New and improved infra-red absorption cross sections and ACE-FTS retrievals of carbon tetrachloride (CCl4)

- New IR absorption cross sections (700-860 cm−1) reported for carbon tetrachloride.
- They cover temperatures from 208 to 296 K and appropriate atmospheric pressures.
- They improve upon previously available CCl4 cross-section datasets.
- They will enable retrievals of CCl4 abundances from IR remote-sensing instruments.
- New and improved ACE-FTS CCl4 retrieval more consistent with ground measurements.

Carbon tetrachloride (CCl4) is one of the species regulated by the Montreal Protocol on account of its ability to deplete stratospheric ozone. As such, the inconsistency between observations of its abundance and estimated sources and sinks is an important problem requiring urgent attention (Carpenter et al., 2014) [5]. Satellite remote-sensing has a role to play, particularly limb sounders which can provide vertical profiles into the stratosphere and therefore validate stratospheric loss rates in atmospheric models.This work is in two parts. The first describes new and improved high-resolution infra-red absorption cross sections of carbon tetrachloride/dry synthetic air over the spectral range 700-860 cm−1 for a range of temperatures and pressures (7.5-760 Torr and 208-296 K) appropriate for atmospheric conditions. This new cross-section dataset improves upon the one currently available in the HITRAN and GEISA databases. The second describes a new, preliminary ACE-FTS carbon tetrachloride retrieval that improves upon the v3.0/v3.5 data products, which are biased high by up to ~20-30% relative to ground measurements. Making use of the new spectroscopic data, this retrieval also improves the microwindow selection, contains additional interfering species, and utilises a new instrumental lineshape; it will form the basis for the upcoming v4.0 CCl4 data product.