Rapid developments of industrialization and urbanization have made anthropogenic aerosols dramatically increased over eastern China in last several decades. The more recent observations (Zhang et al. 2013) show that aerosol mass concentrations in many Chinese large cities are much higher than European and Northern American cities. The heavy aerosols in China not only cause serious environmental hazards but also affect East Asian climate in direct and indirect ways (Li et al. 2010; Sun et al. 2014). Thus, it is essential to investigate the climate impacts of aerosols on East Asian region. To quantify aerosol effects on the climate system, the concept of “radiative forcing (RF)” is commonly used to describe the change in radiation budget that aerosols produce. RFs due to anthropogenic activities, such as greenhouse gases and aerosols, are generally obtained using global climate models. Hence, aerosol RF is usually taken as one of key factors indicating its global and regional impacts and examining aerosol RF is therefore a hot issue for climate community. However, few studies were specially aimed at the time evolution of aerosol RF over East Asia in the context of comparisons with Europe and North America. Therefore scientists at the Institute of Atmospheric Physics (IAP) conducted a study to examine the long-term variation of anthropogenic aerosol RF over East Asia with an emphasis on the role of atmospheric moisture.

IAP scientists first updated the relevant physical processes in the atmospheric general circulation model (AGCM) developed by the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG)/IAP, Chinese Academy of Sciences. Afterwards, anthropogenic aerosol RF was investigated with LASG/IAP AGCM and a long-term simulated aerosol mass dataset provided by NCAR CAM-Chem model. The present global annual mean anthropogenic aerosol RF in this work is comparable to the multi-model mean results in recent IPCC AR5, which show that LASG/IAP model process is reasonable enough for studies associated with aerosol RF.

It's found that:

(1) compared to the pre-industrial era, the calculated strongest global mean direct RF (DRF) of −0.30 W m⁻² at the all-sky top of the atmosphere (TOA) occurs in the 1980s and indirect cloud albedo forcing (CAF) of −0.67 W m⁻² in the 2000s; a maximum atmospheric DRF of 0.48 W m⁻² mainly by black carbon absorption is found in the 2010s; (2) much larger aerosol DRF and CAF values are distributed over East Asia until the 2010s and the negative surface and positive atmospheric DRF in Eastern China is even projected to maintain at a magnitude of 5.0 W m⁻² until the 2030s; (3) increasing East Asian aerosol loading changes anthropogenic aerosol RF centers to lower latitudes in the Northern Hemisphere since the 1980s and this trend is more severe under future mid- and high-range emission scenarios. The results as listed above show that anthropogenic aerosol implications over East Asia are still very notable issues in the near future.

Moreover, the most important and novel result revealed by this work is about the effects of atmospheric moisture on East Asian aerosol DRF. In addition to aerosol loading, aerosol RF is also sensitive to some meteorological factors, especially atmospheric moisture. Atmospheric moisture is an important factor in determining aerosol DRF for some hydrophilic aerosol species including sulfate, nitrate and organic carbon etc. Meanwhile, there are significant differences for climatological atmospheric moisture among East Asia, North America and Europe. During the summer monsoon period, atmospheric moisture is abundant over East Asia, producing favorable conditions that create hygroscopic effects for hydrophilic aerosols. Their study indicates that larger DRF values over East Asia can be partly attributed to relatively higher atmospheric moisture, which remarkably enhances aerosol hygroscopic effect, then strengthens aerosol optical depth and DRF at clear-sky TOA and surface and even influences their long-term changes. The role of atmospheric moisture on aerosol DRF is clearly shown in the following figure. It is seen in the figure that the climatological air moisture is much stronger over East Asia than those over other two regions. Although the maximum sulfate loading occurred over Europe in 1970s, the largest sulfate AOD appears over East Asia in the 2010s. This is because the sulfate hygroscopic effect is stronger and causes higher AOD when sulfate loading over East Asia peaks in the 2010s. Moreover, sulfate is the largest component for anthropogenic aerosols over many eastern China. The total anthropogenic aerosol DRF at clear-sky TOA is therefore similar to the variation of sulfate AOD.

This study shows that present strong aerosol DRF is closely related to regional meteorological fields. In particular, the unique summer monsoon features bring about significant aerosol regional radiative effects. This work also provides valuable reference for the study about the interaction between climatic processes and aerosol effects. The study was published early online in Climate Research in May 2014 at http://www.int-res.com/prepress/c01236.html.