Validation of OMPS Suomi NPP and OMPS NOAA-20 Formaldehyde Total Columns With NDACC FTIR Observations

We validate formaldehyde (HCHO) vertical column densities (VCDs) from Ozone Mapping and Profiler Suite Nadir Mapper (OMPS-NM) instruments onboard the Suomi National Polar-orbiting Partnership (Suomi NPP) satellite for 2012-2020 and National Oceanic and Atmospheric Administration-20 (NOAA-20) satellite for 2018-2020, hereafter referred to as OMPS-NPP and OMPS-N20, with ground-based Fourier-Transform Infrared (FTIR) observations of the Network for the Detection of Atmospheric Composition Change (NDACC). OMPS-NPP/N20 HCHO products reproduce seasonal variability at 24 FTIR sites. Monthly variability of OMPS-NPP/N20 has a very good agreement with FTIR, showing correlation coefficients of 0.83 and 0.88, respectively. OMPS-NPP (N20) biases averaged over all sites are -0.9 (4) +/- 3 (6)%. However, at clean sites (with VCDs < 4.0 x 10(15) molecules cm(-2)), positive biases of 20 (32) +/- 6 (18)% occur for OMPS-NPP (N20). At sites with HCHO VCDs > 4.0 x 10(15) molecules cm(-2), negative biases of -15% +/- 4% appear for OMPS-NPP, but OMPS-N20 shows smaller bias of 0.5% +/- 6% due to its smaller ground pixel footprints. Therefore, smaller satellite footprint sizes are important in distinguishing small-scale plumes. In addition, we discuss a bias correction and provide lower limit for the monthly uncertainty of OMPS-NPP/N20 HCHO products. The total uncertainty for OMPS-NPP (N20) at clean sites is 0.7 (0.8) x 10(15) molecules cm(-2), corresponding to a relative uncertainty of 32 (30)%. In the case of HCHO VCDs > 4.0 x 10(15) molecules cm(-2), however, the relative uncertainty in HCHO VCDs for OMPS-NPP (N20) decreases to 31 (18)%. Plain Language Summary Formaldehyde is a proxy for nonmethane volatile organic compounds (NMVOCs). Formaldehyde observations from satellites have been widely used to investigate NMVOC emissions, ozone production regime with NOx, and the production of secondary organic aerosols. We validate new publicly available satellite formaldehyde observations from two Ozone Mapping and Profiler Suite (OMPS) instruments onboard Suomi NPP and NOAA-20 satellites, called OMPS-NPP and OMPS-N20 hereafter, with ground-based Fourier-Transform Infrared (FTIR) observations. OMPS-NPP/N20 products show good correlation coefficients of 0.83 and 0.88 compared to FTIR and also capture seasonal variability at 24 sites. However, OMPS products have a bias depending on formaldehyde concentrations. In low formaldehyde concentrations, positive biases of 20 (32) +/- 6 (18)% occur for OMPS-NPP (N20). In high concentrations, negative biases of -15% +/- 6% appear for OMPS-NPP, but OMPS-N20 products have a smaller bias of 0.5% +/- 6% due to its smaller ground pixel footprints. This implies that smaller satellite footprint sizes are important in distinguishing small-scale plumes. In addition, we provide a lower limit for the monthly uncertainty of OMPS-NPP/N20 formaldehyde products.

Automated summary

In this study, we validate the formaldehyde vertical column densities (VCDs) measured by OMPS-NPP and OMPS-N20 satellites using ground-based FTIR observations. Both OMPS-NPP and OMPS-N20 products show good agreement with FTIR observations in terms of seasonal and monthly variability. However, biases exist in the satellite products, with positive biases in low concentrations and negative biases in high concentrations.