Abstract
Long-term measurements of aerosol optical depth (AOD) from the Aerosol Robotic Network (AERONET) located in Beijing reveal a strong diurnal cycle of aerosol load staged by seasonal patterns. Such pronounced variability is matter of importance in respect to the estimation of daily averaged direct aerosol radiative forcing (DARF). Polar-orbiting satellites could only offer a daily revisit, which turns in fact to be even much less in case of frequent cloudiness. Indeed, this places a severe limit to properly capture the diurnal variations of AOD and thus estimate daily DARF. Bearing this in mind, the objective of the present study is however to evaluate the impact of AOD diurnal variations for conducting quantitative assessment of DARF using Moderate Resolution Imaging Spectroradiometer (MODIS) AOD data over Beijing. We provide assessments of DARF with two different assumptions about diurnal AOD variability: taking the observed hourly-averaged AOD cycle into account and assuming constant MODIS (including Terra and Aqua) AOD value throughout the daytime. Due to the AOD diurnal variability, the absolute differences in annual daily mean DARFs, if the constant MODIS/Terra (MODIS/Aqua) AOD value is used instead of accounting for the observed hourly-averaged daily variability, is 1.2 (1.3) Wm−2 at the top of the atmosphere, 27.5 (30.6) Wm−2 at the surface, and 26.4 (29.3) Wm−2 in the atmosphere, respectively. During the summertime, the impact of the diurnal AOD variability on seasonal daily mean DARF estimates using MODIS Terra (Aqua) data can reach up to 2.2 (3.9) Wm−2 at the top of the atmosphere, 43.7 (72.7) Wm−2 at the surface, and 41.4 (68.8) Wm−2 in the atmosphere, respectively. Overall, the diurnal variation in AOD tends to cause large bias in the estimated DARF on both seasonal and annual scales. In summertime, the higher the surface albedo, the stronger impact on DARF at the top of the atmosphere caused by dust and biomass burning (continental) aerosol. This indicates that the impact on DARFs estimates is sensitive to assumptions of aerosol type and surface albedo.
Highlights
Atmospheric aerosols affect the Earth’s energy budget directly by scattering and absorbing the solar radiation
The correlation coefficients of these two direct aerosol radiative forcing (DARF) can reach to 0.93 in ATM, 0.94 at top of atmosphere (TOA), and 0.96 at SUR, respectively. These results show that instantaneous DARF estimated by Moderate Resolution Imaging Spectroradiometer (MODIS)/terra and MODIS/Aqua aerosol optical depth (AOD) are both in good agreement with that calculated by Aerosol Robotic Network (AERONET) observed AOD
In this study the influence of the diurnal variation in AOD on the MODIS - derived DARFs at the top of the atmosphere, surface and in the atmosphere has been investigated at the AERONET Beijing site, China, during the year of 2010
Summary
Atmospheric aerosols affect the Earth’s energy budget directly by scattering and absorbing the solar radiation. Estimates of DARF regionally still contain significant uncertainties due to complex aerosol sources, strong diurnal variability, and poorly known morphology (Arola et al, 2013). Reducing these uncertainties requires improved understanding of the spatial and temporal distribution of aerosols properties, such as aerosol optical depth (AOD, Adhikary et al, 2008), single scattering albedo (SSA) and asymmetry factor (ASY). As satellite remote sensing remains the only means of observing the large spatial and temporal variability in aerosol properties, satellite observations have been used extensively to perform global estimates of DARF (Chin et al, 2009; Yu et al, 2006). MODIS acquires data over a same site twice per day due to the limited satellite revisit cycle, and the lack of fine temporal samplings of AOD from MODIS cannot allow us to faithfully reproduce the diurnal variations of AOD (Kuang et al, 2015)
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