Tropospheric ozone and precursors (HCHO and CO) from the NDACC FTIR ground-based network

Ground-based FTIR (Fourier transform infrared) stations contributing to the Network for Detection of Atmospheric Composition Change (NDACC), deliver time-series of ozone and some of its precursors at more than 20 sites, starting from the 90’s for the oldest stations. In the context of the Tropospheric Ozone Assessment Report (TOAR-II), we will present the status of the ground-based FTIR tropospheric ozone, formaldehyde (HCHO), and carbon monoxide (CO) measurements.

From high-resolution solar absorption spectra, O₃, HCHO and CO total columns are obtained with a precision of about 2%, 8%, and 1%, respectively. In addition, the pressure dependence of fully resolved absorption lines allows retrieving low vertical resolution profiles and thus deriving few independent partial columns. For O₃, the degrees of freedom for signal (DOFS) are about 4.5, allowing O₃ amounts to be retrieved in four independent altitude layers: one in the troposphere and three in the stratosphere up to about 45 km, with a precision of 5–6 % for each partial column. For HCHO, the DOFS are only of order 1.0-1.5, with a sensitivity mainly located in the troposphere where most of the HCHO lies. For CO, about 2 DOFS can be obtained, with one of them located in the troposphere.

We will show the variability and trends (when long time-series are available) of O₃, HCHO, and CO tropospheric partial columns at many FTIR stations, covering a wide range of latitudes and pollution conditions. To derive the trends, we use a multiple linear regression model including seasonal cycles and dynamical proxies explaining the species’ variability such as, e.g., the tropopause height, the El Niño-Southern Oscillation (ENSO), or the Quasi-Biennial Oscillation (QBO).