NDACC harmonized formaldehyde time series from 21 FTIR stations covering a wide range of column abundances

Among the more than twenty ground-based FTIR (Fourier Transform infrared) stations currently operating around the globe, only a few have provided formaldehyde (HCHO) total columns time-series until now. Although several independent studies have shown that the FTIR measurements can provide accurate and precise formaldehyde total columns, the spatial coverage has not been optimal for providing good diagnostics for satellite or model validation. Furthermore, these past studies used different retrieval settings, and biases as large as 50 % can be observed in the HCHO total columns depending on these retrieval choices, which is also a weakness for validation studies combining data from different ground-based stations. For the present work, the HCHO retrieval settings have been optimized based on experience gained from the past studies and have been applied consistently at the 21 participating stations, most of them are either part of the Network for the Detection of Atmospheric Composition Change (NDACC), or under consideration for membership. We provide the harmonized settings and a characterization of the HCHO FTIR products. Depending on the station, the systematic and random uncertainties of an individual HCHO total column measurement lie between 11 and 31 %; and between 1 and 11 × 10 14  molec/cm 2 , respectively, with median values among all stations of 14 % and 2.6 × 10 14  molec/cm 2 . This unprecedented harmonized formaldehyde data set from 21 ground-based FTIR stations is presented and its comparison to a global chemistry transport model shows its consistency, in absolute values as well as in seasonal cycles. The network covers very different concentration levels of formaldehyde, from very clean levels at the limit of detection (few 10 13  molec/cm 2 ) to highly polluted levels (7 × 10 16  molec/cm 2 ). Because the measurements can be made at any time during daylight, the diurnal cycle can be observed and is found to be significant at many stations. These HCHO time-series, some of them starting in the 1990's, are crucial for past and present satellite validation, and will be extended in the coming years for the next generation of satellite missions.