Satellite remote-sensing capability to assess tropospheric column ratios of formaldehyde and nitrogen dioxide: case study during the LISTOS 2018 field campaign
Authors: Johnson Matthew S. , Philip Sajeev , Kumar Rajesh , Naeger Aaron , Souri Amir H. , Geddes Jeffrey , Judd Laura , Janz Scott , Sullivan John 
Published: September 2022

Abstract. Satellite retrievals of tropospheric column formaldehyde (HCHO) and nitrogen dioxide (NO2) are frequently used to investigate the sensitivity of ozone (O3) production to concentrations and emissions of nitrogen oxides (NOx) and volatile organic carbon compounds (VOCs). Space-based remote-sensing information of chemical proxies for NOx (i.e., NO2) and VOCs (i.e., HCHO), in particular the ratios of tropospheric column HCHO and NO2 (FNRs), provide insight into the non-linear relationship of O3 formation in the lower troposphere. Ultraviolet–visible (UV/VIS) satellite spectrometers such as the Ozone Monitoring Instrument (OMI) and TROPOspheric Monitoring Instrument (TROPOMI) are capable of providing FNR information with high spatiotemporal coverage, yet a recent study suggested that the biases and noise of satellite retrievals are the largest source of uncertainty for applying satellite-derived FNRs to better understand O3 production sensitivities. To quantify, and inter-compare, the uncertainties in two of the most commonly-applied satellite sensors to investigate O3 production sensitivities, we evaluated OMI and TROPOMI retrievals of NO2 and HCHO tropospheric columns, and resulting FNRs, using Geostationary Trace gas and Aerosol Sensor Optimization (GeoTASO) and GEO-CAPE Airborne Simulator (GCAS) airborne remote-sensing data taken during the Long Island Sound Tropospheric Ozone Study 2018 (LISTOS 2018). Compared to suborbital remote-sensing observations of tropospheric column NO2 and HCHO, the accuracy of OMI (using both the National Aeronautics and Space Administration (NASA) version 4 and the Quality Assurance for Essential Climate Variables (QA4ECV) retrieval algorithms) and TROPOMI were magnitude-dependent with high biases (i.e., satellite tropospheric columns > suborbital tropospheric columns) in clean/background environments and a tendency towards a low bias (i.e., satellite tropospheric columns < suborbital tropospheric columns) in moderate to polluted regions. Campaign-averaged NO2 median biases for OMI, using both the NASA and QA4ECV algorithms, were similar at 0.4±4.1 × 1015 molecules cm-2 (6.3 %) and 0.4±4.5 × 1015 molecules cm-2 (6.8 %), respectively. TROPOMI retrievals of NO2 had a campaign-averaged median bias of -0.3±3.7 × 1015 molecules cm-2 (-4.8 %) and 0.3±3.3 × 1015 molecules cm-2 (5.8 %) when averaged at finer (0.05° × 0.05°) and coarser (0.15° × 0.15°) spatial resolution. The three satellite products (NASA OMI, QA4ECV OMI, and TROPOMI) differed more when evaluating tropospheric column HCHO retrievals. Noise in the HCHO retrievals, likely due to low signal-to-noise ratios and the fact the UV/VIS measurement sensitivity at shorter wavelengths used in HCHO retrievals are low in the troposphere, resulted in low correlations and high oscillation/variability in bias (bias standard deviation) in all three satellite products, with campaign-averaged median biases of 5.1±7.8 × 1015 molecules cm-2 (38.7 %), 2.3±8.9 × 1015 molecules cm-2 (17.3 %), 1.9±6.7 × 1015 molecules cm-2 (12.9 %), and 2.9±4.9 × 1015 molecules cm-2 (23.1 %) for NASA OMI, QA4ECV OMI, and TROPOMI at finer and coarser spatial resolution, respectively. Spatially-averaging TROPOMI tropospheric column HCHO, along with NO2 and FNRs, to coarser resolutions similar to OMI native pixel size proved to reduce the bias standard deviation of the retrieval data. While large median biases, and enhanced variability in bias, were derived for HCHO, errors in both NO2 and HCHO tropospheric columns tended to offset as all three satellite products compared well to observed FNRs with campaign-averaged median biases from NASA OMI, QA4ECV OMI, and TROPOMI of 0.4±3.8 (11.0 %), -0.2±3.3 (-5.4 %), and 0.4±2.3 (13.0 %), respectively. While satellite-derived FNRs had minimal campaign-averaged median biases, the statistical analysis shows that all satellite FNR values still had large bias standard deviation due to unresolved errors in satellite retrievals of HCHO. This result is important as accurate retrievals (minimal median biases) of FNRs from satellites do not suggest the accuracy of the underlying proxy species. The reduction in noise in satellite retrievals of HCHO with additional calibration and improved sensor design and/or improved a priori information of the vertical profiles of HCHO in the troposphere to avoid the impact of the low measurement sensitivity in the shorter UV/VIS wavelengths used to retrieve HCHO is critical for reducing unresolved biases in satellite retrievals of FNRs. Furthermore, this work demonstrates the large impact of a) a priori vertical profiles of NO2 and HCHO for calculations of Air Mass Factors in tropospheric column trace gas retrievals in both OMI and TROPOMI, b) spatiotemporal averaging to increase signal-to-noise, and c) different retrieval algorithms on retrieval errors. Finally, the novel diurnal information of tropospheric FNRs that is expected to be provided by the upcoming NASA geostationary sensor Tropospheric Emissions: Monitoring of Pollution (TEMPO) is investigated and compared to low earth orbiting sensors currently applied to investigate tropospheric FNRs.