Estimation of Nighttime Aerosol Optical Depths Using Atmospheric Infrared Sounder Longwave Radiances.

Aerosol remote sensing typically relies on reflected shortwave radiation and thus lacks nighttime retrievals. Here we made an original attempt to retrieve nighttime aerosol optical depth (AOD) by utilizing longwave measurements in the atmospheric window region from the Atmospheric InfraRed Sounder (AIRS) instrument. A machine‐learning based algorithm is developed using AIRS longwave radiance and auxiliary data as the input and AOD from Moderate Resolution Imaging Spectroradiometer (MODIS) as well as reanalysis surface temperature as the output. Independent validation indicates good agreement with lunar AOD derived from surface photometers. An overall increase in nighttime AOD compared to daytime is also uncovered, which is further corroborated by surface and space lidar measurements. The theoretical basis of the algorithm is further verified using radiative transfer simulations. Our approach substantially extends the potential of hyperspectral longwave measurements and offers valuable insights into nighttime aerosol properties. Plain Language Summary: Aerosol optical depth (AOD) is an important parameter for understanding the effects of aerosols on the Earth's atmosphere and air quality. However, current satellite‐based methods for estimating AOD mainly rely on reflected sunlight, thus lacking the capability of deriving aerosol properties at night. In this study, we develop a novel machine‐learning based method for estimating nighttime AOD using longwave far infrared measurements from the Atmospheric InfraRed Sounder (AIRS) satellite instrument. Our algorithm shows satisfactory performance with good agreements with surface AOD derived from moonlight, as well as from space lidar. Through radiative transfer simulations, we were able to verify the theoretical basis of our method and establish a relationship between AOD and the longwave measurements. This new approach greatly expands our ability to estimate AOD during nighttime and provides valuable insights into day‐time aerosol variability. Key Points: Nighttime AOD is successfully retrieved from satellite longwave radiance measurementsResults agree well with AERONET lunar AOD and CALIPSO nighttime AODNighttime AOD is found to be overall higher than daytime AOD globally [ABSTRACT FROM AUTHOR]