Abstract
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.
Highlights
Climate change has been considered a premier global problem in the scientific community for decades
As explained in the instrumental section, Global Atmospheric Watch (GAW) protocol suggests that the σsp and σbsp be measured at low and controlled humidity, and that is the case for almost all stations considered here, except for those in the IMPROVE network which measure at ambient conditions due to their different monitoring goals
Our study found statistically insignificant trends in aerosol loading for both the high-altitude surface site in China (WLG) and in Taiwan (LLN), perhaps because measurements at both these sites span the aerosol optical depth (AOD) increase/decrease periods mentioned by Zhao et al (2017)
Summary
Climate change has been considered a premier global problem in the scientific community for decades. Aerosols have been recognized as an important active climate forcing agent since the 1970s and, in the last IPCC report (IPCC, 2013), the impact of aerosols on the atmosphere was still considered to be one of the most significant and uncertain aspects of climate change projections and, for the first time, decadal trend analysis of in situ aerosol optical properties around the world was reported. Aerosol optical properties are the relevant parameters that determine the radiative forcing of particulate matter. While some of these optical properties are currently measured by satellite (Choi et al, 2019), airborne and ground-based remote sensing (REM) technologies
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