Consistent weekly cycles of atmospheric NO2, CO, and CO2 in a North American megacity from ground-based, mountaintop, and satellite measurements

Abstract

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 (NO2), carbon monoxide (CO), and carbon dioxide (CO2) 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 NO2, 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 NO2 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 CO2, we show that the weekly cycle of XCO2 enhancement above background from OCO-2 observations has a Sunday decline (15%–20%) consistent with ground-based observations and TCCON. This weekly pattern of CO2 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 CO2 (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.