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

Abstract. Regular aerosol observations based on well-calibrated instruments have led to a better understanding of the aerosol radiative budget on Earth. In recent years, these instruments have played an important role in the determination of the increase of anthropogenic aerosols by means of long-term studies. Only few investigations regarding long-term trends of aerosol optical characteristics (e.g. aerosol optical thickness (AOT) and Ångström exponent (ÅE)) have been derived from ground-based observations. This paper aims to derive and discuss linear trends of AOT (440, 675, 870, and 1020 nm) and ÅE (440–870 nm) using AErosol RObotic NETwork (AERONET) level 2.0 spectral observations. Additionally, temporal trends of coarse- and fine-mode dominant AOTs (CdAOT and FdAOT) have been estimated by applying an aerosol classification based on accurate ÅE and Ångström exponent difference (ÅED). In order to take into account the fact that cloud disturbance is having a significant influence on the trend analysis of aerosols, we introduce a weighted least squares regression 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 industrializing countries in East Asia (weighted trends; +6.23% yr⁻¹ at Beijing) and active agricultural burning regions in South Africa (+1.89% yr⁻¹ at Mongu). On the other hand, insignificant or negative trends for FdAOTs are detected 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, both increase and decrease of CdAOTs (+3.37% yr⁻¹ at Solar_Village and −1.18% yr⁻¹ at Ouagadougou) are observed depending on meteorological conditions.