An algorithm to retrieve peroxyacetyl nitrate from AIRS

Abstract. Herein, we describe an approach to retrieve free tropospheric columns of peroxyacyl nitrates (PANs) from radiances observed by the Atmospheric Infrared Sounder (AIRS). AIRS has provided daily global coverage since its launch in 2002, making the AIRS data a valuable long term record. Although the instrument is very radiometrically stable, the radiance noise level is large enough to present a challenge when retrieving a weak absorber such as PAN. To address this, spectral windows were selected to minimize interference from other species as much as possible and a set of filters was developed to predict whether a PAN value retrieved from AIRS is within 0.2 ppb or 50 % of what would be retrieved from the Cross-track Infrared Sounder (CrIS) and to remove spurious signals caused by specific surface features or clouds. We show that AIRS is capable of retrieving PAN plumes with very high concentrations of PAN (such as those from significant wildfires) that have similar spatial extent as seen by CrIS and that PAN retrieved from AIRS has good correlation with CrIS given sufficient averaging. We conclude with recommendations for users to help ensure that these data are used appropriately.