Formaldehyde (HCHO) in the clean atmosphere is a product of methane oxidation, while high HCHO concentrations appear over regions in the presence of anthropogenic and biogenic volatile organic compounds (VOCs). Satellite measurements of HCHO can be used as a proxy of atmospheric VOCs, and have been used to estimate VOC emissions and examine air quality issues such as ozone and secondary organic aerosol production.
The Ozone Mapping and Profiler Suite (OMPS) Nadir Mapper (NM) instruments onboard the Suomi NPP and NOAA-20 satellites have been used to produce a HCHO record from 2012 to the present. Formaldehyde products from the OMPS-NM instruments were recently updated, compared with TROPOMI observations over 12 geographical regions, and validated with NDACC FTIR HCHO observations at 24 ground stations.
The comparison of OMPS/Suomi NPP and OMPS/NOAA-20 HCHO monthly means exhibits a good agreement with ground-based NDACC FTIR data, with correlation coefficients of R=0.83 and 0.88, respectively. However, OMPS HCHO data have biases that vary with HCHO total column as compared to the FTIR data. In low HCHO conditions (HCHO columns smaller than 4 ´ 1015 molecules cm–2), OMPS-NM products have a positive bias of 20 (32) ± 6 (18) % for OMPS/Suomi NPP (NOAA-20), respectively. But for high HCHO (columns larger than 4 ´ 1015 molecules cm–2), negative biases of −15% ± 4% occur for OMPS/Suomi NPP, and OMPS/NOAA-20 shows a much smaller bias of 0.5% ± 6% due, we conclude, to its smaller ground pixel footprint (OMPS/Suomi NPP: 50 ´ 50 km2, OMPS/NOAA-20: 12 ´ 17 km2). The OMPS-NM products also show a good agreement with TROPOMI, despite differences in their respective retrieval algorithms. OMPS HCHO monthly means are consistent with those from TROPOMI (correlation coefficient, R=0.92) but are 10% ± 16% larger in polluted regions (HCHO total columns > 8 ´ 1015 molecules cm–2).
The high quality of NDACC HCHO observations distributed around the world is a key component of the intensive validation success. The encouraging results obtained by Kwon et al. (2023) increase the credibility of OMPS HCHO products, which can be used for air quality studies such as top-down estimation of non-methane VOC emissions, ozone production, secondary organic aerosol formation, and health impacts of HCHO. A third OMPS-NM instrument was recently launched on NOAA-21 in late 2022, and two more are planned for the JPSS series of satellites with launches into the 2030’s. The series of OMPS-NM instrumented satellites can ensure the continuation of consistent afternoon overpass HCHO observations that began with OMI in 2004.
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Nowlan et al. (2023). Global Formaldehyde Products From the Ozone Mapping and Profiler Suite (OMPS) Nadir Mappers on Suomi NPP and NOAA-20. Earth and Space Science, https://doi.org/10.1029/2022EA002643
Kwon et al. (2023). Validation of OMPS Suomi NPP and OMPS NOAA-20 Formaldehyde Total Columns With NDACC FTIR Observations. Earth and Space Science, https://doi.org/10.1029/2022EA002778
Contacts: hakwon@suwon.ac.kr (University of Suwon) and cnowlan@cfa.harvard.edu (Center for Astrophysics | Harvard and Smithsonian)
Figure 1. Scatter plots (a) between OMPS/Suomi NPP and FTIR data (2012–2020) and (b) between OMPS/NOAA-20 and FTIR data (2018–2020) for monthly mean comparable pairs. A correlation coefficient (R), root mean square error (RMSE), normalized mean bias (NMB), normalized mean error (NME), and the number of pairs (NUM) are given in each panel. Colors of symbols indicate the regional classification of sites shown in Table 2 of Kwon et al. (2023).
Figure 2. Correlation plot of monthly average HCHO vertical column densities for 12 geographic regions shown in Figure 9 of Nowlan et al. (2023) for (a) OMPS/Suomi NPP versus OMPS/NOAA-20, (b) TROPOMI versus OMPS/Suomi NPP and (c) TROPOMI versus OMPS/NOAA-20.