## 1 Introduction  
Air pollution has become a global environmental problem, because of its great adverse impact on human health and the climate [1]. China, especially eastern China, has been facing serious environmental issues like the particulate matter (PM) pollution [2], due to the rapidly expanding economic and industrial developments, causing a prominent increase in aerosol loading over this area [3; 4; 5]. Aerosols can act as cloud condensation nuclei (CCN) or ice nuclei (IN) and have a significant influence on precipitation through aerosol-cloud interactions (ACI). The aerosol effects have been broadly researched based on data from in-situ measurements [6; 7; 8; 9; 10; 11] to spaceborne satellite [12; 13; 14; 15]. It has been reported that, given sufficient contact IN due to air pollutions, more ice particles (IP) can be formed, leading to more frequent glaciation of supercooled clouds as the ice crystals grow rapidly at the expense of the droplets in a high ice supersaturated environment, so that more precipitation is produced [16]. On the other hand, if no ice nuclei are present, it is believed that anthropogenic air pollution can suppress precipitation, because high aerosol concentrations increase CCN while reduce cloud particle sizes and lead to a narrow droplet spectrum that inhibits collision-coalescence processes [17; 18; 19; 20; 21; 22; 23]. However, other scholars also concluded that aerosols can increase updrafts and cause more precipitation, because increased CCN delay the onset of precipitation and strengthen more freezing of cloud drops as well as associated latent heat release [24; 25; 26; 27; 28; 6]. Recent investigations showed that the aerosol impacts on precipitation are dependent on meteorological conditions or systems. Under varying meteorological systems, aerosols can have different, or even opposite effects. [PERSON] et al. [30] studied three different deep convection systems (a tropical oceanic squall, a mid-latitude continental squall, and a midafternoon sea breeze convection system) and found that raindrops reach the surface earlier in clean environment, while rainfall is suppressed at early stage in polluted environment. However, in the mature stage, continental storm precipitation decreases, marine storm precipitation increases, and the sea breeze convection precipitation is nearly invariable in polluted environment. Evaporative cooling is the key to the increase/decrease of precipitation in polluted environment. Stronger evaporative cooling results in stronger cold pool, causing more intense convergence and more precipitation. [PERSON] et al. [31] used the mesoscale model to study deep convection clouds in Florida and found that, compared to clean environments, polluted conditions with mid to high concentrations of CCNs experienced delayed precipitation events, vigorous convection development, and increased peak precipitation rate, but decreased cumulative precipitation. [PERSON] and [PERSON] [32] pointed out that as CCN concentration increases, surface precipitation from isolated convective cells decreases, but precipitation from multi-cell convection storms increases. Several scientists have also studied the effect of aerosols on the intensity of tropical cyclones (TC) [33; 34; 35; 36]. The studies concluded that pollution aerosols encourage the formation of a large number of cloud droplets, and strengthen the outside convections, but weaken the intensity of the TC. Previous studies focused on the effects of aerosols on clouds and precipitation in mesoscale convective systems (MCS). Recently, scholars turned to examine some synoptic-scale systems, like frontal systems. They found that precipitation reduced slightly near the front but increased farther northward as aerosol concentration was increased [37; 38]. To the authors' knowledge, no study has specifically examined the indirect effects of aerosols on the Mei-Yu frontal system, which is the most important system during the summer in eastern China.