GFIT3: a full physics retrieval algorithm for remote sensing of greenhouse gases in the presence of aerosols

Abstract

Abstract. Remote sensing of greenhouse gases (GHGs) in cities, where high GHG emissions are typically associated with heavy aerosol loading, is challenging due to retrieval uncertainties caused by the imperfect characterization of scattering by aerosols. We investigate this problem by developing GFIT3, a full physics algorithm to retrieve GHGs (CO2 and CH4) by accounting for aerosol scattering effects in polluted urban atmospheres. In particular, the algorithm includes coarse- (including sea salt and dust) and fine- (including organic carbon, black carbon, and sulfate) mode aerosols in the radiative transfer model. The performance of GFIT3 is assessed using high-spectral-resolution observations over the Los Angeles (LA) megacity made by the California Laboratory for Atmospheric Remote Sensing Fourier transform spectrometer (CLARS-FTS). CLARS-FTS is located on Mt. Wilson, California, at 1.67 km a.s.l. overlooking the LA Basin, and it makes observations of reflected sunlight in the near-infrared spectral range. The first set of evaluations are performed by conducting retrieval experiments using synthetic spectra. We find that errors in the retrievals of column-averaged dry air mole fractions of CO2 (XCO2) and CH4 (XCH4) due to uncertainties in the aerosol optical properties and atmospheric a priori profiles are less than 1 % on average. This indicates that atmospheric scattering does not induce a large bias in the retrievals when the aerosols are properly characterized. The methodology is then further evaluated by comparing GHG retrievals using GFIT3 with those obtained from the CLARS-GFIT algorithm (used for currently operational CLARS retrievals) that does not account for aerosol scattering. We find a significant correlation between retrieval bias and aerosol optical depth (AOD). A comparison of GFIT3 AOD retrievals with collocated ground-based observations from AErosol RObotic NETwork (AERONET) shows that the developed algorithm produces very accurate results, with biases in AOD estimates of about 0.02. Finally, we assess the uncertainty in the widely used tracer–tracer ratio method to obtain CH4 emissions based on CO2 emissions and find that using the CH4/CO2 ratio effectively cancels out biases due to aerosol scattering. Overall, this study of applying GFIT3 to CLARS-FTS observations improves our understanding of the impact of aerosol scattering on the remote sensing of GHGs in polluted urban atmospheric environments. GHG retrievals from CLARS-FTS are potentially complementary to existing ground-based and spaceborne observations to monitor anthropogenic GHG fluxes in megacities.

A portable reflected-sunlight spectrometer for CO2 and CH4

“…Zhang et al (2015) estimated 10-20 ppm XCO 2 enhancements for CLARS-FTS measurements of the WP target by calculating the differences between CLARS-FTS Spectralon and LA TCCON measurements. Zeng et al (2021) showed typical enhancements of 20 ppm in XCO 2 and 150 ppb in XCH 4 retrieved from CLARS-FTS measurements with the GFIT3 full physics algorithm. Assuming background mole fractions of 420 ppm for CO 2 and 1900 ppb for CH 4 , these typical enhancements amount to relative variations of 5 % (CO 2 ) and 8 % (CH 4 ), and thus they are greater than our VCD b errors.…”

“…The largest potential for improvement lies in the explicit integration of aerosol scattering in the retrieval algorithm, as shown by Zeng et al (2021). This would most likely improve fit quality, reduce uncertainty and pave the way for reliable measurement of near-ground CO 2 and CH 4 concentrations.…”

“…The spectral analysis methods need to take into account that there is a quasi-vertical light path component from the sun to the reflection point on the ground in addition to the quasi-horizontal component above the city and that light scattering by particles in the atmosphere complicates radiative transfer considerations (Zhang et al, 2015;Zeng et al, 2017Zeng et al, , 2020b. Despite these challenges, the CLARS-FTS has been delivering various insights into the spatiotemporal distribution of CO 2 and CH 4 (and carbon monoxide (CO), nitrous oxide (N 2 O) and water vapor isotopologues) above the LA Basin and the related greenhouse gas emission patterns on the ground (Wong et al, 2015(Wong et al, , 2016He et al, 2019;Zeng et al, 2020aZeng et al, , 2021Zeng et al, , 2023Addington et al, 2021).…”

Optimal estimation retrieval of tropospheric ammonia from the Geostationary Interferometric Infrared Sounder on board FengYun-4B

“…The Levenberg-Marquardt modification of the Gauss-Newton method is used to search the solution. The optimal estimation method has been described thoroughly in Rodgers (2000) and applied in several previous studies by the group (Zeng et al, 2017(Zeng et al, , 2021Natraj et al, 2022;Zeng et al, 2023). The parameters in the state vector to be retrieved from the algorithm are listed in Table 1.…”