Observational Requirements for Quantifying the DiurnalCycle of XCO2 From Space

Abstract

Spatiotemporal variations in atmospheric carbon dioxide (CO2) provide a means to quantifysurface fluxes of carbon over a range of space and timescales. NASA's Orbiting Carbon Observatory‐3(OCO‐3), aboard the International Space Station, is the first CO2‐monitoring mission to observe the sunlitportion of the diurnal cycle of total column‐averaged CO2 (XCO2) from space, since OCO‐3 collects data atvarious times of the day. Previous analysis of the climatological diurnal cycle in XCO2 measured from a ground‐based spectrometer in the Total Carbon Column Observing Network (TCCON) suggests that the XCO2 diurnalcycle provides information about local fluxes. Here, we examine the diurnal signal at four TCCON sitesspanning the tropics through midlatitudes. The signal is typically less than 1 ppm even at the peak of the growingseason. Because relatively sparse OCO‐3 data observes a diurnal cycle at a given location only across multipledays, mesoscale transport variations complicate detection of the diurnal signal from the space‐based record. Webootstrap the long‐term records of TCCON XCO2 to quantify the minimum number of OCO‐3 observationsnecessary to infer the climatological diurnal cycle of XCO2, and find that even during the peak growing season, oforder 10 observations per daylight hour for each month are required for robust detection. The number ofrequired observations increases outside the growing season when fluxes are weaker. Our results show that denseand long‐term observation are required to infer the diurnal cycle from OCO‐3 or future CO2‐monitoring satellitemissions.