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Influence of observed diurnal cycles of aerosol optical depth on aerosol direct radiative effect

by A. Arola, T. F. Eck, J. Huttunen, K. E J Lehtinen, A. V. Lindfors, G. Myhre, A. Smirnov, S. N. Tripathi, H. Yu show all authors
Atmospheric Chemistry and Physics ()
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The diurnal variability of aerosol optical depth (AOD) can be significant, depending on location and dom-inant aerosol type. However, these diurnal cycles have rarely been taken into account in measurement-based estimates of aerosol direct radiative forcing (ADRF) or aerosol direct ra-diative effect (ADRE). The objective of our study was to es-timate the influence of diurnal aerosol variability at the top of the atmosphere ADRE estimates. By including all the possi-ble AERONET sites, we wanted to assess the influence on global ADRE estimates. While focusing also in more detail on some selected sites of strongest impact, our goal was to also see the possible impact regionally. We calculated ADRE with different assumptions about the daily AOD variability: taking the observed daily AOD cycle into account and assum-ing diurnally constant AOD. Moreover, we estimated the cor-responding differences in ADREs, if the single AOD value for the daily mean was taken from the the Moderate Reso-lution Imaging Spectroradiometer (MODIS) Terra or Aqua overpass times, instead of accounting for the true observed daily variability. The mean impact of diurnal AOD variability on 24 h ADRE estimates, averaged over all AERONET sites, was rather small and it was relatively small even for the cases when AOD was chosen to correspond to the Terra or Aqua overpass time. This was true on average over all AERONET sites, while clearly there can be much stronger impact in in-dividual sites. Examples of some selected sites demonstrated that the strongest observed AOD variability (the strongest morning afternoon contrast) does not typically result in a sig-nificant impact on 24 h ADRE. In those cases, the morning and afternoon AOD patterns are opposite and thus the impact on 24 h ADRE, when integrated over all solar zenith angles, is reduced. The most significant effect on daily ADRE was induced by AOD cycles with either maximum or minimum AOD close to local noon. In these cases, the impact on 24 h ADRE was typically around 0.1–0.2 W m −2 (both positive and negative) in absolute values, 5–10 % in relative ones.

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