Measuring atmospheric chemistry and ozone levels

Atmospheric short-lived chemical species such as chlorine monoxide (ClO), hypochlorous acid (HOCl), the hydroperoxyl radical (HO₂), and bromine monoxide (BrO) play an important role in changing the composition of the atmosphere and are the cause of ozone (O₃) depletion. A good understanding of halogen and hydrogen chemistry is therefore required to understand climate change more precisely and to aid atmospheric ozone recovery efforts. 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 abundance in the atmosphere.

The Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) is an atmospheric observation instrument that is used to observe submillimeter waves emitted from minor atmospheric constituents, with unprecedented sensitivity, to monitor changing ozone levels. SMILES (see Figure 1) is the first global environment observation instrument in the Japanese Experiment Module onboard the International Space Station (ISS). SMILES was launched on 11 September 2009 and installed on the ISS on 25 September 2009. This mission is a joint project of the National Institute of Information and Communications Technology (NICT) and the Japan Aerospace Exploration Agency.^1–3

Figure 1. Illustration of the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) mission on the International Space Station.¹

We achieve high sensitivity with SMILES by employing superconducting technology. Our instrument sensors consist of superconductor-insulator-superconductor submillimeter-wave receivers that are cooled to 4.2K, and of low-noise high-electron-mobility transistor amplifiers that are cooled to 4K, 20K, and 100K. We use a Joule-Thomson circuit cryocooler and a two-stage Stirling refrigerator to cool the different components of the receiver system to the three different temperatures.

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 so that we can have a full understanding of the atmosphere's variability.

The non-sun-synchronized orbit of the ISS gives us the opportunity to observe the diurnal variation of the species we measure.^{4, 5} We performed atmospheric limb observations for this purpose between 12 October 2009 and 21 April 2010. The measurements ended due to the failure of the local oscillator and cryocooler system. 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 large.²

Figure 2 shows an example of the SMILES observation results from October and November 2009, illustrating the time and altitude dependence of BrO abundance. 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.

Figure 2. SMILES results showing the diurnal variation of bromine monoxide (BrO) in the stratsophere and mesosphere. This data was obtained over the equatorial region (30°N–30°S) between October and November 2009. vmr: Vertical mixing ratio. ppt: Parts per trillion.

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.⁶ This information will be used in future modeling studies of atmospheric chemistry and climate change. We are now working on the development of the Air Pollution Observation (APOLLO) instrument, which is the next Earth observation mission planned for the ISS. With this system we aim to make quantitative observations of short-lived climate pollutants (e.g., black carbon, methane, and tropospheric O₃). In particular, we will use full-spectrum sensing, i.e., from microwave to UV-visible light wavelengths, to discern the high-resolution (1–2km horizontally and 3km vertically) 3D structure of tropospheric ozone.

We thank the whole SMILES team at NICT for their work on this project.