1. Introduction

In recent decades, the Asian continent has experienced significant aerosol pollution due to industrial development and increasing anthropogenic activity (Streets et al., 2000; Chung et al., 2005; Ohara et al., 2007;Ramanathan et al., 2007; Zhu et al., 2012). Significant aerosol pollution, frequent haze days (Wu et al., 2010),decreased hours of average sunshine (Guo and Ren,2006), and decreased visibility (Che et al., 2007; Zhang X. Y. et al., 2012; Han et al., 2013) have seriously affected transportation safety, aviation, and human health.

Air pollution is caused by different kinds of concentration and distribution of aerosols depend on variations in the aerosol emissions (IPCC, 2007, 2014)and the dry deposition and wet scavenging processes in the atmosphere (Zhao et al., 2003; Liu et al., 2011). In addition to these aerosol sources and sinks, the processes of air transport and diffusion via atmospheric circulation play a crucial role in the concentration and distribution of aerosols (e.g., Bao et al, 2008; Yoon et al., 2010; Zhang et al., 2010; Liu et al., 2011; Yan et al., 2011; Zhu et al.,2012; An et al., 2015).

Aerosols vary on different timescales. Using the monthly mean aerosol optical depth (AOD) data obtained from the Moderate Resolution Imaging Spectroradio-meter (MODIS), with a rotated principal component analysis, Bao et al. (2008) revealed that a unique spatial structure exists in the seasonal and interannual variability of AOD and this variability is shown to be closely related to the wind strength at 850 hPa. By using satellite data from MODIS, observational data from the Aerosol Robotic Network (AERONET), and precipitation data from the Global Precipitation Climatology Project (GPCP), Yoon et al. (2010) investigated the transport of aerosols by monsoon circulation and suggested that the AOD increases by 40%–50% and precipitation decreases significantly over the downwind region of China, including the Yellow River, the Korean Peninsula, and the East China Sea.

The aerosols and monsoon circulation patterns interact with each other. On the one hand, the monsoon has profound impacts on the aerosol distribution on different spatial and temporal scales (Zhao et al., 2003; Liu et al.,2011; Yan et al., 2011; Zhu et al., 2012; An et al., 2015).On the other hand, the aerosols, of course, may also contribute to feedback processes driving monsoon circulation variations (e.g., Zhang H. et al., 2009, 2012; Wang et al., 2015; Wu et al., 2015). However, in the present paper, we do not pay attention to the feedback issues. Instead,we focus only on the monsoon influences on aerosols.

Monsoon circulation strongly affects the distribution of aerosols, which can be revealed by use of observation data. Relative to the AOD over northern East Asia, the AOD over the southern parts of East Asia is higher during strong Indian summer monsoon years and lower during weak Indian summer monsoon years, because of the mechanisms of transmission and diffusion associated with the Indian monsoon circulation (Liu et al., 2011).Thus, monsoon circulation variations have significant impacts on the aerosol changes. The changes in the East Asian summer monsoon may lead to different influences on the AOD at various geographical locations via circulation transport, as reported in an observational study using satellite data from MODIS and NCEP/NCAR reanalysis by An et al. (2015).

Influences of the Asian monsoon on aerosol variations have also been widely studied by using numerical models, such as the regional coupled climate–chemistry/aerosol model (RegCM3). Rather than local emissions or dry and wet deposition processes, monsoon circulation has been found to be a predominant factor that determines the regional AOD distribution (Yan et al., 2011). In a simulation study using the GEOS-Chem (GEOS: the Goddard Earth Observing System) model with consideration of sulfate, nitrate, ammonium, black carbon, and organic aerosols, the concentration of surface-layer East China is found to increase by 17.7% in the weakest summer monsoon years than in the strongest summer monsoon years, indicating that the weakened East Asian summer monsoon can induce increases in the aerosol concentration via altering the atmospheric circulation(Zhu et al., 2012). Monsoon circulation also influences the aerosol distribution related to wet and dry carrying out numerical experiments with the Northern Aerosol Regional Climate Model (NARCM), Zhao et al. (2003) found that dust aerosols were primarily removed via dry deposition in the vicinity of their sources and via wet deposition associated with precipitation during the inter-Pacific transport path over the regions downwind of the sources.

The above discussion reveals that most previous studies have focused on the Asian monsoon’s strong influences on aerosol changes in Asia, especially in East China. However, as the Asian monsoon regime consists of two different but related subsystems, i.e., the East Asian monsoon system and the Indian monsoon system(Tao and Chen, 1985; Huang et al., 1998; Chen and Huang, 2006), it is interesting to discover different features of aerosol changes between China and India. In particular, during the seasonal cycle, what are the similarities and differences of aerosol variations between China and India? What are the relationships between aerosol changes and other meteorological quantities? Do the East Asian monsoon and the Indian monsoon have different impacts on aerosol properties? These questions have not been fully answered until now. In the present study, we try to answer these questions.

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