°N, 86.25°E), without altering the overall chemically dominated O 3 evolution.