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

Abstract

Abstract. Among the more than 20 ground-based FTIR (Fourier transform infrared) stations currently operating around the globe, only a few have provided formaldehyde (HCHO) total column time series until now. Although several independent studies have shown that the FTIR measurements can provide formaldehyde total columns with good precision, 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 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 total systematic and random uncertainties of an individual HCHO total column measurement lie between 12 % and 27 % and between 1 and 11×1014 molec cm−2, respectively. The median values among all stations are 13 % and 2.9×1014 molec cm−2 for the total systematic and random uncertainties.This unprecedented harmonized formaldehyde data set from 21 ground-based FTIR stations is presented and its comparison with a global chemistry transport model shows 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 1013 molec cm−2) to highly polluted levels (7×1016 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 1990s, are crucial for past and present satellite validation and will be extended in the coming years for the next generation of satellite missions.

TROPOMI/S5P formaldehyde validation using an extensive network of ground-based FTIR stations

“…profile information (from the model WACCM, Garcia et al (2007)), and a L1 Tikhonov regularization matrix (Tikhonov, 1963), low vertical resolution profiles can be retrieved in principle, as well as total columns. However, as described in Vigouroux et al (2018), the degrees of freedom for signal are very low for HCHO (median value of 1.1 for all FTIR sites), meaning that we essentially have one piece of information. The FTIR total column averaging kernel shows a decrease of the sensitivity at the surface, which is quite similar to the TROPOMI sensitivity.…”

“…Table 1 lists the ground-based FTIR stations, their coordinates and altitude, the spectrometer type, the retrieval code, and the team involved in the measurements and/or the retrievals of HCHO. For more details on the monitoring of FTIR solar absorption spectra at these stations, we refer to Vigouroux et al (2018) and references therein, and, for the FTIR retrieval principles, to e.g. Vigouroux et al (2009).…”

“…The random uncertainty is dominated at all sites by the measurement noise uncertainty, which can vary from site to site depending on the spectrometer. The uncertainty on the retrieved FTIR total columns for individual sites is given in Vigouroux et al (2018) for the 21 sites involved at that time. We obtain a median random uncertainty of 2.3 ×10 14 molec/cm 2 , with a large value of 11.1 ×10 14 molec/cm 2 only at Mexico City where a lower resolution instrument is used (Vertex 80).…”

“…With the inclusion of the smoothing error in the uncertainty budget, the median total random uncertainty is 2.9 ×10 14 molec/cm 2 , which is very close to our empirical uncertainty estimation of 2.8 ×10 14 molec/cm 2 , based on the standard deviation of the differences between two FTIR individual subsequent measurements (within a maximum of 30 minutes interval), confirming our theoretical uncertainty calculation. Since the Vigouroux et al The forward model parameters median systematic uncertainty on the HCHO FTIR total columns is 13% in the network of Vigouroux et al (2018). As already mentioned, the dominating systematic uncertainty sources are the spectroscopic parameters: the line intensities and the pressure broadening coefficients of the fitted HCHO absorption lines.…”

“…Despite the smaller standard deviations usually obtained within a 6 h criterion, we finally choose 3 h to reduce the possible impact of some passing plumes and of the HCHO diurnal cycle on the comparisons. The diurnal cycle at most of the FTIR stations can be found in Vigouroux et al (2018) and its Supplement. At many stations no significant diurnal cycle was observed but, in some cases, mainly polluted sites, we obtained a maximum around noon-1 p.m., close to the TROPOMI overpass time.…”