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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. [ABSTRACT FROM AUTHOR]

The weekday-weekend effect of anthropogenic emissions in cities, driven by the associated weekly changes in human activities, provides a unique opportunity to assess the sensitivity of observation networks (e.g., ground-based and space-borne instruments) on urban emissions. In this study, we focus on the weekly cycle amplitudes of nitrogen dioxide (NO 2), carbon monoxide (CO), and carbon dioxide (CO 2) in the Los Angeles (LA) megacity, where a significant weekly cycle of human activities exists. In addition, abundant observations are being produced continuously from existing ground-based, mountaintop, and satellite platforms to monitor carbon emissions and air quality in LA. From our analysis, significant agreement can be found in observations from different platforms. For NO 2 , a 30%–35% Sunday decline relative to mid-week mixing ratios can be observed from both ground-based and satellite observations. For CO, the Sunday drops from ground-based, mountain-top and satellite observations are 13%–20%. The TROPOMI instrument with its high spatial resolution provides detailed spatial information on the reduction of tropospheric NO 2 and CO columns on Sundays. The spatial pattern is in good agreement with traffic density in LA. Impact due to the prevailing winds from the coast in the afternoon can also be observed. For anthropogenic CO 2 , we show that the weekly cycle of XCO 2 enhancement above background from OCO-2 observations has a Sunday decline (15%–20%) consistent with ground-based observations and TCCON. This weekly pattern of CO 2 in a megacity directly detected by OCO-2 is reported for the first time. In addition, we also investigate the weekly cycles in the stable carbon isotopic composition of CO 2 (δ13C) from ground-based observations, which demonstrates the weekly variation in fossil fuel usage in LA. Finally, using the COVID-19 lockdown period as an example of a short-term perturbation on anthropogenic emissions, we found that the weekly cycle amplitude became larger during the lockdown period primarily because of the traffic volume changes in light-duty vehicles. This study highlights the consistencies and effectiveness of existing observing platforms in monitoring the anthropogenic emissions of the LA megacity. • Weekly cycles of observed atmospheric NO 2 , CO, CO 2 , and δ13C in Los Angeles megacity are assessed. • Weekly cycles from ground-based, mountaintop, and satellite measurements are comparable. • A weekly pattern for CO 2 in a megacity directly detected by OCO-2 is reported for the first time. • Existing observing platforms are consistent in monitoring carbon emissions of LA. [ABSTRACT FROM AUTHOR]

Atmospheric carbon monoxide (CO) is an effective tracer for monitoring atmospheric transport processes and for detecting pollution sources of anthropogenic origin. However, very few observation systems exist that are capable of providing measurements with high spatial and temporal resolution to identify hotspots for emission control purposes. Here we introduce a mountain-top remote sensing observatory, the California Laboratory for Atmospheric Remote Sensing (CLARS), for mapping the enhancement of CO column-averaged mixing ratio (XCO) over the Los Angeles (LA) megacity. Compared to conventional observation network, CLARS is unique in the following ways: (1) it mimics a geostationary satellite observatory for LA with approximately hourly- and kilometer-scale mapping capability; (2) the free tropospheric background atmosphere is simultaneously measured; and (3) the measurements are highly sensitive to anthropogenic emissions due to the long light path along the planetary boundary layer (PBL). The CO slant column density and XCO are retrieved from reflected sunlight measurements in the 2.3 μm CO band and the 1.27 μm oxygen (O 2) band. Data filtering and corrections for aerosol scattering and geometric effects are then implemented to derive the XCO enhancement, which is the XCO excess in the PBL compared to the background value. In the LA megacity, the XCO enhancement shows a distinctive diurnal cycle primarily driven by changes in anthropogenic emissions and sea-breeze circulation. Such diurnal patterns can be reproduced by the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). The enhancement also shows a significant weekly cycle resulting from the weekly pattern in anthropogenic CO emissions. On average, the XCO enhancements on Sunday and Saturday are 16.1% and 4.4%, respectively, lower than weekday values. The weekly XCO enhancement patterns also show high correlation with traffic counts. A seasonal pattern of XCO enhancement with high (low) spatial contrast in summer (winter), resulting from changing sea-breeze circulation, can be observed. These diurnal, weekly, and seasonal patterns of XCO enhancement serve as tracers of the atmospheric pulse of the LA megacity. The CLARS observatory can serve as a testbed for future geostationary missions to track anthropogenic emissions in cities. • A mountaintop remote sensing observatory for mapping XCO in LA is introduced; • Maps of XCO enhancement at hour- and kilometer-scales are generated; • The diurnal, weekly, and seasonal pulses of XCO enhancement are investigated; • WRF-Chem simulations reproduce the observed spatial XCO patterns; • The observatory is a testbed for future geostationary missions to track emissions. [ABSTRACT FROM AUTHOR]