TY - JOUR

T1 - TROPOMI-Sentinel-5 Precursor formaldehyde validation using an extensive network of ground-based Fourier-transform infrared stations

AU - Vigouroux, Corinne

AU - Langerock, Bavo

AU - Augusto Bauer Aquino, Carlos

AU - Blumenstock, Thomas

AU - Cheng, Zhibin

AU - De Mazière, Martine

AU - De Smedt, Isabelle

AU - Grutter, Michel

AU - Hannigan, James W.

AU - Jones, Nicholas

AU - Kivi, Rigel

AU - Loyola, DIego

AU - Lutsch, Erik

AU - Mahieu, Emmanuel

AU - Makarova, Maria

AU - Metzger, Jean Marc

AU - Morino, Isamu

AU - Murata, Isao

AU - Nagahama, Tomoo

AU - Notholt, Justus

AU - Ortega, Ivan

AU - Palm, Mathias

AU - Pinardi, Gaia

AU - Röhling, Amelie

AU - Smale, Dan

AU - Stremme, Wolfgang

AU - Strong, Kim

AU - Sussmann, Ralf

AU - Té, Yao

AU - Van Roozendael, Michel

AU - Wang, Pucai

AU - Winkler, Holger

PY - 2020/7/10

Y1 - 2020/7/10

N2 - TROPOMI (the TROPOspheric Monitoring Instrument), on board the Sentinel-5 Precursor (S5P) satellite, has been monitoring the Earth s atmosphere since October 2017 with an unprecedented horizontal resolution (initially 7 km2-3:5 km2, upgraded to 5.5 km2-3:5 km2 in August 2019). Monitoring air quality is one of the main objectives of TROPOMI; it obtains measurements of important pollutants such as nitrogen dioxide, carbon monoxide, and formaldehyde (HCHO). In this paper we assess the quality of the latest HCHO TROPOMI products versions 1.1.(5-7), using ground-based solar-absorption FTIR (Fourier-transform infrared) measurements of HCHO from 25 stations around the world, including high-, mid-, and low-latitude sites. Most of these stations are part of the Network for the Detection of Atmospheric Composition Change (NDACC), and they provide a wide range of observation conditions, from very clean remote sites to those with high HCHO levels from anthropogenic or biogenic emissions. The ground-based HCHO retrieval settings have been optimized and harmonized at all the stations, ensuring a consistent validation among the sites. In this validation work, we first assess the accuracy of TROPOMI HCHO tropospheric columns using the median of the relative differences between TROPOMI and FTIR ground-based data (BIAS). The pre-launch accuracy requirements of TROPOMI HCHO are 40 % 80 %. We observe that these requirements are well reached, with the BIAS found below 80% at all the sites and below 40% at 20 of the 25 sites. The provided TROPOMI systematic uncertainties are well in agreement with the observed biases at most of the stations except for the highest-HCHO-level site, where it is found to be underestimated. We find that while the BIAS has no latitudinal dependence, it is dependent on the HCHO concentration levels: an overestimation (C26-5 %) of TROPOMI is observed for very low HCHO levels (> 2:5-1015 molec. cm-2), while an underestimation (-30:8%-1:4 %) is found for high HCHO levels (>8:0-1015 molec. cm-2). This demonstrates the great value of such a harmonized network covering a wide range of concentration levels, the sites with high HCHO concentrations being crucial for the determination of the satellite bias in the regions of emissions and the clean sites allowing a small TROPOMI offset to be determined. The wide range of sampled HCHO levels within the network allows the robust determination of the significant constant and proportional TROPOMI HCHO biases (TROPOMI D C1:10-0:05-1015 C0:64-0:03 FTIR; in molecules per square centimetre). Second, the precision of TROPOMI HCHO data is estimated by the median absolute deviation (MAD) of the relative differences between TROPOMI and FTIR groundbased data. The clean sites are especially useful for minimizing a possible additional collocation error. The precision requirement of 1:2-1016 molec. cm-2 for a single pixel is reached at most of the clean sites, where it is found that the TROPOMI precision can even be 2 times better (0.5 0:8-1015 molec. cm-2 for a single pixel). However, we find that the provided TROPOMI random uncertainties may be underestimated by a factor of 1.6 (for clean sites) to 2.3 (for high HCHO levels). The correlation is very good between TROPOMI and FTIR data (R D 0:88 for 3 h mean coincidences; R D 0:91 for monthly means coincidences). Using about 17 months of data (from May 2018 to September 2019), we show that the TROPOMI seasonal variability is in very good agreement at all of the FTIR sites. The FTIR network demonstrates the very good quality of the TROPOMI HCHO products, which is well within the prelaunch requirements for both accuracy and precision. This paper makes suggestions for the refinement of the TROPOMI random uncertainty budget and TROPOMI quality assurance values for a better filtering of the remaining outliers.

