Multidecadal trend analysis of in situ aerosol radiative properties around the world
Authors: Collaud Coen Martine , Andrews Elisabeth , Alastuey Andrés , Arsov Todor Petkov , Backman John , Brem Benjamin T. , Bukowiecki Nicolas , Couret Cédric , Eleftheriadis Konstantinos , Flentje Harald , Fiebig Markus , Gysel-Beer Martin , Hand Jenny L. , Hoffer András , Hooda Rakesh , Hueglin Christoph , Joubert Warren , Keywood Melita , Kim Jeong Eun , Kim Sang-Woo , Labuschagne Casper , Lin Neng-Huei , Lin Yong , Lund Myhre Cathrine , Luoma Krista , Lyamani Hassan , Marinoni Angela , Mayol-Bracero Olga L. , Mihalopoulos Nikos , Pandolfi Marco , Prats Natalia , Prenni Anthony J. , Putaud Jean-Philippe , Ries Ludwig , Reisen Fabienne , Sellegri Karine , Sharma Sangeeta , Sheridan Patrick , Sherman James Patrick , Sun Junying , Titos Gloria , Torres Elvis , Tuch Thomas , Weller Rolf , Wiedensohler Alfred , Zieger Paul , Laj Paolo 
Published: July 2020

Abstract. In order to assess the evolution of aerosol parameters affecting climate
change, a long-term trend analysis of aerosol optical properties was
performed on time series from 52 stations situated across five continents.
The time series of measured scattering, backscattering and absorption
coefficients as well as the derived single scattering albedo, backscattering
fraction, scattering and absorption Ångström exponents covered at
least 10 years and up to 40 years for some stations. The non-parametric
seasonal Mann–Kendall (MK) statistical test associated with several
pre-whitening methods and with Sen's slope was used as the main trend analysis method. Comparisons with general least mean square associated with autoregressive bootstrap (GLS/ARB) and with standard least mean square analysis (LMS) enabled confirmation of the detected MK statistically
significant trends and the assessment of advantages and limitations of each
method. Currently, scattering and backscattering coefficient trends are
mostly decreasing in Europe and North America and are not statistically
significant in Asia, while polar stations exhibit a mix of increasing and
decreasing trends. A few increasing trends are also found at some stations
in North America and Australia. Absorption coefficient time series also
exhibit primarily decreasing trends. For single scattering albedo, 52 % of
the sites exhibit statistically significant positive trends, mostly in Asia,
eastern/northern Europe and the Arctic, 22 % of sites exhibit statistically significant negative trends, mostly in central Europe and central North
America, while the remaining 26 % of sites have trends which are not statistically significant. In addition to evaluating trends for the overall
time series, the evolution of the trends in sequential 10-year segments was also analyzed. For scattering and backscattering, statistically significant
increasing 10-year trends are primarily found for earlier periods (10-year trends ending in 2010–2015) for polar stations and Mauna Loa. For most of
the stations, the present-day statistically significant decreasing 10-year trends of the single scattering albedo were preceded by not statistically
significant and statistically significant increasing 10-year trends. The effect of air pollution abatement policies in continental North America is
very obvious in the 10-year trends of the scattering coefficient – there is a shift to statistically significant negative trends in 2009–2012 for all
stations in the eastern and central USA. This long-term trend analysis of aerosol radiative properties with a broad spatial coverage provides insight
into potential aerosol effects on climate changes.