Diurnal variation of BrO, HO2, HOCl, ClO, and O3 in the range from stratosphere to lower ionosphere observed by SMILES

A good understanding of halogen and hydrogen chemistry is required to understand Earth’s upper atmosphere above stratosphere. The abundance of these highly reactive species varies widely (within the parts per billion to parts per trillion range) on a day-to-day timescale due to photochemical reactions induced by sunlight. Until now, however, it has been difficult to accurately estimate their abundances and behavior. The Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) is an atmospheric observation instrument with unprecedented sensitivity to monitor atmospheric minor compositions. SMILES is the first global environment observation instrument in the Japanese Experiment Module onboard the International Space Station (ISS). This mission is a joint project of the National Institute of Information and Communications Technology (NICT) and the Japan Aerospace Exploration Agency. The observation frequency bands of SMILES range from 624.32-626.32GHz to 649.12-650.32GHz. This range allows us to obtain vertical profiles of several atmospheric constituents simultaneously. We measure the abundance of O3 and its isotopic compositions, and of hydrochloric acid (HCl), ClO, HOCl, HO2, hydrogen peroxide (H2O2), BrO, acetonitrile (CH3CN), nitric acid (HNO3), water vapor, and ice clouds. The non-sun-synchronized orbit of the ISS gives us the opportunity to observe the diurnal variation of the species we measure. We performed atmospheric limb observations between 12 October 2009 and 21 April 2010. With the successful observations, we found that the amplitude of diurnal variations for halogen and hydrogen radicals (i.e., ClO, HOCl, HO2, and BrO) are quite large. Within one day, the abundance of BrO in the stratosphere and mesosphere varies from 0 to 16 parts per trillion. This comprises more than 80% of the total stratospheric bromine inventory. The results from SMILES represent the first studies of atmospheric composition diurnal variation using short-lived radical species such as O3, HCl, ClO, HOCl, HO2, and BrO. This information will be used in future modeling studies of atmospheric chemistry and climate change.