First Evidence of Middle Atmospheric HO2 Response to UV variability during 27-day Solar Cycles From Satellite Observations

Abstract:

HO₂ and OH, also known as odd oxygen HO_x, play an important role in middle atmospheric chemistry, in particular O₃ destruction through catalytic HO_x reaction cycles. Due to their photochemical production and short chemical lifetimes, HO_x species are expected to show rapid responses to solar irradiance changes, resulting in variability in the corresponding O₃ chemistry. While the OH response to solar cycles has been investigated, HO₂ studies have been limited by the lack of reliable observations. The recently developed “offline” HO₂ data product from the Aura Microwave Limb Sounder (MLS) makes it possible, for the first time, to investigate HO₂ variability and the behavior of OH/HO₂ partitioning, which plays an important role in O₃-destroying HO_x cycles. Here we present the first evidence of global mean HO₂ variability during solar 27-day cycles by investigating the new MLS HO₂ data. We focus on the most recent data from 2012 - 2015, when solar irradiance variability is strong near the peak of Solar Cycle 24. The features of mesospheric HO₂ variability are found to correlate well with those of solar Lyman-α variability. The strongest HO₂ solar cycle signals occur in the pressure range 0.01 – 0.068 hPa. When continuous MLS OH observations are not available, the new HO₂ data could be a promising alternative for investigating HO_x variability and the corresponding impacts on O₃ and the climate.