Trend analysis of aerosol optical thickness and Ångström exponent derived from the global AERONET spectral observations

[摘要] Regular aerosol observations based on well-calibrated instruments have ledto a better understanding of the aerosol radiative budget on Earth. Inrecent years, these instruments have played an important role in thedetermination of the increase of anthropogenic aerosols by means oflong-term studies. Only few investigations regarding long-term trends ofaerosol optical characteristics (e.g. aerosol optical thickness (AOT) andÅngström exponent (ÅE)) have been derived from ground-basedobservations. This paper aims to derive and discuss linear trends of AOT(440, 675, 870, and 1020 nm) and ÅE (440–870 nm) using AErosol ROboticNETwork (AERONET) level 2.0 spectral observations. Additionally, temporaltrends of coarse- and fine-mode dominant AOTs (CdAOT and FdAOT) have beenestimated by applying an aerosol classification based on accurate ÅE andÅngström exponent difference (ÅED). In order to take intoaccount the fact that cloud disturbance is having a significant influence onthe trend analysis of aerosols, we introduce a weighted least squaresregression depending on two weights: (1) monthly standard deviation(σ_t) and (2) number of observations per month (n_t).

Temporal increase of FdAOTs (440 nm) prevails over newly industrializingcountries in East Asia (weighted trends; +6.23% yr⁻¹ at Beijing) andactive agricultural burning regions in South Africa (+1.89% yr⁻¹ atMongu). On the other hand, insignificant or negative trends for FdAOTs aredetected over Western Europe (+0.25% yr⁻¹ at Avignon and−2.29% yr⁻¹ at Ispra) and North America (−0.52% yr⁻¹ for GSFC and−0.01% yr⁻¹ at MD_Science_Center). Over desert regions, bothincrease and decrease of CdAOTs (+3.37% yr⁻¹ at Solar_Village and−1.18% yr⁻¹ at Ouagadougou) are observed depending on meteorologicalconditions.

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