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Improvement of OMI ozone profile retrievals by simultaneously fitting polar mesospheric clouds
Bak, Juseon,Liu, Xiong,Kim, Jae H.,Deland, Matthew T.,Chance, Kelly Copernicus GmbH 2016 Atmospheric measurement techniques Vol.9 No.9
<P><p><strong>Abstract.</strong> The presence of polar mesospheric clouds (PMCs) at summer high latitudes could affect the retrieval of ozone profiles using backscattered ultraviolet (UV) measurements. PMC-induced errors in ozone profile retrievals from Ozone Monitoring Instrument (OMI) backscattered UV measurements are investigated through comparisons with Microwave Limb Sounder (MLS) ozone measurements. This comparison demonstrates that the presence of PMCs leads to systematic biases for pressures smaller than 6<span class='thinspace'></span>hPa; the biases increase from ∼ −2<span class='thinspace'></span>% at 2<span class='thinspace'></span>hPa to ∼ −20<span class='thinspace'></span>% at 0.5<span class='thinspace'></span>hPa on average and are significantly correlated with brightness of PMCs. Sensitivity studies show that the radiance sensitivity to PMCs strongly depends on wavelength, increasing by a factor of ∼ <span class='thinspace'></span>4 from 300 to 265<span class='thinspace'></span>nm. It also strongly depends on the PMC scattering, thus depending on viewing geometry. The optimal estimation-based retrieval sensitivity analysis shows that PMCs located at 80-85<span class='thinspace'></span>km have the greatest effect on ozone retrievals at ∼ <span class='thinspace'></span>0.2<span class='thinspace'></span>hPa ( ∼ <span class='thinspace'></span>60<span class='thinspace'></span>km), where the retrieval errors range from −2.5<span class='thinspace'></span>% with PMC vertical optical depth (POD) of 10<sup>−4</sup> to −20<span class='thinspace'></span>% with 10<sup>−3</sup> POD at backscattering angles. The impacts increase by a factor of ∼ <span class='thinspace'></span>5 at forward-scattering angles due to stronger PMC sensitivities. To reduce the interference of PMCs on ozone retrievals, we perform simultaneous retrievals of POD and ozone with a loose constraint of 10<sup>−3</sup> for POD, which results in retrieval errors of 1-4 × 10<sup>−4</sup>. It is demonstrated that the negative bias of OMI ozone retrievals relative to MLS can be improved by including the PMC in the forward-model calculation and retrieval.</p> </P>
Kwon, Hyeong-Ahn,Park, Rokjin J.,Jeong, Jaein I.,Lee, Seungun,Gonzá,lez Abad, Gonzalo,Kurosu, Thomas P.,Palmer, Paul I.,Chance, Kelly Copernicus GmbH 2017 Atmospheric Chemistry and Physics Vol.17 No.7
<P>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. </P>