Chlorine nitrate (ClONO₂) plays an important role in catalytic ozone destruction, and thus monitoring it is essential for informing our understanding of trends in ozone depletion and recovery. In this study, the implementation of a ClONO₂ retrieval for a ground-based Fourier Transform InfraRed spectrometer (FTIR) located in Hefei, China (31.91^o N, 117.17^o E, 34.5 m above sea level) is detailed. The FTIR is a commercial Bruker 125HR installed in 2017 that measures in the mid- and near-infrared on alternating days. The mid-infrared measurements used for this retrieval cover the spectral region between 600 to 4500 cm^-1 with 0.005 cm^-1 spectral resolution. The Hefei station is part of the Network for the Detection of Atmospheric Composition Change (NDACC).

The ClONO₂ retrievals were performed using the SFIT4 v0.9.4.4 retrieval package, which is based on the Optimal Estimation Method (OEM) of Rodgers. The retrieval is performed on a 48 layer grid, spanning from the surface to 120 km, with temperature, pressure and water vapour information coming from the National Centers for Environmental Prediction (NCEP), and the a priori profile coming from Whole Atmosphere Community Climate Model (WACCM) v7 model. Due to the low concentration of atmospheric ClONO₂, the Norton-Beer function and first-order Tikhonov–Phillips regularization are used to retrieve profile information about the gas. An example spectral fit is shown in Figure 1, while the mean annual profile of ClONO₂ is shown in Figure 2.

Figure 1: Spectral fit of ClONO₂ and interfering species.

Figure 2: Mean annual distribution of ClONO₂ for the period from 2017 to 2024. N is the number of measurements per year.

At present, ACE-FTS is the only satellite instrument that provides profile observations of ClONO₂. In order to compare these satellite measurements to those from the ground-based FTIR, the former are smoothed with the a priori and averaging kernel of the latter to account for differences in their vertical resolutions. Measurements are deemed coincident if they are made within 5 degrees latitude and 10 degrees longitude of each other, and within 2 days. Measurements are compared as partial columns, with the lower vertical bound chosen as 20 km, to ensure the ACE-FTS profiles cover the range, and the upper bound chosen to be 42 km, to ensure that the FTIR profiles have sufficient sensitivity. For the 66 collocated pairs of measurements, the mean relative difference is 14.47 ± 17.41%, with the individual comparisons shown in Figure 3.

Figure 3: Relative difference between the ACE-FTS and FTIR partial columns, calculated as (ACE-FTS - FTIR)/FTIR.