Abstract
Abstract. We examine upcoming geostationary satellite observations of formaldehyde (HCHO) vertical column densities (VCDs) in East Asia and the retrieval sensitivity to the temporal variation of air mass factors (AMFs) considering the presence of aerosols. Observation system simulation experiments (OSSE) were conducted using a combination of a global 3-D chemical transport model (GEOS-Chem), a radiative transfer model (VLIDORT), and a HCHO retrieval algorithm developed for the Geostationary Environment Monitoring Spectrometer (GEMS), which will be launched in 2019. Application of the retrieval algorithm to simulated hourly radiances yields the retrieved HCHO VCDs, which are then compared with the GEOS-Chem HCHO VCDs as true values for the evaluation of the retrieval algorithm. In order to examine the retrieval sensitivity to the temporal variation of AMF, we examine three AMF specifications, AMFm, AMFh, and AMFmh, using monthly, hourly, and monthly mean hourly input data for their calculation, respectively. We compare the retrieved HCHO VCDs using those three AMFs and find that the HCHO VCDs with AMFh are in better agreement with the true values than the results using AMFmh and AMFm. AMFmh reflects diurnal variation of planetary boundary layer and other meteorological parameters, so that the results with AMFmh show a better performance than those with AMFm. The differences between AMFh and AMFm range from −0.76 to 0.74 in absolute value and are mainly caused by temporal changes in aerosol chemical compositions and aerosol vertical distributions, which result in −27 to 58 and −34 to 43 % changes in HCHO VCDs over China, respectively, compared to HCHO VCDs using AMFm. We apply our calculated AMF table together with OMI aerosol optical properties to OMI HCHO products in March 2006, when Asian dust storms occurred, and find −32 to 47 % changes in the retrieved HCHO columns due to temporal changes in aerosol optical properties in East Asia. The impact of aerosol temporal variability cannot be neglected for future geostationary observations.
Highlights
Formaldehyde (HCHO) is mainly produced by the oxidation of hydrocarbons with minor direct emissions from fuel combustion, vegetation, and biomass burning (DiGangi et al, 2012)
We examined the sensitivity of retrieved HCHO vertical column densities (VCDs) to air mass factors (AMFs) temporal specifications
Retrieved VCDs with three AMF specifications were consistent with the true values, but the result using AMFh showed the best agreement with the true values
Summary
Formaldehyde (HCHO) is mainly produced by the oxidation of hydrocarbons with minor direct emissions from fuel combustion, vegetation, and biomass burning (DiGangi et al, 2012). Successive instruments, including SCIAMACHY (Wittrock et al, 2006), OMI (Kurosu et al, 2004; González Abad et al, 2015), GOME-2 (De Smedt et al, 2012), and OMPS (Li et al, 2015; González Abad et al, 2016), onboard sun-synchronous satellites have observed global HCHO column concentrations with re-visiting between 1 and 6 days. VLIDORT yields derivatives of radiances with respect to optical thicknesses of interfering gases that are used to calculate AMFs. we apply our retrieval algorithm to the observed radiances to obtain the satellite observed HCHO columns. We apply our retrieval algorithm to the observed radiances to obtain the satellite observed HCHO columns This retrieval process begins by fitting a simple Lambert– Beer model that explains the absorption of trace gases and the scattering by molecules in the atmosphere to the observed radiances by using a non-linear least square method (Chance et al, 2000). We use AMF values at 346 nm, which is in the middle of the HCHO fitting window
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