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.
Highlights
Through reactions with hydroxyl radical (OH) and NOx (NO + NO2), the volatile organic compounds (VOCs) exert a strong influence on the oxidizing capacity of the atmosphere
While the global emission budgets are in general consistent (Stavrakou et al, 2015), there are large differences in the top-down estimates on a regional scale, e.g. differences up to nearly 50 % are observed over Amazonia between SCIAMACHY and OMI (Barkley et al, 2013) and up to nearly 25 % between GOME-2 and OMI (Stavrakou et al, 2015)
The small number of stations and the differences in bias associated with the different retrieval strategies made it difficult to use the FTIR network as a coherent tool for satellite or model validation
Summary
Through reactions with hydroxyl radical (OH) and NOx (NO + NO2), the volatile organic compounds (VOCs) exert a strong influence on the oxidizing capacity of the atmosphere. Validation studies of HCHO satellite products have taken place at a few locations only, mainly using aircraft data (Martin et al, 2004; Barkley et al, 2013; Zhu et al, 2016), the MAX-DOAS (Multi-Axis Differential Optical Absorption Spectroscopy) technique (Wittrock et al, 2006; De Smedt et al, 2015) and the FTIR (Fourier transform infrared) technique (Jones et al, 2009; Vigouroux et al, 2009; De Smedt et al, 2015) This is not sufficient to provide a good picture of the accuracy of the satellites, especially given the high geographical variability of formaldehyde.
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