AB - TROPOMI (the TROPOspheric Monitoring Instrument), on board the Sentinel-5 Precursor (S5P) satellite, has been monitoring the Earth s atmosphere since October 2017 with an unprecedented horizontal resolution (initially 7 km2-3:5 km2, upgraded to 5.5 km2-3:5 km2 in August 2019). Monitoring air quality is one of the main objectives of TROPOMI; it obtains measurements of important pollutants such as nitrogen dioxide, carbon monoxide, and formaldehyde (HCHO). In this paper we assess the quality of the latest HCHO TROPOMI products versions 1.1.(5-7), using ground-based solar-absorption FTIR (Fourier-transform infrared) measurements of HCHO from 25 stations around the world, including high-, mid-, and low-latitude sites. Most of these stations are part of the Network for the Detection of Atmospheric Composition Change (NDACC), and they provide a wide range of observation conditions, from very clean remote sites to those with high HCHO levels from anthropogenic or biogenic emissions. The ground-based HCHO retrieval settings have been optimized and harmonized at all the stations, ensuring a consistent validation among the sites. In this validation work, we first assess the accuracy of TROPOMI HCHO tropospheric columns using the median of the relative differences between TROPOMI and FTIR ground-based data (BIAS). The pre-launch accuracy requirements of TROPOMI HCHO are 40 % 80 %. We observe that these requirements are well reached, with the BIAS found below 80% at all the sites and below 40% at 20 of the 25 sites. The provided TROPOMI systematic uncertainties are well in agreement with the observed biases at most of the stations except for the highest-HCHO-level site, where it is found to be underestimated. We find that while the BIAS has no latitudinal dependence, it is dependent on the HCHO concentration levels: an overestimation (C26-5 %) of TROPOMI is observed for very low HCHO levels (> 2:5-1015 molec. cm-2), while an underestimation (-30:8%-1:4 %) is found for high HCHO levels (>8:0-1015 molec. cm-2). This demonstrates the great value of such a harmonized network covering a wide range of concentration levels, the sites with high HCHO concentrations being crucial for the determination of the satellite bias in the regions of emissions and the clean sites allowing a small TROPOMI offset to be determined. The wide range of sampled HCHO levels within the network allows the robust determination of the significant constant and proportional TROPOMI HCHO biases (TROPOMI D C1:10-0:05-1015 C0:64-0:03 FTIR; in molecules per square centimetre). Second, the precision of TROPOMI HCHO data is estimated by the median absolute deviation (MAD) of the relative differences between TROPOMI and FTIR groundbased data. The clean sites are especially useful for minimizing a possible additional collocation error. The precision requirement of 1:2-1016 molec. cm-2 for a single pixel is reached at most of the clean sites, where it is found that the TROPOMI precision can even be 2 times better (0.5 0:8-1015 molec. cm-2 for a single pixel). However, we find that the provided TROPOMI random uncertainties may be underestimated by a factor of 1.6 (for clean sites) to 2.3 (for high HCHO levels). The correlation is very good between TROPOMI and FTIR data (R D 0:88 for 3 h mean coincidences; R D 0:91 for monthly means coincidences). Using about 17 months of data (from May 2018 to September 2019), we show that the TROPOMI seasonal variability is in very good agreement at all of the FTIR sites. The FTIR network demonstrates the very good quality of the TROPOMI HCHO products, which is well within the prelaunch requirements for both accuracy and precision. This paper makes suggestions for the refinement of the TROPOMI random uncertainty budget and TROPOMI quality assurance values for a better filtering of the remaining outliers.

KW - MAX-DOAS OBSERVATIONS

KW - TROPOSPHERIC NO2

KW - HIGH-RESOLUTION

KW - RETRIEVAL

KW - SATELLITE

KW - TROPOMI

KW - GOME-2

KW - MODEL

KW - FTIR

KW - OMI

UR - http://www.scopus.com/inward/record.url?scp=85088494513&partnerID=8YFLogxK

U2 - 10.5194/amt-13-3751-2020

DO - 10.5194/amt-13-3751-2020

M3 - Article

AN - SCOPUS:85088494513

VL - 13

SP - 3751

EP - 3767

JO - Atmospheric Measurement Techniques

JF - Atmospheric Measurement Techniques

SN - 1867-1381

IS - 7

ER -