What can high temporal resolution measurements from geostationary satellites contribute to the estimate of aerosol radiative forcing?

Advances in polar-orbiting satellite measurements during the past decade have significantly contributed to constraining the estimate of aerosol direct radiative forcing in cloud-free conditions. An underlying assumption in such estimates is that the measured aerosol optical depth and other optical properties over the satellite overpass time are well representative of daily average aerosols. However, large daytime variations of AOD can occur on regional scale. Because of the dependence of aerosol direct forcing on solar zenith angle, the daytime variation of aerosol properties doesn't directly translate into that of direct radiative forcing. Our goal is to assess what uncertainty is introduced to the direct forcing calculation by neglecting aerosol daytime variations and what benefits can be brought in by the high temporal resolution measurements from future geostationary satellites. We calculated cloud-free aerosol direct radiative forcing at the top of atmosphere and surface by using hourly aerosol measurements from 45 Aerosol Robotic Network (AERONET) sites over North and South Americas (i.e., within the planned geographical coverage of the Geostationary Coastal and Air Pollution Events or GEO-CAPE mission) in different scenarios. We found that the use of constant AOD observed near Terra or Aqua overpass time for aerosol direct forcing calculations can introduce substantial biases to the instantaneous DRF, depending on sites and season. This suggests that high temporal resolution measurements from geostationary platform are necessary for studying how aerosols influence the evolution of the atmospheric boundary layer and photochemistry. However when integrating over the day, the overall bias in the daily direct forcing becomes smaller. We also found that the use of aerosol observed in early morning or late afternoon generally introduces larger bias in the daily direct forcing than the use of aerosol observed in other time does. Furthermore, the use of two or three hourly aerosol measurements over a day can improve the accuracy of estimated DRF. High temporal resolution measurements from GEO-CAPE could also contribute to the estimate of aerosol direct forcing in cloudy condition and understanding of aerosol-cloud interactions.